An ancient north polar ocean on Mars has been proposed [ Parker et al., 1989] and we use MOLA data to test the hypothesis. Of the two proposed contacts/shorelines, the younger Contact 2 shows the closest approximation to an equipotential surface; vertical variations along this surface occur in areas with post‐contact‐formation geological activity or suspected changes in the position of an equipotential surface (e.g., Tharsis) with time. The surface of Mars is smoother at all scales below Contact 2 than above. The volume of the region below Contact 2 (∼1.5×10 7 km³) is between the minimum estimated total outflow channel discharge and the maximum estimated megaregolith pore space. These results are consistent with the hypothesis that a large standing body of water occupied the northern lowlands in the past history of Mars.
This volume of reports is the 2000 Annual Report of the International Very Long Base Interferometry (VLBI) Service for Geodesy and Astrometry (IVS). The individual reports were contributed by VLBI groups in the international geodetic and astrometric community who constitute the permanent components of IVS. The IVS 2000 Annual Report documents the work of the IVS components for the period March 1, 1999 (the official inauguration date of IVS) through December 31, 2000. The reports document changes, activities, and progress of the IVS. The entire contents of this Annual Report also appear on the IVS web site at http://ivscc.gsfc.nasa.gov/publications/ar2000. This book and the web site are organized as follows: (1) The first section contains general information about IVS, a map showing the location of the components, information about the Directing Board members, and the report of the IVS Chair; (2) The second section of Special Reports contains a status report of the IVS Working Group on GPS phase center mapping, a reproduction of the resolution making IVS a Service of the International Astronomical Union (IAU), and a reprint of the VLBI Standard Interface (VSI); (3) The next seven sections hold the component reports from the Coordinators, Network Stations, Operation Centers, Correlators, Data Centers, Analysis Centers, and Technology Development Centers; and (4) The last section includes reference information about IVS: the Terms of Reference, the lists of Member and Affiliated organizations, the IVS Associate Member list, a complete list of IVS components, the list of institutions contributing to this report, and a list of acronyms. The 2000 Annual Report demonstrates the vitality of the IVS and the outstanding progress we have made during our first 22 months.
Beginning with the earliest days of space exploration, satellites have been used to chart the gravity and magnetic fields of the earth. As a continuation of these studies NASA is proposing to launch a new geopotential fields exploration system called the Geopotential Research Mission (GRM). Two spacecraft will be placed in a circular polar orbit at 160 km altitude. Distances between these satellites will vary from 100 to 600 km. Both scalar and vector magnetic fields will be measured by magnetometers mounted on a boom positioned in the forward direction on the lead satellite. Gravity data will be computed from the measured change in distance between the two spacecraft. This quantity, called the range‐rate, will be determined from the varying frequency (Doppler shift) between transmitter and receiver on each satellite. Expected accuracies (at the one sigma level) are: gravity field, 1×10 −5 m s −2 (1 milliGal). 5 cm geoid height; magnetics, scalar field 2 nT, vector to 20 arc seconds (96 microradians), both resolved to less than 100 km. With these more accurate and higher resolution data we will be able to investigate the earth's structure from the crust (with the shorter wavelength gravity and magnetic anomalies) through the mantle (from the intermediate wavelength gravity field) and into the core (using the longer wavelength gravity and magnetic fields).
A spherical harmonic solution of the Mars gravity field to degree and order 80, Goddard Mars Model 2B (GMM‐2B), has been developed using X band tracking data of Mars Global Surveyor (MGS) from October 1997 to February 2000 and altimeter crossovers formed from the Mars Orbiter Laser Altimeter (MOLA) data between March and December 1999. During the mapping mission, MGS was located in a near‐polar (92.9° inclination) and near‐circular orbit at a mean altitude of 400 km. The tracking data from this orbit provide a detailed, global, and high resolution view of the gravity field of Mars. Mars gravity solutions are stable to 60×60 even without application of a Kaula power law constraint. The Valles Marineris is resolved distinctly with lows reaching −450 mGals. Olympus Mons and its aureole are both separately resolved, and the volcano has a peak anomaly of 2950 mGals. The global correlation of the GMM‐2B gravity coefficients with MOLA‐derived topography is 0.78 through degree 60, and the correlation remains above 0.6 through degree 62. The global gravity anomaly error predicted from the GMM‐2B error covariance through 60×60 is 11 mGal. The global geoid error from GMM‐2B through 60×60 is 1.8 m. MGS orbit quality using GMM‐2B, as measured by overlapping orbital arcs, is 1 m in the radial direction and 10 m in total position.
The masses of Mars and its satellites, Phobos and Deimos, have been estimated from the Mariner 9 and Viking 1 and 2 Orbiter tracking data. These spacecraft were sensitive to the gravitational force of Mars as well as to its satellites. Although the satellite masses are eight orders of magnitude smaller than Mars, their regular effect on the orbits of the spacecraft is evident in the tracking data and has enabled us to derive their masses simultaneously with that of Mars. Our method for estimating the satellite masses uses the many “distant encounters” of the spacecraft with these small bodies rather than the few “close encounters” used in previous studies. The mass estimate for Phobos leads to a mean density of 1530±100 kg m −3 based on a volume of 5748±190 km³ (Thomas, 1993), while the mass estimate of Deimos leads to a poorly constrained mean density of 1340±828 kg m −3 based on a volume of 1017±130 km³ (Thomas, 1993). Our analysis confirms, within the bounds of error, the anomalously low density of Phobos using an independent method and data set. If the result is valid within several times the estimated error (1σ), then factors other than composition, i.e., porosity, a thick regolith and/or a significant interior ice content, are required to explain the observed mass of this body.
