Gravity Evidence for Extinct Magma Chambers on Mars: Tyrrhena Patera and Hadriaca Patera

The Mars Global Surveyor mission has vastly improved our knowledge of the topography and gravity of Mars [1,2,3], permitting detailed geophysical modeling of subsurface structures such as magma chambers for the first time. I have recently presented a gravity model for Syrtis Major [4] and showed that the gravity data requires the presence of a buried, high-density body. The spatial association between the caldera and the buried structure indicates that the subsurface structure is most likely due to dense cumulate minerals in an extinct magma chamber. The Syrtis Major magma chamber is approximately 300 by 600 km across, broader than the topographic caldera but narrower than the overall volcanic edifice. Like the caldera, the magma chamber is elongated in the north-south direction. The minimum magma chamber thickness is 3.6 km. In this work, I build on the Syrtis Major study by examining two additional highland volcanos, Tyrrhena Patera and Hadriaca Patera. Model The gravity models are calculated for spherical harmonic degrees 2 through 50, corresponding to a half-wavelength spectral resolution of 213 km. This ensures that the signal-to-noise ratio remains high and thus that the observed gravity anomalies are robustly determined. The RMS uncertainty in the gravity models for this part of Mars is 10-11 mGal [2], which is a small fraction of the observed peak amplitude for either Tyrrhena or Hadriaca Patera. Flexural support of the surface topography is calculated using a spherical thin-elastic shell model [5] using elastic constants and a load density appropriate for basalt. Pyroclastic volcanism has been proposed at both Tyrrhena and Hadriaca [6,7], which suggests that a smaller load density might be appropriate. By choosing a large density for the surface topography, I maximize the gravity expected from the topography and thus minimize the required buried load. The effects of smaller topographic load densities will be investigated in future work. I model the subsurface structures as buried vertical cylinders. Bounds on the cylinder radius, R, are determined from the width of the gravity anomaly. The depth of the cylinder, D, and the density contrast between the cylinder and the surrounding crust, δρ, are adjusted to fit the anomaly amplitude. Results in this abstract are based on single cylinder fits to each region. Models with multiple cylinders may improve the fit to the observations and are currently in development. The cylinders are assumed to be uncompensated to set a lower bound on the required density …