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Water on Mars

Heat and matter · level 6, lesson 6 of 6

A question to start

Mars has dried-up river valleys, and signs of huge floods. Yet today an open cup of water there would freeze and boil away. Where is Mars's water now, and could any of it be liquid?

The idea

Long ago, Mars had liquid water on its surface. NASA says some features suggest huge floods about 3.5 billion years ago, when the air was probably thicker.

Today, almost all of it is ice: in the polar caps and just under the ground near the poles. Phase diagrams explain why: the pressure is near the triple point, and the temperature is usually far below 0 °C.

Salt changes things. Salt lowers the freezing point: seawater on Earth freezes at about −2 °C, and some salts let water stay liquid far colder than that. NASA's Mars facts page describes briny (salty) water that seasonally flows down some hillsides and crater walls. Salt also lowers the vapour pressure, so brine evaporates more slowly.

Water today Mars Earth
Liquid at the surface rare, salty, short-lived oceans cover about 70%
Main store ice in the polar caps and the ground the oceans
Pressure compared with the triple point just above, or below far above
dusty soilicea trench reaches the ice

Worked examples

1. Phoenix's ice. The ice lumps in Phoenix's trench sublimed over four sols. Subliming 10 g of ice takes \(0.010 \times 2{,}830{,}000 = 28{,}000\) J, which the Sun and the warm soil around it supplied.

2. Why salt helps. Seawater freezes at −2 °C, not 0 °C. Stronger brines freeze much colder still, so on a Mars afternoon some salty water might stay liquid when pure water would freeze.

3. A habitat. Inside, at Earth-like pressure and room temperature, water behaves just as it does on Earth. The phase diagram only bites outside.

Try it

  1. Where is most of Mars's water today?
  2. What evidence shows Mars once had liquid water?
  3. How does salt help water stay liquid on Mars?
  4. Why can water in a habitat stay liquid when water outside cannot?
Answers
  1. Frozen, in the polar caps and under the ground near the poles.
  2. Dried-up river valleys and features that suggest huge floods about 3.5 billion years ago.
  3. It lowers the freezing point, and lowers the vapour pressure so the water evaporates more slowly.
  4. The habitat keeps its air at a pressure far above the triple point, and its rooms warm.

Common mistake

"Mars is dry, so it never had water." Its surface is dry today, but its ice holds a lot of water, and its old valleys show it once had liquid water flowing.

Where it leads