Making a Splash on Mars

The massive Hellas impact basin in the southern hemisphere of Mars is nearly 9 kilometres deep and 2,100 kilometres across. The air pressure at the bottom of the basin is about twice the global average. In this false-color image based on measurements from the Mars Global Surveyor laser altimeter, red and white colours denote high elevations and blue denotes low.

Conditions would be favourable for liquid water only during the martian day. The temperature falls precipitously at night, so any liquid would re-freeze. At the Viking lander sites, for example, instruments registered temperatures as high as -17 C in the air and +27 °C in the soil on sunlit summer days. After sunset, thermometer readings plunged back to -60 °C or below. [click for more information about martian temperatures]

"One thing we have to be careful of is our everyday experience that water always freezes at zero degrees," noted Hoover. "It doesn't. Water containing dissolved salts freezes at a significantly lower temperature. Don Juan Pond in Antarctica is a good example. It's a high salinity pond with liquid water at temperatures as low as -24 °C."

"Salts have the potential to significantly lower the freezing point of water," agrees Steve Clifford of the Lunar and Planetary Institute. "Indeed, there are some combinations of salts that can lower the freezing point by as much as 60 °C. However, thermodynamic and chemical stability arguments (arising from work by Benton Clark) suggest that, on Mars, the most potent freezing point-depressing brines are likely to be based on NaCl (common table salt)."

An analysis of a Martian meteorite by Arizona State University scientists suggests that ancient martian oceans - if they existed - contained a mix of salts similar to those in Earth's oceans today. That wasn't the first clue that Mars was salty, though. In 1976 the two Viking landers analyzed martian soil and found that it probably contained 10 to 20 percent salts. Martian rocks, like those on Earth, react to form salt and clay minerals when exposed to water. On our planet this process gives rise to a variety of brines in the western salt lakes of North America. The detailed chemistry of the brines depends on the composition of local rocks.