Abstract
Due to its significance in astrobiology, assessing the amount and state of liquid water present on Mars today has become one of the drivers of its exploration. Subglacial water was identified by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard the European Space Agency spacecraft Mars Express through the analysis of echoes, coming from a depth of about 1.5 km, which were stronger than surface echoes. The cause of this anomalous characteristic is the high relative permittivity of water-bearing materials, resulting in a high reflection coefficient. A determining factor in the occurrence of such strong echoes is the low attenuation of the MARSIS radar pulse in cold water ice, the main constituent of the Martian polar caps. The present analysis clarifies that the conditions causing exceptionally strong subsurface echoes occur solely in the Martian polar caps, and that the detection of subsurface water under a predominantly rocky surface layer using radar sounding will require thorough electromagnetic modeling, complicated by the lack of knowledge of many subsurface physical parameters. Higher-frequency radar sounders such as SHARAD cannot penetrate deep enough to detect basal echoes over the thickest part of the polar caps. Alternative methods such as rover-borne Ground Penetrating Radar and time-domain electromagnetic sounding are not capable of providing global coverage. MARSIS observations over the Martian polar caps have been limited by the need to downlink data before on-board processing, but their number will increase in coming years. The Chinese mission to Mars that is to be launched in 2020, Tianwen-1, will carry a subsurface sounding radar operating at frequencies that are close to those of MARSIS, and the expected signal-to-noise ratio of subsurface detection will likely be sufficient for identifying anomalously bright subsurface reflectors. The search for subsurface water through radar sounding is thus far from being concluded.
Highlights
In spite of the theoretical difficulties in reconciling the presence of liquid water with the known characteristics of the SPLD, recent observations acquired by MARSIS over the same region, and analyzed using signal processing procedures commonly applied on Earth to discriminate between wet and dry subglacial areas, are in agreement with the earlier detection of subglacial water, and provide evidence for other wet areas in its surroundings, suggesting the presence of a complex hydrologic system [35]
The investigation leading to the discovery of subsurface liquid water on Mars through radar sounding was prompted by the detection of echoes, coming from a depth of about 1.5 km, which were stronger than surface echoes [32]
The present analysis clarifies that the conditions causing exceptionally strong subsurface echoes occur solely in the Martian polar caps, and that the detection of subsurface water under a predominantly rocky surface layer will require thorough electromagnetic modeling, complicated by the lack of knowledge on many subsurface physical parameters
Summary
Most of the ice present on Mars today is located in the polar regions, which are covered by ice sheets extending for millions of square kilometers and possessing a thickness of thousands of meters These deposits are constituted by several geologic units differing in origin, composition, and age. Martian atmosphere is much higher than the one observed in ancient rocks such as the Martian meteorites, confirming the loss of a large quantity of water inferred from observed atmospheric loss rates [14,15] It has been estimated [16] that the total water present on the surface of Mars 4.5 Gyr ago must have been 6–7 times the quantity existing today
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