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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

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  • RE: Response to Orosei et al., Radar evidence of subglacial liquid water on Mars, by Hecht, Fisher, Catling and Kounaves
    • Roberto Orosei, Research Scientist, Istituto Nazionale di Astrofisica
    • Other Contributors:
      • Sebastian E. Lauro, Research Scientist, Università degli Studi Roma Tre
      • Elena Pettinelli, Assistant Professor, Università degli Studi Roma Tre
      • Andrea Cicchetti, Technologist, Istituto Nazionale di Astrofisica
      • Marcello Coradini, Professor Emeritus, Agenzia Spaziale Italiana
      • Barbara Cosciotti, Research Scientist, Università degli Studi Roma Tre
      • Federico Di Paolo, Research Scientist, Istituto Nazionale di Astrofisica
      • Enrico Flamini, Professor Emeritus, Università degli Studi "G. d'Annunzio" Chieti - Pescara
      • Elisabetta Mattei, Research Scientist, Università degli Studi Roma Tre
      • Maurizio Pajola, Research Scientist, Istituto Nazionale di Astrofisica
      • Francesco Soldovieri, Senior Research Scientist, Consiglio Nazionale delle Ricerche

    We wish to thank the authors of the letter for the opportunity to clarify the points raised by their comments. We will reply by quoting each sentence from the letter in their original order.

    > "The widely publicized interpretation (Orosei et al. 2018) of a synthetic aperture radargram as evidence of liquid water at the base of the martian south polar layered deposits (SPLD) draws on two lines of argument:"

    This sentence contains some factual inaccuracies. The paper is based on the analysis of data from 29 orbits, in which the subsurface was probed simultaneously at two different frequencies (see Table S1 in the Supplementary Materials). Our identification of liquid water as the most plausible explanation for the observed strong basal echoes in an area about 20 km across centered at 193°E, 81°S, is based on data from nine intersecting orbits, for a total of 18 radargrams in which the strong basal reflector was consistently observed at different frequencies, altitudes and illumination conditions. Also, quoting from the Supplementary Materials, "data processing on Earth consisted of range compression and geometric calibration to compensate for altitude variations. In our analysis, SAR (synthetic aperture radar) processing was not performed because of the smoothness of the SPLD in this area, which causes surface echoes to originate solely from the specular direction; in this case, SAR processing would be reduced to a simple moving average of nad...

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    Competing Interests: None declared.
  • RE: Radar evidence of subglacial liquid water on Mars (Orosei et al.)
    • Michael Hecht, Assoc. Director for Research Management, MIT Haystack Observatory

    The widely publicized interpretation (Orosei et al. 2018) of a synthetic aperture radargram as evidence of liquid water at the base of the martian south polar layered deposits (SPLD) draws on two lines of argument: the brightness of the reflection itself, and a model by the authors of this letter that argues for the plausibility of briny sludge lubricating the base of the North PLD (Fisher et al. 2010, J. Geophys. Res. Planets 115, E00E11). Without the model the radar evidence does not stand alone, as a bright reflection below a stratified medium like the SPLD can derive from any number of scattering phenomena. Unfortunately, the application of our model to the SPLD observation has not been done correctly. Orosei et al. assume a surface temperature of 160K and the postulated perchlorate brine located 1.5 km below the surface requires a temperature >200K. This implies a thermal gradient of at least 26.7K/km, nearly twice the value of 15 K/km used in our model. In calculating dT/dz = G/k, we took the heat flow G to be 0.03 W/m^2 and the thermal conductivity k to be 2.0 W/m-K. The resulting value of 15 K/km is itself an upper limit, as a more recent global mean estimate for G is 0.020 W/m^2, with regional variations unlikely to exceed 0.006 W/m^2 (Parro et al. 2017, Sci. Rep. 7: 45629), and k is likely to be closer to 3.2 W/m-K, the value for pure ice over the range 160-200K. Periodic climate change can exaggerate the gradient, but the temperature will not change by more...

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    Competing Interests: None declared.