Elsevier

Acta Astronautica

Volume 146, May 2018, Pages 144-150
Acta Astronautica

Shallow transient liquid water environments on present-day mars, and their implications for life

https://doi.org/10.1016/j.actaastro.2018.02.027Get rights and content

Highlights

  • Shallow transient liquid brines water can support life.

  • Transient brines occur on Mars and are likely widespread in the present day.

  • Portions of the martian shallow subsurface are capable of supporting microbial life.

  • A strategy of ‘following the salt’ to identify habitable environments is proposed.

Abstract

The identification and characterisation of subsurface liquid water environments on Mars are of high scientific interest. Such environments have the potential to support microbial life, and, more broadly, to develop our understanding of the habitability of planets and moons beyond Earth. Given our current state of knowledge of life on Earth, three pre-requisites are necessary for an environment to be considered ‘habitable’ and therefore capable of supporting terrestrial-like life: energy, biogenic elements, and liquid water with a sufficiently high water activity. The surface of Mars today is predominately cold and dry, and any liquid water exposed to the atmosphere will vaporise or freeze on timescales of hours to days. These conditions have likely persisted for much of the last 10 million years, and perhaps longer. Despite this, briny liquid water flows (Recurrent Slope Linea) have been observed in a number of locations in the present-day. This review examines evidence from the Phoenix Lander (2008) and the Mars Science Laboratory (2012-current), to assess the occurrence of habitable conditions in the shallow Martian regolith. It will be argued that shallow, transient, liquid water brines are potentially habitable by microbial life, are likely a widespread occurrence on Mars, and that future exploration aimed at finding present-day habitable conditions and potential biology should ‘follow the salt’.

Section snippets

Introduction & background

All known active life requires liquid water. This observation, and the remarkable adaptations shown by life in even the most inhospitable environments where liquid water is available, has guided the search for life on other planets. For terrestrial-like life to exist in the harsh conditions that dominate the surfaces of other rocky planets, our current understanding suggests that minimum fundamental requirements of liquid water, nutrients, and a gradient in chemical energy must be met. Within

Phoenix

The Phoenix Lander (PL) was active in the northern arctic from May-November 2008, during which time it provided the first compelling evidence of present-day liquid water on Mars. Observations by PL significantly shifted the understanding of liquid water availability in the shallow regolith and the importance of salts in enabling liquid to persist in Mars' significantly sub-zero conditions. In early images, spherical droplets were detected on the landing struts with their size correlated with

Habitability of transient brines

The ability of salts on Mars to temporarily liquefy by the absorption of atmospheric liquid water provides an important source of water for potential biology within the subsurface diffusion depth of water vapour, and potentially within longer term liquid water environments at greater depth. Although carbonate and sulfate brines may be the most abundant (due to those salts occurring in higher concentrations, Table 2), limited information is currently available on their water activity. It is

Conclusion

There is now strong evidence that shallow transient liquid water exists at a broad range of latitudes and seasons on Mars. In situ observations by the Phoenix Lander and Curiosity Rover demonstrate that salts readily form transient hydrates and liquid films on Mars throughout the top tens of centimetres of the martian subsurface, and likely persistent brines at greater depth. These brines accumulate to sufficient volume to influence surface features, as indicated by the formation of recurrent

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