The presence of a scattered contribution in the atmosphere of Mars is a major problem for spectroscopic observations of the surface in the infrared since the main mineralogical absorptions have a typical depth of 1% and could be easily masked or subdued by atmospheric scattering. An estimate of the aerosol contribution between 0.77 and 2.6 microns was previously derived above Tharsis from ISM imaging spectroscopic data acquired from the Phobos 2 spacecraft in 1989. It is used here to investigate the effect of the scattering on the criteria that allow the mineralogical characterization of the surface

Erard, S.

English

NASA Technical Reports Server

18 Results from the MSA TT 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 delI'Universita I1, 00185 Roma, Italy.
=
Introduction: The presence of a scattered contribution in the
present, but it is entirely piled up into large transverse dunes. In atmosphere of Mars is a major problem for spectroscopic observa-
Hellespontus, 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.This is in contrast with craters in Iow-albedo regions like absorptions have a typical depth of 1% and could be easily masked
Mare Cimmerium or Margaritifer Sinus, where there are many _or 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
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 low-albedo 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] Cults J. A. and Smith R. S. U. (1973) JGR, 78. 4139-4154.
[3] Arvidson R. E. (1974) Icarus, 21, 12-27. [4] Peterfreund A. R.
(I 981 ) Icarus, 45,447-467. [5 ] Christensen P. R. (1983) Icarus, 56,
496--518. [6] Edgett K. S. (1990) M.S. thesis, Arizona State Univ.
[7] Edgett K. S. and Christensen P. R. (1991)LPSC XXH, 335-336.
[8] Edgett K. S. and Christensen P. R. (1992) LPSCXXIII, 325-326.
[9] Edgett K, S. and Christensen P. R. (1993) JGR, submitted.
[ I0] Haberle R. M. and Jakosky B. M. ( 1991 ) Icarus, 90, 187-204.
[ I I] Paige D. A. et al. (1993) JGR, submitted. [ 12] Martin T. Z. and
Richardson M. I. (1993) JGR, 98, 10941-10949. [13] Thomas P.
(1984) Icarus, 57, 205-227. [ 14] Edgett K. S. and Christensen P. R.
(1991) JGR, 96, 22765-22776. II51 Pye K. and Tsoar H. (1990)
Aeolian Sand and Sand Dunes, 218-220, Unwin and Hyman, Lon-
don. [ 16] Lancaster N. (I 989) The Namib Sand Sea, 111-112, A. A.
Balkema, 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.
Phobos 2 spacecraft in 1989 [IT.'h is used here to investigate the
effect of the scattering on the criteria that allow the mineralogical
: characterization of the surface.
Aerosols Cordribui|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-lR,
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
(a)
I&$
I
Q. 1
..... I ........ , ...... i ...... I ....... i
LyblQ
s,';','r.t,
.........,
O. 0 _ ..... i ....... i i ........ i ......... i . i i .... i
115 1.0 1.5 ;r.O 25 3,0
Wavelength (Jzm)
(b) I ............................
# Ill'
it- p7
............... i ...... i I ..... i _
0.$ 1.0 1.5 2.0 l.._ 3,0
+Wo,,-ele,,_lhC_)
Fig. 1. Average spectra of the six main spectral units in the Syrtis Major.lsidis
Planitia image-cube: (a) calibrated spectra:(b) after removal of the estimated
scattering.
https://ntrs.nasa.gov/search.jsp?R=19940028698 2020-06-16T12:15:59+00:00Z
LPl Technical Report 93-06, Part 1 19
TABLE 1. Range of variation of the spectral criteria in the Syris-lsidis image cube.
Surface + Scattering Surface Alone
(Measured) (Modeled)
Min Max Range/tins Noise Min Max Range/nns Noise
Reflectance at 1.09 pm 0.115 0.33
Spectral slope reflectance
units/pm -0.025 0.0002
2-pm pyroxenes (band depth) 0.97 1.00
0.9- pm Fe_"(band depth) 0.965 1.055
1-gm Fe2.(band depth) 0.0 0.40
1200 0.08 0.30 1200
35 -0.010 0.020 30
20 0.98 1.01 20
50 0.955 1.055 60
85 0.0 0.70 150
[1]. The main hypotheses in this procedure are that (1) the aerosol
contribution is assumed to be zero at 2.6 pm, (2) the photometric
function of the surface is assumed independent of the wavelength,
and (3) the difference between extinction and forward scattering is
supposed to be small compared to backscattering. Although there is
some uncertainty about the absolute level of the spectra, the result
is reasonable in terms of opacity ('c = 0.23 at 1.9 pm) and is con-
sistent with the efficace radius of 1.2 pm derived from Phobos 2/
Auguste [31.
The image cube covering Syrtis Major and Isidis Planitia (400 x
3000 km2), which is the more contrasted one, was used to test the
effect of the scattering on the spectra. Statistical analysis of these
3000 spectra showed that the vertical scattering contribution can be
considered uniform on the whole region, with a value of 0.75x the
estimate derived on Tharsis, and no altimetric dependence. This
correction is probably overestimated on the darkest areas.
Surface Properties: The six main spectral criteria related to the
surface materials are the reflectance at 1.09 pro; the spectral slope,
estimated here as the derivative between 1.84 and 2.35 pm; the
strength of the 2-pm pyroxene band; an equivalent of the depth of
the 0.88-pm ferric oxide band; and the surface of the Fe -'* band,
band area is reduced by almost a factor of 2, although the transfor-
mation is almost a linear stretch of the scale. This reduction may
help explain why the mafic features are not always observed from
the ground.
We measured the centers of the l-pro band on the spectra of
Fig. I. On the corrected spectra they are systematicaly shifted by
some tens of nanometers toward the short wavelengths, but the shift
is not large enough to change dramatically the mineralogical inter-
pretation of the surface: The absorptions are still compatible with
hematite on bright regions (center at 0.86 pm) and with calcic
pyroxenes in Syrtis Major (center at 0.94 pro), though probably less
rich in Ca than previously inferred from ISM I71 (Ca/(Fe + Ca +
Mg) = 0.20 _+0.08 instead of 0.275 + 0.075). Very calcic pyrox-
enes could have their bands shifted up to 1.05 pm by the addition of
a steep scattered component. In this case they could be mistaken for
olivine, so no detection of this mineral could be validated unless
made under very low opacity.
References: [I ] Erard et al. (1993) LPS XXIV, 445.12] Dros-
sart et al. ( 1991 ) Annal. Geophys., 9, 754. [3] Korablev et al. (1993)
Icarus, 102, 76. [4] Erard et al. (1993) LPSXXIV, 443. [5] Clancy
and Lee (1991) Icarus, 93, 135. [6] Erard et al. (1992) LPSXXIII,
integrated from 0.86to 1.09 pm. They were computed onthe whole 335. [7] Mustard et al. (1993) JGP oR "_'_f_7
image cube for the calibrated spectra and for the spectra corrected /:;#e,.:7. N94- 33205from scattering (estimate of the surface alone); their values are given -._/,_: _.ff/" - .
in Table I. The last three criteria were also used to establish a A /_ODEL FOR THE E_IOLUTION OF CO z ON MARS.
classification of the calibrated spectra [4]; the main spectral types are R.M. Haberle t, D. Tyler 2, C. P. McKay I, and W. L. Davis I , 1NASA
given in Fig. la. The spectra of the same pixels were averaged after
subtraction of the aerosols contribution, and are given in Fig. lb.
Discussion" The level of the scattered spectrum below 2 pm
ranges from 0.02 to 0.05, which represents 5-15% of the albedo of
the bright areas and 15-30% of that of the dark regions. This large
contribution of scattering is similar to that infered for "clear atmos-
phere" from IRTM observations I5] and independent ISM analysis
[6]. The negative spectral slope often ascribed to dark materials
appears to be largely due to the aerosol scattering continuum,
although intrinsic variations are recognized on the data. The oppo-
sition between the eastern and western parts of Syrtis Major is
enhanced by the correction.
The three criteria measuring the main surface absorptions are
relatively insensitive to the addition of a low-opacity scattered com-
ponent. Although they are subdued and slightly shifted to longer
wavelengths, their spatial distribution and relative contrast are not
deeply modified. The Fe 3+ and 2-pro pyroxene bands appear to be
very insensitive to small variations of opacity. Conversely, the Fe -'+
Ames Research Center, Moffett Field CA 94035-1000, USA, -'San
Jose State University, San Jose CA 95192, USA.
Our MSATr work has focused on the evolution of CO 2 on Mars.
We have constructed a model that predicts the evolution of CO 2 on
Mars from a specified initial amount at the end of the heavy bom-
bardment to the present. The model draws on published estimates
of the main processes believed to affect the fate of CO 2 during this
period: chemical weathering, regolith uptake, polar cap formation,
and atmospheric escape. Except for escape, the rate at which these
processes act is controlled by surface temperatures that we calculate
using a modi fled version of the Gierasch and Toon [ 1] energy balance
model. The modifications account for the change in solar luminosity
with time, the greenhouse effect, and an equatorial (as well as polar)
energy budget. Using published estimates for the main parameters,
we find no evolutionary scenario in which CO 2is capable of produc-
ing a warm (global mean temperatures >250K) and wet (surface
pressures >30 mbar) early climate, and then evolves to present


Aerosols scattering and near-infrared observations of the Martian surface

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