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NASA NTRS2001

Radiometry Measurements of Mars at 1064 nm Using the Mars Orbiter Laser Altimeter

Measurements by the Mars Orbiter Laser Altimeter (MOLA) on board the Mars Global Surveyor (MGS) may be used to provides a radiometric measurement of Mars in addition to the topographic measurement. We will describe the principle of operation, a mathematical model, and the receiver calibration in this presentation. MOLA was designed primarily to measure Mars topography, surface roughness end the bidirectional reflectance to the laser beam. To achieve the highest sensitivity the receiver detection threshold is dynamically adjusted to be as low as possible while keeping a predetermined false alarm rate. The average false alarm rate 29 monitored in real time on board MOLA via a noise counter, whose output is fed to the threshold control loop. The false alarm rate at a given threshold is a function of the detector output noise which is the sum of the photo detector, shot noise due to the background light seen by the detector and the dark noise. A mathematical model has been developed that can be used to numerically solve for the optical background power given the MOLA threshold setting and the average noise count. The radiance of Mars can then be determined by dividing the optical power by the solid angle subtended by the MOLA receiver, the receiver optical band-width, end the Mars surface area within the receiver field of view. The phase angle which is the sun-Mars-MOLA angle is available from the MGS database. MOLA also measures simultaneously the bidirectional reflectance of Mars vie its 106-lum loser beam at nadir with nearly zero phase angle. The optical bandwidth of the MOLA receiver is 2um full width at half maximum (FWHM) and centered at 106-lum. The receiver field of view is 0.95mrad FWHM. The nominated spacecraft altitude is 100km and the ground track speed is about 3km/s. Under normal operation, the noise counter are read and the threshold levels are updated at 1Hz. The receiver sensitivity is limited by the detector dark noise to about 0.1nW, which corresponds to less than 2% the maximum radiance during daytime from the brightest area on Mars. The results from the mathematical model agree well with the prelaunch measurements at several calibrated optical power levels. The radiance of sunlit Mars estimated with this technique correlates well with the measurement from the MGS. Thermal Emission Spectrometer (TES) and the Hubble Space Telescope at similar wavelength.

Sun, Xiao-Li↗
NASA NTRS1998

Martian Polar Region Impact Craters: Topographical Perspectives from the Mars Orbiter Laser Altimeter

The Mars Orbiter Laser Altimeter (MOLA) has acquired over 100 topographic cross-sections of impact landforms in the polar regions of Mars as part of Mars Global Surveyor (MGS) Science Phasing Orbit observations during the period from April to July, 1998. These MOLA topographic profiles offer the first three-dimensional perspectives of high latitude craters on Mars yet available, and provide evidence of landform geometries not previously recognized. Indeed, the relatively poor quality of Viking Orbiter images of many high northern latitude regions has allowed the MOLA data to provide insights into the cavities and ejecta topologies of non-degraded impact landforms that have clearly experienced interactions with condensates, either as part of their formation, or as a post-modification stage effect. Here we report a preliminary summary of the results associated with topographic measurements for a statistically significant population of impact features all of which lie north of 60N latitude. MOLA sampled four impact features with frost-related interior deposits, including the 81 km (diameter) Korolev feature. In several cases, there is evidence from near-centerline MOLA cross-sections of crater interior features (i.e., central peak or ice-dust deposits) that are anomalously large relative to the crater cavity. Central structures that make up more than 50% of the volume of a crater cavity are observed, suggesting that either substantial accumulation of mantling materials has occurred, or that crater excavation triggered production of volume-enhancing materials (ice?). Pedestal craters sampled by MOLA also attest to enhanced production of ejecta materials in high latitude terrains. For example, many of the pedestal craters suggest a volume of ejecta (Ve) to volume of cavity (Vc) ratio far in excess of 1.0 (i.e., over 3.0), even in cases where the floor of the cavity appears unfilled. Finally, the well-defined transitions between simple and complex craters observed in MOLA data for non-polar latitude impact features is not preserved at high northern latitudes (for non-degraded craters). This further suggests the role of target materials (i.e., frozen volatiles?) in the cratering process on Mars in such regions.

Garvin, J. B.↗
NASA NTRS2001

A New Era in Geodesy and Cartography: Implications for Landing Site Operations

The Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) global dataset has ushered in a new era for Mars local and global geodesy and cartography. These data include the global digital terrain model (Digital Terrain Model (DTM) radii), the global digital elevation model (Digital Elevation Model (DEM) elevation with respect to the geoid), and the higher spatial resolution individual MOLA ground tracks. Currently there are about 500,000,000 MOLA points and this number continues to grow as MOLA continues successful operations in orbit about Mars, the combined processing of radiometric X-band Doppler and ranging tracking of MGS together with millions of MOLA orbital crossover points has produced global geodetic and cartographic control having a spatial (latitude/longitude) accuracy of a few meters and a topographic accuracy of less than 1 meter. This means that the position of an individual MOLA point with respect to the center-of-mass of Mars is know to an absolute accuracy of a few meters. The positional accuracy of this point in inertial space over time is controlled by the spin rate uncertainty of Mars which is less than 1 km over 10 years that will be improved significantly with the next landed mission.

Duxbury, T. C.↗
NASA NTRS2003

Polar Dunes Resolved by the Mars Orbiter Laser Altimeter Gridded Topography and Pulse Widths

The Mars Orbiter Laser Altimeter (MOLA) polar data have been refined to the extent that many features poorly imaged by Viking Orbiters are now resolved in densely gridded altimetry. Individual linear polar dunes with spacings of 0.5 km or more can be seen as well as sparsely distributed and partially mantled dunes. The refined altimetry will enable measurements of the extent and possibly volume of the north polar ergs. MOLA pulse widths have been recalibrated using inflight data, and a robust algorithm applied to solve for the surface optical impulse response. It shows the surface root-mean-square (RMS) roughness at the 75-m-diameter MOLA footprint scale, together with a geological map. While the roughness is of vital interest for landing site safety studies, a variety of geomorphological studies may also be performed. Pulse widths corrected for regional slope clearly delineate the extent of the polar dunes. The MOLA PEDR profile data have now been re-released in their entirety (Version L). The final Mission Experiment Gridded Data Records (MEGDR's) are now provided at up to 128 pixels per degree globally. Densities as high as 512 pixels per degree are available in a polar stereographic projection. A large computational effort has been expended in improving the accuracy of the MOLA altimetry themselves, both in improved orbital modeling and in after-the-fact adjustment of tracks to improve their registration at crossovers. The current release adopts the IAU2000 rotation model and cartographic frame recommended by the Mars Cartography Working Group. Adoption of the current standard will allow registration of images and profiles globally with an uncertainty of less than 100 m. The MOLA detector is still operational and is currently collecting radiometric data at 1064 nm. Seasonal images of the reflectivity of the polar caps can be generated with a resolution of about 300 m per pixel.

Gregory A Neumann↗
NASA NTRS1993

Bench checkout equipment for spaceborne laser altimeter systems

This paper addresses the requirements for testing and characterizing spaceborne laser altimeter systems. The Bench Checkout Equipment (BCE) system, test requirements, and flow-down traceability from the instrument system's functional requirements will also be presented. Mars Observer Laser Altimeter (MOLA) and the MOLA BCE are presented as representative of a 'typical' laser altimeter and its corresponding test system. The testing requirements of other or future laser altimeter systems may vary slightly due to the specific spacecraft interface and project requirements. MOLA, the first solid-state interplanetary laser altimeter, was designed to be operational in Mars orbit for two Earth years. MOLA transmits a 7.5 ns pulse at a wavelength of 1.064 microns with a 0.25 mr beam divergence and a pulse repetition rate of 10 Hz. The output energy is specified at 45 mj at the beginning of mapping orbit and 30 mj at the end of one Martian year. MOLA will measure the laser pulse transit time from the spacecraft to the Mars surface and return to a resolution of 1.5 meters.

Smith, James C.↗
NASA NTRS1998

An Overview of Observations of Mars' North Polar Region From the Mars Global Surveyor Laser Altimeter

Since its arrival at Mars on September l5, 1997, the Mars Global Surveyor (MGS) has been in a near-polar elliptical orbit, with the orbital eccentricity decreasing during orbital periapse passes where the spacecraft aerobrakes through the martian atmosphere. The Mars Orbiter Laser Altimeter (MOLA), an instrument on the MGS, has the ability to range to the martian surface during nonaerobraking passes. MOLA can operate whenever the range from the spacecraft to the surface is less than 786 km, with the limit determined by the number of bits encoded for the range measurement During the capture orbit, aerobraking hiatus, and science phasing orbit (SPO) mission phases, MOLA acquired approximately 200 profiles across the northern hemisphere of Mars and provided more than 2,000,000 measurements of the radius of the planet. These observations cover the region from the north pole to about 10 degrees S latitude with a precision of a few tens of centimeters and an accuracy (at present) of about 30 in. Absolute accuracy of the elevations is limited by the knowledge of the MGS orbits; these should improve later in the mission due to a more optimal tracking geometry, an improved gravitational field, and the use of the high-gain antenna once the spacecraft achieves its approximately 400-km-altitude circular mapping orbit. MOLA measurements so far show a planet with a low, flat high-latitude region in the north and a higher, topographically rougher terrain nearer the equator. The north polar cap stands approximately 2-3 km above the surrounding terrain and displays deep chasms and complex structure. MOLA measurements of elevation, 1064-nm reflectivity, and backscattered pulse width indicate that the layered terrains are composed mainly of ice.

Smith, D. E.↗
NASA NTRS1999

Orbit Determination for Mars Global Surveyor During Mapping

The Mars Global Surveyor (MGS) spacecraft reached a low-altitude circular orbit on February 4, 1999, after the termination of the second phase of aerobraking. The MGS spacecraft carries the Mars Orbiter Laser Altimeter (MOLA) whose primary goal is to derive a global, geodetically referenced 0.2 deg x 0.2 deg topographic grid of Mars with a vertical accuracy of better than 30 meters. During the interim science orbits in the' Hiatus mission phase (October - November 1997), and the Science Phasing Orbits (March - April, 1998, and June - July 1998) 208 passes of altimeter data were collected by the MOLA instrument. On March 1, 1999 the first ten orbits of MOLA altimeter data from the near-circular orbit were successfully returned from MGS by the Deep Space Network (DSN). Data will be collected from MOLA throughout the Mapping phase of the MCS mission, or for at least one Mars year (687 days). Whereas the interim orbits of Hiatus and SPO were highly eccentric, and altimeter data were only collected near periapsis when the spacecraft was below 785 km, the Mapping orbit of MGS is near circular, and altimeter data will be collected continuously at a rate of 10 Hz. The proper analysis of the altimeter data requires that the orbit of the MGS spacecraft be known to an accuracy comparable to that of the quality of the altimeter data. The altimeter has an ultimate precision of 30 cm on mostly flat surfaces, so ideally the orbits of the MGS spacecraft should be known to this level. This is a stringent requirement, and more realistic goals of orbit error for MGS are ten to thirty meters. In this paper we will discuss the force and measurement modelling required to achieve this objective. Issues in force modelling include the proper modelling of the gravity field of Mars, and the modelling of non-conservatives forces, including the development of a 'macro-model', in a similar fashion to TOPEX/POSEIDON and TDRSS. During Cruise and Aerobraking, the high gain antenna (HGA) was stowed on the +X face of the spacecraft. On March 29, 1999 the HGA will be deployed on a meter long boom which will remain Earth-pointing while the instrument panel (including the MOLA instrument) remains pointed at nadir. The tracking data must be corrected for the regular motion of the high gain antenna with respect to the center of mass, and the success of the MGS determination during Mapping will depend on correctly accounting for this offset in the measurement model.

Lemoine, F. G.↗
NASA NTRS1992

The Mars Observer laser altimeter investigation

The primary objective of the Mars Observer laser altimeter (MOLA) investigation is to determine globally the topography of Mars at a level suitable for addressing problems in geology and geophysics. Secondary objectives are to characterize the 1064-nm wavelength surface reflectivity of Mars to contribute to analyses of global surface mineralogy and seasonal albedo changes, to assist in addressing problems in atmospheric circulation, and to provide geodetic control and topographic context for the assessment of possible future Mars landing sites. The principal components of MOLA are a diode-pumped, neodymium-doped yttrium aluminum garnet laser transmitter that emits 1064-nm wavelength laser pulses, a 0.5-m-diameter telescope, a silicon avalanche photodiode detector, and a time interval unit with 10-ns resolution. MOLA will provide measurements of the topography of Mars within approximately 160-m footprints and a center-to-center along-track foot print spacing of 300 m along the Mars Observer subspacecraft ground track. The elevation measurements will be quantized with 1.5 m vertical resolution before correction for orbit- and pointing induced errors. MOLA profiles will be assembled into a global 0.2 deg x 0.2 deg grid that will be referenced to Mars' center of mass with an absolute accuracy of approximately 30 m. Other data products will include a global grid of topographic gradients, corrected individual profiles, and a global 0.2 deg x 0.2 deg grid of 1064-nm surface reflectivity.

Zuber, M. T.↗
NASA NTRS1999

Implications of the Improved Understanding of the Gravitational and Topography Fields

The Mars Orbiter Laser Altimeter (MOLA) has been successfully mapping the shape and topography of Mars on an intermittent basis since the Mars Global Surveyor (MGS) spacecraft arrived at Mars in Sept. 1997. In early April, MGS reached it's mapping orbit and began the mapping phase of the mission. Since that time MOLA has been operating continuously. MOLA is a laser altimeter with a pulse rate of 10 Hz and a precision of about 30 to 40 cm over flat surfaces. The laser pulses are approximately 330 meters apart on the surface of Mars and each pulse illuminates an area approximately 150 meters in diameter. The orbital accuracy of MGS is a few meters in the radial direction enabling the topography to be interpreted at the few meter accuracy level. The initial data have provided a new geodetic description of the size and shape of Mars, including accurate estimates for the equatorial and polar radii, and the planetary flattening. The data have revealed the northern hemisphere to be very low in elevation, very flat, and almost devoid of craters. The north polar icecap lies in a basin about 5 km below the average altitude and reaches to a height of 3 km. The heavily cratered southern hemisphere is high and the terrain is rough. The south polar icecap is about 5 km above the average planetary altitude and 2 km above the surrounding terrain. The MOLA profile data show the gradual but steady increase in altitude of the surface of Mars from the north pole to the south pole. This is a consistent feature for all north-south profiles and reflects the 3 km separation of the center of figure for Mars from the center of mass along the rotation axis.

Smith, David E.↗
NASA NTRS2000

Vertical Roughness of the Polar Regions of Mars from Mars Orbiter Laser Altimeter Pulse-Width Measurements

The sub-kilometer scale vertical roughness of the martian surface in the polar regions can be investigated using calibrated, optical pulse width data provided by the Mars Orbiter Laser Altimeter (MOLA). Garvin and others have previously discussed initial observations of what we have called "total vertical roughness" or TVR, as derived from MOLA optical pulse width observations acquired during the pre-mapping phases of the Mars Global Surveyor (MGS) mission. Here we present the first assessment of the Mars polar region properties of the TVR parameter from more than nine months of continuous mapping by MOLA as part of the MGS mapping mission. Other than meter-scale surface properties directly inferred from Mars Orbiter Camera (MOC) images, MOLA measurements of footprint-scale TVR represent the only direct measurements of the local vertical structure of the martian surface at approx. 150 m length scales. These types of data have previously been shown to correlate with geologic process histories for terrestrial desert surfaces on the basis of Shuttle Laser Altimeter (SLA) observations. Additional information is obtained in the original extended abstract.

Garvin, J. B.↗
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 NTRS2003

Research Participation in the Mars Orbiter Laser Altimeter Experiment

This report describes the tasks that have been completed by the Principal Investigator, Gordon Pettengill, and his team during the first year of this grant. Dr. Pettengill was assisted by Dr. Peter Ford and Ms. Joan Quigley. Our main task has been to analyze the polar clouds detected by MOLA (Mars Orbiter Laser Altimeter) during the nominal mission of the Mars Global Surveyor (MGS) in 1999-2001 and to correlate the results with other data sets, in particular that from TES, the MGS thermal emission spectrometer. Starting with the Martian cloud database that we constructed prior to the start of this grant, we have examined all TES footprints that overlap MOLA clouds in time and space, correlating the thermal signature against specific categories that we assign to MOLA clouds on the basis of visual inspection. We are particularly interested in clouds in the region of "cold spots", areas of anomalously low thermal brightness temperature that have been detected in the polar winter by several instruments beginning with IRIS on Mariner 9. They are thought to indicate regions of active CO2 sublimation or snowfall, and it is hoped that MOLA measurements may tell us more about these regions.

Pettengill, Gordon H.↗
NASA NTRS1990

Radiation effects on various optical components for the Mars Observer spacecraft

The performance of selected optical parts of the Mars Observer Laser Altimeter (MOLA) is considered. Test results indicate that Schott RG-830, RG-850 filter glass, and the Rolyn Optics Neutral Density filters are essentially immune to levels of radiation an order of magnitude larger than that expected for the MOLA spacecraft. The Corion LG-840 filters are shown to be relatively safe for this application, but exposures to levels higher than 15 kilo-rads(Si) have a severe effect on this material. The BK-7 prism appears to be acceptable for the relatively benign environment required for MOLA. It is concluded that the optical performance of the components tested is not degraded by exposure to the whole life dose expected for the Mars Observer.

Lowry, Jay H.↗
NASA NTRS2001

A Mars' Year of Topographic Mapping With The Mars Orbiter Laser Altimeter

Since the end of February 1999 the Mars Orbiter Laser Altimeter (MOLA) has been mapping the planet continuously except for a 2 month period around solar conjunction in June 2000. At the end of January 2001 the Mars Global Surveyor Spacecraft (MGS) had completed its prime Mission, one Mars year of observing the planet, and begun the Extended Mission of slightly more than 14 months. MOLA will had acquired over 530 million altimetric measurements by early 2001, and continued to work perfectly. During the Extended Mission the main objective for MOLA will be observations of the seasonal variations in the locations and altitudes of clouds, the changes in the elevations of the polar icecaps due to the deposition and sublimation Of CO2, as well as supporting NASA's search for suitable future landing sites.

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 NTRS2003

MGS/TES-Odyssey/THEMIS-IR Analysis of Localized Low Albedo Regions in Valles Marineris

We are conducting a systematic analysis of small (approximately 10's of km), localized regions in Valles Marineris that display significant albedo differences relative to their surroundings. This analysis is based on a finding that the locations of the hematite deposits identified by [1] in the interior layered deposits of Valles Marineris typically coincide with regions having a low MGS/TES visible bolometric albedo [1,2]. Until recently, it was difficult to identify the morphology or geologic context of the regions containing the hematite deposits. However, with the recent advent of high-resolution (1/128 /pixel) MOLA grided topography and Mars Odyssey s THEMIS-IR instrument, it has been possible to better understand the morphologic context of TES observations. This analysis combines the use of PDS-released data from the MGS/TES visible bolometer and infrared spectrometer, the Odyssey/THEMIS Infrared imager, and MOLA grided topography. First, the TES infrared bolometer is used to identify regions of interesting albedo variability, and is overlaid on Viking controlled photomosaics for context. THEMIS-IR data, in conjunction with MOLA topography, is then used to: 1) identify the context and morphology of the area; and 2) identify spectrally unique regions at the km scale. In preparation for the latter, all the THEMIS planes are coregistered using an autocorrelation routine, the data are converted to brightness temperature and then each plane is normalized to the brightness temperature of the third plane (1261 cm-1). We then perform a 3-band search for color variations and a Principle Components Analysis (PCA) of the 8 unique bands in the THEMIS-IR dataset. Any variability is then investigated using both THEMIS-IR and TES spectra of the same regions. In both cases, the spectra are ratioed to near-simultaneously acquired spectra of adjacent or "average" regions that do not show this albedo variation, therefore allowing us to identify spectral variability unique to the area of interest. This procedure also allows us to account for calibration problems in THEMIS-IR data, and for any atmospheric effects in both the THEMIS-IR and the TES data.

E Z Noe Dobrea↗
NASA NTRS2003

Mars Digital Image Model 2.1 Control Network

USGS is currently preparing a new version of its global Mars digital image mosaic, which will be known as MDIM 2.1. As part of this process we are completing a new photogrammetric solution of the global Mars control network. This is an improved version of the network established earlier by RAND and USGS personnel, as partially described previously. MDIM 2.1 will have many improvements over earlier Viking Orbiter (VO) global mosaics. Geometrically, it will be an orthoimage product, draped on Mars Orbiter Laser Altimeter (MOLA) derived topography, thus accounting properly for the commonly oblique VO imagery. Through the network being described here it will be tied to the newly defined IAU/IAG 2000 Mars coordinate system via ties to MOLA data. Thus, MDIM 2.1 will provide complete global orthorectified imagery coverage of Mars at the resolution of 1/256 deg of MDIM 2.0, and be compatible with MOLA and other products produced in the current coordinate system.

Archinal, B. A.↗
NASA NTRS2012

Using GRIDVIEW to Better Understand the Early Bombardment History of the Moon, Mars and Earth

For more than a decade we have used GRIDVIEW to help analyze topographic and related data for Mars and more recently for the Moon. Our focus has been to employ the stretching, contouring, profiling, circle-fitting and other capabilities of GRIDVIEW to search for Quasi-Circular Depressions (CTAs) in MOLA, LOLA and other topographic data, and for Circular Thin Areas (CTAs) in Mars and Moon model crustal thickness data. Both QCDs and CTAs likely represent buried or obscured impact craters not readily visible in image data. We found clear evidence for a much larger population of buried impact craters in the northern lowlands of Mars (Frey et al. 2002), suggesting that part of the Red Planet is not significantly younger than the southern highlands. Edgar and Frey (2008) found that the N(300) crater retention ages of both areas were essentially identical, a conclusion confirmed by Wyatt (unpublished data) using more recent crustal thickness data for Mars. MOLA topographic data and MOLA-derived crustal thickness data were used to both identify a large number of previously unrecognized very large impact basins (D> 1000 km) on Mars and to determine relative crater retention ages for them (Frey, 2008). The distribution of N(300) CRAs suggested most formed in a relatively short interval of time. This dating also suggested the main magnetic field of Mars disappeared during this period (Lillis et al., 2008), because only the youngest basins systematically lack a remagnetized signature. Similar QCD and CTA analysis of first Clementine (Frey, 2011) and more recently LOLA topographic and LOLA-derived crustal thickness data for the Moon (Frey et al., 2011) revealed a significantly larger population of impact basins > 300 km in diameter than previously known. N(50) CRAs suggest a two-peak distribution of ages (Frey, 2012). An improved counting process confirms the two peaks, perhaps indicating both a pre-Nectaris Early Heavy Bombardment (EHB) as well as a Late Heavy Bombardment (LHB) on the Moon (Frey and Burgess, 2012, this meeting), with obvious implications for the early bombardment history of the Earth.

Frey, Herbert↗