Mars, Above and Below

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Science  23 Dec 2005:
Vol. 310, Issue 5756, pp. 1865
DOI: 10.1126/science.310.5756.1865d

The Mars Express satellite carries an instrument called MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding), which has been imaging Mars with radar waves. The radar waves penetrate the surface, including the kilometer-thick polar ice caps, to reveal subsurface features. As described by Picardi et al. (p. 1925, published online 30 November), the data reveal the base of icy deposits near the martian north pole, showing that the crust there is rigid, and a buried circular crater, 250 km in diameter, in the Chryse Planitia lowlands. The radar echoes also reveal information about the martian ionosphere. Gurnett et al. (p. 1929, published online 30 November) show that reflections occur where there are sharp changes or gradients in electron density, and with characteristic frequency signatures. In many scans of the ionosphere, Gurnett et al. record a range of echo types, including oblique signals in regions where the relic magnetic field preserved in Mars' crust is strong.

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