Progress in the following research areas is discussed: (1) volatile ice sublimation in a simulated Martian polar environment; (2) a global synthesis of Venusian tectonics; (3) a summary of nearly a decade of field studies of eolian processes in cold volcanic deserts; and (4) a model for interpretation of Martian sediment distribution using Viking observations. Some conclusions from the research are presented

Malin, M. C.

English

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N92-10940
PLANETARY GEOMORPHOLOGY RESEARCH: FY 1990-1991
M. C. Malin, Department of Geology, Arizona State University, Tempe, AZ 85287-1404
Progress in research supported by NAGW-1 included completion of the dissertation of Jeffrey
Moore (volatile ice sublimation in simulated martian polar environment), continuation of
research of Robert Sullivan (hillslope processes on Mars), and completion of a global
syntheses of venusian tectonics (M. Malin with R. Phillips and R. Grimm of Southern
Methodist University). Work also continued on completing papers summarizing nearly a
decade of field studies of eolian processes in cold volcanic deserts (M. Malin) and on a new
model for interpretation of martian sediment distribution using Viking observations (M. Malin
with P. Christensen, ASU).
Moore's dissertation research examined the effects of non-volatile mantling materials on the
sublimation rate of water ice under simulated martian conditions. Using the Mars Environment
Simulator chamber described in previous progress reports, Moore examined the effects of
single layers of either "dust" (silt -10 microns in diameter) or "sand" (modal size -125
microns) on the rate of sublimation of water ice granules at 3 mb 95% CO2 and 5% Earth
atmosphere (i.e., 4% N2, 0.6% 02, 0.25% Ar, etc.). In order to observe effects within the
timescale available for experiments, temperatures were elevated with respect to martian
conditions, and results then scaled to Mars. Moore was also able to examine the effect on
sublimation of atmospheric vapor transport using as a circulation pump. The principal variable
examined was mantle thickness, which was increased from 1 mm to 5 cm in roughly
logarithmic increments.
Three major conclusions can be drawn from the first set of experiments performed. First, the
rate of mantle-free ice sublimation was nearly an order of magnitude smaller than that predicted
by Ingersoll's 1970 model, which has served as the basis for most subsequent treatments.
More recent models (but not yet published, e.g., that by Haberle and Clow) appear more in
concert with the experimental results. Second, atmospheric transport of even a small amount
of latent heat and water vapor can play an important role in ice sublimation when ice is exposed
or mantled by thin layers of sand. Differences between the effects of dust and sand mantles on
"wind-induced" sublimation were observed and are interpreted to arise from the control of
diffusion rate by pore size and partial pressure. Third, variations in sublimation rates resulting
both from mantle thickness and from mantle particle size were investigated--in general, mantles
.greater in thickness than 5 cm, of either grain-size material, effectively insulated the underlying
ice from sublimation. This result can be used to understand the role non-volatile mantles play
in the evolution of the martian polar deposits.
The work of Sullivan will be reported at a later date by Sullivan and Jonathan Fink, who is
now Sullivan's research and thesis advisor.
In an attempt to balance the "plate"-oriented view of venusian tectonics espoused by J. Head
and co-workers over the past five years, R. Phillips, R. Grimm, and M. Malin prepared a
synthesis ,,°f the SMU/ASU. research, for publication, in Science. (3 May. 1991 issue) . This" p re -
Magellan work outhnes a vertmal tectomcs model whereto venus_an hot spots create plateaus
of uplifted and underplated crust that evolve into tesserae with time. Deformation is distributed
both laterally and vertically (as opposed to occurring in specific, narrow zones). The model
provides an integrated view of venusian structure and geophysics.
A synthesis of results of a 4-year study of eolian processes in the cold volcanic desert of central
Iceland was completed for publication. By judicious choice of study sites, information on a
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broad rangeof topicspertainingtoeolianabrasionby basalticmaterialswas gathered.Material
propertiesas a functionof heightvarydramatically,both incompositionand caliber,depending
on wind speed and surfaceconfiguration.Only the simplestsurface(onecomposed entirelyof
well-sortedeolian sand) appears to be successfullyrepresentedby present analyticaland
numericalmodels. Sand transportas a functionof tirncishighlyepisodic.Flux mcasm'cn_nts
suggestthatperiodsof sand transportwere relativelyshortand separatedby long periodsof
littleor no transport. Abrasion appears to have occurred during periods of wind speeds
demonstrably higher than those necessary to initiateand sustainmotion. Together, these
observationssuggest thateolianprocesses may bc relativelycatastrophic,reflectingbrief,
intensewinds. For rocks protrudingabove the surfaceby only a few centimeters,abrasionis
very low because particleimpact speedsarcsmall.Some evidence suggestsitin_s rapidly
to a maximum, and then decreases with increasingheight. However, thispatternappears
sensitive to many factors, and the stochastic nature of air flow and particle motion greatly affect
what happens at a given location. Combined with effects associated with the physical
properties of the target rock material itself, including variations in the properties on the scale of
individual sand grains, it is unlikely that a simple model of rock abrasion with height can be
applied to more than an arbitrarily small and hence um'epresentative number of cases. The
implications for Mars of these and other observations were also considered. They can, for
example, be used to support the conclusion that infrequent winds of sufficient magnitude to
cause major abrasion is the primary cause of the apparent lack of abrasion of martian rocks as
seen in the Viking Orbiter images.
Finally, a model for the interpreting the elastic sediment distribution on Mars has been
developed with P. Christensen. This model is based on three major assumptions: 1) surface
albedo is proportional to the fraction of the surface covered by a thin deposit of high albedo
dust, 2) thermal inertia, in particular the "fine-component" inertia (average minus rock
contribution), reflects the effective "mean" particle size of wind-transportable fine particles, and
3) "rock fraction" is directly related to the thickness of the mantle of fine particles as they fill-in
and cover non-wind-transportable materials. The model is used to investigate sediment
transport and depositional patterns in both regional and global context. It is found, for
example, that the global mean thickness of transportable fines, averaged over 1 degree square
areas (3600 km 2) is about 35 cm, and that the thickest deposit on that scale is less than a meter
thick. It is thus possible, using this model, to reconcile thermophysical interpretations of major
martian surface features (e.g., the low inertia of the Tharsis volcanos) with photogeological
analyses.
Owing to funding limitations in FY 1990, no funds were available after October 1990 to
support research other than for student stipends. FY 1991 funds have not been received as of
this date (22 May 1991), and owing to the change in location of the Principal Investigator, the
prognosis for funding in the near future is poor. Future work in Planetary Geomorphology,
contingent upon receiving funding, and following recommendations of the LGPRP, will focus
on examining the thermophysical properties of deposits of non-volatile particulates using the
Mars Environmental Simulator Chamber to which most of the funding derived from NAGW-1
has been devoted the past three years. The results of this study will be used to develop
observational strategies and predictive tests for remote sensing observations of Mars using the
Mars Observer Thermal Emission Spectrometer, on which the proposer is Co-Investigator.
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Planetary geomorphology research: FY 1990-1991

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