The potential is described of a candidate Mars Observer altimeter for determining dielectric properties of Mars regolith. It is pointed out that it is straightforward to use the time between altimeter pulse trains for passive radiometry (hence dielectric properties) and roughness can be derived. Given the mission plan the whole surface can be mapped at least three times, yielding data on seasonal variability

Frey, H. V.

Garvin, J. B.

Smith, D. E.

Solomon, S.

Ulaby, F. T.

Zwally, H. J.

English

NASA Technical Reports Server

N89 - 1078 9 :
37
MARTIAN SURFACE PHYSICAL PROPERTIES TO BE DERIVED BY RADAR rl
ALTIMETER ON THE MARS OBSERVER SPACECRAFT
Garvin J. B. #, F.T. Ulaby @, D.E. Smith #, H.V. Frey #, S. Solomon $, and
H. J. Zwally#
#NASA/Goddard, Greenbelt, MD 20771; @Radiation Lab, Univ. Michigan,
Ann Arbor, MI 48109; $Dept. Earth and Planetary Sci., MIT, Cambridge,
MA 02139.
Two of the primary objectives of the Mars Observer (MO) mission are
global monitoring of those surface physical properties which can be linked
with the recent climatic history of Mars, as well as characterization of
planet-wide surface composition and mineralogy. While the main emphasis in
orbital radar altimetry is quite naturally the determination of the topography
of the planet, information concerning meter-scale roughness and the dielect-
ric properties of the upper surface layer can be inferred from analysis of
doppler and time-delay altimeter waveforms. In addition, it is relatively
straightforward to use the time between altimeter pulse trains for passive
microwave radiometry from which complementary data concerning surface emis-
sivity (and hence dielectric properties) and roughness can be derived. The
Pioneer Venus (PV) radar altimeter (17 cm wavelength), although in a highly
elliptical orbit, was able to acquire valuable information concerning the
roughness, radar reflectivity (r), and emissivity (e) of the venusian surface
at coarse spatial resolution (30-100 km) for virtually all of the planet
[I-3]. A 13.6 GHz (Ku-band or 2.2 cm wavelength) radar altimeter/radiometer
(FLAR), a candidate instrument for the Mars Observer [4], is capable of prov-
iding approximately I0 km resolution data on the surface roughness (C),
microwave brightness temperature (Tb), emissivity (e), Fresnel reflectivity
(r), and dielectric permittivity (k') for the entire martian surface. The
complete waveform resulting from spreading of the altimeter pulses is saved
(after being compressed) and used to derive the properties mentioned above
by means of both a Hagfors scattering model (e.g. as used by Pettengill [I]
and colleagues for PV, and to be used for Magellan altimetry), and an empir-
ical scattering law (based upon the solution of an inverse scattering matrix
problem as proposed by Tyler and Simpson for Magellan [5]).
As the MO is planned to be in a 360 km circular polar orbit, the deriv-
ed altimeter surface properties can be obtained for the entire planetary
surface at least three times (on the basis of the mapping orbit groundtracks),
and thus seasonal effects can be directly assessed. This may shed new light
on the proposition that seasonal variations in radar reflectivity (observed
from Earth) in certain regions of Mars (e.g. Solis Lacus) are caused by the
presence of liquid water on or near the surface [6,7]. Figure 1 is a theor-
etical volume mixing model approach for considering how much liquid water
would be required to cause the observed seasonal variation in the radar refl-
ectivity (i.e. a factor of 2) [8,9]. The model assumes that the water would
occur as either grain coatings (as in clays) or as a pore fluid in the reg-
olith or bedrock, and applies in general to radar reflectivity observations
above 1GHz [8,9]. As can be seen from the mixing curves ( one for each type
of surface, from very porous regolith to dense basalt [porosity less than
8 %]), between 8 to I0 vol. % liquid water would be required to cause the
observed reflectivity variations. While regolith porosities of 8-10 % may
be quite reasonable, saturation of the regolith by liquid water may not be
https://ntrs.nasa.gov/search.jsp?R=19890001418 2020-03-20T06:08:02+00:00Z
38
MARTIAN SURFACE PHYSICAL PROPERTIES
Garvin J. B. et al.
feasible on Mars except in regions where surface permafrost is dominant. For
average porosity igneous and sedimentary rocks, between 1 and 5 vol. % liquid
water would be required to account for the observed variations; thus, low
vesicularity basalts with a few vol. % pore water could explain the increase.
It should be noted however, that a few vol. % of a high permittivity phase
such as ilmenite, rutile, pyrite, or even simple hematite could cause the ob-
served reflectivity magnitudes [2,3,9]. For instance, 5-10 vol. % hematitic
coating on average regolith grains (perhaps basaltic ash) would easily lead
to reflectivities above 0.16 (the maximum observed), although a seasonal var-
iation would not be expected. Perhaps the deposition and subsequent removal
of porous eolian material (low permittivity) from a surface of lower porosity
and greater enrichment in alteration products (e.g. hematite or pyrite) would
serve as an alternate explanation to the seasonally varying reflectivity.
Whatever the case, surface-layer permittivities derived from radar altimeter
measurements on a seasonal basis should provide new insights on temporal
electrical property variations (and porosity?), and perhaps on the activity
of liquid water, especially if visual-IR mapping spectrometry can distinguish
iron oxides from "wet regolith".
The surface properties derived from a Ku-band RAR in circular martian
orbit are complementary and synergistic with other synoptic MO datasets such
as thermal emission spectroscopy, Visual-IR mapping spectrometry, ultra high-
resolution imaging (e.g. 1 m resolution), mm-wave radiometry, and HF-VHF
sounding radar (to be flown on the PHOBOSmission in 1988 [I0]). The depth
penetration capabilities of 2.2 cm radar depend on the loss properties of the
upper surface layer (and hence porosity and composition), but for Mars should
vary from about 2 cm to 30 cm (for loss tangents between 0.I and O.OOl).
More interesting is the possibility of detecting high permittivity regions
which could be associated with near-surface concentrations of liquid water
(as mentioned above and illustrated in Fig. I), or to deposits of metallic-
oxides or sulfides perhaps resulting from volcanism or weathering (or even
to very high Ti basalts such as those discovered on the Moon). The multi-
temporal seasonal coverage should pemlit some separation of mechanisms resp-
onsible for high permittivity surfaces to be established. The radiometer mode
, as with PV, will help to refine estimates of the magnitude of high dielect-
ric properties [2].
Surface physical/electrical properties derived from a RAR instrument on
MO are a potentially important global and seasonal dataset which will com-
plement measurements from other MO experiments (TEMS, VIMS, MOC etc.), and
lead to a better understanding of the interaction of geological and climat-
ological processes on the surface of Mars. This report merely illustrates
some of the possibilities for answering fundamental questions with such
measurements.
REFERENCES:
[I] Pettengill G. et al. (1980) JGR 85, 8261. [2] Ford P. and G. Petten-
gili(1983) Science 20, 1379. [3] Garvin J. et ai.(1985) JGR 90, 6859. [4]
Smith D. et ai.(1985) A Radar Altimeter for the Topography and Surface Prop-
erties of Mars, Proposal to NASA for AO no, 0SSA-2-85, the Mars Observer.
[5] Pettengill G. et ai.(1985) Magellan Sc_, Expt. Plan, 19-22. [6] Zisk S.
and P. Mouginis-Mark (1980) Nature 288, 735. [7] Roth L. et ai.(1985) LPSC
XVI, 712. [8] Sen P. et aI.(1981) Geo_hys. 46, 781. [9] Garvin J. and J. Head
(1986) LPSC XVII, 255. [I0] Armand N. (1986) NASA TM-88413, 30 pp. (translat-
ion of "Performance data on the Soviet PHOBOS mission "GRUNT" radar",
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Martian surface physical properties to be derived by radar altimeter on the Mars observer spacecraft

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