Mars Geodesy/cartography Working Group Recommendations on Mars Cartographic Constants and Coordinate Systems

NASA's Mars Geodesy/Cartography Working Group (MGCWG), established in 1998 and chaired since 2000 by one of us (TCD), consists of leading researchers in planetary geodesy and cartography at such diverse institutions as JPL, NASA Ames and Goddard Centers, Purdue and Ohio State Universities, Malin Space Science Ssystems, the German Center for Aerospace Research DLR, and the US Geological Survey, as well as representatives of the current and future Mars mission teams that are the customers for Mars maps. The purpose of the group is to coordinate the activities of the many agencies active in Mars geodesy and cartography in order to minimize redundant effort and ensure that the products needed by mission customers are generated. A specific objective has been to avoid repeating the experience of the 1970s–80s, when competing researchers produced geodetic control solutions and maps of Mars that were mutually inconsistent. To this end, the MGCWG has recently assembled a set of preferred values for Mars cartographic constants, based on the best available data. These values have been transmitted to the International Astronomical Union and appear in the report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites as the officially recommended constants for Mars(Seidelmann et al., 2002). The MGCWG has also recommended to NASA that the USGS adopt the IAU-approved coordinate system of planetocentric latitude and east longitude for future maps of Mars, in place of the (also IAU-approved) planetographic system with west longitude positive. This recommendation has recently been approved by NASA. In this paper we present the preferred values for Mars cartographic constants with discussion of the process by which they were derived, then discuss the rationale and implications of the use of east/planetocentric coordinates in future Mars maps. * Corresponding author. 1. Cartographic Constants The parameters that must be defined in order to carry out mapping of a planet such as Mars with spacecraft data are the orientation of the spin axis (including precession), rotation rate, rotational orientation at a specified time, and size and shape of an ellipsoidal reference surface. In addition to these parameters and their uncertainties, the IAU/IAG Working Group reports, where available, statistics on the deviation of the planetary surface from an ellipsoidal model. 1.1 Spin Axis and Rotation Period. Parameters describing Mars’ rotation rate and direction of its polar axis in space have been adopted from those values determined by Folkner et al. ( 1997), based on tracking of Mars Pathfinder and the Viking 1 and 2 landers. The right ascension α and declination δ in degrees at a given time t are given by the expressions: α = 317.68143o 0.1061o/century * T δ = 52.88650o 0.0609o/century * T where T is the number of Julian centuries of t from the standard epoch of J2000.0 TDB. The uncertainties in these values estimated by Folkner et al. are 0.00001o in offset and 0.0007o/century in rate for α, and 0.00003o in offset and 0.0004o/century in rate for δ. The rotation rate of Mars is assumed to be W_dot = 350.89198226o/day, for which Folkner et al. cite an uncertainty of 0.00000008o/day. 1.2 Orientation at Epoch. The orientation of the prime meridian of Mars in space at a given epoch t is specified by the angle W, which is measured along the equator to the east between the 0o (or prime) meridian and the equator’s intersection with the celestial equator. It is given (in degrees) by the expression: