Density variations within the south polar layered deposits of Mars

The south polar layered deposits (SPLD) constitute the largest known reservoir of water on Mars. Previous studies solved for the best fit uniform density of the deposits using a forward approach. Here we invert for the lateral density variations in the layered deposit using gravity data from radio tracking of Mars Reconnaissance Orbiter, topography from MOLA on board Mars Global Surveyor, and radar sounding data from MARSIS on board Mars Express. We use the gravity anomalies outside the SPLD to construct a Wiener filter, which is applied to the gravitational signature of the SPLD to remove the short-wavelength anomalies over the SPLD that are spectrally consistent with an origin in the crust or mantle. We then use a constrained inversion for the vertically averaged density within the SPLD as a function of position. The results suggest significant density variations within the SPLD. An inverse relationship between the density and thickness of the SPLD suggests that thicker portions of the cap contain less dust. Alternatively, the Dorsa Argentea Formation may extend beneath the SPLD and result in the observed high gravity anomaly in the marginal area of the SPLD. We find these conclusions to be robust against the choice of inversion constraint and perturbations to the applied filter. A synthetic test is also performed to verify the recoverability of the density variation in our approach.