A post-Pathfinder evaluation of areocentric solar coordinates with improved timing recipes for Mars seasonal/diurnal climate studies

Abstract

The accurate determination of the Mars pole vector derived from Pathfinder and Viking Lander radio data ( Folkner et al., 1997. Science 278, 1749–1752), together with the VSOP87 representation of planetary orbits (Bretagnon and Francou, 1988. Astron. Astrophys. 202, 209–315), have been applied to a new evaluation of the right ascension of the “Fictitious Mean Sun” (FMS) at Mars. With Δ t J2000 the elapsed time in days from the J2000 epoch (JD 2451545.0 TT), α FMS=270°.3863+0.5240384(°/d)·Δ t J2000−4×10 −13(°/d 2)·Δ t 2 J2000 represents a best least-squares quadratic fit of the FMS, including aberration, to each instance of the four equinox and solstice passages for each of 134 Mars orbits spanning the calendar years 1874–2126. The implied tropical orbit period for Mars, 686.9726 d, closely agrees with the recent evaluations by Surán (1997. Planet. Space Sci. 45, 705–708) and Allison (1997. Geophys. Rev. Lett. 24, 1967–1970). Together with the Pathfinder radio determination of the Mars sidereal rotation, the derived FMS rate corresponds to a mean solar day (or “sol”) of 1.02749125 d. The new FMS determination would serve to define the Mean Solar Time at Mars to the nearest tenth-second, according to historical conventions originally established for terrestrial time-keeping, once the Mars prime meridian defined by the crater Airy-0 is determined within inertial space to the same accuracy. For convenient reference to current epochs, 2000 January 06 00:00:00 UTC (=MJD 51549.000 UTC) corresponds to a coincidence of α FMS and the rotation angle of the crater Airy-0 measured with respect to the Mars equinox (i.e. “mean solar midnight” on the planet’s prime meridian), to within the current uncertainty in the locational definition of the planet’s cartographic grid. As a further result of the analysis, the consistently derived Mars obliquity of date is ϵ=25°.192+3.45×10 −7(°/d)·Δ t J2000. An improved analytic recipe for the calculation of the solar areocentric longitude ( L S) of Mars to an accuracy better than 0°.01 is also provided, accounting for the primary perturbations of Earth, Jupiter, and Venus, which may in turn be applied to an efficient evaluation of Mars Local True Solar Time (LTST) to within the uncertainty of the inertial position of the Mars prime meridian. For specific applications to the data archives for landed Mars spacecraft, simple conversion formulae are given for the determination of the Viking “Local Lander Time” and the Pathfinder “Local True Solar Time” in terms of the terrestrial calendar date and UTC.