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NASA NTRS2000

Evidence for a Hematite Ore Body on Mars

The Mars Global Surveyor (MGS) spacecraft was launched from Cape Kennedy in November 1996. MGS was put into orbit around Mars in September of 1997 and has since been sending back data from a suite of instruments, including the Thermal Emission Spectrometer (TES). The TES instrument is an interferometric spectrometer designed to map the surface mineralogy of Mars by measuring the midinfrared emitted radiation over the spectral region of about 1600 to 200 cm(exp -1) (about 6 to 50 microns). This mineralogically sensitive technique utilizes the characteristic intra- and inter-molecular vibrations of minerals that are manifested in the midinfrared spectra. These spectral "fingerprints" are unique because they are dependent upon chemical composition, crystal structure, crystal orientation, and other factors. Midinfrared spectral data received from the MGS-TES instrument have indicated the presence of a large deposit of hematite (alpha-Fe2O3) in Sinus Meridiani, Mars. This hematite ore body, that is accompanied by basalt, is really extensive, encompassing an area about 350 by 500 km. To better understand the geologic context of this large deposit, a detailed laboratory spectroscopic investigation was conducted using more than 20 hematite samples so that their spectra could be compared to the martian spectra. The samples included red and gray polycrystaline hand samples, gray single-crystal hand samples, and red and gray fine- and coarse-grained particulates. The laboratory analyses provided thermal emissivity spectra that, when compared to the hematite emissivity spectra from Mars, suggest the Sinus Meridiani hematite is possibly an exposure of oriented hematite grains. These grains are likely coarser that 10 microns (and may be much larger) and gray in color. The characteristic of oriented grains is suggested by the apparent crystal axis-dependence of the energy emitted from the surface of Mars. The strong degree of crystal alignment exhibited in the emissivity spectra of Mars suggests that these oriented hematite crystals most likely occur as bedrock of aligned specular hematite grains (possibly schistose in texture) or as a secondary coating on bedrock, rather than as discrete particles. We are investigating the nature of this vast hematite deposit in order to understand better the geologic setting and infer past conditions and geological evolution on Mars.

Morris, Richard V.↗
NASA NTRS2000

Evidence for a Hematite Ore Body on Mars

The Mars Global Surveyor (MGS) spacecraft was launched from Cape Kennedy in November 1996. MGS was put into orbit around Mars in September of 1997 and has since been sending back data from a suite of instruments, including the Thermal Emission Spectrometer (TES). The TES instrument is an interferometric spectrometer designed to map the surface mineralogy of Mars by measuring the midinfrared emitted radiation over the spectral region of approximately 1600 to 200 per centimeter (appjroximately 6 to 50 microns). This mineralogically sensitive technique utilizes the characteristic intra- and inter-molecular vibrations of minerals that are manifested in the midinfrared spectra. These spectral "fingerprints" are unique because they are dependent upon chemical composition, crystal structure, crystal orientation, and other factors. Midinfrared spectral data received from the MGS-TES instrument have indicated the presence of a large deposit of hematite (alpha-Fe2O3) in Sinus Meridiani, Mars. This hematite ore body, that is accompanied by basalt, is areally extensive, encompassing and area approximately 350 by 500 km. To better understand the geologic context of this large deposit, a detailed laboratory spectroscopic investigation was conducted using more than 20 hematite samples so that their spectra could be compared to the martian spectra. The samples included red and gray polycrystaline hand samples, gray single-crystal hand samples, and red and gray fine- and coarse-grained particulates. The laboratory analyses provided thermal emissivity spectra that, when compared to the hematite emissivity spectra from Mars, suggest the Sinus Meridiani hematite is possibly an exposure of oriented hematite grains. These grains are likely coarser that 10 microns (and may be much larger) and gray in color The characteristic of oriented grains is suggested by the apparent crystal axis-dependence of the energy emitted from the surface of Mars. The strong degree of crystal alignment exhibited in the emissivity spectra of Mars suggests that these oriented hematite crystals most likely occur as bedrock of aligned specular hematite grains (possibly schistose in texture) or as a secondary coating on bedrock, rather than as discrete particles. We are investigating the nature of this vast hematite deposit in order to understand better the geologic setting and infer past conditions and geological evolution on Mars.

Lane, M. D.↗
NASA NTRS1999

Implications of MOLA Global Roughness, Statistics, and Topography

New insights are emerging as the ongoing high-quality measurements of the Martian surface topography by Mars Orbiter Laser Altimeter (MOLA) on board the Mars Global Surveyor (MGS) spacecraft increase in coverage, resolution, and diversity. For the first time, a global characterization of the statistical properties of topography is possible. The data were collected during the aerobreaking hiatus, science phasing, and mapping orbits of MGS, and have a resolution of 300-400 m along track, a range resolution of 37.5 cm, a range precision of 1-10 m for surface slopes up to 30 deg., and an absolute accuracy of topography of 13 m. The spacecraft's orbit inclination dictates that nadir observations have latitude coverage of about 87.1S to 87.1N; the addition of observations obtained during a period of off-nadir pointing over the north pole extended coverage to 90N. Additional information is contained in the original extended abstract.

Aharonson, O.↗
NASA NTRS1999

The Removal of Periodic Gravitational Perturbations from Mars Global Surveyor's Science Phasing Orbits Applied to the Study of the Martian Exosphere

The martian exosphere and upper atmosphere exert measurable drag on the Mars Global Surveyor (MGS) spacecraft. Using the Goddard Space Flight Center's GEODYN orbit determination software, the science phasing orbits (SPO) were analyzed to determine the atmospheric drag and hence, measure the average density near the orbits' perifocus (170-180 km altitude above high northern latitudes). Future work will include the gravity calibration and mapping periods as well. Additional information is contained in the original extended abstract.

Tracadas, P. W.↗
NASA NTRS2000

Shape and Topography of Mars

Observations by the Mars Orbiter Laser Altimeter (MOLA) on the Mars Global Surveyor (MGS) spacecraft are showing a new planet in its range of topography and in the detail of the geological features. MGS arrived at Mars in Sept 1997 and since Feb. 28, 1999 the laser altimeter has operated continuously. By the end of April 2000 MOLA had acquired over 350 million measurements of the planet's shape and topography. These observations show a strong down-hill topographic gradient from the south pole to the north pole with an extremely low and flat basin encompassing most of the northern hemisphere. Large outflow channels are seen in the Chyrse region that strongly suggest considerable quantities of water once flowed into the lower northern hemisphere from the south. The northern icecap, which rises to a height of three kilometers above the surrounding terrain, is shown to be largely composed of water ice and at the center of a large basin. The southern hemisphere is about five kilometers higher than the low northern plains and dominated by the Hellas impact basin whose ejecta is the major contributor to the topography of the hemisphere and the crustal dichotomy. The south polar icecap, which is part of a much larger region of layered terrain composed of water ice and dust, reaches an elevation of nearly five kilometers and except for the large Tharsis volcanoes is the highest part of the planet.

Smith, David E.↗
NASA NTRS2000

Spring and Summer Changes at the South Pole as Seen by the Mars Orbiter Camera

The Mars Orbiter Camera (MOC) on the Mars Global Surveyor (MGS) spacecraft has been able to follow individual features as the CO2 frost disappears and exposes the material underneath. Because the orbit of MGS is inclined at an angle of 93 degrees relative to the equator, the spacecraft gets especially good coverage of the ring at 87 degrees latitude. The following is a list of phenomena that have been seen during the spring and summer at the South Pole: (1) Circular depressions that are approximately ten meters deep and hundreds of meters in diameter. They are found only within the residual polar cap, the part that survives the summer. The high areas between the depressions are flat-topped mesas whose sides are concave circular arcs. In some places the depressions form patterns that exhibit north-south symmetry, suggesting some control by sunlight; (2) Dark layers that are exposed on the walls of the mesas. Each layer is at most a few meters thick. The dark layers might accumulate during climatic episodes of high atmospheric dust content, or they might accumulate during the annual cycling of dusty CO2; (3) Albedo differences that develop during the summer within the residual cap. These include subtle darkening of the floors of the depressions relative to the mesas and occasional major darkening of the floors, especially near the edge of the cap. The floors and mesas form a distinct stratum, suggesting they represent a distinct compositional boundary. For instance the floors may be water and the mesas may be CO2; (4) Small dark features that appear in spring on the seasonal frost outside the residual cap. Some of the features have parallel tails that are clearly shaped by the wind. Others are more symmetric, like dark snowflakes, with multiple branching arms. After the CO2 frost has disappeared the arms are seen as troughs and the centers as topographic lows; (5) Polygons whose sides are dark troughs. Those that are outside the residual cap seem to disappear when the frost disappears. The polygons and the dark snowflakelike structures may be related, and suggest that CO2 frost may form cohesive slabs; (6) Irregular depressions outside the residual cap. They look like degraded versions of the circular depressions inside the residual cap, and may be a remnant of the cap's changing location; and (7) Areas of burial and exhumation of circular depressions. Thomas et al. give an example with a sharp boundary: On one side the depressions are buried and on the other side they are exposed. In other cases there are rounded troughs up to one kilometer wide, which are dark in summer and appear to have eroded down below the floor of the circular depressions.

Ingersoll, A. P.↗
NASA NTRS2000

An Overview of the Topography of Mars from the Mars Orbiter Laser Altimeter (MOLA)

The Mars Global Surveyor (MGS) spacecraft has now completed more than half of its one-Mars-year mission to globally map Mars. During the MGS elliptical and circular orbit mapping phases, the Mars Orbiter Laser Altimeter (MOLA), an instrument on the MGS payload, has collected over 300 million precise elevation measurements. MOLA measures the range from the MGS spacecraft to the Martian surface and to atmospheric reflections. Range is converted to topography through knowledge of the MGS spacecraft orbit. Ranges from MOLA have resulted in a precise global topographic map of Mars. The instrument has also provided measurements of the width of the backscattered optical pulse and of the 1064 nm reflectivity of the Martian surface and atmosphere. The range resolution of the MOLA instrument is 37.5 cm and the along-track resolution of MOLA ground shots is approx. 300 m; the across-track spacing depends on latitude and time in the mapping orbit. The best current topographic grid has a spatial resolution of approx. 1/16 deg and vertical accuracy of approx. one meter. Additional information is contained in the original extended abstract.

Smith, David E.↗
NASA NTRS2000

Structure and Dynamics of the Polar Regions of Mars from MGS Topography and Gravity

The Mars Global Surveyor (MGS) spacecraft has been engaged in systematic mapping of Mars since insertion into Mars orbit in September, 1997. The objectives of the MGS mission are to globally map Mars as well as to quantify seasonal changes on the planet. MGS geophysical/geodetic observations of topography from the Mars Orbiter Laser Altimeter (MOLA) and gravity from the Radio Science investigation are providing significant new insights on both static and time-varying aspects of the polar regions of Mars. These observations have implications for polar processes on diurnal seasonal and climatic timescales. Thus far, MOLA has collected over 300 million precise measurements of Martian topography and cloud heights. The instrument has also provided measurements of the width of the backscattered optical pulse and of the 1064 nm reflectivity of the Martian surface and atmosphere. The along-track resolution of MOLA ground shots is approx. 300 m and the across-track spacing in the polar regions is a maximum of about four kilometers. The vertical accuracy of the topography is determined by the precision recovery of spacecraft orbits from the Radio Science investigation, which includes MOLA altimetry in the form of crossovers. This accuracy is currently approx. one meter. The gravity field is derived from X-band Doppler tracking with typical accuracy of 0.03 to 0.05 mm/s averaged over ten seconds. Current Mars gravity fields are to approximately degree and order 80 but are interpretable to the approximate degree and order 60 (spatial resolution < 180 km), which represents an estimate of the approximate coefficient limit of a field that can be produced without a power law constraint on the gravitational field inversion, which is commonly imposed for solution stability. Additional information is contained in the original extended abstract.

Zuber, Maria T.↗
NASA NTRS2000

Observed Changes in the Gravity Field of Mars Due to Seasonal Atmospheric Processes

The atmosphere of Mars deposits approximately 30% of its mass at the winter pole as part of its seasonal cycle of CO2 exchange and sublimes it back to the atmosphere in the spring, thus creating an annual hemispheric cycle of mass re-distribution. Using X-band tracking data of the Mars Global Surveyor (MGS) spacecraft, we have detected the signature of changes in the low degree gravity field from March 1999 through August 2000, corresponding to about three-quarters of a Martian year. The observed variations show a general resemblance to predicted variations from general circulation models. Also observed are irregular changes that appear to be due to transient phenomena in the Martian atmosphere such as large dust storms that provide significant heat into the lower atmosphere, even in the polar regions. In addition, we have identified a change in the rate of rotation of Mars over this same period that appears somewhat smaller than anticipated, but in general agreement with general circulation models.

Smith, David E.↗
NASA NTRS2001

Lithospheric Thickness Variations from Gravity and Topography in Areas of High Crustal Remanent Magnetization on Mars

Large regions of intense crustal re- manent magnetization were fortuitously discovered on Mars by the Mars Global Surveyor (MGS) spacecraft. Gravity and topography admittance studies are used to examine lithospheric structure in the areas of intense magnetization. Areas with positively magnetized crust appear to have thinner crust and elastic lithosphere than negatively magnetized crust. Additional information is contained in the original extended abstract.

Smrekar, S. E.↗
NASA NTRS2000

Mars Global Surveyor Ka-Band Frequency Data Analysis

The Mars Global Surveyor (MGS) spacecraft, launched on November 7, 1996, carries an experimental space-to-ground telecommunications link at Ka-band (32 GHz) along with the primary X-band (8.4 GHz) downlink. The signals are simultaneously transmitted from a 1.5-in diameter parabolic high gain antenna (HGA) on MGS and received by a beam-waveguide (BWG) R&D 34-meter antenna located in NASA's Goldstone Deep Space Network (DSN) complex near Barstow, California. The projected 5-dB link advantage of Ka-band relative to X-band was confirmed in previous reports using measurements of MGS signal strength data acquired during the first two years of the link experiment from December 1996 to December 1998. Analysis of X-band and Ka-band frequency data and difference frequency (f(sub x)-f(sub ka)/3.8) data will be presented here. On board the spacecraft, a low-power sample of the X-band downlink from the transponder is upconverted to 32 GHz, the Ka-band frequency, amplified to I-W using a Solid State Power Amplifier, and radiated from the dual X/Ka HGA. The X-band signal is amplified by one of two 25 W TWTAs. An upconverter first downconverts the 8.42 GHz X-band signal to 8 GHz and then multiplies using a X4 multiplier producing the 32 GHz Ka-band frequency. The frequency source selection is performed by an RF switch which can be commanded to select a VCO (Voltage Controlled Oscillator) or USO (Ultra-Stable Oscillator) reference. The Ka-band frequency can be either coherent with the X-band downlink reference or a hybrid combination of the USO and VCO derived frequencies. The data in this study were chosen such that the Ka-band signal is purely coherent with the X-band signal, that is the downconverter is driven by the same frequency source as the X-band downlink). The ground station used to acquire the data is DSS-13, a 34-meter BWG antenna which incorporates a series of mirrors inside beam waveguide tubes which guide the energy to a subterranean pedestal room, providing a stable environment for the feed and electronics equipment. A dichroic plate is used to reflect the X-band energy and pass the Ka-band energy to another mirror. The RF energy for each band is then focused onto a feed horn and low-noise amplifier package. After amplification and RF/IF downconversion, the IF signals are sent to the Experimental Tone Tracker (ETT), a digital phase-lock-loop receiver, which simultaneously tracks both X-band and Ka-band carrier signals. Once a signal is detected, the ETT outputs estimates of the SNR in a I -Hz bandwidth (Pc/No), baseband phase and frequency of the signals every I -sec. Between December 1996 and December 1998, the Ka-band and X-band signals from MGS were tracked on a regular basis using the ETT. The Ka-band downlink frequencies described here were referenced to the spacecraft's on-board USO which was also the X-band frequency reference (f(sub ka)= 3.8 f(sub x)). The ETT estimates of baseband phase at I -second sampled time tags were converted to sky frequency estimates. Frequency residuals were then generated for each band by removing a model frequency from each observable frequency at each time tag. The model included Doppler and other effects derived from spacecraft trajectory files obtained from the MGS Navigation Team. A simple troposphere correction was applied to the data. In addition to residuals, the USO frequencies emitted by the spacecraft were estimated. For several passes, the USO frequencies were determined from X-band data and from Ka-band data (referred to X-band by dividing by 3.8) and were found to be in good agreement. In addition, X-band USO frequency estimates from MGS Radio Science data acquired from operational DSN stations were available for comparison and were found to agree within the I Hz level. The remaining sub-Hertz differences were attributed to the different models and software algorithms used by MGS Radio Science and KaBLE-11. A summary of the results of a linear fit of the USO frequency versus time (day of year) is presented in Table I for an initial segment of passes.

Morabito, D.↗
NASA NTRS2001

The Mars Orbiter Altimeter (MOLA) Investigation of the Shape and Topography of Mars

The Mars Orbiter Laser Altimeter (MOLA) is an instrument on the Mars Global Surveyor (MGS) spacecraft that has been orbiting Mars since September 1997. After some preliminary observations in Sept/Oct, 1997 and in the spring and summer of 1998, the MGS spacecraft entered its mapping orbit of 400 km above the surface of Mars in February 1999. MGS began a 2 year program of systematically mapping the planet with a camera (MOC), thermal emission spectrometer (TES), magnetometer (MAG), laser altimeter (MOLA), and a radio science investigation for gravity and radio occultations. MOLA has a 48mJ, 1064 nrn ND:YAG, diode pumped laser with a 8 nanosecond pulse width, a pulse rate of 10 Hz, and a range precision of less than 40 cm. MOLA has been operating almost continuously for over two years and obtained over 600 million measurements of the radius of Mars. Using very precise orbits for the MGS spacecraft derived from the Doppler and range tracking of MGS by the Deep Space Network a topographical map of Mars has been developed with an average radial accuracy of a meter and a horizontal accuracy of 100 meters. This topographical map has revealed a new Mars, a planet with some of the flattest areas in the solar system and one of the largest impact basins. MOLA has revealed clear evidence of the effect of past fluid action on the surface and found icecaps that contain as much water ice today as the icecap of Greenland.

Smith, David E.↗
NASA NTRS2001

Mapping Mars with a Laser Altimeter

In November 1996 the Mars Global Surveyor (MGS) spacecraft was launched to Mars. One of the instruments on the spacecraft was a laser altimeter, MOLA, for measuring the shape and topography of the planet. The altimeter has a diode pumped Q-switched ND:YAG laser at 1064nm, operating at 10Hz with an 8 nsec pulse width. The pulse energy is 48mJ, and the instrument has a 37cm ranging precision. The laser illuminates a spot on the surface of Mars approximately 160 meters in diameter and the instrument has accumulated over 600 million range measurements of the surface since arrival at Mars in September 1997. MOLA has operated continuously for over 2 years and has mapped the planet at a horizontal resolution of about 1 km and a radial accuracy of about a meter. MOLA has measured the shape of the planet, the heights of the volcanoes, the depths of the canyons, and the volumes of the polar icecaps. It has detected carbon dioxide clouds and measured the accumulation of seasonal CO2 on the polar icecaps. This new remote sensing tool has helped transform our understanding of Mars and its geological history, and opened a new door to planetary exploration.

Smith, David E.↗
NASA NTRS2002

Rotational Dynamics and Time-variable Gravity of Mars and Implications for Volatile Cycling and Atmospheric Structure

The seasonal motion of atmospheric material, its deposition and sublimation at the poles, represent a minute redistribution of the planet's mass which has been detected in the motion of the Mars Global Surveyor (MGS) spacecraft. These observations have enabled us to estimate the quantity of material involved. Additional information is contained in the original extended abstract.

Zuber, M. T.↗
NASA NTRS2002

The Crustal Thickness of Mars: Accuracy and Resolution

Since the arrival of the Mars Global Surveyor (MGS) spacecraft at Mars and its entry into its mapping orbit in February 1999, the radio tracking and altimetry data from the mission have been part of the systematic mapping of the planet and used to develop very precise models of the gravity field and topography of Mars. Until the altimetry function of Mars Orbiter Laser Altimeter (MOLA) failed on June 30, 2001, the instrument had acquired close to 700 million measurements of the planet's radius, the majority of which have been used to develop a model of the topography with horizontal resolution of about 500 m and radial accuracy of better than 1 m. Concurrently, Doppler and range tracking of MGS by the Deep Space Network at X-band frequencies, with accuracies of about 50 microns/s and about 5 m respectively, have provided orbital knowledge of MGS to the few meter level and enabled the gravity perturbations of the spacecraft to be used to develop a improved gravity models of Mars. The recent models have horizontal resolutions of about 200 km, or degree 65, when expressed in spherical harmonics, and have accuracies of the order of a few mGals at the poles and about 10 mGals at the equator at the highest resolution.

Smith, David E.↗
NASA NTRS2003

Aeroheating Thermal Model Correlation for Mars Global Surveyor (MGS) Solar Array

The Mars Global Surveyor (MGS) Spacecraft made use of aerobraking to gradually reduce its orbit period from a highly elliptical insertion orbit to its final science orbit. Aerobraking produces a high heat load on the solar arrays, which have a large surface area exposed to the airflow and relatively low mass. To accurately model the complex behavior during aerobraking, the thermal analysis needed to be tightly coupled to the spatially varying, time dependent aerodynamic heating. Also, the thermal model itself needed to accurately capture the behavior of the solar array and its response to changing heat load conditions. The correlation of the thermal model to flight data allowed a validation of the modeling process, as well as information on what processes dominate the thermal behavior. Correlation in this case primarily involved detailing the thermal sensor nodes, using as-built mass to modify material property estimates, refining solar cell assembly properties, and adding detail to radiation and heat flux boundary conditions. This paper describes the methods used to develop finite element thermal models of the MGS solar array and the correlation of the thermal model to flight data from the spacecraft drag passes. Correlation was made to data from four flight thermal sensors over three of the early drag passes. Good correlation of the model was achieved, with a maximum difference between the predicted model maximum and the observed flight maximum temperature of less than 5%. Lessons learned in the correlation of this model assisted in validating a similar model and method used for the Mars Odyssey solar array aeroheating analysis, which were used during onorbit operations.

Amundsen, Ruth M.↗
NASA NTRS2003

Ten-Meter Scale Topography and Roughness of Mars Exploration Rovers Landing Sites and Martian Polar Regions

The Mars Orbiter Camera (MOC) has been operating on board of the Mars Global Surveyor (MGS) spacecraft since 1998. It consists of three cameras - Red and Blue Wide Angle cameras (FOV=140 deg.) and Narrow Angle camera (FOV=0.44 deg.). The Wide Angle camera allows surface resolution down to 230 m/pixel and the Narrow Angle camera - down to 1.5 m/pixel. This work is a continuation of the project, which we have reported previously. Since then we have refined and improved our stereo correlation algorithm and have processed many more stereo pairs. We will discuss results of our stereo pair analysis located in the Mars Exploration rovers (MER) landing sites and address feasibility of recovering topography from stereo pairs (especially in the polar regions), taken during MGS 'Relay-16' mode.

Ivanov, Anton B.↗
NASA NTRS2003

Investigating the Martian Gullies for Possible Brine Origin: A Preliminary Search for Evaporite Minerals Using THEMIS Data

Photographs taken by both the Mars Orbiter Camera (MOC) aboard the Mars Global Surveyor (MGS) spacecraft and the Thermal Emission Imaging System (THEMIS) aboard the Mars Odyssey (MO) spacecraft have shown the presence of young gullies on Mars. These gullies occur at middle and high latitudes (predominantly in the southern hemisphere) in the walls of both impact craters and canyons. They are thought possibly to be formed by the melting of ground ice, groundwater seepage (possibly as brines), surface runoff, or even liquid CO2, activated sporadically as a result of oscillations in Mars orbit. For this work, the hypothesis of gully formation being related to the outflow of brines will be investigated through the observation and study of spacecraft data. Brine rich fluids expunged from underground onto the walls of canyons and craters would either evaporate or freeze and sublimate. Removal of water from a brine by evaporation or sublimation would cause the solutes to precipitate as evaporite minerals on the canyon and crater walls or at the base of the walls, and possibly on the canyon and crater floors. Hence, the gully sites are ideal target areas to search for evaporites using THEMIS data. The objective of this work is to survey the recently acquired THEMIS data for spectral evidence of evaporite minerals, with a focus on areas of gully formation. Identifying salt mineral residues could provide chemical evidence in support of the brine origin of the Martian gullies.

Lane, M. D.↗