It has been known since 1972 that many Martian craters have dark features on their floors, and that when seen at higher image resolution, some of the dark units are dune fields. Interpretations of thermal inertia derived from Viking Infrared Thermal Mapper (IRTM) data have been used to suggest that many dark intracrater features, including those where dunes are not observed in images, contain some amount of sand or particles in the range 0.1-10 mm. However, it has never been known if all these dark features consist of dunes. We assembled a set of 108 carefully constrained Viking IRTM observations for dark crater-floor units. The data and selection criteria are described in detail elsewhere. Studied in conjunction with Mariner 9 and Viking orbiter images of each crater, these data indicate that the dark crater-floor units in some regions have different thermal properties than those in other regions. Thermal inertias were computed using the Viking thermal model of H. H. Kieffer and corrected for atmospheric CO2 effects using the relationship for a dust-free atmosphere shown by Haberle and Jakosky

Christensen, P. R.

Edgett, K. S.

English

NASA Technical Reports Server

LPI Technical Report 93-06, Part l 17
as much as a billion years or more for the martian groundwater
system to achieve hydrostatic equilibrium, and the -2--4 km eleva-
tion difference between the outflow channel source regions and the
northern plains, the water confined beneath the frozen crust of the
northern plains should have been under a significant hydraulic head.
Thus, the existence of a hydraulic pathway between the ponded
flood waters above the northern plains and the confined aquifer
lying beneath it would not have led to the infiltration of flood water
back into the crust, but rather the additional expulsion of groundwa-
ter onto the surface.
A more detailed discussion of the fate of flood waters discharged
by the martian outflow channels is currently in preparation.
References: [I] Clifford S. M. (1993)JGR, 98, 10973-11016.
12] Carr M. H. (I 979) JGR, 84, 2995-3007. [3] Brace W. F. (1980)
Int. J. Rock Mech. Min. Sci. Geochem. Abstr., 17. 241-251.
N94-33203
REGIONAL SEDIMENTOLOGICAL VARIATIONS
AMONG DARK CRATER FLOOR FEATURES: TOWARD
A MODEL FOR MODERN EOLIAN SAND DISTRIBU-
TION ON MARS. K.S. Edgett and P. R. Christensen, Department
of Geology, Arizona State University, Tempe AZ 85287-1404,
USA. : = :
It has been known since 1972 tlaat ma-ny n_artian craters (_<25kin
diameter) have dark features on their floors, and that when seen at
higher image resolution, some of the dark units are dune fields [ t-3]."
Interpretations of thermal inertia derived from Viking Infrared Ther- -
mal Mapper (IRTM) data have been used to suggest that many dark -
intracrater features, including those where dunes are not observed
in images, contain some amount of sand or particles in the range 0.1-
l0 mm [4,5]. However, it has never been known if all these dark
features consist of dunes.
We assembled a set of 108 carefully constrained Viking 1RTM
observations for dark crater-floor units. The data and selection _
criteria are described in detail elsewhere [6-9]. Studied in conjunc-
tion with Mariner 9 and Viking orbiter images of each crater, these
data indicate that the dark crater-floor units in some regions have
different thermal properties than those in other regions [9'-9]. Fig-'__
00
Total (108}
2OO
Margadtifer Sinus (8} _ =
_--U"---_ lapygia (11)
¢ , Oxi* (6)
_-_t1---.--_ MareCimmerium(_)
_-----==tl---_ Mare Erytlvaeum(14)
MareSirenum('29)
Hellespontus(7)
_--I1------* SouthwesternArabia{3)
NorthwesternArabia(2)
300 400 500 600 700
Thermal Inertia (J m -2 s-°5 K-t)
Fig. I. Mean and standard deviation oftherrnal inenias for dark intracrater
features in different regions. Numbers in parentheses indicate total craters
examined. To convert thermal inertia to units of 10-_ cal cm -2 s_-_ K-J. divide
by 41.84.
particle size inferred for each, but sand (0.06-2 mm) is probably the
dominant particle size [5,9,14].
In an unvegetated environment, the form and scale ofeolian sand
deposits are functions of sand availability, grain size distribution,
wind energy and directional variability, the presence of topographic
obstacles (e.g, crater walls), and climatic variations that might affect
any of these factors [ 15]. In terms of sand-deposit morphology, grain
size is probably not a significant control except among zibars [ 16].
Barchan and transverse dunes are typical of unidirectional wind
regimes; their differences are largely considered a function of"sand
supply": the amount of loose sand available for eolian transport in a
region [17]. Barchans form in areas of low sand supply, though
transverse dunes do not form exclusively in places of high sand
supply [I 8]. Transverse dunes on Mars can be large deposits like
ure I shows thermal inertia means and standard deviations for dark those in Hellespontus or they can be small and difficult to identify
intracrater units in nine different regions. Thermal inertias were _ without high-resolution images (an example occurs in Pettit Crater
computed using the Viking thermal model of H. H. Kieffer and _ [19]).
corrected for atmospheric CO 2 effects using the relationship for a To first order, the areal coverage of eolian sand (dunes, drifts,
dust-free atmosphere shown by Haberle and Jakosky [10ff Th_:_sheets) may be the main factor causing the observed regional differ-
thermal inertias and interpreted particle sizes in Fig. I are regarded ences in the thermal properties of Iow-albedo inlracrater units [8,9].
as upper limits, with lower limits (due to suspended dust in the
atmosphere) perhaps 50-200 J m -2 s --n.5K-1 less than shown [ 10, I I].
However, because the atmosphere had a nearly uniform dust opacity
from L, 344°--125 ° over the regions examined [12], the relative
differences between regions in Fig. I are genuine [9].
The thermophysical differences illustrated in Fig. I [also see 7-
9] are probably related to regional variations in the amount of
surface covered by sand and perhaps dunes. In two of these regions,
Hellespontus and Oxia, the thermal differences are consistent with
an observed difference in the morphology of dunes comprising the
dark features. In Hellespontus there are large transverse dunes
while in Oxia there are fields of small barchans [2,9,13]. The
regional differences are independent of the exact thermal inertia and
The percentage of dune cover [8] may be similar among craters
within a given region, but different between regions. The amount of
sand transported and deposited is likely related to two main factors:
sand supply and wind regime.
Sand sources that are regional in extent might include pyroclastic
deposits laid down over a vast area, or perhaps fluvial and lacustrine
strata. Detection of such sources will require high-resolution imag-
ing or investigation on the planet's surface to find deposits or out-
crops of volcaniclastic or fiuviolacustrine sandstones from which
dark sand might have eroded. Location of sand sources will also
require remote multispectral observations to determine the miner-
alogy of the sediment and to trace wind-worked sand back to source
areas.
https://ntrs.nasa.gov/search.jsp?R=19940028697 2020-06-16T12:16:01+00:00Z
]8 Results from the MSATT Program
Regional wind regimes are important because the wind distrib-
utes sand over an area, removes material from a source, and forms
deposits such as dunes. Sand has been or is being distributed in the
classical albedo regions of Mars in different ways. In Hellespontus,
for example, there appears to be a considerable volume of sand
N94- 33204
AEROSOLS SCATTERING AND NEAR-INFRARED OB-
SERVATIONS OF THE MARTIAN SURFACE. S. Erard,
IAS-Planetologia, viale dell'Universita I I, 00185 Roma, Italy.
Introduction: The presence of a scattered contribution in the
present, but it is entirely piled up into large transverse dunes. In
loose sand from the surrounding areas and deposit the sand in
craters.
In contrast, some of the sand from small barchans in Oxia has been
removed from the craters to form dark streaks [20]. In places, the
streaks coalesce to make larger, Iow-albedo units like Oxia Palus
(9°N, 16°W). Barchans indicate that the amount of loose sand avail-
able for transport in Oxia is small relative to Hellespontus, yet the
winds might have been more vigorous in order to facilitate the
movement of sand in and out of craters and prevent the accumulation
of fine, bright dust over the whole region. The coalescence of dark
streaks forming Oxia Palus may provide a model for the formation
Of Iow-albedo regions like Mare Cimmerium, Mare Sirenum, or
Margaritifer Sinus, where the winds may be stronger [21] and the
sand supply might be greater than in Oxia, thus causing sand to be
more widely distributed. Alternatively. the sources of sand in Iow-
albedo regions might be more ubiquitous. Perhaps sediment beds
are currently eroding and feeding the present-day dark eolian depos-
its found throughout the Iow-atbedo regions.
Which of the two influences, broadly termed "wind regime" and
"sand supply," has had the greatest influence on the nature of
eolian sediment deposition and removal in different martian re-
gions remains to be understood. One additional factor is the question
of climatic change; for example, were the huge piles of sand in the
Hellespontus craters deposited under climate conditions that were
different from the present?
References: [I] Sagan C. et al. (1972) Icarus, 17, 346-372.
[2] Cutts J. A. and Smith R. S. U. (1973) JGR. 78, 4139-4154.
[3] Arvidson R. E. (1974) Icarus, 21, 12-27. [41 Peterfreund A. R.
( 1981 ) Icarus, 45, 43,7-467. [5] Chrlstensen P. R. (1983) Icarus, 56,
496--518. [61 Edgett K. S. (1990) M.S. thesis, Arizona State Univ.
[71 Edgett K. S. and Christensen P. R. (1991 ) LPSC XXII, 335-336.
[8] Edgett K. S. and Christensen P. R. (I 992) LPSC XXIll, 325-326.
[9] Edgett K. S. and Christensen P. R. (1993) JGR, submitted.
[ 101 Haberle R. M. and Jakosky B. M. ( 1991 ) Icarus, 90, 187-204.
[ I 1] Paige D. A. et al. (1993) JGR, submitted. [ 121 Martin T. Z. and
Richardson M. I. (1993) JGR, 98, 10941-10949. [131 Thomas P.
(I 984) Icarus, 57,205-227. [ 14] Edgett K. S. and Christensen P. R.
(i991) JGR, 96, 22765-22776. [151 Pye K. and Tsoar H. (1990)
Aeolian Sand and Sand Dunes, 218-220, Unwin and Hyman, Lon-
don. [ 161 Lancaster N. (1989) The Namib Sand Sea, I I I- I 12, A. A.
Baikema, Rotterdam. [ 17] Wasson R. J. and Hyde R. ( 1983)Nature,
304, 337-339. [18] Rubin D. M. (1984) Nature, 309, 91-92.
[ 19] Zimbelman J. R. ( 1986)Icarus, 66, 83-93. [20] Thomas P. et al.
(1981) Icarus. 45, 124-153. [21 ] Greeley R. et al. (1993) JGR, 98,
3183-3196.
atmosphere of Mars is a major problem for spectroscopic observa-
Heilespontus, there appears to be little dark material that is not piled tions of the surface in the infrared since the main mineralogical
up in dunes.Thisis incontrast with craters inlow-albedo regions like absorptions have a typical depth of 1% and could be easily masked
Mare Cimmerium or Margaritifer Sinus, where there are many _-- Zor subdued by atmosphere scattering. An estimate of the aerosol
extracrater dark deposits. The configuration of dunes in Hellespontus contribution between 0.77 and 2.6 pm was previously derived above
might have resulted from winds that were strong enough to strip Tharsis from ISM imaging spectroscopic data acquired from the
Phobos 2 spacecraft in 1989 [l].h is used here to investigate the
effect of the scattering on the criteria that allow the mineralogical
characterization of the surface.
-- Aerosols Conlr_bui|on: Under low opacity and near-normal
viewing geometry (the conditions of ISM observations) multiple
atmospheric scattering can be neglected. Because the martian aero-
sols are very bright and strongly forward-scattering in the near-IR,
the radiance factor can be further approximated as the sum of the
surface reflectance and the backscattering [2]. This model was used
to derive an estimate of the scattered spectrum, taking advantage of
the overlap between two image cubes in the region ofPavonis Mons
g
0.0
(b)
0
$_,rll* [
sl_t ,-_
JY
_/u_o©, + I¢o'_ll'lng}
$ _.0 |.$ _.0 2.$ _kO
Wovelength (,u.m)
°3=::
\V/
I _:_t;7- .--'-"_ _" /- _' - '
|
Wovelength (k_m)
Fig. 1. Average spectraofthe six main spectral units in the Syrtis Major-lsidis
Planitia image-cube: (a) calibrated spectra:(b) after removal of the estimated
scanering.
..¢.


Regional sedimentological variations among dark crater floor features: Toward a model for modern eolian sand distribution on Mars

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