Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p41b1615b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P41B-1615
Physics
[5462] Planetary Sciences: Solid Surface Planets / Polar Regions, [5714] Planetary Sciences: Fluid Planets / Gravitational Fields
Scientific paper
How accurately do we need to measure seasonal variations in Mars gravity, in order to significantly contribute to an understanding of the seasonal climate cycle? It has long been understood that seasonal cycles of volatile mass transport on Mars, mainly involving CO2 exchange between the atmosphere and the polar caps, will change the gravitational field by measurable amounts. In recent years, the gravitational field models, which are obtained from measured Doppler shifts in the tracking data for Mars-orbiting satellites, have become accurate enough that they can resolve some seasonal variations. However, the present models only resolve seasonal cycles for two parameters, nominally J2 and J3, which are zonal components of degree 2 and 3, respectively. In fact, what is actually observed is an unresolved linear combination of even degree zonals, in the guise of J2, and a similar combination of odd degree zonals for J3. Mars climate models are currently constrained mainly by the surface atmospheric pressure measurements made at the two Viking Lander sites. Wood and Paige (1992) showed that the observed seasonal pressure cycles at these two locations can be very well simulated by a simple one-dimensional surface thermal balance model, when its 6 free parameters (separate values for albedo and emissivity for each polar cap, and a soil thermal inertia for each hemisphere ) are properly chosen. However, it also emerged that the preferred values for albedo and emissivity are quite different from those obtained via optical remote sensing. It thus appears that the 1-D climate model yields aliased estimates of these parameters. It seems clear that, if we had sufficiently accurate gravity measurements, it would be equivalent to having a global grid of effective Viking Lander pressure measurements, with the number of grid points related to the spatial resolution of the gravity measurements. For example, if the seasonal variations were seen in a full Nth degree and order gravity model, that would comprise M = (N+1)2 -4 separate time series (M = 437 for N = 20), and would dramatically decrease the aliasing of thermal parameters in the climate models. To partially address this question, we have used the MarsWRF GCM to compute an annual cycle of surface and atmospheric mass values, on a 5x5 degree surface grid, at 10 day time steps, and then converted the resulting mass distributions into equivalent gravitational potential spherical harmonic coefficients. We can then compare the corresponding signal amplitude spectrum to the estimated gravitational model error amplitude spectrum, for various future mission measurement scenarios. From these simulations, it appears that resolving the seasonal cycle in a full N = 20 gravity model with 30 day time steps, will require substantial improvements beyond the current generation of Mars gravity models. The job of finding solutions to the associated technical problems is still in an early phase. However, it appears that adaptations to Mars of Earth-orbiting gravity missions, such as GRACE or GOCE, should suffice.
Bills Bruce G.
Mischna Michael A.
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