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Liquid saline water on Mars

Salts depress the freezing temperature and vapor pressure of moisture, allowing aqueous solutions to form.
18 June 2009, SPIE Newsroom. DOI: 10.1117/2.1200906.1661

Liquid water is a basic ingredient for life as we know it. An important goal of space exploration is, therefore, to determine whether liquid water exists, or ever existed, beyond Earth. Mars is an important candidate in the search for liquid water because it is the most Earthlike world known and will probably be the first other planet to be visited by humans.

The Phoenix Mars Mission landed on 25 May 2008. It was designed to study the habitability potential of the Martian arctic's ice-rich soil and the history of the planet's water.1 The lander was sent to the polar region because the neutron spectrometer on board the orbiting Mars Odyssey spacecraft had been instrumental in uncovering evidence that the top meter of the soil in this region contains about 30–50% ice by mass.2,3 This was confirmed by the discovery of ice in craters produced by the Phoenix spacecraft's landing thrusters, and then in shallow trenches excavated using its robotic arm.1,4 An unexpected result of the Phoenix mission was the discovery of physical evidence for liquid saline water.4 This was the first time that liquid water was detected and photographed on another planet.

The vapor pressure of ~600Pa at the triple point of water (the temperature and pressure at which the gas, liquid, and solid phases of a substance coexist in thermodynamic equilibrium) is below the present-day atmospheric pressure in many regions of Mars, including at the Phoenix landing site, where it is ~700–800Pa. However, although the planet's low surface temperature (~180–250K) and dry air inhibit the presence of pure liquid water near the surface, the presence of liquid saline water is possible because many salts depress the freezing temperature to below present-day surface temperatures.5 In fact, even the presence of salts with relatively high freezing temperatures—such as sodium chloride—might allow liquid water to form sporadically on most of Mars' surface.5 On Earth, except in some extremely dry deserts, salts are leached from the crust and carried to the subsurface or sea, but on Mars leaching is inhibited by its dry climate. Instead, salts absorb water from the atmosphere and subsurface ice, forming liquid aqueous solutions.

In addition to reporting evidence for liquid saline water (brine) in areas disturbed by the Phoenix lander,4 it is also thought that segregated (layered) ice found in two trenches excavated by Phoenix's robotic arm formed in the presence of brine. This is significant because it suggests that liquid saline water may also be present in undisturbed areas of Mars. This idea was supported by chemical soil analysis in Phoenix's Wet Chemistry Laboratory, which revealed that it contains large amounts of perchlorates,6 salts that can produce liquid aqueous solutions at temperatures recorded at the landing site.

Figure 1. Spheroids on a strut of the Phoenix leg on Martian days eight (left), 31 (center), and 44 (right) after landing. Note that of the green spheroids, the one on the left appears to either gradually fall or partially merge (left) with the one on the right (center), and the resulting spheroid (right) moves to the right. The spheroids moved most likely because liquid growth was inhibited in the material left behind by the spheroid that appears to have merged with its neighbor, indicating that it carried most of the deliquescent material with it. (Credit: NASA/Jet Propulsion Laboratory-California Institute of Technology/University of Arizona/Max Planck Institute.)

More recently, it has been proved experimentally that sodium perchlorate, a salt found in significant amounts by the Phoenix lander, grows by deliquescence7 (melting into a liquid aqueous solution) while absorbing water from the atmosphere at the environmental conditions prevalent at the Phoenix landing site. We also showed that because deliquescence is an exothermic process, it lessens freezing even at temperatures below 225K. This result eliminates almost any doubt that the spheroids photographed on a strut of the lander's leg (see Figure 1) were made of liquid saline water and grew by deliquescence.

The recent discovery of ancient microbes (alive beneath an Antarctic glacier) in frigid water as saline as that found by Phoenix suggests that development of microbial life might be possible on Mars. Moreover, in the Atacama desert, a cold and extremely dry region in Chile, salts absorb atmospheric moisture and deliquesce, forming highly saline but habitable environments.8 Therefore, the discovery of liquid saline water on Mars has important implications for the planet's habitability.

Our future work will try to quantify the amount of liquid water and, in particular, the natural processes that form pockets of concentrated saline water. We will therefore develop instruments to search for such pockets on Mars. We are also studying the thermodynamics of life, aimed at understanding what conditions could allow bacterial life to exist on Mars today.

The author was supported by NASA's Phoenix Mars Mission and National Science Foundation/Atmospheric Sciences award ATM 0622539.

This work was presented in the conference Instruments, Methods and Missions for Astrobiology at the SPIE Optics + Photonics symposium in August 2009 in San Diego.

Nilton Renno
Department of Atmospheric, Oceanic and Space Sciences (AOSS)
University of Michigan
Ann Arbor, MI

Nilton Renno graduated from the Massachusetts Institute of Technology with a PhD in atmospheric sciences in 1992, and is currently a professor. He was one of the lead scientists of the Phoenix Mars Mission.