SAMPLE RETURN AND CLIMATE.

Michael H. Carr
U.S. Geological Survey
MenloPark, CA

Understanding the climate history of Mars is essential for unravelling the geologic history of the planet and assessing its potential for biologic evolution. Suspicion that climates in the past have at times been very different from those that presently prevail is based largely on surface morphology, particularly the presence of the seemingly water-worn valley networks. The climate history is, however, very uncertain because of ambiguities in both the geomorphic interpretations and the ages of the relevant features. Possibly the strongest evidence for climate change is the unambiguous change in erosion rates at the end of heavy bombardment. Many Noachian craters tens of kilometers across are highly degraded suggesting erosion rates comparable to terrestrial rates. In contrast craters only a few kilometers across that formed after the Noachian are mostly well preserved, suggesting that erosion rates dropped by two to three orders of magnitude at the end of the Noachian. The evidence for climate change from the valley networks is less clear. Noachian units are the most heavily dissected. Over 70% of the valleys are incised into materials of this age. But the valleys continued to form throughout Mars' history as shown by the dissection of young volcanoes. The general consensus is that the valley networks formed by slow erosion of running water. If so, then warm climates must have occurred late in Mars' history. But many attributes of the valleys suggest other processes such as small floods and mass wasting may have contributed to their formation so that the evidence for climate change from the valleys, while suggestive, is not compelling. The large floods are also of climatologic interest for they indicate the presence on the planet of large amounts of water, and these very large events may have induced changes in climate. Modelling studies show that it would have been very difficult to induce major climate changes on early Mars because of the lower energy output of the Sun. These studies suggest that cloud formation in a CO2/H2O atmosphere would prevent significant greenhouse warming at the surface. Later in Mars' history a 3-5 bar CO2 atmosphere could raise surface temperatures above 273 K and so allow liquid water at the surface. Carbonate deposits have not however been detected and these should be close to the surface if thick atmospheres had been present late in Mars' history. Thus the climate history of Mars is extremely uncertain.

Many of the uncertainties could be resolved with returned samples. Samples of the atmosphere, the regolith and the rock record would all contribute. The atmosphere contains a record of it origins, subsequent additions, exchange with the surface, and losses to various sinks such as space and carbonate deposits. The record is mostly in the isotopic composition of the noble gases and various atmophile elements such as C, N, and O. Losses to space appear to have been the dominant factor in setting the isotopic composition of the present atmosphere but other processes have clearly been involved. Precise determination of composition of the present atmosphere will allow more precise determination of its evolution. The composition is important in itself and also as a reference against which to compare the same gases trapped in polar deposits, weathering products, ancient glasses, and so forth. For the first sample return, elaborate sample mechanisms are not needed. Gases trapped in the head space of containers for other materials will be adequate.

The regolith must also contain information on climate. The source of the fine-grained air-deposited materials at the two Viking landing sites is not known. It is thought to be largely weathered material, but there must be other components such as volcanic dust and meteoritic debris. Because of the low erosion rates, and by inference low weathering rates, throughout Mars' history, the weathered material is suspected to have mostly originated early in Mars history. Some indication as to whether this is true may be shown by model ages for the weathered materials. The chemistry and mineralogy of the regolith materials will provide us with indications of past climatic conditions. Weathering involves chemical interaction of the atmospheric species with silicate rocks. The isotopic composition of the weathered products have, therefore, the potential for indicating the isotopic composition of the atmosphere at the time of weathering. D/H and O isotopes are of particular interest. A seemingly salt-rich layer, or duricrust, was observed at both Viking landing sites. The layer indicates that vertical migration of salts has taken place within the soil profile. It is not clear that this migration can occur under present conditions, although it might, so the duricrust may provide information on climatic excursions that have taken place since the regolith materials were deposited. The requirements for a regolith sample on the first mission could be very simple. One to two scoops amounting to 100 gm should be adequate. The soil could also be used as a filler in rock-sample containers. Vertical profiles through the duricrust could be acquired on subsequent missions.

Three main types of rocks are probably available for sampling: igneous rocks, breccias and sediments. For climate studies igneous rocks are probably the least desirable. They may, however, contain information on weathering subsequent to deposition and on the composition of rock-altering fluids. They can also be used to calibrate the crater ages, thereby enabling the dating of events of climatological interest, such as large floods. Impact breccias from the era of heavy bombardment are of considerably more interest. They date from the era for which we have the best evidence for climate change. They consist of clasts of different types of rocks, and so provide a means of sampling the diversity of Noachian rocks, which could include sediments. Moreover, impacts can implant contemporary atmospheric gases in rocks, so that the breccias may provide direct evidence on the composition of the early atmosphere. Sediments and precipitates, being the products of climate-sensitive processes may provide a more direct indication of past climates. Various kinds of sediments are probably accessible. Included are alluvium on outwash plains of large floods, lake deposits at the ends of outflow channels, fluvial terraces and bars, layered canyon deposits, and sediments in lakes fed by valley networks. Precipitates might include carbonates and other salts such as sulfates and nitrates. Of all these possibilities ancient lake beds are probably the most attractive. They will reveal information on chemical and physical conditions within the lake, the nature of contemporary weathering, the episodicity and carrying capacity of the feeder streams, the composition of contemporary volatiles and so forth. Any ancient lake deposits are likely to be covered with younger deposits but samples should be available around impact craters. Impact craters are useful also in that they probably excavate materials from a variety of depths. For an early mission not knowing the exact position of the samples in a section is less important that acquiring variety. Lake deposits from outflow channels are also of interest for climate studies. Some may contain thick ice deposits that formed shortly after the flood event. Other types of sediments such as fluvial terraces and bars may be better studied by rover missions than sample return missions because much of the interest is in macrostructures such as cross-bedding rather than in the characteristics of small samples.

The need for mobility at the sample sites deserves emphasis. At any landing site we need to be able to view the scene and sample the diversity of the site. At a lake site, for example, we will need to go to crater to sample material excavated by the crater. The mobility needs will depend in part on landing accuracy. The size and number of samples needed cannot be specified with assurance but multiple samples each weighing on the order of at least a hundred grams are likely to be needed. Should rock chips be present at the site then a rake sample would be useful.

In conclusion, the prime sample sites for climate studies are ancient lake beds and other ancient highland rocks. Simple atmosphere and regolith samples are also needed. A variety of small rock samples should be collected at each site, which probably implies having a rover.

Vugraphs:

VG 1: Geologic Evidence for Climate Change

VG 2: Erosion Rates

VG 3: Valley Networks

VG 4: Climatic Implications of Valley Networks

VG 5: Climatic Implications of Valley Networks (Cont'd)

VG 6: Glaciation, Lakes and Oceans

VG 7: Loose Air-Deposited Surface Materials

VG 8: Polar Deposits

VG 9: Conclusions

VG 10: Regolith and Climate

VG 11: Regolith and Climate (Cont'd)

VG 12: Massive Air Fall Deposits and Climate

VG 13: Sediments and Climate

VG 14: Sediments and Climate (Cont'd)

VG 15: Chemical Precipitates

VG 16: Polar Deposits

VG 17: Other Materials

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