Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p53a1430d&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P53A-1430
Physics
5430 Interiors (8147), 5450 Orbital And Rotational Dynamics (1221), 5455 Origin And Evolution, 5494 Instruments And Techniques
Scientific paper
The LaRa instrument is a coherent transponder using one uplink and one downlink in X-band and proposed to be part of the Humboldt Payload (on the fixed platform lander) of the ExoMars ESA mission. LaRa will measure the variation of Mars' rotation rate (related to the length-of-day) and the orientation of Mars' rotation axis in space (precession and nutations), by measuring Doppler shifts resulting from the motion of Mars relative to the Earth, through monitoring a radio signal between the ExoMars lander and the tracking stations from ESA (ESTRACK stations) and NASA (the Deep Space Network, DSN) on Earth. The primary objective of LaRa is a precise measurement of precession, nutations, and length-of-day. Comparing the data with theoretical modeling allows inferring knowledge on Mars' interior and on the global circulation in its atmosphere. Precession and nutations are induced by the well-known gravitational forcing of the Sun on Mars. Nutations depend on the internal properties of Mars, in particular on the state and dimension of the core. Length-of-day variations are deviations from the uniform rotation rate of the planet. They are mainly related to the dynamics of the geophysical fluids of the system, mainly the atmosphere of Mars. The seasonal condensation/sublimation of the icecaps induces a large change in the length-of-day at the seasonal periods. These phenomena were previously estimated from the tracking data of the Viking (S-band) and of the Pathfinder (X-band) Landers as well as from the time variations of the gravity field of Mars determined from radioscience on spacecraft orbiting around Mars. The expected precision of LaRa will be at least a factor two better in the known quantities, even a factor four for the precession value for a very conservative 180 day mission life-time and better for a long stay at the surface of Mars. The improvement in the precision of the moment of inertia determination deduced from the precession observation realized by LaRa, even without further independent improvement in the mantle mineralogy, will reduce the uncertainty on the core radius. LaRa will thus provide more accurate values or new estimates of Mars' precession and nutations and Mars' length-of-day variations. Those geodetic quantities will substantially increase our knowledge on the interior structure of Mars and on the global circulation in the atmosphere related to the seasonal CO2 sublimation and condensation.
Attilio R.
Dehant V. M.
Folkner William
LaRa Team
Le Maistre Sébastien
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