2025-11-25 NASA
This map shows the approximate area where in 2018 ESA’s Mars Express detected a signal the mission’s scientists interpreted as an underground lake. The red lines show the path of NASA’s Mars Reconnaissance Orbiter, which flew both directly overhead as well as over an adjacent region.Credit: Planetary Science Institute
＜関連情報＞
- https://www.nasa.gov/missions/mars-reconnaissance-orbiter/nasa-orbiter-shines-new-light-on-long-running-martian-mystery/
- https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025GL118537
火星の氷床下に存在するとされる液体水の高周波レーダーによる観測 High Frequency Radar Perspective of Putative Subglacial Liquid Water on Mars
Gareth A. Morgan, Matthew R. Perry, Bruce A. Campbell, Nathaniel E. Putzig, Jennifer L. Whitten, Fabrizio Bernardini
Geophysical Research Letters Published: 17 November 2025
DOI:https://doi.org/10.1029/2025GL118537
Abstract
Anomalously bright radar reflections from the base of Mars’ south polar cap raise the tantalizing possibility of present-day liquid water. Orosei et al. (2018, https://doi.org/10.1126/science.aar7268) first reported bright subsurface echoes from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) prompting studies of whether the high reflectivity, stems from liquid water or dry scattering interfaces. A key challenge has been the prior inability of the higher-frequency Shallow Radar (SHARAD) to detect this basal zone, hindering potential diagnostic cross-frequency comparisons. Due to a novel spacecraft maneuver, SHARAD has now obtained a basal return associated with the putative body of water. Modeling of the radar response is not consistent with the liquid water explanation, instead suggesting a localized, low roughness region of dry rock/dust beneath the ice could explain the SHARAD response. Reconciling the divergent responses of SHARAD and MARSIS remains essential to determine the nature of this anomalous south polar region.
Plain Language Summary
An investigation by Orosei et al. (2018, https://doi.org/10.1126/science.aar7268) of Mars’ southern polar ice cap using the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) orbital radar discovered unusually bright radar returns from the ice base, suggesting liquid water. Due to implications for habitability, the discovery has motivated significant debate regarding the need for liquid water to explain the bright reflections. One of the big challenges is that the Shallow Radar (SHARAD), which uses higher frequencies than MARSIS, couldn’t probe deep enough to detect the base of the ice cap at the location of the possible lake. By undertaking a special type of maneuver that increases the SHARAD signal strength, we were able to observe the putative water body. The SHARAD results are not consistent with the presence of liquid water in this location and an alternative explanation, such as very smooth basal materials, is needed.
Key Points
- We report on a specialized spacecraft maneuver to boost the SHARAD signal strength over the site of a putative subglacial lake on Mars
- SHARAD has detected a basal reflector associated with a potential subglacial water body
- Modeling of the radar response at the lake site is inconsistent with the presence of liquid water
