The objective of this study was to support the rebuild and implementation of the Mars Orbiter Laser Altimeter (MOLA) investigation and to perform scientific analysis of current Mars data relevant to the investigation. The instrument is part of the payload of the NASA Mars Global Surveyor (MGS) mission. The instrument is a rebuild of the Mars Observer Laser Altimeter that was originally flown on the ill-fated Mars Observer mission. The instrument is currently in orbit around Mars and has so far returned remarkable data

Zuber, Maria T.

English

NASA Technical Reports Server

MASSACHUSETTS INSTITUTE OF TECHNOLOGY
1 - Grant Title:
,_/s -/e--c#'&_
Cambridge, Massachusetts 02139-4307
_ C,,:r----
DEPARTMENT OF EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES / _ c_' _;_
Mars Orbiter Laser Altimeter
2 - Type of Report: Summary of Research
3 - Principal Investigator: Maria T. Zuber
4 - Period of Performance: 11/1/95- 9/30/97
5 - Recipient's Institution: Massachusetts Institute of
Technology
Department of Earth,
Atmospheric and Planetary
Sciences
Building 54, Room 918
77 Massachusetts Avenue
Cambridge, MA 02139
6 - Grant Number: NAG5-2222
https://ntrs.nasa.gov/search.jsp?R=19980203568 2020-06-16T00:19:11+00:00Z
Summary of Research
Grant Title: Mars Observer Laser Altimeter (NAG5-2222)
Principal Investigator: Maria T. Zuber
Introduction
The objective of this study was to support the rebuild and implementation
of the Mars Orbiter Laser Altimeter (MOLA) investigation and to perform scientific
analysis of current Mars data relevant to the investigation. The instrument is part
of the payload of the NASA Mars Global Surveyor (MGS) mission. The
instrument is a rebuild of the Mars Observer Laser Altimeter that was originally
flown on the ill-fated Mars Observer mission. The instrument is currently in orbit
around Mars and has so far returned remarkable data.
Work Completed
The following tasks were accomplished as part of the investigation:
- Participated in initial planning of the Mars Global Surveyor mission after the
loss of Mars Observer, including instrument re-design and rebuild.
- Attended meetings and reviews in support of the MOLA investigation.
- Assisted in preparation of budgets, schedules and documentation.
- Participated in the development of a new gravitational field for Mars from re-
analysis of Mariner 9 and Viking Orbiter Doppler tracking data.
- Performed re-analysis of Mariner 9 and Viking Orbiter occultations to develop
an new shape of Mars.
- Participated in simultaneous estimation of the masses of Mars and its natural
satellites Phobos and Deimos.
- Determined a formalism to remove the attraction of the Tharsis bulge from the
Mars gravity field. Published results.
- Performed simulation of the mass variation of the polar caps over the Martian
seasonal cycle. Submitted results for publication.
- Supported MGS orbit insertion activities and mission planning in spacecraft
aerobraking and elliptical orbit.
- Programmed instrument to collect data in initial calibration pass and during
aerobraking hiatus period.
- Analyzed data.
- Participated in preparation of initial report of data results, submitted to Science,
January, 1998.
- Submitted 11 abstracts on results to Lunar and Planetary Science Conference.
- Prepared releases for 3 press conferences.
- Participated in numerous public outreach activities discussing investigation
results.
Refereed publications
Smith, D.E., F.J. Lerch, R.S. Nerem, M.T. Zuber, G.B. Patel, S.K. Fricke, and
F.G. Lemoine, An improved gravity model for Mars: Goddard Mars Model-
1 (GMM-1), J. Geophys. Res., 98, 20,871-20,899,1993.
Smith, D.E., and M.T. Zuber, New gravity field for Mars fuels new research, EOS
Trans. Am. Geophys. Un., 75, 97, 1994.
C
Smith, D.E., F.G. Lemoine, and M.T. Zuber, Simultaneous estimation of the
masses of Mars, Phobos, and Deimos from spacecraft distant encounters,
Geophys. Res. Lett., 22, 2171-2174,1995.
Smith, D.E., and M.T. Zuber, The shape of Mars and the topographic signature
of the hemispheric dichotomy, Science, 271, 184-188, 1996.
Zuber, M.T., and D.E. Smith, Mars without Tharsis, J. Geophys. Res., 102,
28,673-28,685,1997.
Smith, D.E., M.T. Zuber, R.M. Haberle, D.D. Rowlands, and J.R. Murphy, The
Mars seasonal CO 2 cycle and the time variation of the gravity field: A
General Circulation Model simulation, submitted to J. Geophys. Res.,
1997.
Smith, D.E., M.T. Zuber, H.V. Frey, J.B. Garvin, J.W. Head, G.H. Pettengill, R.J.
Phillips, S.C. Solomon, H.J. Zwally, W.B. Banerdt, T.C. Duxbury,
Topography of the northern hemisphere of Mars from the Mars Orbiter
Laser Altimeter, submitted to Science, 1998.
Budget Summary
To be provided by MIT Office of Sponsored Programs.
\
Topography of the Northern Hemisphere of Mars from the
Mars Orbiter Laser Altimeter
D.E. Smith, 1. M.T. Zuber, H.V. Frey, J.B. Garvin, J.W. Head, D.O. Muhleman,
G.H. Pettengill, R.J. Phillips, S.C. Solomon, H.J. Zwally,
W.B. Banerdt ,T.C. Duxbury
Submitted to: Science
January 15, 1998
The first 18 tracks of laser altimeter data across the northern
hemisphere of Mars show latitudes above 50 ° to be unusually flat,
with regional slopes and surface roughness increasing toward the
equator. The polar layered terrain shows evidence of being a thick
ice-rich formation with a non-equilibrium planform indicating
ablation near the periphery. Many Impact craters show quantitative
evidence for ejection of fluidized surface material. Slope
relationships Indicate a significant uplift contribution to regional
topography in the northern Tharsis province. A profile across the
Ares Vallis channel Indicates a much greater discharge than
previously estimated. Comparison with Viking albedos shows
significant 1-1zm atmospheric opacities, particularly in low-lying
areas such as Valles Marineris.
Characterization and modeling of the processes that govern planetary evolution require
accurate knowledge of surface topography over a broad range of spatial scales. On
September 15, 1997 the Mars Orbiter Laser Altimeter (MOLA) (1), an instrument on the
Mars Global Surveyor (MGS) spacecraft (2), acquired its first pass across the surface of
Mars. The altimeter obtained measurements of topograpfiy (3), surface reflectivity (4), and
backscattered laser pulse width (5). This profile was followed by a further 17 tracks
1D.E. Smith, M.T. Zuber, J.B. Garvin, H.V. Frey, H.J. Zwally, Earth Sciences Directorate,
NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA.
M.T. Zuber, G.H. Pettengill, Massachusetts Institute of Technology, Cambridge, MA 02139,
USA.
J.W. Head, Brown University, Providence, RI 02912, USA.
D.O. Muhleman, California Institute of Technology, Pasadena, CA 91125, USA
R.J. Phillips, Washington University, St. Louis, MO 63130, USA.
S.C. Solomon, Department of Terrestrial Magnetism, Carnegie Institution of Washington 20015,
USA.
W.B. Banerdt, T.C. Duxbury, Jet Propulsion Laboratory, Pasadena, CA 91109, USA.
* To whom correspondence should be addressed.
\
\
The Mars seasonal COz cycle and the time variation of the
gravity field: A General Circulation Model simulation
David E. Smith l, Maria T. Zuber 2.j, Robert M. Haberle 3, David D. Rowlands l, James
R. Murphy 3
Submitted to: Journal of Geophysical Research -- Planets
December 16, 1997
Abstract. We investigate the time variation of the long wavelength gravitational
field of Mars due to mass re-distribution associated with the annual cycle of CO2
exchange between the atmosphere and polar caps. Our_malysis utilizes simulated
monthly estimates of atmospheric pressure and CO2 polar frost as calculated by the
NASA/Ames General Circulation Model (GCM) for a "typical" year. We show that
the deposition and sublimation of CO2 pol_:frost is expected to be the dominant
effect on the gravity field at all low degrees. Variationsin Me amplitude and phase
of time variations of gravity are sensitive to multiple factors, including polar
elevation and the eccentricity of the Martian orbit. Phase effects associated with
additive and competing influences at the summer and winter poles dictate that odd
numbered harmonic coefficients produce an annual signal in the gravity field while
even-numbered coefficients show a semi-annual signal. The predicted changes in
the planetary flattening and pear-shaped terms of the field are at or above the noise
level of expected X-band Doppler tracking observations from Mars Global
Surveyor, and so could conceivably be detected from an orbital spacecraft.
Introduction
Carbon dioxide is the primary ingredient of the atniosphere of Mars, constituting about 95%
at the surface [Owen et al., 1977], though the amount is seasonally variable. The atmosphere exhibits
an annual cycle in the COz concentration which represents primarily an exchange between the
atmosphere and the seasonal polar caps. The polar caps consist mainly of COs frost in which the
solid/vapor equilibrium represents a balance, at the poles, of absorbed solar energy and the re-
radiation of that energy [Leighton and Murray, 1966]. As much as 25%-30% of the atmospheric
COs is transported from one pole to the other during the annual cycle [Hess et al., 1980; James et al.,
1992], and this large-scale movement results in changes in the distribution of atmospheric mass, and
hence, planetary mass. As illustrated schematically in Figure l, carbon dioxide, and to a much lesser
extent water [Haberle and Jakosicy, 1990], sublime at the summer pole and move toward the equator
via the zonal mean circulation [Leovy, 1979; 1985], decreasing the mass at that pole while increasing
,!:
The Shape of Mars and the Topographic
Signature of the Hemispheric Dichotomy
David E. Smith and Maria T. Zuber
Reanalysls of occultation data from the Mariner g and Viking Orbiter spacecraft to de-
termine the shape of Mars indicated that the hemispheric dichotomy is not a fundamental
feature of the shape of the planet._lt is a consequence of an approximately 3-kilometer
offset between Mars's center of mass and center of figure, and the boundary, along most
of its length, consists of broad, gradual surface slopes over distances of thousands of
kilometers. This result was supported by analysis of high spatial resolution Earth-based
radar topographic profiles. Any successful model for the odgin of the dichotomy must
explain a planet with an ellipsoidal shape and a long wavelength gradual to_)graphic
transition between the northern and southem hemispheres.
The surface of Mars is distinctly different
in the northern and southern hemispheres.
The south is old and heavily cratered,
whereas the north is younger and lightly
cratered and was probably volcanically re-
surfaced early in Mars's history (1, 2). This
hemispheric dichotomy is characterized by
a geologic boundary between the hemi-
spheres that is expressed as knobby and
fretted terrains and detached plateaus (2-4)
distributed over a relatively limited width of
_700 km (5). Along the boundary, eleva-
D. E. Smith, Laboraton/for Terrestrial Physics, NASA/
Goddard Space Right Center, Greenbelt, MD 20771,
USA.
M,T.Zubef,DepartmentofEarth,AtmosphericandPlan-
etarySciences,MassachusettsInstituteof Technology,
Cambridge,MA02139-4307,USA,andLaboratoryfor
TerrestrialPhysics,NAS/MGoddardSpaceRightCenter,
Greenbelt,MD20771,USA.
tions have been interpreted to decrease
from south to north by -1 to 3 km (1, 6,
7), and the change in topography has been
correlated with geologic features (8).
The formation of the dichotomy has
been attributed to internal processes, such
as postaccretional core formation (9), and
to crustal de[amination (in the northern
hemisphere) and underplating (in the
southern hemisphere) by vigorous mantle
convection (10). It has also been proposed
that the low northern hemisphere was the
result of a massive impact (5, 11) or im-
pacts (12), and this region may have been
the site of an early martian ocean (13).
The lack of gravity anomalies along the
boundary (14) may indicate thick crust be-
neath the southern highlands and thinner
crust beneath the northern lowlands (l, 15).
In addition, the boundary region has been
proposed as the site of relic plate boundaries
(16). Consequently, understanding the ori-
gin of the hemispheric difference has impli-
cations for the evolution and internal struc-
ture of Mars.
Most analyses of the origin of the di-
chotomy have been based on global topo-
graphic models (8) with poor long-wave-
length accuracy (17). The topography has
also been studied by means of higher spa-
tial resolution measurements derived from
photoclinometry, stereo imaging, and ul-
traviolet spectra (7, 18). Although these
methods yield information on relative
heights within an individual image frame
or mosaic, the heights cannot be placed in
a global reference frame and are of limited
utility in relating local structure to global
structure.
Radio occultation measurements (19-
22) provide estimates of the radius of the
planet at the time (and location) when
the radio signal from a spacecraft is lost
(occulted) behind the planet or emerges
from behind the planet in its orbit (23).
Occultation data formed the basis of sev-
eral early determinations of Mars's topog-
raphy (21, 22, 24) and were included in
more recent U.S. Geological Survey
(USGS) digital elevation models (DEMs)
(8), but the data have not been analyzed
since the 1970s. Here, we reanalyzed these
data by using improved spacecraft orbital
information (25), the latest planetary eph-
emerides and dynamical information (26),
and revised atmospheric refraction correc-
184 SCIENCE • VOL. 271 • 12JANUARY 1996
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. El2, PAGES 28,673--28,685, DECEMBER 25, 1997
Mars without Tharsis
Maria T. Zuber 1
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology. Cambridge
David E. Smith
Laboratory for Terrestrial Physics, NASA/Goddard Space night Center, Greenbelt, Maryland
Abstract. The significant power in the Martian gravitational field due to the Tharsis rise may
mask or modify gravitational signatures that contain important information on Martian
geophysical processes. In order to isolate gravity signals in regions where thi_ field is significantly
affected by Tharsis as well to investigate characteristics of the global field, we present a numericai
technique to "remove" Tharsis from the Martian gravitational field. Our analysis will show that to
first order Tharsis can be represented as a sixth-degree spherical harmonic zonal gravitational feature
in a reference frame in which the axis of symmetry runs through the center of the province. We
produced the "Mars without Tharsis" (MW'I) field by subtracting the gravitational signature of
Tharsis from the full field and rotating the spherical harmonics back to the original coordinate
system, in which the z-axis is coincident with the planetary rotation axis. Our study yields limits
on the moment of inertia factor of 0.361<C/MR2<0.366 for the range of allowable non-hydrostatic
contributions to the flattening. The analysis indicalg_s that the formation of Tharsis may have
resulted in re-orientation of the spin axis, though if the planet retained rotational fossil bulge due
to a thick elastic lithosphere then changes in the global figure could have been inhibited. The
zonal and tesseral components of the degree 2 field in the GMM- 1 and rotated configurations
indicates that Tharsis is -5 times more axisymmetric about its "pole" than Mars as a whole is
about the rotation axis. With the removal of Tharsis' long wavelength gravitational power,
regional structures become better defined in a spatial sense while maintaining their local dynamic
range. The Olympus Mons volcanic shield displays clear evidence of a gravitational flexural moat.
Our method for removing Tharsis may also be applied to analyze other global-scale geophysical
features that may be considered axisymmetric to first order, such as mantle plumes, major volcanic
constructs, or large impact basins.
1. Introduction
The Martian gravitational field contains significantly more
power at long wavelengths than does Earth's [Phillips and
Lambeck, i980; Balmino et al., 1982]. The difference reflects
the fact that topographic excursions on Mars are greater than
on Earth, because Mars is a smaller planet with a smaller mass.
Topographic variations are characterized by large gravity and
geoid anomalies, of which the most prominent planet-scale
feature is the Tharsis bulge. Tharsis, which is conspicuous in
the global Martian geoid in Figure 1, is the dominant locus of
volcanism and tectonism on Mars and rises up to I0 km above
its surroundings [Cart, 1981]. The widespread and diverse
distribution of tectonic features and the high topography
represent evidence that Mars has undergone complex thermal
and stress histories which have been influenced on both the
global and regional spatial scale by Tharsis [Banerdt et al.,
1992]. The large amplitude, long wavelength gravity and
geoid anomalies associated with the Tharsis rise must certainly
IAIso at Laboratory for Terrestrial Physics, NASA/Goddard Space
Hight Center, Greenbelt, Maryland.
Copyright 1997 by the American Geophysical Union.
Paper number 97JE02527.
0148-0227/97/97JE-02527509.00
influence both long and short wavelength gravitational
signatures that are relevant to a range of geophysical
processes. In order to isolate gravity signals in regions where
the field is especially affected by Tharsis, as well as to
investigate aspects of the global field that have been
influenced by Tharsis, we have developed a numerical
technique to "remove" Tharsis from the Martian gravity field.
Our methrd exploits the ability to quantify an axisymmetric
signal in an arbitrary coordinate system in a spherical
harmonic gravity model.
Other workers have also recognized the utility of isolating
the gravitational influence of Tharsis, and our approach has
both similarities and some important differences compared to
previous work. Reasenberg [1977], Kaula [1979] and Kaula et
al. [1989] accounted for the contribution of Tharsis to the
second degree (flattening term) of the gravity field by
assuming that the non-hydrostatic contribution to the moment
of inertia is axisymmetrically distributed along an axis in the
direction of the province. In a study concerned with re-
orientation of the planetary rotation axis, Melosh [1980]
removed the attraction of Tharsis by deleting point masses
from a then-current gravity model [Sjogren et al., 1975]. In
order to estimate the contribution of Tharsis to the Martian
flattening, Willeraann and Turcotte [1982] employed an
approach that is conceptually similar to that invoked here:
they moved the center of Tharsis to Mars' rotation pole using
the spherical harmonic addition theorem [Kaula, 1968].
28,673


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