Liquid Saltwater Likely Present on Mars

What happens when you have salt water on a surface in cold weather?  On a human body, it comes out of pores as sweat and if the conditions are right, it condenses on the skin as droplets of salty water.  The saltiness means that they probably wouldn’t freeze to your skin until it gets very, very cold — which is why if you exercise outside in the winter, sweat droplets don’t turn into ice blobs right away, even if they land somewhere other than on your warm skin.

It turns out that this tendency of salt water to freeze more slowly in cold weather is behind the discovery of water droplets on the legs of the Mars Phoenix Lander which is sitting silent in the Martian north polar region.

Scientists at the University of Michigan have analyzed an an image taken during the mission and say that this is proof of liquid water on Mars.  This is the first time liquid water has been detected and photographed anywhere but Earth.

So, how could this be?

The common wisdom is that water exists on Mars only as ice or vapor. This is because of the planet’s low temperature and atmospheric pressure. Any water ice that did get exposed would probably vaporize in the low pressure and dry conditions, but it wasn’t likely to just simply melt.  At least, that was the thinking before the Mars Phoenix mission landed.

It came to rest in a place where an interesting confluence of conditions exist to make the droplets of salt water found on the Phoenix lander leg possible. First, temperature fluctuation in the arctic region of Mars where Phoenix landed and salts in the soil could create pockets of water too salty to freeze in the climate of the landing site.

The droplets you see on the leg in the image grew during the polar summer. Based on the temperature of the leg and the presence of large amounts of “perchlorate” salts detected in the soil, scientists think that the droplets were most likely salty liquid water and mud that splashed on the spacecraft when it touched down. The lander was guided down by rockets whose exhaust melted the top layer of ice below a thin sheet of soil.

Some of the mud droplets that splashed on the lander’s leg appear to have grown by absorbing water from the atmosphere, Images suggest that some of the droplets darkened, then moved and merged—physical evidence that they were liquid.

So, where did the salt water come from?  The wet chemistry lab on Phoenix found evidence of perchlorate salts, which likely include magnesium and calcium perchlorate hydrates. Mix these with water vapor released during landing and you have compounds with freezing temperatures of about -90 and -105 Fahrenheit respectively. The temperature at the landing site ranged from approximately -5 to -140 Fahrenheit, with a median temperature around -75 Fahrenheit. Temperatures at the landing site were mostly warmer than this during the first months of the mission.

The Michigan team did thermodynamic calculations, which provide additional evidence that salty liquid water can exist where Phoenix landed and — this is important — elsewhere on Mars.And the fact that the droplets seem to grow as time went by implies more salty water available than the amount melted when the lander came to rest on the surface.

Now, this is quite interesting of course because water is one of those things that we’re pretty sure that life needs to survive in the long term. And, we know that life can exist in pretty salty environments (the so-called halophiles). So, the discovery that liquid salty water can exist on Mars is a big boost for the search for places where life could exist on Mars (now or in the present).  Looking at it another way, knowing that such conditions exist on Mars widens the number of places where we know conditions are suitable for life.