Primary objectives for exploration of Mars include determination of: (1) the distribution, abundance, and sources and sinks of volatile materials, and (2) the interaction of surface materials with the atmosphere. Both objectives fall within the purview of planetary surface weathering studies and require documented samples of weathered materials, including rock surfaces, soils, and sediments. Major issues to be addressed in selecting and studying Martian samples in this context are summarized

Gooding, J. L.

English

NASA Technical Reports Server

N89- 18320 
88 
MARTIAN WEATHERING PRODUCTS AS TRACERS OF CLIMATE CHANGE AND 
ATMOSPHERE/HYDROSPHERE EVOLUTION ON MARS. J. L. Gooding, 
SN2/Planetary Materials Branch, NASA/Johnson Space Center, Houston, TX 77058. 
Introduction. Primary objectives for exploration of Mars include 
determination of (a) the distribution, abundance, and sources and sinks of 
volatile materials, and (b) the interaction of surface materials with the 
atmosphere [l]. Both objectives fall within the purview of planetary surface 
weathering studies [2] and require documented samples of weathered materials, 
including rock surfaces, soils, and sediments. Major issues to be addressed 
in selecting and studying Martian samples in this context are summarized in 
the followin review. 
elements are located in the regolith? Reversible sinks include 
physically condensed species (ground ice, frosts and adsorbed gases) that can 
recirculate under the influence of diurnal, seasonal, or climatic temperature 
cycles. In contrast, irreversible sinks (hydrated silicates, carbonates, 
oxides, etc.) are resistant to surface temperature cycles and will return 
volatiles to global circulation only under extreme duress (e.g., catastrophic 
meteoroid impacts or volcanic intrusions). First-order goals are to determine 
the abundance and distribution of ground ice and volatile-bearing secondary 
minerals as a measure of whether Mars' atmophile-element endowment was 
sufficient to have once supported a thicker, more Earth-like atmosphere and 
hydrosphere. Carbonates, for example, are theoretically crucial but unproven 
sinks for Martian CO [3]. Efforts to detect Martian carbonates by Earth- 
based remote sensingZhave not been successful [4] .although calcite has been 
found in at least one SNC meteorite (EETA79001) [ 5 ] .  Mars Observer may be 
able to detect and map carbonates but direct laboratory study of soils and 
sediments is the only effective way to determine the times and modes of 
carbonate formativn. 
2. Do weathering products of various ages indicate systematic 
climate changes? In the absence of abundant biogenic fossils, Martian 
paleoclimates must be deduced from the mineralogical and stable-isotopic 
compositions of fossil weathering products in soils. Under current conditions 
on Mars, gas-solid weathering reactions should predominate and few genuine 
clay minerals should be forming [ 6 ,  71. Some evidence suggests, though, that 
clay mineral(oid)s might be major components in Martian soils [8], indicating 
metastable preservation after formation in environments dominated by liquid- 
solid, water-based reactions. Critical issues are how and when liquid water 
became available to drive weathering, whether silt- and clay-sized materials 
contain true phyllosilicates or simply palagonite/allophane phases, and uJI,c;.lp; s:ub;ta a ti ~2 quantities of zeolites were formed [9]. Ratios of D/H, 
O/ 0 in secondary minerals should permit calibration of 
weathering temperatures, provided that isotopic compositions of other 
participating volatile reservoirs (ground ice, atmospheric water vapor and 
COz, etc.) can also be determined. Exchangeable ions in clay minerals and 
zeolites would reveal the chemistry of the solutions with which they were last 
equilibrated. In addition, gases occluded in the same phases might provide 
samples of the Martian atmosphere as it existed durink vffious weathering 
epochs. Absence of nitrates and absence of significant N/ N fractionations VJ f-,egolith phases must be confirmed as tests for models for atmospheric 
N/ N fractionation and their implications for loss of the early Martian 
atmosphere. Finally, analysis of cosmic-ray-produced particle tracks and 
nuclides in rock surfaces and layered regolith samples would permit inference 
of how atmospheric shielding (i.e., density) has changed over time [lo]. To 
date climatic periods, ages of old soils and sediments might be obtained by 
the Sm-Nd chronometer (Rb-Sr might be too susceptible to disturbance by 
weat ering) whereas younger samples might be dated by the U-Th disequilibrium 
and "Cl methods 
3. Did mechanisms and rates of chemical weathering change through 
time and did they fractionate surface compositions? Martian 
surface materials receive high doses of ultraviolet radiation [ll] and, as 
1. What a % undances of potential atmosphere- or ocean-forming 
https://ntrs.nasa.gov/search.jsp?R=19890008949 2020-03-20T04:07:33+00:00Z
MARTIAN *WEATHERING PRODUCTS AS CLIMATE INDICATORS 
Gooding, J. L. 
89 
either a consequence or coincidence, contain trace quantities of oxidants that 
react vigorously with water and simple organic compounds [12]. It has been 
speculated, though not established, that ultraviolet-catalyzed reactions have 
dominated chemical weathering on Mars [13], including formation of unusual 
peroxides or superoxides that would destroy Martian organic compounds. 
Clearly, though, W-driven reactions can only be important in the outer few 
micrometers of the Martian surface so that it is imperative to understand 
whether different weathering processes operate at depth and how styles of 
weathering might have changed through time. Independent of W influence, 
low-temperature weathering can drastically fractionate K from other 
lithophiles [ 141 , producing misleading impressions of the 40K contents (an 
index for internal heat generation) of the parental materials. Mars Observer 
will provide global maps of K abundance that can be correctly interpreted only 
if weathering effects are understood through sample studies. 
4 .  What samples are needed? Paleoclimate studies must distinguish soils 
from sediments [2], requiring regolith drill-core samples that document the 
zonal structures and parent/daughter material relationships that are essential 
features of soils. Interpretation of duricrust genesis (cyclical vs. recent- 
only processes) must also rely on search for multiple duricrust layers in 
continuous cores. Process-oriented weathering studies will require chips (or 
short drill cores) of natural surfaces from large rocks of various types and 
ages so that contemporary rock weathering can be distinguished from pedogenic 
or fossil weathering recorded in the regolith. At the minimum, Noachian-, 
Hesperian- , and Amazonian-age units should be sampled to establish basic 
weathering and volatile-sink trends through time. In addition, atmospheric 
dust should be sampled to establish its possible traceability to known units 
or weathering epochs. 
Stable-isotopic and trapped-gas studies of ground ice are essential 
complements to similar studies of soils. It is doubtful, though, that ground 
ice can be sampled within a few meters of the surface at any location outside 
the polar circles. Based on geomorphic evidence, ground ice is rare to absent 
within & 30' latitude of the Martian equator but increases in abundance toward 
the poles [13]. According to model calculations, ground ice could survive on 
Mars for 10 y if covered by at least 10 m of ultrafine-grained and 
effectively non-porous soil [16]. For all geologically reasonable values of 
regolith particle-size and porosity, though, diffusion of water vapor from 
buried ice into the atmosphere should preclude survival of temperate-latitude 
ground ice over Martian geologic history at any depth shallower than 100 m 
[17]. Accordingly, acquisition of ground ice samples will probably require a 
mission to a polar site. 
All samples must be protected from thermal degradation and from both 
pro- and retro-grade, mineral/volatile reactions. Important paleoclimate 
indicators are expected to be metastable mineral assemblages with stable- 
isotopic signatures that are susceptible to further atmospheric exchange 
reactions. An upper limit of 263' K for stora e should preserve thermally 
labile compounds and ice but a design goal of 240 K should be implemented to 
control desorption and reactivity of atmospheric gases. References : 111 
Comnittee on Planetary and Lunar Exploration (1978) Strategy for Exploration of the Inner Planets: 1977- 
1987, Space Science Board, National Academy of Sciences, Washington, D.  C., 97 p. 121 Gooding J. L. (1986) 
I n  M. G. Kivelson (4.) The Solar System: Observations and Interpretations, Prentice-Hall, 208-229. 131 
Kahn R. (1985) Icarus, 62, 175-190. 141 Blaney D. L .  and McCord T.  B. (1987) Lunar Planet. Sci. XVIII, 
Lunar and Planetary Institute, Houston, 85-86. 151 Gooding J. L . ,  Uentworth S. J . ,  and Zolensky M. E. 
(1987) Lunar Planet. Sci. XVIII, Lunar and Planetary Institute, Houston, 345-346. 161 Gooding J. L. (1978) 
Icarus, 33, 483-513. 171 Gooding J. L. and Keil K. (1978) Geophys. Res. Lett., 5, 727-730. 181 Banin A. 
and Hargulies L. (1983) Nature, 305, 523-525. 191 Gooding J. L .  (1986) I n  Carr M., James P., Leow C., 
Pepin R. and Pollack J. (eds)., Workshop on the Evolution of the Martian Atmosphere, L P I  Tech. Rpt. 86-07, 
Lunar and Planetary Institute, Houston, 17-18. 1101 Arvidson R. E., Hohenberg C. M. and Schirck J.  R. 
(1981) Icarus, 45, 250-262. 1111 Levine J. S., Kraemer, D .  R., and Kuhn W. R.  (1977) Icarus, 31, 136. 
1121 Klein H. P. (1978) Icarus, 34, 666-674. 1131 Huguenin R.  L. (1976) Icarus, 28, 203-212. 1141 Gooding 
J. L. (1986) Geochim. Cosmochim. Acta, 50, 2215-2223. 1151 Squyres S. and Carr M. H. (1986) Science, 231, 
249-252. 1161 Smoluchowski R.  (1968) Science, 159, 1348-1350. 1171 Cl i f ford S. M. and H i l l e l  D. (1983) 
J. Geophys. Res., 88, 2456-2474. 
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Martian weathering products as tracers of climate change and atmosphere/hydrosphere evolution on Mars

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