Radar evidence of subglacial liquid water on Mars

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Science  03 Aug 2018:
Vol. 361, Issue 6401, pp. 490-493
DOI: 10.1126/science.aar7268
  • Fig. 1 Maps of the investigated area.

    (A) Shaded relief map of Planum Australe, Mars, south of 75°S latitude. The map was produced using the MOLA topographic dataset (11). The black square outlines the study area. (B) Mosaic produced using infrared observations by the THEMIS (Thermal Emission Imaging System) camera (13), corresponding to the black square in (A). South is up in the image. The red line marks the ground track of orbit 10737, corresponding to the radargram shown in Fig. 2A. The area consists mostly of featureless plains, except for a few large asymmetric polar scarps near the bottom right of (B), which suggest an outward sliding of the polar deposits (34).

  • Fig. 2 Radar data collected by MARSIS.

    (A) Radargram for MARSIS orbit 10737, whose ground track is shown in Fig. 1B. A radargram is a bi-dimensional color-coded section made of a sequence of echoes in which the horizontal axis is the distance along the ground track of the spacecraft, the vertical axis represents the two-way travel time of the echo (from a reference altitude of 25 km above the reference datum), and brightness is a function of echo power. The continuous bright line in the topmost part of the radargram is the echo from the surface interface, whereas the bottom reflector at about 160 μs corresponds to the SPLD/basal material interface. Strong basal reflections can be seen at some locations, where the basal interface is also planar and parallel to the surface. (B) Plot of surface and basal echo power for the radargram in (A). Red dots, surface echo power; blue dots, subsurface echo power. The horizontal scale is along-track distance, as in (A), and the vertical scale is uncalibrated power in decibels. The basal echo between 45 and 65 km along-track is stronger than the surface echo even after attenuation within the SPLD.

  • Fig. 3 Maps of basal topography and reflected echo power.

    (A) Color-coded map of the topography at the base of the SPLD, computed with respect to the reference datum. The black contour outlines the area in which bright basal reflections are concentrated. (B) Color-coded map of normalized basal echo power at 4 MHz. The large blue area (positive values of the normalized basal echo power) outlined in black corresponds to the main bright area; the map also shows other, smaller bright spots that have a limited number of overlapping profiles. Both panels are superimposed on the infrared image shown in Fig. 1B, and the value at each point is the median of all radar footprints crossing that point.

  • Fig. 4 Results of the simulation and retrieved permittivities.

    (A) Output of the electromagnetic simulations computed at 4 MHz (figs. S4 and S6). The blue shaded area is the envelope of all curves incorporating different amounts of H2O ice and dust along with various basal temperatures for the SPLD. The blue line is the curve for a single model (basal temperature of 205 K and 10% dust content), shown for illustration, and the black horizontal line is the median normalized basal echo power at 4 MHz from the observations. (B) Normalized basal echo power distributions inside (blue) and outside (brown) the bright reflection area, indicating two distinct populations of values. These distributions, together with the chart in (A), are used to compute the basal permittivity; for example, the intersection between the blue curve and the black line gives a basal permittivity value of 24. (C) Basal permittivity distributions inside (blue) and outside (brown) the bright reflection area. The nonlinear relationship between the normalized basal echo power and the permittivity produces an asymmetry (skewness) in the distributions of the values.

Supplementary Materials

  • Radar evidence of subglacial liquid water on Mars

    R. Orosei, S. E. Lauro, E. Pettinelli, A. Cicchetti, M. Coradini, B. Cosciotti, F. Di Paolo, E. Flamini, E. Mattei, M. Pajola, F. Soldovieri, M. Cartacci, F. Cassenti, A. Frigeri, S. Giuppi, R. Martufi, A. Masdea, G. Mitri, C. Nenna, R. Noschese, M. Restano, R. Seu

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Supplementary Text
    • Figs. S1 to S6
    • Table S1
    • References

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