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Earth-Science Reviews 67 (2004) 313–337

Mars chronology: assessing techniques for quantifying

surficial processes

Peter T. Dorana,*, Stephen M. Cliffordb, Steven L. Formana, Larry Nyquistc,Dimitri A. Papanastassioud, Brian W. Stewarte, Neil C. Sturchioa, Timothy D. Swindlef,

Thure Cerlingg, Jeff Kargelh, Gene McDonaldd, Kunihiko Nishiizumii,Robert Poredaj, James W. Ricek, Ken Tanakah

aDepartment of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, IL 60607-7059, USAbLunar and Planetary Institute, Houston, TX, USA

cNASA, Lyndon B. Johnson Space Center, Houston, TX, USAdDivision of Earth and Space Science, CALTECH Jet Propulsion Lab, Pasadena, CA, USA

eDepartment of Geology and Planetary Sciences, University of Pittsburgh, Pittsburgh, PA, USAfLunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

gDepartment of Geology and Geophysics, University of Utah, Salt Lake City, UT, USAhU.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, AZ, USAiSpace Sciences Laboratory, University of California, Berkeley, CA, USA

jDepartment of Earth and Environmental Science, University of Rochester, Rochester, NY, USAkDepartment of Geological Sciences, Arizona State University, Tempe, AZ, USA

Received 22 July 2003; accepted 2 April 2004

Abstract

Currently, the absolute chronology of Martian rocks, deposits and events is based mainly on crater counting and remains

highly imprecise with epoch boundary uncertainties in excess of 2 billion years. Answers to key questions concerning the

comparative origin and evolution of Mars and Earth will not be forthcoming without a rigid Martian chronology, enabling the

construction of a time scale comparable to Earth’s. Priorities for exploration include calibration of the cratering rate, dating

major volcanic and fluvial events and establishing chronology of the polar layered deposits. If extinct and/or extant life is

discovered, the chronology of the biosphere will be of paramount importance. Many radiometric and cosmogenic techniques

applicable on Earth and the Moon will apply to Mars after certain baselines (e.g. composition of the atmosphere, trace species,

chemical and physical characteristics of Martian dust) are established. The high radiation regime may pose a problem for

dosimetry-based techniques (e.g. luminescence). The unique isotopic composition of nitrogen in the Martian atmosphere may

permit a Mars-specific chronometer for tracing the time-evolution of the atmosphere and of lithic phases with trapped

atmospheric gases. Other Mars-specific chronometers include measurement of gas fluxes and accumulation of platinum group

elements (PGE) in the regolith. Putting collected samples into geologic context is deemed essential, as is using multiple

techniques on multiple samples. If in situ measurements are restricted to a single technique it must be shown to give consistent

results on multiple samples, but in all cases, using two or more techniques (e.g. on the same lander) will reduce error. While

0012-8252/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.earscirev.2004.04.001

* Corresponding author. Tel.: +1-312-413-7275; fax: +1-312-413-2279.

E-mail address: [email protected] (P.T. Doran).

P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337314

there is no question that returned samples will yield the best ages, in situ techniques have the potential to be flown on multiple

missions providing a larger data set and broader context in which to place the more accurate dates.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Mars; Chronology; Surficial processes; Dating

1. Introduction 2. Background and key chronology targets

Fig. 1. Martian epochs and the model age uncertainty for their lower

(older) boundaries as determined by Hartmann and Neukum (2001).

Epochs and their crater densities defined in Tanaka (1986) and

Tanaka et al. (1992). The uncertainties arise from discrepancies in

crater density data acquired by various workers and from

uncertainties in the relative crater production flux between the

Moon and Mars. Note that the uncertainty reaches 2 billion years

during the middle part of Mars’ history. The Noachian lower

boundary is not shown as it is largely undefined geologically.

Noachian model ages probably have uncertainties of only a few

hundred million years, as that part of the cratering history is much

better defined relative to the Moon.

The exploration strategy for Mars is focused on the

origins and evolution of possible extinct or extant life,

the evolution of the climate and atmosphere, including

the role of water, the geologic history of the Martian

interior and surface features and the planet’s habit-

ability for human exploration (McCleese et al., 2001).

Accurately assessing the chronology of past Martian

events is critical to answering the questions associated

with this strategy. However, the current surface chro-

nology of Mars is based primarily on crater counting,

and chronological uncertainties are large. As used in

this paper, an absolute age can be expressed in time

units before present, whereas a relative age indicates

only whether a geologic unit or surface feature is

younger or older than another feature. Crater counting

can be both a relative and an absolute chronologic

technique, but the calibration for absolute chronology

for Mars would require the determination of radio-

metric ages of Martian rocks of known provenance.

The commonly used alternative, namely comparison

to the dated lunar crater record, is imprecise at best.

The ability to determine absolute ages of Martian

materials and surface features of known provenance

would be a significant contribution toward the above

exploration goals, particularly with respect to the

history of stable liquid water on or near the Martian

surface. Such data would help to constrain the time

period during which life could have originated and

evolved on Mars, which in turn would help to

constrain models for prebiotic chemistry, protobiology

and potential transfer of life by meteorites from Mars

to Earth.

This paper addresses: (1) key scientific questions

related to Martian chronology; (2) chronological

techniques applicable to Martian materials; (3) pro-

cesses unique to Mars that could be used to obtain

rates, fluxes, and ages; and (4) sampling issues (in situ

and sample return) for the techniques considered.

Geologic mapping of the Martian surface and

crater statistical analysis provide the basis for relative

age determinations and global correlations (Tanaka,

1986; Scott and Tanaka, 1986; Greeley and Guest,

1987; Tanaka and Scott, 1987; Tanaka et al., 1992;

Hartmann and Neukum, 2001; Ivanov, 2001). How-

ever, the absolute chronology of Martian rocks and

events remains highly uncertain (Fig. 1) due to un-

certainty involving the cratering rate and lack of

directly dated samples. Justification for improved

chronology lies not only in improving the Martian

P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337 315

geologic time scale, but also in being able to better

describe the nature of surface processes.

We identify a number of key areas where improved

chronology is needed. These follow in no particular

order.

2.1. Cratering rates and ages

An important objective of Martian chronology

studies is to establish the evolution of the planet,

from formation to potentially recent magmatic activ-

ity. Determining radiometric ages for a few critical

samples would permit calibration of a long-term

cratering flux rate on Mars and its use as a planet-

wide dating tool. Current models attempt to establish

an absolute chronology on Mars via a cratering rate

relative to the Moon, which has a reconstructed

cratering record based on the absolute dating of

returned lunar samples (Hartmann and Neukum,

2001; Wilhelms, 1987). However, uncertainties for

Mars versus the Moon, involving impactor size-fre-

quency populations and velocities through time and

their resulting crater diameters, result in large uncer-

tainties in Martian chronology (see, for example, the

large range in absolute ages for the Hesperian–Am-

azonian boundary in Fig. 1) (Tanaka, 1986; Hartmann

and Neukum, 2001; Ivanov, 2001). Igneous rocks on

surfaces with well defined cratering stratigraphies

would be the most desirable materials for age deter-

mination. We also propose dating large impacts (e.g.

Hellas) by measuring metamorphic/brecciation/recrys-

tallization radiometric ages. Doing so may provide the

potential to provide global stratigraphic markers

(boundaries). Finally, measuring young ( < 1 Ma)

crater ages will address surface gardening and other

resurfacing effects (e.g. Hartmann et al., 2001).

2.2. Thermal/geochemical evolution

The early evolution of Mars is important from

several aspects:

(1) Differentiation of the whole planet may result

in a metal core. Preservation of a molten core and the

development of a dynamo could provide a source for

the observed magnetized blocks on the surface of

Mars (Acuna et al., 1999; Connerney et al., 1999);

(2) An early crust–mantle differentiation of Mars

would result in the transport of incompatible trace

elements, including large-ion lithophile elements, to

the Mars crust. This would have the effect of remov-

ing the heat sources from the interior of the planet and

determining a potentially more rapid subsequent ther-

mal cooling of the interior of the planet, especially if

convection became an active process.

(3) An early melting of the planet could have

resulted in degassing the planet of volatile compo-

nents, including water. Subsequent evolution of the

atmosphere and near surface water would enhance the

development of life but could be affected by the

cratering history on the surface of Mars. For example,

a ‘‘terminal cataclysm’’ on Mars, similar to what has

been established for the Moon, could result in the loss

of the early-formed atmosphere on Mars and also

affect the water preservation on Mars and the subse-

quent evolution of an atmosphere with a different

composition.

Orbital thermal emission spectroscopic (TES)

measurements and in situ alpha proton X-ray spec-

trometer measurements appear to identify an extreme-

ly limited variety of materials (basaltic andesite, basalt

and coarse, gray hematite contained in very small

areas) (McSween et al., 1999; Christensen et al.,

2000a,b). The TES data may well at least be partially

obscured by the fine-grained eolian material that

blankets the surface. However, one conclusion from

the TES data is that there is no extensive chemical

weathering in the dark region of Cimmeria Terra,

where the basaltic component was identified (Chris-

tensen et al., 2000b). However, even if there are

weathered components on the surface of Mars, it

may be possible to determine the mean age and

provenance (crustal versus mantle) of their source

materials. Data from SNC (Mars-derived) meteorites

(e.g. Jagoutz et al., 1994; Lee and Halliday, 1997;

Blichert-Toft et al., 1999; Brandon et al., 2000;

Nyquist et al., 2001) suggest that impact ejecta can

provide samples suitable for determining the geo-

chemical evolution of Mars.

2.3. Atmospheric evolution

The CO2-dominated present atmosphere of Mars is

clearly different, in several respects, from any plausi-

ble original Mars atmosphere. The major constituents

of the Martian atmosphere, CO2 and H2O, are less

abundant now than they evidently were in the past


(Jakosky and Phillips, 2001), and the history of when

the abundance of the gases changed is crucial to

understanding overall planetary history. Also impor-

tant are diagnostic changes in the isotopic composi-

tions of the major atmospheric volatiles N, H and O

(i.e. 15N/14N, D/H, 18O/16O and 17O/16O) as a result of

partitioning between atmospheric and sedimentary

reservoirs and as a result of loss of atmospheric

gaseous species to space. The ratio 15N/14N is of

particular interest because higher values occur in the

atmosphere of Mars than in any plausible primordial

material. This ratio has increased in response to loss of

atmospheric nitrogen to space, so that, in effect, the

history of Martian 15N/14N is the history of exospheric

loss. A comprehensive review of the evolution of

stable isotopes and of noble gases on Mars has been

published recently (Bogard et al., 2001).

In addition, planetary atmospheric isotopic ratios40Ar/36Ar and 129Xe/132Xe vary from one planet to

another, as a result of addition of the decay products

of 40K and 129I, respectively. The evolution of radio-

genic isotope ratios provides unique constraints on

planetary evolution because such ratios depend both

on the history of the atmosphere and on the differen-

tiation history of the solid planet, which is the original

location of the parent elements.

The scientific goal is thus to identify the abundance

of major atmospheric species and the values of key

isotopic ratios as a function of time in the Martian

past. In general, it is important to compare these

records for igneous and sedimentary rocks and as a

function of age.

2.4. Hydrologic evolution

There is considerable geomorphic evidence (e.g.

Fig. 2) that Mars is water-rich and, at various times

throughout its history, substantial quantities of liquid

water have flowed across its surface. The first visual

evidence of fluvial features that were likely carved by

water was obtained by the Mariner mission (Sagan et

al., 1973). Based on a conservative estimate of the

discharge required to erode these features, and the

probable extent of their original source region, Carr

(1986) has estimated that Mars may possess a global

inventory of water equivalent to a global ocean 0.5–1

km deep. The vast bulk of this reservoir is now

thought to reside beneath the planet’s surface as

ground ice and groundwater. Given the probable long

residence time of any water in the Martian crust, it is

expected to be highly mineralized. Salts high in K

and Rb (perhaps suitable for radiometric dating) may

have precipitated from the waters in the associated

fluvial and lacustrine sediments. Thus, K–Ar and

Rb–Sr dating techniques might be applicable to

precipitated salts and aqueously altered sedimentary

rocks.

2.4.1. Valley networks

Although a combination of low atmospheric pres-

sure and subfreezing temperatures preclude the stable

existence of liquid water on the surface of Mars

today, the presence of quasi-dendritic networks of

small valleys (resembling terrestrial runoff channels)

and the degraded state of the planet’s oldest terrains

suggests that the climate may have been considerably

different in the past (Masursky et al., 1977; Pollack et

al., 1987; Carr, 1999). This belief is supported by the

morphologic characteristics of some of the valley

networks, which are suggestive of a distributed

source of water (such as rainfall or snow melt)—an

observation that implies the potential existence of an

Earth-like hydrological cycle on early Mars (Masur-

sky et al., 1977; Pollack et al., 1987). Groundwater

sapping as been presented as an alternative explana-

tion for the networks (Sharp and Malin, 1975; Ahar-

onson et al., 2002). The vast majority of valley

networks occur within the ancient (f 4 billion years

old) heavily cratered Martian highlands (Carr and

Clow, 1981).

2.4.2. Outflow channels

Evidence that Mars is water-rich is also provided by

the outflow channels—huge scoured channels, tens of

kilometers wide and f 102–103 kilometers long that

are incised up to a kilometer into the Martian surface

(Baker et al., 1992). The abrupt emergence of these

channels from regions of collapsed and disrupted

terrain, and the enormous dimensions of the braided

and streamlined forms found within their beds, testify

to an origin by catastrophic floods, apparently fed by

the artesian discharge of subpermafrost groundwater

(Carr, 1979; Baker, 1982). Such discharges may have

been triggered by a wide variety of potential mecha-

nisms and local conditions. For this reason, the outflow

channels are not necessarily diagnostic of the nature of

Fig. 2. Examples of evidence for water on Mars. (1) Portion of MOC image (M15-00538) taken May 8, 2000 and centered near 37.9jS,170.2jW shows gullies emanating from a south-facing wall of a trough in the Gorgonum Chaos region (Malin et al., 2000), (2) portion of Mars

Orbital Camera (MOC) image (AB1-02306) obtained during Mars Global Surveyor (MGS) aerobraking maneuvers on October 18, 1997 at

15:42 PST, centered at 5.5jS, 340.7jW shows a feature hypothesized to be a playa deposit (beyond the black arrow; image credit: NASA/JPL/

Malin Space Science Systems). (3) MOC image (AB1-07707) taken December 29, 1997 at 13:19 PST, centered at 65.1jS, 15.1jW shows

evidence of groundwater seepage (Malin et al., 1997), (4) mosaic of six MOC images acquired between 1999 and 2002 showing evidence of

large-scale flooding Athabasca Vallis in the Cerberus region, south of the Elysium volcanoes (Malin et al., 2002), (5) portion of MOC image

(M03-02709) acquired on July 14, 1999 and centered at 70.7jS, 355.7jW shows similar gullies in a polar pit wall (Malin et al., 1999), (6)

portion of MOC image (E16-00043) acquired in May 1, 2002 shows gullies on the layered north wall of a crater in Newton Basin near 41.8jS,158.0jW (image credit: NASA/JPL/Malin Space Science Systems).


the climate that existed when they were formed and

could well have formed under environmental condi-

tions resembling those of today.

Channel ages, inferred from the density of super-

posed craters, range from the Early Hesperian to as

recently as the Late Amazonian (Tanaka, 1986; Parker

et al., 1989; Mouginis-Mark, 1990; Rotto and Tanaka,

1991; Baker et al., 1992). But as with the case of the

valley networks, the range of uncertainty associated

with these ages is substantial.

2.4.3. Paleolakes/oceans

The possibility that a large ocean once occupied the

northern plains of Mars was first raised by Parker et al.

(1989, 1993), who identified evidence of potential

shorelines in Viking Orbiter images. This interpreta-

tion has recently received additional support from

elevation measurements made by the Mars Orbiter

Laser Altimeter (MOLA). These measurements indi-

cate that at least one of the putative shorelines lies

along a boundary of near constant elevation—a result


that is most easily explained by erosion associated with

a fluid in hydrostatic equilibrium (Head et al., 1998,

1999). Additional evidence in support of a northern

ocean, as well as smaller transient bodies of water

(ranging from lakes to seas) found elsewhere on the

planet, has been identified (Lucchitta et al., 1986;

Nedell et al., 1987; Rice et al., 1988; Baker et al.,

1991; Goldspiel and Squyres, 1991; McKay and

Davis, 1991; Scott et al., 1991a,b; Costard and Kargel,

1995; Cabrol and Grin, 1999, 2001).

There are several ways in which these fluvial

episodes might be dated. They include fluvial sedi-

ments interlayered with lava flows/ash, lava flow

encroaching upon a lake floor, or lava flows cut by

channels. Other situations could involve the fluvial

erosion of stratigraphic marker units such as specific

lithologic bed types or ejecta blankets. Other potential

targets for sampling and dating include carbonates,

evaporites, and clays.

From an astrobiological standpoint, perhaps the

most important chronologic target would be the North

Polar Basin. If an ocean filled this basin on early Mars,

this ocean would have represented a significant frac-

tion of the planet’s total inventory of water. Reliable

dates of the age of any lacustrine sediments present in

this basin would allow a much tighter constraint on

how long this sizable body of liquid water was present

on the surface, a critical factor in understanding how

long conditions may have been suitable for the origin

and evolution of life. The fact that the basin is still very

flat means that at least some of the ocean sediments,

the oldest sediments, are still there. If deep drilling is

ever a possibility on Mars, acquiring at least a basal

date from these sediments would be a high priority.

But even if only shallow sediments are retrieved and

dated, the contribution to our understanding of the

history of Mars would be immense.

2.4.4. Modern, high-latitude ‘‘seepage’’ features

Published images taken by the high-resolution

camera aboard the Mars Global Surveyor spacecraft

have revealed the presence of a new class of rare,

young and apparently fluvial landforms (which in-

clude small gullies, alluvial fans, and debris flows)

that are found emanating from scarps at shallow

(f 100 m) depths at latitudes above 30j in both

hemispheres (Malin and Edgett, 2000a). The youth-

fulness of these features is supported by their fresh

appearance, the lack of craters on surrounding ter-

rain, and their superpositional relationship to sand

dunes and other landforms that are widely recog-

nized as having a young ( < 107 years) age. If these

‘‘seepage’’ features were truly formed by the flow of

liquid water, then both their shallow depth of origin

and their young age pose a significant challenge to

our present understanding of the recent hydrologic

evolution of Mars.

2.5. Tectonics

Evidence of recent volcanism on Mars (Berman

and Hartmann, 2002; Hartmann et al., 1999) permits

the possibility that recent and perhaps ongoing tecto-

nism is also an important part of Martian geology.

Generally, evidence for faulting seems to indicate a

decrease over geologic time, but rates of tectonic

deformation and concomitant levels of seismicity

remain poorly constrained (Banerdt et al., 1992;

Golombek et al., 1992). The absolute calibration of

the Martian cratering history therefore will assist in

providing a much more accurate knowledge of tec-

tonic history.

Dohm et al. (2001a,b) report an investigation of a

comprehensive paleotectonic data set for the western

hemisphere of Mars that shows numerous regional

and local centers of magmatism-related tectonic

activity with varying and often overlapping ages.

They argue that these features may imply distinct

episodes of magmatic activity accompanied by vol-

canic rise and tectonic center development, volcano

construction, volcanic plains formation and exten-

sional fault growth. They argue further that such

regional events may be similar to some observed for

Earth and Venus and may have triggered climatic

variations (Baker et al., 1991). Geochronology of

igneous rocks will be important in clarifying the role

that pulses of magmatic activity may have had in

triggering climatic variations on Mars, and the

influence this may have had on potentially develop-

ing ecosystems.

2.6. Mars system science and global change

Chronology on Mars is unlike chronology on the

moon in that Mars is a dynamic body with an

atmosphere, active wind erosion (e.g. Kuzmin et al.,

Fig. 3. Martian polar layered deposits showing deformed layers and

angular unconformities. Left image is a portion of MOC image SP2-

44503, right image is a portion of SP2-44504. Both images were

obtained in July 1998 and near 84.4jN, 118.2jW (Malin et al.,

1998).


2001), apparently frequent mass movements (e.g.

Sullivan et al., 2001), active periglacial features

(e.g. Mustard et al., 2001), polar ice caps (e.g.

Clifford et al., 2000), recent volcanism (e.g. Hart-

mann et al., 1999) and possibly even the existence

of near surface liquid water (e.g. Malin and Edgett,

2000a). The planet apparently was even more

dynamic in the past, with the potential for an active

hydrologic cycle that may have included periods of

continental glaciation (e.g. Kargel and Strom, 1992;

Kargel et al., 1995). Recent evidence even points to

global climate change occurring today (Malin et al.,

2001a). Therefore, numerous deposits from these

various processes are in place, and assessing their

chronology will yield information on rates of surfi-

cial processes and any cyclicity imbedded in the

global system. This research will be comparable to

ongoing Earth system science investigations and

will allow for independent verification of hypothe-

sized forcings on Earth’s climate. For instance, what

is the cause of glacial/deglacial cycles on Mars if

they exist, and how do they compare to those on

Earth?

As on Earth, the polar ice caps on Mars and layered

terrain (Fig. 3) should be important storehouses of

global change information. On Earth, ice caps provide

a reliable layered sequence of accumulated precipita-

tion and trapped gas, which has been used very

successfully to evaluate atmospheric conditions over

the past ca. 500 ky (e.g. Bender et al., 1994; Petit et al.,

1999). On Mars, the situation may not be so straight-

forward, as the center of the ice caps are not always

areas of net accumulation. Theoretically, we may

expect that the caps accumulate mass during the low

to mid obliquity epochs, and lose mass during high

obliquity. Depending on the overall mass balance, the

caps may disappear completely during high obliquity,

but at the very least we would expect large uncon-

formities in the record (e.g. Murray et al., 2001).

Nevertheless, the preservation of large sequences of

annual accumulation bounded by unconformities can

reasonably be expected in the current caps. Layer

counting akin to that done on Earth (e.g. Alley et al.,

1997; Meese et al., 1997) could be performed within

these sequences given imaging resolution down to

10� 3 to 10� 5 m. Absolute techniques (e.g. radiomet-

ric or cosmogenic) would be needed to put these

sequences in perspective.


3. Applicable dating techniques

Martian planetary history events are datable by one

or more chronometers discussed in the next section.

3.1. Techniques for dating old ( >10 Ma) events

Over the past f 50 years, and especially as part of

the Apollo program, geologists and geochemists have

developed a large suite of geochronological tools

based primarily on the decay of radioactive isotopes

of a number of elements. Dating techniques based on

radionuclides depend on the ability to measure pre-

cisely the isotopic composition of the elements of

interest. In applying these techniques to terrestrial and

extraterrestrial samples, there has been a drive toward

reducing the required sample size as well as improv-

ing the temporal resolution of the techniques through

higher analytical precision. Issues involved in reduc-

ing sample size pertain to reducing blank contribu-

tions from sample processing as well as increasing

analytical sensitivity. These issues are particularly

important when planning for the analysis of Martian

samples because sample size is likely be an important

limiting factor. This is particularly relevant for sample

return but is also of concern for in situ analyses, for

which power and weight requirements of instrumen-

tation will limit the amount and number of samples

that can be analyzed. At present, standard analytical

techniques for the Rb–Sr, Sm–Nd, Lu–Hf, Re–Os,

U–Th–Pb isotopic systems require sophisticated

chemical separation techniques followed by precision

mass spectrometry. Until recently, thermal ionization

mass spectrometry (TIMS) has been the technique of

choice for high precision isotopic analysis. To this

must be added the recent and ongoing development of

multi-collector-inductively coupled plasma source-

mass spectrometry (MC-ICP-MS) (Halliday et al.,

1998). Advancements in techniques of isotopic anal-

ysis, including further development and application of

microbeam techniques such as secondary ion mass

spectrometry and laser ablation MC-ICP-MS, will be

essential for maximizing the scientific return from

limited amounts of sample. The K–Ar system (see

Section 3.2.1) does not require as much chemical

separation or quite as high isotopic precision, since

the noble gas Ar can simply be degassed and experi-

ences large changes in isotopic ratios, but improve-

ments in blanks and sensitivity (including microbeam

techniques) are also needed for this system. Based on

this, we consider that a healthy mix of current state-of-

the-art techniques (and future improvements) will be

necessary for the radiogenic isotope analysis of

returned or in situ samples from Mars. The possible

use of such an extensive ‘‘mix’’ of instrumentation

and laboratory-based expertise is a main driver for

sample return missions.

Standard chronometers based on the decay of

radionuclides depend on the fractionation of the

parent from the daughter element during a natural

(geologic) process, the timing of which is the event

that is dated by that particular chronometer. Further,

the application of these chronometers requires com-

plete isotopic equilibrium at the time of the fraction-

ation event, and weathering or low temperature

alteration can complicate results. Because parent and

daughter isotopes of various chronometer systems

have different geochemical affinities, they are chem-

ically fractionated from each other to varying degrees

by different processes. Therefore, it is highly desirable

to analyze multiple samples with more than one

technique in an attempt to obtain concordant ages

and a clear understanding of the nature of the chem-

ical differentiation processes being dated.

Many currently developed chronometers are based

upon decay of long-lived radioisotopes and can be

applied towards dating igneous crystallization events

older than a few million years. However, there are also

short-lived chronometers (e.g. 146Sm–142Nd, half life

f 103 Ma; 182Hf–182W, half life f 9 Ma) that can

be applied to early planetary events such as core–

mantle separation and crust–mantle differentiation

(Regelous and Collerson, 1996; Schoenberg et al.,

2002). Initial ratio isotopic systematics provided by

several chronometers can also be used to address

larger questions of cumulative crust–mantle differen-

tiation and planetary evolution.

3.2. Techniques for dating young (<10 Ma) events

3.2.1. Potassium–argon and argon–argon dating

K–Ar (including 40Ar–39Ar) dating is based on

the radioactive decay of 40K to 40Ar. Due to the low

abundance of 40K in basalt and andesitic rocks, and

the long half-life of 40K (1.26 billion years), the useful

window for K–Ar dating will be for rocks >105 years


in age. The most promising scenario is to date

volcanic flows whose stratigraphic position is well

enough known to use them to calibrate the absolute

chronology of the surface (see Section 2.1). However,

the system could also be used for dating young

volcanic lava flows, or dating widespread ash depos-

its, for example, in Polar Layered Deposits if ash were

to be found there. Additionally, it is possible that

fluvial events would create K-rich salts that could be

dated by these techniques. For K–Ar age determina-

tions, separate measurements of K and Ar must be

made. For Ar–Ar dating, exposure to a high neutron

fluence is required before measurement. The require-

ment for a high neutron fluence means that Ar–Ar

measurements will have to be made on samples

returned to Earth. K–Ar dating could potentially be

done in situ, though with much less precision than

would be possible on returned samples. For any

measurement, the possibility of inherited atmospheric40Ar will have to be considered and corrections

applied, if possible.

3.2.2. Uranium/thorium–helium dating

U/Th–He dating, based on the buildup of alpha

particles (4He) produced by the decay of actinides,

might be effective on young samples. Lower temper-

atures on Mars mean that He diffusion is less likely

to be a problem than it is on Earth. Impact induced

helium loss may complicate the interpretation of the

U/Th–He age for old surfaces but also will provide

important relative age information in most surficial

deposits. U/Th–He dating requires the determination

of actinide abundance and 4He abundance. Actinides

are trace elements, which makes their detection

more difficult, but, at sufficiently high concentra-

tions they might be detectable through gamma ray

spectroscopy.

3.2.3. Uranium and thorium decay-series methods

The U- and Th-decay-series methods of age deter-

mination are based on radioactive disequilibrium that

occurs when a parent nuclide is separated from its

daughter nuclides by a chemical or physical process

such as melting, crystallization, dissolution, adsorp-

tion, precipitation, evaporation or condensation.

Obtaining information about the time scale of a given

process depends on knowing the initial state of

radioactive (dis)equilibrium and whether the process

is episodic or continuous. Commonly used decay-

series methods exploit nuclides having a wide range

of decay constants (from days through 105 years) and

chemical characteristics (e.g. Th, U, Ra, Pb, Rn), thus

allowing a wide range of geochronologic applications

(reviewed in Ivanovich and Harmon, 1982). On Mars,

specific applications of U- and Th-series methods

could include: (1) determination of ages of young

lava flows or pyroclastic deposits; (2) determination

of ages of waterlain spring deposits, evaporites, hy-

drothermal deposits and hydrothermal alterations; and

(3) determination of the atmospheric residence times

of eolian particles.

Nuclide measurement in U–Th decay series is

normally accomplished by counting decay events

(alpha, beta and gamma) using solid-state detectors

or by mass spectrometry (TIMS or MC-ICP-MS). The

potential for practical in situ applications of U- and

Th-series decay methods on Mars is limited at this

time to those nuclides emitting gamma rays that can

be measured by solid state detectors in the range of

10–2000 keV. Other methods (e.g. alpha counting and

mass spectrometry) require sample dissolution and

wet chemical purification of U, Th or other elements,

followed by electrodeposition onto plates or filaments

prior to measurement, which would be difficult in situ.

For maximum information retrieval using U-series

methods, sample return is preferable.

3.2.4. Cosmogenic nuclides

Cosmogenic nuclides are produced by cosmic ray

nuclear interactions with target nuclei in rocks, soils,

ice and the atmosphere. The cosmogenic nuclides,

both stable noble gases like 3He, 21Ne and 36,38Ar and

radionuclides like 10Be, 14C, 26Al and 53Mn, have

been widely used for investigation of solar system

matter for several decades (Reedy et al., 1983) and are

now routinely used on Earth for dating alluvial fans,

marine terraces, outburst floods, recent volcanic ac-

tivity, eolian deposits and glacial deposits (e.g. Cerl-

ing and Craig, 1994). Cosmogenic nuclides on Mars

could address similar questions. The cosmic ray flux

on Mars is similar to that on the Moon and meteorites,

which is f 1000 times that on the Earth (Masarik and

Reedy, 1995). Stable nuclides build up over time as

the surface is exposed to cosmic rays and integrate

over the entire exposure. The concentrations of cos-

mogenic radionuclides, such as 53Mn (half-life = 3.7

P.T. Doran et al. / Earth-Science322

My), 10Be (1.5 My), 26Al (0.705 My) and 14C (5730

years) only build up until they reach saturation values

after a few half-lives. The 1/e nuclear attenuation

length of cosmic ray interactions on Mars is about

165 g/cm2 cm below 100 g/cm2. In the < 107 year

time window, cosmogenic nuclides can give ages on

events such as young impact craters using exposure

ages of ejecta (e.g. Nishiizumi et al., 1991), erosion

(e.g. by floods, landslides and glaciation), deposition

ages of previously unirradiated materials excavated

from depth (e.g. by impact events, glacial processes

and floods), faulting (tectonic scarps), and young

volcanic events. The study of bedrock surfaces under

a steady state of erosion may give information on

long-term erosion rates of the Martian surface. Depth

profiles of cosmogenic nuclides provide rate and size

of long- and short-term regolith mixing processes

(Langevin and Arnold, 1977). Since all cosmogenic

nuclides studied in meteorites can be measured in

Martian surface rocks, the application potential is

much wider than for terrestrial cosmogenic nuclide

studies.

Cosmogenic nuclides may have their greatest util-

ity in the polar regions. Fine grain material likely

composed of micrometeorites, cosmic dust and wind-

blown surface material gives the Polar Layered

Deposits their distinctive signature. The decay of

cosmogenic radionuclides in dust exposed on the

surface of Mars and then trapped and shielded from

irradiation in Polar Layered Deposits may allow the

determination of the age of the deposits. Another

potential method involves the use of cosmic ray

produced 3He and 21Ne. The 3He/21Ne production

rate ratio in meteorites is well-studied (e.g. Eberhardt

et al., 1966; Masarik et al., 2001). The ratio changes

with cosmic ray shielding conditions as well as with

chemical composition. After deposition in the Polar

Layered Deposits and burial below a depth of about

1–2 m, 3He should begin to diffuse out of the fine-

grained minerals, so the 3He/21Ne ratio will drop. The

major disadvantages to the technique will be the

difficulty in obtaining a mineral separate remotely

and the need to sort out the diffusional release

mechanisms for variable grain size and temperature.

Sample collection and preservation is also a key issue.

Experiments with mineral analogs in the laboratory

may allow for the estimate of the diffusion rate of 3He

to within a factor of two.

3.2.5. Dosimetry-based methods

Luminescence dating is a developing suite of

techniques which on Earth has greatest utility in

dating eolian deposits spanning the past 100 ka

(Aitken, 1998; Forman et al., 2000; Wintle, 1997).

The principle of the method is to quantify the rate of

accumulation of free electrons from exposure to

natural ionizing radiation within mineral grains as a

function of time using the stimulated luminescence as

a metric of the absorbed dose. Over time, ionizing

radiation from the decay of naturally occurring radio-

nuclides and from cosmic rays liberates charge car-

riers within mineral grains. The charge carriers can

subsequently become localized at crystal defects lead-

ing to the accumulation of ‘‘trapped’’ charge popula-

tion. When exposed to solar radiation, the trapped

charge population is depleted, thereby resetting the

clock. Recombination of the charge carriers results in

photon emission (luminescence). The measurement

process can be initiated by thermal or optical stimu-

lation, termed thermoluminescence (TL) and optically

stimulated luminescence (OSL), respectively. The

intensity of the luminescence emitted from the sample

is usually proportional to the dose absorbed since the

last exposure to sunlight, which is proportional to the

time since burial of the sediment grains (i.e. the

depositional age).

The limiting age for the TL or OSL is determined

by saturation of the luminescence signal as a function

of dose and the local ionizing radiation dose rate.

Saturation occurs because there is only a finite con-

centration of defects in the material at which charge

can be localized. For typical dose rates found on Earth

one can expect maximum ages on the order of

100,000 to 300,000 years. Minimum ages are dictated

by sample sensitivity and equipment design issues and

are < 1000 years. On Mars, natural dose rates due to

U, Th and K are anticipated to be lower than on Earth,

but cosmic ray fluxes in near-surface deposits will be

f 1000 times higher. Thus, for shallow Martian

deposits exposed primarily to cosmic rays, the upper

and lower age bounds may be reduced. For deposits

buried rapidly to depths of several meters, irradiation

will be dominantly from the decay of radionuclides

within the deposit, and the maximum age limit may be

increased. However, a challenge for effective applica-

tion of luminescence dating is the need to know the

burial and erosion rates and to calculate the attenua-

Reviews 67 (2004) 313–337


tion of cosmic ray dose in the upper f 2 m. Addi-

tional knowledge of the magnitude of variations in

cosmic dose through geologic time is needed to

resolve credible ages.

Other techniques related to luminescence that may

have applications on Mars are Electron Spin Reso-

nance (ESR) or Electron Paramagnetic Resonance

(EPR). EPR spectroscopy has been applied in broad

ranges of disciplines such as geochronology, geomor-

phology and archaeology. There have been numerous

publications (e.g. Fain et al., 1991) on the subject

which indicate varying degrees of success. The EPR

dating method is based on the dosimetric character-

istics of a sample, which can retain a record of the

natural radioactivity that originated from within the

sample or from the surrounding environment. Nori-

zawa et al. (2000) have recently proposed the use of

ESR to date Martian polar ice.

4. Potential chronology techniques that exploit

processes on Mars

4.1. Variation of atmospheric stable isotopes

The unique isotopic composition of nitrogen in

the Martian atmosphere provides an opportunity to

develop a Mars-specific ‘‘chronometer’’ of potential

importance for tracing the time-evolution of the

atmosphere and of lithic phases in which portions

of the atmosphere become trapped. Theoretical mod-

els (Jakosky and Jones, 1994; Pepin, 1994) predict a

nearly linear increase of d15N from near zero to the

present-day value of 620F 160x (Nier and McEl-

roy, 1977). The rate of change of d15N could be

calibrated by measuring the nitrogen isotopic com-

position of N2 trapped in lithic phases of rocks (e.g.

impact-produced glass) for which the ages were

measured by standard radiometric techniques. Alterna-

tively, N2 may have been trapped in fluid inclusions of

datable mineral phases precipitated from fluids satu-

rated with atmospheric gases. These measurements

would serve to determine the theoretical parameters

used to model the evolution of the nitrogen isotopic

fractionation. Once this had been achieved, measure-

ment of d15N in a nitrate, for example, would determine

the time of nitrate deposition without the necessity of a

direct radiometric age determination. The secular var-

iation in the isotopic composition of other atmospheric

gases (e.g. O, C, H and Ar) could also be used in the

samemanner to determine independent estimates of the

deposition times. This promising approach could build

on measurements on the Martian meteorites as well as

theoretical developments to correlate potential isotope

fractionation effects for different elements.

4.2. Radiometric dating of ice

The probable presence on Mars of various reser-

voirs of ice and/or permafrost provides potential

opportunities to apply radiometric dating techniques

to environments and materials not normally so used in

terrestrial geology. That is, such ice reservoirs are apt

to remain unmelted over periods long enough to allow

buildup of measurable quantities of some radiogenic

decay products, principally 40Ar from decay of 40K in

dust or chemical precipitates. The probable utility of

these methods is currently unknown because neither

the retentivity of Ar in ice at relevant temperatures nor

the abundance of K in candidate materials is known.

Alternatively, if crystals with very low abundances of

radioactive trace elements (e.g. olivine) are trapped in

frozen ‘‘brine’’, then recoil from isotopes decaying

within the ice could produce identifiable nuclear

tracks on the outer skin of these crystals or grains.

These nuclear tracks could be used for dating based

on the surface density of tracks and the concentrations

of U and Th in the ice.

4.3. Extraterrestrial material flux

Measuring the abundance of platinum group ele-

ments (PGE) in Martian soils also would provide a

measure of the accumulated influx of micrometeorites

to the Martian surface. The initial inventory of PGE

on Mars should have been mostly partitioned into the

Martian core at the time of early differentiation of the

planet. The lack of crustal recycling on Mars would

allow the accumulation of PGE solely from micro-

meteorites/extraterrestrial material on the Martian sur-

face over long periods of time. This approach was

used successfully for soil samples returned from the

Moon, as pristine (igneous) rocks on the Moon are

highly depleted in PGE (see Righter et al., 2001; a

recent compilation of the work on PGE). The princi-

pal conclusions of the work were: (1) lunar soils and


breccias are greatly enriched in siderophile elements,

relative to lunar basalts (by factors of 102 to 103); (2)

there is a range of siderophile element enrichments,

with highland sites having higher enrichments than

mare sites; (3) the siderophile abundances in igneous

rocks can be used to differentiate pristine lunar rocks

(unaffected by meteorite impacts) from rocks pro-

duced by impact melts near the lunar surface; and

(4) the relative abundances of siderophile elements

could be used to identify distinct types of impactors.

In contrast to the analytical work on Apollo sam-

ples, which employed instrumental and radiochemical

neutron activation analyses, it is now possible to use

much higher analytical sensitivity as provided by

negative ion TIMS (Creaser et al., 1991; Chen et al.,

1998). These techniques have been applied recently to

lunar samples (Chen et al., 2001). Models for Martian

surface gardening would be required to convert mea-

sured PGE abundances to the total fluence of micro-

meteorites. Again, these data could be compared to

lunar data to compare the meteoroid fluxes to the

surfaces of the two bodies and to obtain information

about relative cratering rates. Analyses of the PGE at

the required sensitivity can only be done on returned

samples.

Measurements of secondary neutron fluence effects

in Martian soils and in rocks exposed near the surface

of Mars could also be done. These measurements

would take advantage of the large neutron capture

cross sections of 157Gd and 155Gd for thermal neu-

trons and of 149Sm for epithermal neutrons. In these

cases, the neutrons are produced in the upper f 2.5 m

of the Mars surface, through the bombardment by the

attenuation of primary galactic cosmic rays. These

effects were used extensively on the Moon to address

the gardening rate of surface materials as well as the

contributions of unirradiated materials excavated from

deeper layers in the Moon (Eugster et al., 1970; Russ

et al., 1971; Woolum et al., 1975; Curtis and Wasser-

burg, 1977).

4.4. Volatile flux rates

The goal of flux investigations is to establish the

rates of volatile transfer from the Martian crust to the

atmosphere. In the near surface (1–10 m), the dom-

inant mechanism of gas transfer will most likely be

molecular diffusion. By establishing the concentration

gradients with depth in the Martian soil for a number

of volatile species, such as He, Rn, CO2 and H2O, the

flux then will follow Fick’s Law (i.e. a diffusing

substance moves from where its concentration is

larger to where its concentration is smaller at a rate

that is proportional to the spatial gradient of concen-

tration). The flux measurement can then be used as a

prospecting tool to define areas of recent hydrologic

processes either in the form of deep saline ground

water or cooled hydrothermal fluids trapped beneath

the ice.

Water and CO2 play an important role in Martian

climate. The major identifiable reservoirs for these

two phases are the polar ice caps. However, there are

indications that the Martian subsurface also may be a

significant repository for water and CO2. Measure-

ment of a water and carbon dioxide flux over the

annual cycle of ice cap growth and shrinkage would

establish the importance of the subsurface reservoir

and whether the deep subsurface is providing a

unidirectional flux of these volatiles to the surface.

In addition, measurement of 14CO2/CO2 in the atmo-

sphere would constrain the exchange rate of CO2

among the atmosphere, ice caps, rocks and soils

(Jakosky et al., 1996).222Radon is a short-lived (half-life = 3.8 days) pro-

duct of 238U decay. A noble gas, it is released from

soils at rates that range between 10% and 90% of the

production. On Earth, Rn is detected at levels between

1 and 1000 Bq/m3 of air with the concentration a

function of the U content, permeability of the soils

and the temperature distribution. Although U concen-

trations on Mars are thought to be about 10 times

lower than on Earth, detectable Rn activity could be

measured by processing sufficient quantities of Mar-

tian soil gas. Because of the short half-life, Rn is

sensitive to processes occurring in the upper few

meters of the crust and would be important in quan-

tifying the rates of transport from the soils.

4.5. Direct dating of Martian organic material

If organic material is found in a Martian sample,

those organic compounds could possibly be dated by

at least two methods. Organic material no more than a

few thousand years old could possibly be dated by 14C

analysis. The age range over which this method would

be useful would depend on the production rate of 14C


in the Martian atmosphere and regolith (Jakosky et al.,

1996). Martian organic material that is biogenic and

contains amino acids could yield chronometric infor-

mation by measuring the extent of amino acid race-

mization (Bada and McDonald, 1995). By analogy

with terrestrial life, a biological system should initially

contain only one enantiomer (either the L- or D-

enantiomer) of amino acids. Once the organism dies,

these amino acids will racemize over time until nearly

equal amounts of L- and D-enantiomers are present.

The rate of this racemization is dependent on the

thermal and water exposure histories of the deposit,

which would have to be known.

5. Sampling

5.1. In situ versus sample return

It bears emphasis that in situ analysis and sample

return are synergistic approaches to understanding

the geology of Mars. In situ techniques could be

used to ‘‘prescreen’’ both landing sites and individ-

ual samples for the sample return missions. Both for

sample return and for in situ investigations, it has

been recognized that surface mobility is highly

desirable, as is the ability to characterize the miner-

alogy and chemistry of components on Mars. Many

potentially interesting sites on Mars display a com-

plicated depositional history and are likely to include

materials from several different geologic units trans-

ported to the site by volcanic, fluvial or impact

processes. Appropriate in situ instrumentation would

provide the opportunity to preselect samples, thereby

improving the likelihood of obtaining a comprehen-

sive set of appropriate samples to meet specific

scientific goals. However, because of the limited

nature of in situ instrumentation and because of

limited analytical capabilities of in situ instruments,

a simple, first sample return consisting of soils and

small rocks (f 0.5 cm in diameter, obtained by use

of a rake) would permit detailed analyses on Earth

and calibration of in situ instrumentation.

In situ chronologic measurements can potentially

offer many advantages for investigation of Mars

surface geology, provided that sufficiently capable

instruments can be developed and deployed. The

immediate, and perhaps most obvious, advantage is

that it is clearly less risky at our current stage of

technology development to pursue one-way, landed

missions, provided that there is a high probability of

meaningful science return. There are also direct sci-

entific advantages to an in situ mission scenario. It is

important for an in situ platform to have capabilities

for multiple measurements and for extensive coverage

of the Martian surface and shallow subsurface. That

is, the measurement can be repeated on the same,

originally sampled surface unit, or further experiments

may be conducted on related materials at the site. The

capability for repeat measurements means that the

results from one measurement can be used to guide

further measurements or follow-on missions. Exten-

sive coverage of the planetary surface is also possible

with in situ missions, through rover-based measure-

ments, multiple landers deployed from a single space-

craft and multiple missions.

There are also some clear disadvantages to mount-

ing an in situ program without sample return. An in

situ instrument will of necessity use the technology

available when the mission was designed, and there-

fore cannot utilize advances in measurement science

that might be developed later. In the same way, the

samples are not available for the application of exten-

sive analysis using other techniques not implemented

for the mission. It is widely believed that in situ

instruments will have limited precision and accuracy,

due to engineering limitations on size, power and

sample preprocessing and preparation. While this is

a general discussion, it is important not to simply talk

of in situ versus return sample missions. One needs to

address the specific instruments involved, their

expected analytical precision and the adequacy of

the instrumentation to address specific science ques-

tion with the needed precision.

5.2. Concerns common to in situ and sample return

Mars poses several problems for chronological

studies performed either in situ or on samples returned

from the planet. These problems include: geological

complexity of the surface, the potential presence of

surface weathering products, the effects of impact

metamorphism and brecciation, and contributions

from surface materials such as dust and atmosphere.

Individual dating techniques are variably sensitive to

these problems.


In order for any chronological measurement to be

scientifically useful, its geological context must be

understood to the greatest extent possible. This would

require answering such questions as: (1) Was the

sampled rock formed in place or transported to its

present position? (2) What is the relationship of the

rock to the surrounding rock, soil or sedimentary units?

(3) What is the (apparent) rock type of the sampled

unit? A high-quality imaging system associated with

the lander or rover is obviously essential for this task.

For example, layer counting will be essential in the

Polar Layered Deposits, the suspected sedimentary

layering on the side walls of canyons and the suspected

volcanic-sedimentary interbedding in other sites.

In addition to imaging, requirements for simple

measurements of the characteristics of the strata can

be anticipated. For the Polar Layered Deposits, such

measurements might include density, conductivity,

dust size spectra, mineralogic analysis and presence

of clathrates of CO2 or methane. Additional instru-

ments of greater sophistication can also be envisaged

for deployment in future (e.g. mass spectrometers or

biomolecular analyzers).

For either, in situ measurements or sample return,

sample collection and handling is critical. Not only do

the tools for such handling need to be demonstrated,

but they should be designed to be clean enough to not

interfere with the measurement to be made. Further-

more, different applications require different tools,

potentially including corers, scoops, rock crushers,

magnetic separators and sieves, as well as tools to

move samples from the collection point to the instru-

ment or the sample return container.

5.3. Candidates for in situ measurement and specific

issues

Certain measurements are more generally appro-

priate to in situ instrumentation. Such measurements

may be a large-scale surface characterization, or

involve a volatile or unstable component, or be a

time-varying signal. In addition, there may be other

questions that are simple enough, or for which in situ

measurements would be sufficiently precise to be

useful. The specific measurements need to be identi-

fied and the needs must drive the technology devel-

opment effort. What is absolutely essential is that both

in situ and sample return missions be considered as

complementary and that instrumentation be developed

both for in situ work and improved for laboratory

investigations. An example of a complimentary chro-

nology approach would be an in situ ‘‘dater’’ being

used to preselect rocks for more precise sample return

chronology or other age-specific analysis.

A complete working system has not been demon-

strated for any of the in situ techniques described,

with present systems ranging from concepts to proto-

types. Further development is clearly needed. Since

some of the traits required for spacecraft (low mass,

power, volume) would also be useful in terrestrial

field work, such applications should be considered.

5.3.1. Cosmogenic radionuclides

Short-lived (e.g. 22Na, 60Co) to medium-lived

(26Al) radionuclides in meteorites are measured by

nondestructive ‘‘counting’’. Gamma-radiation requires

no prior chemical preparation, whereas beta and alpha

decay schemes for geochronology will require chem-

ical separation from other elements prior to counting

due to the low activity of natural and cosmogenic

radionuclides. Alpha counting was performed with the

Alpha Proton X-ray Spectrometer (APXS) on the

Pathfinder Mission; however, counting was of alpha

particles from a 244Cm source. The APXS instrument

design would not be adequate to measure natural

alpha decay of U and Th. Problems of chemical

separation may preclude measuring nuclides whose

decay schemes require alpha or beta counting methods

unless chemical separation is very simple. All count-

ing methods must contend with the very high cosmic

ray flux which produces secondary particles that give

high background counting rates. On Earth, lead

shielding and anti-coincidence counters are used to

reduce the background due to cosmic radiation. This

will be a significant limitation to counting methodol-

ogies on Mars but could be addressed by deploying

detectors a few meters underground. Because produc-

tion rates of cosmogenic nuclides on Mars are high,

the activities of some cosmogenic radionuclides can

be detectable on Mars. 26Al is an immediate candi-

date, but other nuclides such as 22Na, 60Co, 40K and

U–Th can be measured. The gamma-ray detector

system can be also used for in situ neutron activation

analysis using a 252Cf neutron source.

Ge gamma-ray detectors would have to be cooled

to f 100 K when in use. A passive radiator, like that

P.T. Doran et al. / Earth-Science R

proposed for a Ge gamma-ray spectrometer on a

Rosetta comet lander, could be used at night. The

poles would be good sites for Ge detectors due to the

environmental conditions. An alternative to Ge detec-

tors may be Cd–Zn–Te room temperature solid state

detectors. The energy resolution (f 2%) will not be

as good as in the Ge detectors, but it could suffice for

use in a coincidence system and would be lighter and

more compact (Nishiizumi and Reedy, 2000).

5.3.2. Luminescence

Additional community input on methodological

and geologic limitations and possibilities is needed

before miniaturization of luminescence technology

for missions to Mars. The required instrumentation

for in situ dating on Mars by luminescence (i.e. a

light sensor, an optical stimulation source, a heater,

optical elements, and an irradiation source for cali-

bration) can conceivably be miniaturized. Lepper

and McKeever (2000) show that Mars soil simulant

(JSC Mars-1) is suitable material for luminescence

techniques. However, significant uncertainty exists

on the utility of luminescence on Mars where the

mineralogy is not well known and cosmic ray

exposure is three orders of magnitude higher than

on Earth. The cosmic radiation regime on Mars in

particular may prove to be an intractable challenge

for luminescence dating to overcome because of the

needed prior knowledge of sedimentation and ero-

sion rates (Fig. 4).

5.3.3. Noble gas-based techniques

Methods based on the buildup of noble gases are

promising for in situ systems (e.g. Fig. 5). For either

K–Ar age determinations or cosmic ray exposure age

determinations using 3He, 21Ne and 36,38Ar, all that is

required is a determination of the abundance of one or

a few major or minor elements, a method to heat the

sample sufficiently to degas it and a determination of

the noble gas thus released. No chemical preparation

of the sample is required. With cosmic ray exposure

ages based on 3He, 21Ne or 38Ar it should, in princi-

ple, be possible to determine surface ages as low as

103 years. Several mass spectrometers capable of

making the required noble gas measurements exist,

as do some methods for chemical analysis and heat-

ing. The critical development questions appear to

involve integration.

5.3.4. Radiogenic techniques

Some of the ‘‘standard’’ techniques long used for

chronology on a >107 year time scale could be

adaptable for in situ analysis of Martian igneous

rocks. The requirements for wet chemistry and chem-

ical separation could be overcome by currently avail-

able techniques such as laser ablation and miniature

mass spectrometry technology. One design concept

currently being developed involves a combination of

laser ablation, resonance ionization and miniature

mass spectrometer detection for the Rb–Sr geochro-

nologic system (Fig. 6 and Cardell et al., 2002). Other

configurations and parent–daughter systems are with-

in the realm of possibility. Clearly, in situ application

of these techniques in the Martian environment would

yield ages with much greater uncertainties than ages

obtained from Earth-based laboratories, so choosing

the appropriate samples and terrain for deployment is

critical. As with the noble gas-based techniques, a

significant infusion of funding will be required to

pursue these developments in a realistic fashion and to

achieve full miniaturization and integration of such an

instrument.

5.4. Sample return requirements and issues

Mars is a complex planet, so a comprehensive

series of sample return missions is desirable. Howev-

er, as part of a series of missions it is also possible that

a useful first sample return can be simple and consist

of samples obtained from a lander with a scoop, a rake

and possibly a simple coring device. In order to gain

meaningful chronologic information on Martian sur-

face rocks, the first mission will need to have some

way to sample rocks.

Subsequent missions will need more sophisticated

sampling devices with roving capability and coring

devices. At this stage, a variety of rocks at each

landing site can be sampled, and a preliminary culling

of samples for return can be made using instruments

carried on the rover. A wider variety of samples will

be obtained from each landing site.

5.4.1. Selecting rocks

The rocks and minerals of the surface provide the

primary record of geological processes and environ-

ments that have influenced the history of the planet.

Unaltered igneous rocks in the regolith will yield

eviews 67 (2004) 313–337 327

Fig. 4. Comparison of using a luminescence geochronometer on Earth versus Mars. The luminescence of silicate crystals is reduced to a low

definable level with exposure to sunlight and galactic cosmic ray (GCR) exposure during sediment deflation and transport. Upon burial and

shielding from sunlight exposure, luminescence is acquired by the trapping of electrons in crystal-charge defects with exposure to ionizing

radiation. On Earth, atmospheric attenuation causes GCR to be an insignificant component (often < 1%) of ionizing radiation at the surface. On

Mars, even when the crystal is shielded from sunlight exposure, it will continue to receive a dominant GCR dose up to at least 1 m depth.

Electron traps will be variably filled and emptied in near surface environments of Mars that are dominated by GCR flux. Therefore, to use

luminescence as a chronometer on Mars, prior knowledge of burial and erosion rates (by another chronometric technique) are required to

establish the attenuation of GCR dose, which dominates the acquisition of electron charge in mineral grains.


invaluable information about magmatic processes,

ages and the tectonic history of the planet. Although

there is no reason to expect metamorphic rocks at the

surface of Mars (other than those caused by impact

metamorphism), their presence would greatly revise

our concepts about crustal processes and recycling on

Mars. In addition, the cosmic ray exposure history of

the Martian surface is almost completely unknown.

Recent discoveries of sedimentary rocks on Mars

(Fig. 7) introduce the same challenge for assessing the

chronology of these rock types as we have on Earth;

that is, absolute ages for many will be unobtainable

due to lack of appropriate chronometers.

5.4.2. Fines

Existing models and new Global Surveyor data

suggest that much of Mars’ surface is mantled with

‘‘fines’’ of mostly unknown thickness and composi-

tion. The fines may provide a global average of

weathering products. If so, isotopic analysis could

Fig. 5. (a) Plot of K–Ar ages of Martian meteorites, plotted as a function of crystallization ages (the latter from Nyquist et al., 2001).

Uncertainties in stratigraphic boundaries can be found in Fig. 1 for comparison. Concordant (‘‘correct’’) ages would fall on the diagonal line.

Although some K–Ar ages, including many of those of the shergottite meteorites, are poorly defined, K–Ar ages that are reasonably precise

(including the shergottites in the inset) are concordant, or nearly so, with the sole exception of ALH84001, which clearly experienced impact

resetting f 4.0 Gy ago (K–Ar ages from Swindle, 2001). (b) With modern analytical techniques designed for in situ measurements, a 3 mm3

crucible, such as those shown here (two are on the arm just above the ball-point pen given for scale) can contain enough material from a Martian

rock to give K–Ar or cosmic ray exposure ages with 1r measurement uncertainties of 10–20%. Image is of the sample-handling portion of an

instrument under development by the University of Arizona, Los Alamos National Laboratory and the Jet Propulsion Laboratory. Samples will

be loaded through screen on right and heated in oven in a vacuum system that will be attached where clear plastic sleeve is on left. A miniature

quadrupole mass spectrometer array will analyze the evolved noble gases and elemental compositions will be determined by laser-induced

breakdown spectroscopy.


Fig. 6. Measurement precision required to achieve Rb–Sr age uncertainties of F 100, F 250, and F 500 My for an igneous rock, compared to

standard measurement precision in a terrestrial laboratory. The curves are calculated for a theoretical two-point isochron of plagioclase and

clinopyroxene (common basaltic minerals). The uncertainty depends not only on the measurement accuracy but also on the age of the rock being

analyzed. Analysis of additional points or of minerals with a greater spread in Rb–Sr (e.g. orthopyroxene) would significantly improve the age

uncertainty of the in situ measurement.


provide clues about the proportions of crustal and

mantle material exposed at the Martian surface, and

cosmic exposure ages would provide the mean resi-

dence time of dust at the surface of Mars.

5.4.3. Sample handling

The samples will experience a long transport

history before they reach earth-based laboratories. It

will be necessary to consider and minimize contam-

ination of samples during this stage and to keep

careful record of the time/temperature/ambient atmo-

sphere conditions of samples in order to understand

how isotopic systems might become compromised.

The materials used for collecting, transporting and

curating samples must be well characterized with

respect to elemental and isotopic compositions rele-

vant to chronology. The first samples released from

the Martian sample receiving facility will likely be

sterilized, either by heat or radiation. It will be

necessary to consider the effects of these processes

on the isotopic systems being studied. For instance,

the diffusion of He is highly temperature-dependant

and heating beyond 40 jC will promote helium loss

from the sample and produce error when dating

minerals by the accumulation of 4He from U and Th

decay (Farley, 2001).

5.4.4. Technology development

The first sample return will perhaps place more

stringent sample size requirements on laboratory

analytical techniques than subsequent missions. As

the first samples may only consist of dust and small

rocks, there will be a premium on techniques that can

use the smallest amount of sample. Because of the

small size of rock fragments, extreme care will need

to be taken to separate the effects of primary crystal-

lization from those from later weathering, shock and

dust contamination. Subsequent sample returns with

the capability of returning drill cores or larger chunks

of coarser grained rock are likely to bring back

Fig. 7. Left image: sedimentary rocks on the floor of an impact crater located near the equator in northwestern Schiaparelli Basin (0.15jN,345.6jW). MOC image (E03-00728) was taken in April 7, 2001 and is approximately 3 km across (Malin et al., 2001b). Right image: a mosaic

of MOC high resolution images showing an ancient, eroded, and exhumed sedimentary distributary fan located in a crater at 24.3jS, 33.5jW,

northeast of Holden Crater. The area covered is 14 km wide and the scene is illuminated by sunlight from the left (image credit: NASA/JPL/

Malin Space Science Systems, MGS MOC Release No. MOC2-543a). Many sedimentary rocks have been discovered by the Mars Global

Surveyor (Malin and Edgett, 2000b) suggesting that Mars has a complex sediment stratigraphy. Layered sediments like these will help establish

relative ages and, like on Earth, will be a challenge for absolute dating.


samples that can be more easily dealt with using

current state-of-the-art techniques in mineral separa-

tion and mass spectrometry.

Microbeam techniques, such as secondary ion

mass spectrometry with multiple detectors and laser

ablation MC-ICP-MS hold great promise for dealing

with small fine-grained samples. Both techniques

are limited in sensitivity to f 0.1%, which needs

to be improved. Laser resonant ionization is one

way to substantially improve sensitivity and is very

efficient at minimizing interferences, but current

applications of the technique have been limited by

laser repetition rates, which limit the number of

ions that can be counted in a reasonable time and

result in relatively low precision of isotope ratio

measurements.

Multi-collector plasma source mass spectrometers

now have isotope ratio precisions approaching that of

TIMS, but very high precision in both techniques is

limited to cases where ion beam currents are high

enough to use Faraday cup detectors. There is a need

for improvement in the stability of more efficient

detectors so that high precision measurements can

be made on smaller number of atoms.

The ultimate limit to precision of isotope ratio

measurement is imposed by the number of atoms in

a sample. Current microbeam methods are at least

three orders of magnitude away from the atom-count-

ing limit, and techniques involving mineral separa-

tion, chemical separation and purification and thermal

ionization or solution plasma mass spectrometry are

considerably less efficient than microbeam methods.

Microbeam methods using laser resonant ionization

now operate at about 10% of the atom-counting limit,

but precision in isotope ratio measurement is not

adequate for most chronology applications (Nicolussi

et al., 1997).

6. General conclusions and recommendations

Mars has a history of dynamic surficial evolution

with various processes that continue to reshape the

planet even up to the present. Many techniques used


to assess chronology on returned lunar samples can

also be used on Mars, but the task is significantly

different as there are more events and different types

of processes and deposits on Mars to assess. On Mars,

the questions to be asked can go beyond the age of the

formation and differentiation of the planet and will

include investigation of the timing of atmospheric and

climate change events. Chronological approaches

used on Mars will parallel those used on Earth as

much as those used on the Moon.

A number of general conclusions and recommen-

dations can be made from our review:

(1) To get useful chronometric information, whether on

recent or past processes, geological context is cri-

tical to understanding what is being dated. Know-

ing the chemistry of the sample is also critical for

most applications. In addition, there are certain

baselines that will be important if not critical, such

as the present composition of the atmosphere

(including any trace species of interest).

(2) For any chronometric determination, an age on a

single sample determined by a single technique

has an unavoidable degree of ambiguity. To have

full confidence in the results, it is preferable to

measure ages by multiple techniques on multiple

samples. The use of multiple parent–daughter

systems is necessary to address possible evidence

of thermal and shock metamorphism and of

chemical alteration, as different parent–daughter

systems have a differing response and sensitivity

to these processes. At least, if a single technique is

to be used (e.g. in situ), we must be confident that

it will give consistent results on multiple samples.

The confidence in the results would be signifi-

cantly greater if a second technique could be

applied (e.g. on the same rover).

(3) Although we have identified interesting questions

and promising techniques, we would not be able

to design a viable in situ mission based entirely on

existing technology. Development work is re-

quired particularly if an in situ mission is planned

to address the recent history of Mars. The most

promising chronometric techniques we have

identified all require substantial development, as

discussed in Section 5.3. For those, not only is

funding required, but development should proceed

at an accelerated pace, so that the systems

developed can be tested fully on analog materials

before being placed on a spacecraft. The level of

development required ranges from integration of

technology (noble gases) to development of

techniques (other long-lived radiometric chronom-

eters), and several projects have at least begun

(e.g. Cardell et al., 2002; Swindle et al., 2003), but

even integration of tools designed for other

applications is far from a trivial task.

(4) For a sample return mission, the questions of

environmental requirements for the samples to be

returned must be studied with regards to the

dangers of compromising chronological techni-

ques (see Gooding, 1990; Neal, 2000). Examples

include contamination of the sample by extrane-

ous materials (which disturb the parameters to be

measured) and changes in pressure and tempera-

ture conditions which likewise disturb the param-

eters to be measured.

(5) Returned sample size for a full isotope chronology

could be as low as 0.1 g per ‘‘rock’’. By this it is

meant that each rock sample should be adequate in

size to permit its age determination by at least two

methods and to provide enough additional mate-

rial to permit its complete characterization via

mineralogical/petrological and geochemical inves-

tigations by a number of investigators. The sample

size estimate presented here assumes such inves-

tigations require about 10 times the amount of

sample required for a basic age determination with

a high probability of success for randomly

selected rock types. Under favorable conditions,

an age determination by a single method on a

favorable rock type can be made with much

smaller samples, down to sample sizes of a few

milligrams for a single mineral grain.

(6) Investments in upgrades of terrestrial laboratory

facilities and capabilities should focus on reducing

sample size requirements. Reduction in sample

size could be realized by increasing the sensitivity

of the instruments used for the basic measure-

ments. It should be borne in mind, however, that

‘‘full isotope chronology’’ of the type described

above will require samples of adequate size so that

a number of analytical methods can be applied to a

single sample.

(7) We consider dating of returned samples and dating

of samples in situ on the surface of Mars to be


complementary options, depending on overall

mission planning. A decision to employ a specific

dating system must be based on a clear under-

standing of the sensitivity of the proposed system

and on the required sensitivity and precision. The

applicability and the final choice of a specific

dating technique and instrumentation will also be

critically dependent on the nature of the terrain to

be sampled. Furthermore, instruments based on

techniques whose systematics are well understood

for Martian environments will have a greater

chance of success.

Acknowledgements

This paper is based on a workshop held at the

University of Illinois at Chicago in July 2000. We

would like to thank the other participants: P.

Beauchamp, L. Borg, C. Budney, G. Cardell, F.

Carsey, J. Christensen, J. Cutts, A. Davis, W. Davis,

S. Guggenheim, D. Kossakovski, J. Lawson, K.

Lepper, S. McKeever, C. McKay, R. Morris, M.

Taylor and M. Wadhwa. The workshop was supported

by the Mars Program Office, Jet Propulsion Labora-

tory (Contract #1215592). The writing of this paper

was partially supported by the NASA Exobiology

Program (NAG5-9427).

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Peter T. Doran is an Associate Professor of

Earth and Environmental Sciences at the

University of Illinois at Chicago. His inter-

ests are geochronology, extreme environ-

ments, climate change and hydrogeology.

His current research projects include long-

term ecological research, paleoclimate stud-

ies from Antarctic dry valley lake cores and

using dry valley lakes as analogs to past

water bodies on Mars.

Stephen Clifford is a staff scientist at the

Lunar and Planetary Institute in Houston,

TX, where he conducts research on the

hydrology and climate of Mars. Recent

research has included studies of the stability

and replenishment of ground ice; glacial

flow and polar evolution; and large-scale

groundwater transport. He is also a U.S.

Participating Scientist on the European

Space Agency’s Mars Express mission.

Steven L. Forman is a Professor of Earth

and Environmental Sciences at the Univer-

sity of Illinois at Chicago. His main re-

search interests include Arctic climate and

environmental change, Northern Hemi-

sphere hydrologic variability, extreme cli-

matic events, including megadroughts and

experimental and applied geochronology.

Laurence E. Nyquist manages the Thermal

Ionization Mass Spectrometry Laboratory at

the Johnson Space Center. His interests

include the origins, chronology, and evolu-

tion of achondritic meteorites, including

Martian meteorites; the chronology and pet-

rogenesis of lunar mare and crustal samples;

the systematics of isotopic equilibration and

resetting of radiometric ages, and the cos-

mic ray exposure history of meteorites.

D. A. Papanastassiou is a Senior Research

Scientist at the Jet Propulsion Laboratory,

and a Faculty Associate in Geochemistry,

Caltech. His interests are in the mass spec-

trometry of positive and negative ions, and

in the study of preserved nucleosynthetic

components in the early solar system and of

the evolution of primitive meteorites and

protoplanets.

B. W. Stewart is an Associate Professor of

Geology and Planetary Science at the Uni-

versity of Pittsburgh. His research is focused

on the application of radiogenic isotopes to a

wide variety of problems, including Quater-

nary paleoclimate, evolution of the Archean

atmosphere and oceans, formation of acid

mine discharges, and meteorite petrogene-

sis. He is also a member of the Penn State

Astrobiology Research Center.

Neil C. Sturchio is Professor and Head of

Earth and Environmental Sciences at the

University of Illinois at Chicago. He re-

ceived his PhD in Earth and Planetary

Sciences from Washington University in

1983, for studying the petrology and geo-

chemistry of metamorphic rocks in the

Eastern Desert of Egypt. He was staff at

Argonne National Laboratory for 15 years

pursuing studies of trace element and iso-

topic behavior in rock–water systems.


T. D. Swindle is a Professor of Planetary

Sciences and Geosciences at the University

of Arizona. His research background is in

noble gas mass spectrometry of extrater-

restrial materials, including Martian mete-

orites, and he is currently P.I. of a project

to develop a spacecraft instrument for in

situ noble-gas-based geochronology.

Thure E. Cerling is a Professor in the

Department of Geology and Geophysics at

the University of Utah. His research is in

isotope geochemistry of processes occur-

ring at or near Earth’s surface and the

geological record of ecologic and climatic

change.

Jeffrey S. Kargel is a Geologist at the U.S.

Geological Survey. His research concerns

ice in the Solar System, ranging from

Earth’s cryosphere to the Martian cryo-

sphere and cryomagmatism on icy satel-

lites of the outer planets. He is the principle

investigator of the 24-nation consortium

called Global Land Ice Measurements from

Space (GLIMS) and is an expert on the

chemistry and physical chemistry of aque-

ous and other volatile systems. He also has

made contributions to the geomorphology of planetary landscapes.

Dr. Gene McDonald is a Research Scientist

in the Astrobiology Research Element at

the Jet Propulsion Laboratory. His research

interests are in exobiology and prebiotic

chemistry, organic geochemistry and bio-

geochemistry, and development of analyt-

ical methods for astrobiology.

K. Nishiizumi is a Senior Space Fellow of

Space Sciences Laboratory at the Univer-

sity of California, Berkeley. His main re-

search interests include exposure histories

of lunar, meteoritic, and terrestrial materi-

als including Martian meteorites and his-

tories of cosmic rays using cosmogenic

nuclides. He is also on the Genesis mission

Science team.

Robert Poreda has been a professor of

Earth and Environmental Science at the

University of Rochester since 1987. He

has spent the past 25 years researching the

noble gas signature in the earth’s oceans,

atmosphere, crust and mantle. He has a

Geology undergraduate from Yale Univer-

sity and PhD from the Scripps Institution

of Oceanography. He has established a

noble gas laboratory devoted to under-

standing the processes that shape our

planet and has published over 60 papers on topics that range from

the degassing of the earth’s interior to the evolution of the Antarctic

Dry Valley lakes.

James W. Rice, Jr. is a Science Team

Member on the Mars Exploration Rover

Missions and Sr. Mission Operations Ana-

lyst on the THEMIS instrument onboard

Mars Odyssey at the Mars Space Flight

Facility/Department of Geological Scien-

ces Arizona State University. His research

specialties are Mars geologic studies and

periglacial, fluvial and lacustrine geomor-

phology. He has also been involved in

Mars landing site selection and certification

for numerous missions (Pathfinder, Polar Lander, MER Rovers).

Kenneth L. Tanaka is a planetary geolo-

gist for the Astrogeology Team of the

U.S. Geological Survey in Flagstaff, Ari-

zona, USA. His main research interests

include geologic mapping and evolution

of Mars. He is involved with research

programs and data dissemination for past,

present, and upcoming missions to Mars

and other planets.

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