# Would poured water boil or freeze on Mars?

Short answer: It depends on the altitude and season.

Longer answer: It’s a bit of a lie that water freezes at 0 °C and boils at 100 °C – in reality that only happens at sea level with standard atmospheric pressure. If you went up a mile in altitude to Denver your water would boil at 95 °C. This means that the phase of water is determined by both the temperature and the pressure. Below is a phase diagram which can be used to determine the phase of water for any given temperature and pressure:

Ice IX has no relation to Kurt Vonnegut.

Martian atmospheric pressure averages about 600 Pascals, which is just below the lowest pressure where liquid water can exist, called the triple point. Put another way, this means that below this pressure there is a direct transition between solid and gaseous states, like what you observe with dry ice (solid and gaseous carbon dioxide) on earth.

So let’s assume we have a bottle of liquid water at room temperature, which is about 300 Kelvin. Let’s also assume we’re on Mars. What happens when we take the lid off and pour it out? According to our phase diagram, it should rapidly boil and form vapor. If you’re wondering where this is on the diagram, it’s just above the first ‘o’ in “Solid/Liquid/Vapour triple point.”

“Commander, what is your position? Over.”
“Houston, we’re 6 kilometers north of the ‘e’ in ‘Uncharted’ and 200 meters above where our eyes would boil without helmets. Over.”

I don’t want anyone to assume this means that liquid water can’t exist anywhere on Mars. I just mean to say that water at room temperature is too hot to be liquid. In fact, when I told you that atmospheric pressure at the surface is  600 Pascals, I lied a bit. That 600 Pascals is the average surface air pressure – the topography has highs and lows where the pressure will vary and since Mars doesn’t have seas it makes it difficult to define sea level. In fact, prior to 2001 the zero elevation was defined as the height where ambient pressure is 610.5 Pascals, which is where liquid water could exist. And even though lows get as cold as −140 °C (−220 °F) , surface temperature can reach highs of 35 °C (95 °F) in the summer. This means that any elevation below “Martian sea level” with the right conditions could potentially host liquid water in the warm Martian summer.

asked by /u/baseball_guy
image credit: Wikimedia Commons

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