New measurements of the topography of the Moon from the Lunar Orbiter Laser Altimeter (LOLA)[1] provide an excellent base-map for analyzing the large crater population (D.20 km)of the lunar surface [2, 3]. We have recently used this data to calculate crater size-frequency distributions (CSFD) for 30 lunar impact basins, which have implications for their stratigraphy and sequence. These data provide an avenue for assessing the timing of the transitions between distinct crater populations characteristic of ancient and young lunar terrains, which has been linked to the late heavy bombardment (LHB). We also use LOLA data to re-examine relative stratigraphic relationships between key lunar basins.

Radio tracking of the MESSENGER spacecraft has provided a model of Mercury's gravity field. In the northern hemisphere, several large gravity anomalies, including candidate mass concentrations (mascons), exceed 100 milli-Galileos (mgal). Mercury's northern hemisphere crust is thicker at low latitudes and thinner in the polar region and shows evidence for thinning beneath some impact basins. The low-degree gravity field, combined with planetary spin parameters, yields the moment of inertia C/MR(2) = 0.353 ± 0.017, where M and R are Mercury's mass and radius, and a ratio of the moment of inertia of Mercury's solid outer shell to that of the planet of C(m)/C = 0.452 ± 0.035. A model for Mercury's radial density distribution consistent with these results includes a solid silicate crust and mantle overlying a solid iron-sulfide layer and an iron-rich liquid outer core and perhaps a solid inner core.

DATA: PREDICTIONS FOR THE LCROSS IMPACT. T. J. Stubbs , Y. Wang , E. Mazarico , G. A. Neumann , D. E. Smith , M. T. Zuber , and M. H. Torrence ; 1 Goddard Earth Sciences and Technology Center, University of Maryland, Baltimore County, Baltimore, MD 21228; 2 NASA Goddard Space Flight Center, Greenbelt, MD 20771; 3 Oak Ridge Associated Universities, NASA Postdoctoral Program; 4 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139; 5 Stinger Ghaffarian Technologies, Greenbelt, MD 20770. Correspondance to: Timothy.J.Stubbs@nasa.gov

LUNAR RECONNAISSANCE ORBITER DATA. E. Mazarico1,2, O. S. Barnouin3, G. Salamuniccar4,5, and Maria T. Zuber1. 1Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Cambridge MA (mazarico@mit.edu); 2NASA Goddard Space Flight, Planetary Geodynamics Laboratory, Greenbelt MD; 3John Hopkins University Applied Physics Laboratory, Laurel MD; 4AVL-AST d.o.o., Av. Dubrovnik 10/II, HR-10020 Zagreb-Novi Zagreb, Croatia; 5Faculty of Electrical Engineering and Computing, University of Zagreb, Unska 3, HR-10000 Zagreb, Croatia.

We analyzed X-band radio tracking observations of the Mars Odyssey spacecraft during its aerobraking phase (October 2001–January 2002). Using the precision orbit determination softwareGEODYN,we obtained estimates of the spacecraft orbital energy lost during eachperiapsis pass due to atmospheric drag.Wealso recovered atmospheric density values at each periapsis, assuming simple exponential atmospheric models. Our measurements are in good agreement with the time series from the Odyssey accelerometer instrument, but they are dependent on the a priori scale height used. Using the accelerometer-derived periapsis densities and the precision orbit determination-derived frictional loss of orbital energy, we calculated new scale heights. Each represents the effective scale height of the atmosphere near periapsis for each aerobraking pass. Our results are consistently 1:7 0:7 km greater than the published accelerometer values. The accelerometermeasurements have higher spatial and temporal resolutionwhen they are available, but these results provide a data set useful for engineering and navigational purposes, to assess variability in the Martian middle atmosphere.

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.

Laser altimeters are presently operating on spacecraft at Mars (MOLA), at the asteroid 433 Eros (NLR), and an earlier system operated at the Moon (Clementine) several years ago. These systems have all advanced our understanding of the evolution of the primary body and several more laser altimeter systems will be launched in the next several years around Earth and other planets to address a wide range of scientific problems. Laser technology for precision altimetry and atmospheric lidar is still in its infancy but the promise of the technology and its demonstrated results already show that laser altimetry/lidar will play an important role in future space observations. To date, lasers have mapped the Moon, Mars, and an asteroid but in a short while they will help measure the planetary librations of Mercury, the tidal distortions of Europa, and tree heights, upper atmosphere winds and the icecaps of planet Earth. Major areas of interest for the immediate future are the development of long-life lasers that can withstand the rigors of long planetary missions in extreme thermal and radiation environments and continue to operate successfully for many years.

Mars Orbiter Laser Altimeter (MOLA) has operated at Mars for a full Mars year and provided a new geodetic and geophysical view of the planet. As the spacecraft enters into the Extended Mission, MOLA will concentrate its observations on the seasonal variability of the icecaps and martian clouds. Additional information is contained in the original extended abstract.

Tracking of the MGS spacecraft in orbit at Mars by the Deep Space Network since last September has provided doppler and range measurements that are being used to improve the model of the Mars gravity field. During most of October 1997, April 1998, and June thru August 1998 high quality tracking data were obtained while the periapse was in the northern hemisphere at altitudes in the 170 to 190 km range. The eccentric orbit had a period of about 11.5 hrs and an inclination of about 96.2 degrees so that low altitude tracking was obtained over most of the northern hemisphere, including the north polar icecap. Data from the earlier Mariner 9 and Viking missions have been added to the MGS data and a series of experimental gravity models developed from the combined datasets. These models have generally been of degree and order 70 and are a significant improvement over earlier models that did not include the MGS data. Gravity anomalies over the north polar cap region of Mars are generally less than 50 to 100 mgals and show no obvious correlation with the topography. Successive MGS orbits derived using these new models are showing agreement at the 100 meter level, and this has been confirmed with the laser altimeter (MOLA) on MGS These comparisons are expected to improve significantly as more tracking data get included in the solution and the MGS orbit becomes more circular giving a more balanced geographical distribution of data at low altitude. This will happen early in 1999 as the orbit approaches the mapping configuration of a circular orbit at about 400 Km.