X-ray studies of planetary surfaces

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X-ray studies of planetary surfaces

Introduction:

Our solar system is believed to be nearly 4.6 billion years old. Soon after the formation ofthe Sun, the residual gas and dust that surrounded the early Sun, slowly cooled and

gravitationally settled to form other large bodies in the solar system. These include thecurrent eight planets with their satellites, minor planets, asteroids and comets. As the

material in the solar system cooled, metal-rich rocky bodies formed in the inner solar

system (as they have high melting points) while the outer solar system produced gas-rich,

metal-poor giant planets.

Exploration of the solar system initiated by the early Pioneer and Voyager series of

spacecrafts began as flybys, maturing to orbiting missions such as the Luna, Apollo, ,Viking, Venera, Galileo, Magellan, Clementine, Cassini, SMART-1, etc. They provided

initial global data on solar system bodies. These steps were most critical to sustain theeffort to understand the origin, composition and evolution of solar system bodies. Remotesensing of planetary bodies were carried out using sensors across the full electromagnetic

spectrum as well as particle detectors (charged and neutral) and electric and magnetic

field sensors for detailed studies of plasma in the interplanetary medium. These have

immensely increased our understanding of these bodies and at the same time generatednew questions. Here we shall concentrate on the specific role of X-ray observations to

study planetary chemistry.

How do we know the chemistry from X-rays?

A few decades after the discovery of X-rays by Wilhelm Rontgen, it was shown byMoseley in 1913 that when pure substances are irradiated with a beam of X-rays from an

X-ray tube, they produce new X-ray photons with an energy, characteristic of the

substance. With the nearly simultaneous developments in the quantum theory of theatom, it became clear that these characteristic X-ray lines arise as follows. When an X-

ray photon hits an atom, it may either be absorbed or scattered by it. If the energy of the

incident photon is greater than the binding energy of an atomic shell, an electron will be

ejected (called photoelectron). This process is called photoelectric effect and the energyof the photoelectron will be equal to the difference between the incident photon energy

and the binding energy of the respective shell. The vacancy is then filled by an electron in

a higher shell. This electron transition results in the emission of an X-ray photon (X-ray

fluorescence - XRF) of energy characteristic to the atom. This X-ray photon will have anenergy equal to the difference in energy between the two shells. Since the atomic energy

levels of each element in the periodic table are distinct, the fluorescent X-ray emissionfrom atoms can be used to directly identify the presence of the element. This forms the

basis for remotely sensing the chemistry of planetary surfaces.


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No:ofX-

rays

Energy of XRF photons

(a)

(b) .1

How do

planetary

surfaces emit X-rays?

In laboratories, X-ray generators or radioactive

isotopes are used to illuminate the sample under

study. But these are too weak to excite enough XRFphotons (that can be detected by an X-ray detector)

from the planetary surface if carried onboard an

orbiting spacecraft.

On the other hand, the Sun can be the source of X-

rays. The surface temperature of the Sun is ~ 6000K, which implies that the resulting blackbody emission is most

intense at visible wavelengths. The Sun also emits X-rays. The changes in solar magnetic

configurations give rise to sporadic release of energy. Often this results in acceleration of

particles. Accelerated electrons that strike cool gas atoms on the solar surface, radiatesenergy in the form of X-rays. This is referred to as a solar flare where the X-ray emission

suddenly increases, reaches a maximum and decays over a time scale of minutes to hours(Fig. 2). Flares are often classified according to increasing intensity as belonging to A, B,C, M or X class (see table 1 for details), each class being ten times more intense than the

previous.

Table 1: GOES classification of solar flares

1 (angstroms) = hc/E=12.398/E (keV)

Figure 1: (a) decay of excited atom results in emission of fluorescent X-ray photon of a specific

characteristic to the atom. (b) XRF spectrum from a stainless steel plate showing lines from the

elements present in stainless steel.

Figure 2: Time profile of X-ray emission from the Sun

observed by the SOXS experiment on GSAT-2. Note t

extreme sudden onset of the flare compared to the sl

decay and how the X-ray intensity varies at the two differe

energy bands.


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\

When the solar X-rays strike the atoms on the surface of the planetary body, as describedin the previous section, XRF photons are emitted from the upper few microns. 2 An X-ray

detector on the orbiting spacecraft can then detect them and measure its energy to

generate a spectrum.

How do we determine elemental abundance from X-ray spectra?

Each element emits XRF photons at a unique energy and thus provides an unmistakable

identification of the same. The technique is thus very efficient in identifying the elementspresent in the sample. Determining the composition also means that we need to know the

proportion in which each element is present in the sample. The conversion of the numberof X-ray photons of a given energy (X-ray line flux) into elemental abundances is not a

straightforward process. This is because, in addition to the true elemental abundances, the

observed X-ray line flux from a planetary body also depends on :

the incident solar spectrum S(E) 3: The solar X-ray emission is highly variable

over periods as short as a few seconds. Hence it is important to continuouslymeasure the solar spectrum.

inter-element effects M(a function of S and E): The observed X-ray fluorescent

line intensity from a sample depends on the mass fraction of other elements in thesample.

particle size : In regions where the mean particle size making up the target is

larger than the penetration depth of X-rays (which is the case of lunar regolith),

particle size affects the X-ray fluorescence line intensity. Laboratory experimentshave shown that the X-ray fluorescence line intensity for a given element

decreases with increasing particle size.

incidence angle (): The angle subtended by solar X-rays with the planetarybodys surface.

phase angle () : The angle between the source (Sun), planetary body and the

detector.

Clearly, the observed line strength (IObs) of an element is not a direct measure of the true

abundance. However, one can define a relationship between IObs(i) and a corrected line

intensity I0(i) (which can be directly related to the true abundance of element i) as

2 For example in the case of Moon, the upper 2 microns of regolith is sampled when we map Mg, Al and Si

abundance3 S represents the distribution of photon energies (E) coming from the Sun

Class Flux (W/m2) .(1.5-12.5 keV)

A < 10-7

B 10-7 to 10-6

C 10-6 to 10-5

M 10

-5

to 10

-4

X > 10-4


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I Obs(i) = f (S(E),i) {I0(i) * M(S(E), E) * P(E, ,,i)}

where f (S(E),i) represents a function of the incident solar spectrum S(E) for element i .

P (E, , ,i) includes cumulative effects of mean particle size and * represents

convolution.

XRF experiments in space

ISRO has over the course of many years established one of the most successful andcomprehensive remote sensing programs in the world for resource mapping of Earth.

Though these efforts have involved observations at microwave, infra-red and visible

bands, X-ray observations have not been deployed for Earth observations because

atmosphere absorbs X-rays. However, it is possible to derive the surface chemistry ofsolid/rocky solar system objects with no atmosphere through X-ray remote sensing

techniques. Moon and Mercury form ideal examples to carry out such studies. There have

also been XRF experiments on the Mars rovers and Venus landers which did in-situ

measurements. It is to be noted that solar X-rays do not provide adequate flux at distancesbeyond the orbit of Mars and hence orbital elemental mapping using X-rays cannot be

performed on outer solar system bodies

Though the first such experiment was flown on Luna 10 in 1966, Apollo-15 and 16 X-

ray fluorescence experiments were the first to clearly demonstrate the usefulness of this

technique. They mapped the abundance of Mg and Al relative to Si for about 10% of thelunar surface at the equator.

The Chandrayaan-1 XRF experiment

The Chandrayaan-1 X-ray Spectrometer (C1XS) isan XRF experiment designed to carry out mapping

of the major rock forming elements such as Mg, Al,Si, Ca, Ti and Fe on the lunar surface. The

complementary experiment alongside C1XS, the X-

ray solar monitor (XSM), provides continuousobservation of the solar X-rays. C1XS was

developed as a collaborative experiment involving

Figure 4: History of XRF experiments for solar system bodies

Figure 5: C1XS flight instrument


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Rutherford Appleton Laboratory, ISRO Satellite Centre and the European Space Agency.

The X-ray Solar Monitor was provided by University of Helsinki in Finland.

C1XS is currently performing well in lunar obit X-ray fluorescence lines from Mg, Al

and Si have been measured from the lunar surface even during the four relatively weak

solar flares that occurred in the past few months.

What does elemental abundance tell us?

Elements are the building blocks of the variety of minerals that compose a planetary body

and hence elemental abundance provides important clues to understand its origin andthermal history. To give an example, as the body originates and evolves, a cation which

goes into the crystal structure of a mineral depends on the environment in which the

mineral crystallize. Moon is believed to have supported an initial partially melted phasefrom the accretional heating arising from the liberation of gravitational potential energy

during its early evolution. Soon after formation, it gradually cooled and differentiated

over time. The upper layer of this magma which formed the Moons crust, is mainlycomposed of a mineral called plagioclase feldspar (which is found in the bright areas seen

with the naked eye). As the magma cooled and this mineral crystallized, the early

formations seem to have accepted Mg into its crystalline structure while the later ones

contain more Fe. Thus, greater the proportion of Mg, more primitive is the rock. HenceMg/Fe ratio is a much sought after parameter which can be derived only from elemental

mapping.

Prior to Apollo, we knew little about the unusual properties of lunar regolith, the absolutelack of hydrous minerals, lack of volatile elements, the tenuous lunar atmosphere and

many other aspects which are unique to an environment like that on the Moon. The most

accepted theory of Moons origin and the magma ocean hypothesis which tries to explainthe apparent dichotomy on the lunar surface is a direct result of mapping the lunar surface

chemistry.

Figure 6: C1XS XRF spectrum during a B-flare on 10 Feb 2009 (Dotted line is the nominal

detector background)

Al

Mg S

i


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The strength of the XRF technique mainly lies in the fact that the elemental signatures are

absolutely unmistakable unlike mineral identification using absorption spectra at infra-

red wavelengths where blending of absorption signatures of individual minerals lead tonon-unique interpretation of data. Mineral maps from IR observations combined with X-

ray elemental maps, form a powerful technique to derive a more quantitative mineral

compositional map of atmosphere-less planetary bodies.

Shyama Narendranath. K.C, S. Athiray and P.Sreekumar

Space Astronomy & Instrumentation Division, ISAC

X-ray studies of planetary surfaces

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