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Full text of "NASA Technical Reports Server (NTRS) 19890008951: Mars sampling strategy and aeolian processes"

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92 


N89 - 18322 

MARS SAMPLING STRATEGY AND AEOLIAN PROCESSES; 
Ronald Greeley, Dept, of Geology, Arizona State University, Tempe, AZ 85287 

It is critical that the geological context of planetary samples (both in situ 
analyses and return samples) be well known and documented. Apollo experience 
showed that this goal is often difficult to achieve even for a planet on which surficial 
processes are relatively restricted. On Mars, the variety of present and past surface 
processes is much greater than on the Moon and establishing the geological context 
of samples will be much more difficult. In addition to impact gardening. Mars has 
been modified by running water, periglacial activity, wind, and other processes, all 
of which have the potential for profoundly affecting the geological intregity of 
potential samples. 

Aeolian, or wind, processes are ubiquitous on Mars. In the absence of liquid 
water on the surface, aeolian activity dominates the present surface as documented 
by frequent dust storms (both local and global), landforms such as dunes, and 
"variable features"-albedo patterns which change their size, shape, and position 
with time in response to die wind. Aeolian processes involve the erosion, 
transportation, and deposition of material by the wind and have the potential for: 

• "sorting" some materials by size, density, and composition 

• "homogenizing" some materials via dust storms 

• altering rock compositions by the formation of desert "varnish" 

• forming deposits which range in thickness from a few |im to tens of 
meters 

• eroding rocks through abrasion and deflation 

In some cases, these processes have a negative effect on sampling goals; in other 
cases they may help achieve sampling goals. In all cases, the potential effects of 
aeolian processes must be taken into account in developing sampling strategies. 

Wind sorting. In some cases, wind is an effective agent for separating 
materials. Depending upon wind strength, fine material is deflated, transported by 
the wind, and redeposited in other areas. In the process, windblown materials 
become sorted by size and density. The surface which remains is also altered; large 
or high-density particles may remain as a lag deposit, contributing to the 
development of desert "pavement" surfaces. 

Homogenization. The fine material carried into the atmosphere via dust 
clouds has the potential for becoming thoroughly mixed. Estimated to be less than a 
few microns in diameter, martian dust-cloud material is globally transported. 


MARS SAMPLING STRATEGY AND AEOLIAN PROCESSES 
R. Greeley 


93 

Thus, dust derived from diverse geological sources may become homogenized in 
the dust cloud and, as such, may reflect a type of planetary compositional average. 

Desert varnish. In many desert regions on Earth, rocks are commonly 
coated with a veneer of dark material, termed desert varnish. Although its origin is 
somewhat controversial, one commonly accepted model involves windblown dust 
which adheres to surfaces and reacts with dew to form a layer which may be a 
centimeter or more thick. Compositional measurements obtained via remote 
sensing or through shallow surface sampling thus may represent only the varnish 
and may be markedly different from the "host" rock. Although desert dew or its 
equivalent is unlikely on Mars today, a type of martian varnish may form through 
other processes. For example, laboratory experiments simulating Mars show that 
under the high winds required to transport material in the low-density martian 
atmosphere, some particles impact rocks and adhere to the surfaces. Although the 
amount of water in the martian atmosphere is miniscule, over the long periods of 
exposure typical for martian surfaces, reactions may occur in which the adhering 
particles react with the host rock to develop a desert-like varnish. 

Sedimentary deposits. Smooth plains deposits are found in many regions 
of Mars, including the layered terrain of the polar areas, mantling deposits found in 
some equatorial regions, as intercrater plains deposits, and as crater-filling units. 
Although the origin of some of these units is generally accepted as aeolian (e.g., the 
polar layered material), the sources and evolution of some of the smooth plains is 
highly controversial. For example, mantling material in the Memnonia region has 
been proposed to be widespread ignimbrite deposits. As such, they could be 
extremely important sampling sites. Alternatively, these materials could be 
deposits of windblown dust. Similarly, other smooth plains deposits on Mars have 
also been suggested as volcanic in origin, yet could also be aeolian. 

Erosion and deflation. Erosion of the surface by the wind and deflation of 
weathered materials in some areas may enhance sampling efforts. Low-albedo 
surfaces, including those found in the "wake" zones of some craters, may represent 
eroded and deflated bedrock surfaces and could be high-priority sites for obtaining 
relatively fresh samples. 

In summary, aeolian activity dominates the present surface of Mars and 
appears to have played an important role in the evolution of its surface. 
Recognizing the occurrence of windblown deposits and the potential for aeolian 
processes in modifying the surface must be taken into account in developing 
sampling strategies.