Scientists unveil new evidence of salt water on Mars

Mars Orbiter Laser Altimeter (MOLA) topographic map of the area studied at Ultimi Scopuli. Dotted lines are MARSIS observations. The blue region indicates the geographic location of the main light area. Observations in the light gray shaded area were not used for data inversion, as they pass through areas of high and low basal reflectivity and cannot be attributed to either bright or bright datasets. not shiny. Credit: Nature Communication (2022). DOI: 10.1038/s41467-022-33389-4

It may be known as a rocky, red planet, but there is growing evidence that salt water exists at the base of polar deposits on Mars.

Professor Graziella Caprarelli from the University of Southern Queensland is part of an international team studying light reflection signals below the Martian surface, first spotted in data acquired between 2010 and 2019 by the MARSIS radar sounder at aboard the Mars Express.

The mostly Italian team proposed that the reflections pointed to a mosaic of salt lakes, publishing their research in Science in 2018 and in natural astronomy in 2021. Recently, a new collaboration between the Italian team and US-based researchers has provided new evidence further supporting this interpretation.

The results of these studies were recently published in the journals Nature Communication and the Journal of Geophysical Research: Planets.

Professor Caprarelli said new laboratory experiments and simulations have ruled out alternative interpretations.

“We explored questions such as ‘is it possible that strong radar signals could be produced by other types of materials like clays or salt ice, or by constructive interference,'” she said. .

“The latest papers address the long-standing issue of temperatures at the base of the southern polar ice cap: until now, these were considered too low even for the brines to be liquid.”

(a) Phase diagram of Ca(ClO₄)₂ with colored outlines of bulk brine (parula color map) and hydrate (pink color map) concentrations. For example, a Ca(ClO of 700 mM (15.1% by weight)₄)₂ the sample at 185 K (point A) has a hydrate content of about 12% by volume. At the eutectic temperature, the hydrate and the ice melt to form a brine of eutectic concentration (Point B_{salt water}) and with a liquid content of about 14% by volume. At 240 K, the amount of liquid brine in the salt-H₂The O mixture is about 22% (point C), while the brine concentration is 40% by weight (point C_{salt water}). The sample then melts completely at 268.4 K (point D). (b) Percent volume of 100, 300 and 1000 mM brine versus temperature. Eutectic temperatures for Ca(ClO₄)₂Mg(ClO₄)₂and CaCl₂ are ∼197.3, 216, 223 K, respectively. Credit: Journal of Geophysical Research: Planets (2022). DOI: 10.1029/2022JE007398

Professor Caprarelli, who is an assistant at the Center for Astrophysics at the University of Southern Queensland, developed the thermal models and calculated the temperature range at the base of Mars’ south polar cap, beneath the south polar stratified deposits ( SPLD).

“We decided to study the physical properties of the deposits themselves, by modeling the propagation of radar waves through water ice and dust.”

The new calculations constrain the percentage of dust inclusions in the deposits to between 5% and 12%, further setting an upper limit of 230 K (-43°C) for the temperature at the base.

“Our studies show that the temperature at the base of the SPLD calculated so far by other researchers (around 170-180 K) has been greatly underestimated and can easily reach 200 K (-73°C), which is in the melting temperature range of perchlorate brines,” said Professor Caprarelli.

“New laboratory experiments conducted at the laboratories of Roma Tre University (Italy) and the Southwest Research Institute (USA) further demonstrate that the physical properties of brines at these revised temperatures are fully compatible with the strength of the signals radar acquired from the base of the Martian south polar fields.”