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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: pdoran@uic.edu (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
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337316
(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).
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337 317
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
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337318
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).
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337 319
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.
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337320
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
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337 321
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
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337 323
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
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337324
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
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337 325
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.
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337326
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.
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337328
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.
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337 329
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.
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337330
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.
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337 331
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
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337332
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
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337 333
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.
P.T. Doran et al. / Earth-Science Reviews 67 (2004) 313–337 337
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.