The evolution and present state of water and carbon dioxide on Mars are discussed. Researchers wished to determine how effectively questions regarding the distribution of water and carbon dioxide on Mars may be addressed with orbital gamma ray spectrometer data. Several simple, multi-layer models of the Martian surface were formulated to address problems such as the ice/dust ratio of layered deposits; the distribution, depth and concentration of ground ice; the thickness of north polar perennial ice; the thickness of the carbon dioxide layer over the south polar cap; the thickness of the seasonal carbon dioxide frost cap; and the water content of the seasonal frost cap. The results indicate that the Mars Observer gamma ray spectrometer will be a powerful tool for investigating the distribution and stratigraphy of volatiles on Mars

Evans, Larry G.

Squyres, Steven W.

English

NASA Technical Reports Server

A-RAY SPEC- 
TROSCOPY 
Steven W. Squyres, Cornell University, Ithaca, NY 14853, and Larry G. Evans, Computer 
Sciences Corporation, Beltsville, RID 20705. 
Earth-based and spacecraft investigations of Mars have shown that Hz0 and C 0 2  are the two 
most important volatile species on the planet. While major advances have been made concerning 
the evolution and present state of H 2 0  &nd C 0 2  on Mars, many unanswered questions remain. 
The upcoming Mars Observer mission will include in its payload a gamma-ray spectrometer. 
This instrument will measure the spectrum of gamma radiation emitted by the planet. F'rom 
the measured gamm&ray spectrum, it is possible to infer a considerable amount of composi- 
tional information. Particularly, the experiment will enable study of the spatial and temporal 
distribution of H20  and GO2 in the near-surface martian regolith. 
The presence of H20  can be investigated directly by me~urement  of the 2.223 MeV gammsray 
line of H. Because H is the most important element for moderating the neutrons produced by 
cosmic-ray interactions with the surface, it can also be investigated by the direct or indirect 
determination of the fast to  thermal ratio of the neutron albedo flux. In general, for materials 
with high H concentrations, the total neutron flux consists of proportionally more thermal 
neutrons and the flux peaks at shallower depths than for materials with low H concentrations. 
The neutron fastlthermal ratio may be determined indirectly from the gamma-ray spectrum. 
Prompt capture lines result primarily from interactions of nuclei with thermal neutrons, while 
inelastic scattering lines result primarily from interactions with fast neutrons. Some elements, 
such as Si and Fe, emit strong lines of both types. One may therefore examine ratios of inelastic 
scatter to prompt capture line strengths for these elements, and acquire information regarding 
the H distribution that is similar to what one would acquire directly from fast/thermal neutron 
ratios. The 2.223 MeV gamma-ray flux is an indicator of the amount of H in the upper few 
tens of g ~ m - ~ ,  while the inelastic/capture ratio for Si or Fe is related to the amount of H 
in the upper w 100 g or more. It is therefore possible to obtain information about the 
vertical distribution of H. C 0 2  is more difficult to detect, but it may be possible to determine 
the thickness of a layer of C 0 2  frost by direct detection of C or by inference from attenuation 
of gamma rays from underlying material. 
We wish to determine how effectively questions regarding the distribution of Hz0 and C02  on 
Mars may be addressed with orbitd gamma-ray data. Our approach is straightforward. We 
have identified several unanswered questions regarding martian H20  and C02. These include: 
(1) What is the iceldust ratio of the polar layered deposits? (2) What is the thickness of the 
polar perennial ice? (3) What is the thickness of the C02  that covers the southern perennial 
ice? (4) What is the thickness of the seasonal C02  frost caps? (5) How much W 2 0  is there 
in the seasonal frost cap? (6) How does the distribution of subsurface ice vary with latitude 
and geologic material? (7) What is the degree of hydration of minerals making up the martian 
surface? (8) Are there near-surface liquid water "oasesn in Solis Lacus or elsewhere? For 
each question, we have formulated a simple multi-layer model of the martian surface. In each 
model, the martian surface is composed of one or two horizontal layers. If one surface layer 
is present, it is taken to be semi-infinite; if two surface layers are present, the upper one has 
some variable thickness and the lower one is semi-infinite. A layer may be composed of HzO, 
C02,  or soil with a composition like that at the Viking landing sites, or some homogeneous 
mixture of two of these materials. In two-layer models in which one layer is a mixture of two 
materials, the other layer is always composed of a single material. Each model therefore can 
be completely described by no more than two parameters. As an example, the problem of 
detection of subsurface ground ice is modeled with a variable-thickness upper layer of pure soil, 
https://ntrs.nasa.gov/search.jsp?R=19870013987 2020-03-20T10:24:53+00:00Z
and a semi-infinite Bower layer of intermixed soil and H20. In this case, the two parameters 
are the thicknws of the upper layer and the soil/HzO mass ratio of the lower layer. For each 
model, we calculate the g-+ray spectrum that will be observed by a spacecraft in orbit. 
All of the calculations include atmospheric attenuation and emission. We then examine ways 
in which the observed spectrum can be interpreted to yield unique determinations of values for 
the parameters of interest. 
In order to predict the expected gamma-ray fluxes from the surface of Mars and the sensitivity 
of the Mars Observer gamma-ray spectrometer experiment, the neutron spatial m d  energy 
distributions in the near-surface materials must be modeled. We use a primary E osalc-ray 
spectrum and resulting secondary neutron source distribution like that for the Moon. We then 
calculate the resulting neutron spectrum and distribution for the surface and atmosphere under 
consideration using the ANISN code. This is a discrete ordinate code that solves the neutron 
transport equation by evaluating the flux for discrete directions, positions, and energies. The 
code is one-dimensional, and uses 100 neutron energy groups and 100 spatial intervals with a 
spacing of 5 g cm-l. 
Once the neutron spatial and energy distributions have been determined, we calculate the 
discrete line gamma-ray fluxes produced by neutron interactions. For capture lines, the cal- 
culated thermal flux is used with the appropriate thermal neutron capture cross-sections and 
gamma-ray yields to calculate the gamma-ray line flux. Inelastic scatter gamma-ray fluxes are 
calculated in a similar manner with the inelastic scatter cross-sections integrated over energy at 
each depth. Production of gamma rays by natural radionuclides is also calculated. All gamma- 
ray line strengths are corrected for attenuation in the martian surface and atmosphere. The 
gamma-ray background is calculated from the observed lunar background, scaled to Mars from 
gamma-ray transport results. 
Combining line strengths, background strength, detector characteristics, and counting time, 
we may estimate the quantitative uncertainty of the measurements to be made. For our cal- 
culations, we used the area, energy resolution, and efficiency of a laboratory 120-cm-~ HPGe 
detector, and a counting time of 38 hr. This period corresponds to the total integration time 
for a single gamma-ray spectrometer resolution element (about 300 km diam.) near the poles 
over one martian year. 
We have considered a total of five models: 
Model 1: Intermixed Soil and H20: This model can be used to address problems such as the 
ice/dust ratio of the layered deposits, the dust content of the north polar perennial ice, and 
the degree of hydration of hydrated silicates. Sample results are given in Figure 1, where we 
plot two observed line strength ratios as a function of the mass fraction of H20 present. For 
small amounts of H20,  optimum results are given by the ratio of Si inelastic capture to prompt 
capture line strengths, while for larger amounts of H20,  optimum results are obtained by using 
the ratio of H (2.223 MeV) to Fe (capture). For a simple model such as this, it should be 
possible to determine the H20  content to a high degree of accuracy. 
Model 2: Soil over Soil + H20: This model consists of two layers: a pure soil layer of variable 
thickness over a semi-infinite layer of soil + H20 with a variable HzO mass fraction. This 
model can be used to address the distribution, depth, and concentration of ground ice (or 
liquid). Figure 2 plots Si(inelastic/capture) vs. H/Fe(capture). Solid curves are contours of 
constant H20  mass fraction in the lower layer, and dashed curves are contours of constant 
thickness of the upper layer. For data falling in the region where the curves are separated, it 
will be possible to determine both the thickness of the upper layer and the ice mass fraction of 
the lower layer. 
Model 3: Ha O over Soil + H2 0: This model can be used to addresa the thickness of the north 
polar perennial ice, which lies atop the layered deposits. Calculations like those in Figure 2 show 
that unique solutions are possible, but the uncertainties are large. However, if we assume that 
we know the ice fraction of the lower layer a priori, (as might be possible from an independent 
measurement of the ice content of exposed layered deposits) determination of the ice layer 
thickness can be improved very substmtially. 
Model 4: C02  over Soil + H20: This model can be used to address two problems. In the 
extreme of very high H20 mixing ratio in the lower layer, it can be used to investigate the 
thickness of tile C02  layer atop the south polar perennial ice. In the exereme of a very low or 
zero HzO mixing ratio in the lower layer, it can be used to address the problem of the thickness 
of the seasonal C02  frost cap. Again, unique solutions are possible, but the uncertainties are 
large in some cases. However, if we know the H20  content of the lower layer a priori (as would 
be the case if it were determined when the C 0 2  layer was absent), uncertainties again can be 
very substantially reduced. 
Model 5: C02  + H 2 0  over Soil: This model can be used to address the H20 content of the 
seasonal frost cap. Unique solutions are found for both adjustable parameters, with small 
uncertainties. 
These results indicate that the Mars Observer gamma-ray spectrometer will be a very powerful 
tool for investigating the distribution and stratigraphy of volatiles on Mars. It is important 
to note that the results here do not include the substantial additional information about the 
neutron energy spectrum that will result from the GRS's neutron mode. With this mode, it 
should be possible to determine fast/therm,al ratios directly, providing an independent check 
on the gamma-ray results, and increasing the overall certainty in the results. 
INTERMIXED SOIL' H20 
38 Hr. 
"b 20 40 60 80 100 
PERCENT H20 
40. 
g z 
.- 4; .- 
V) 
c g? 4.0~ 
w -  
I- 
z 
- 
Figure 1 (left) 
Figure 2 (above) 
..- 
. 8 , . . . . I 1  I I 
SOIL OVER (SOIL+H20) : 
38 Hr : 
- 
- 
g/crn2 
: 5% 
W 
z 
4 
W 
Kc 
0.i 
- 
-Percent H20 
- ------Soil Thickness 
- 
- 
I I I L 
~CT' i oO 10 102 103 
RELATIVE INTENSITY H/Fe (capture) 


Investigation of Martian H2O and CO2 via gamma-ray spectroscopy

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