This week NASA announced that it has discovered evidence that liquid water is flowing on Mars. Not just in the past as previously announced, but now.
The existence of liquid water on the surface of Mars is rather unexpected. To understand why, look at the following phase diagram chart, showing the states of water as a function of pressure (vertical axis) and temperature (horizontal axis):
On the surface of the Earth, we live at 1 atm. This means that water exists as a solid (“ice”) at a temperature below 0 degrees Celsius, as a liquid (“water”) between 0 and 100 degrees Celsius, and as a gas (“water vapor”) above 100 degrees Celsius. You can see this in the diagram above by following the horizontal dotted line from left to right at P = 1 atm.
Let’s look at Mars. The atmospheric pressure on the surface of Mars is about 0.006 atm. Look at the chart above. At an atmospheric pressure of 0.006 atm, liquid water is just on the knife-edge of being able to exist at all. If the pressure were slightly lower than 0.006 atm, water would transition directly from a solid to a gas with an increase in temperature. (An example with which you’re familiar on Earth is carbon dioxide, which transitions directly from a solid we call “dry ice” to a gas at 1 atm). Slightly higher, liquid water could exist, but only in a very narrow band of temperatures.
Atmospheric pressure and temperatures make the existence of liquid water on Mars an extreme challenge. At the very least, the above phase diagram shows why finding liquid water on the surface of Mars is very surprising. Perhaps minerals and salts dissolved in the water shift the phase diagram enough to permit liquid water to exist under a broader range of temperatures. It is certainly worth exploring further, in person.
Speaking of exploring in person, Eric Berger (@chronsciguy) of the Houston Chronicle wrote an article about why it is so hard to get to Mars. Definitely worth reading.