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Isotopes on Mars

Radiation and the atom · level 4, lesson 6 of 6

A question to start

Billions of years ago, Mars had rivers and lakes. Today its air is thin and dry. Where did the air and water go, and how can anyone tell?

The idea

Isotopes leave a trail. Light atoms are a little easier to knock away into space than heavy ones. So if a planet loses gas to space over billions of years, what is left becomes richer in the heavier isotope.

Argon. NASA's MAVEN orbiter measured two isotopes of argon, argon-36 and argon-38, high in Mars's air, and Curiosity measured them at the surface. Argon never joins with anything, so it cannot hide in rocks; the only way to lose it is to space. From how much the heavier isotope had built up, the team worked out that about 65% of the argon Mars ever had has gone. They concluded that most of Mars's carbon dioxide was lost to space too.

Water. Some water molecules contain a deuterium atom instead of ordinary hydrogen. Telescopes on Earth measured both kinds in Mars's air. Mars's water is far richer in deuterium than water in a Mars meteorite 4.5 billion years old. From this, NASA scientists worked out that Mars has lost about 87% of its early water.

Gases in the air (by volume) Mars Earth
Argon 1.94% 0.934%
Water vapour 0.021% typically about 1%
light atoms escapemore easilylightheavy

Worked examples

1. What is left. Suppose Mars started with 100 units of argon and 65% escaped. \(100 - 65 = 35\) units are left: about a third.

2. The early ocean. NASA's estimate is enough water to cover the whole of Mars about 137 m deep. 87% of it is gone, so what is left would cover it about \(137 \times 0.13 \approx 18\) m deep. (Most of that is now ice.)

3. Argon and air. Mars has about twice Earth's share of argon in its air: \(1.94 \div 0.934 \approx 2.1\). Mars's air is so thin, though, that the actual amount of argon above each square metre is much less than on Earth.

Try it

  1. Why does gas escaping to space leave behind more of the heavy isotope?
  2. Why is argon a good gas for measuring how much air escaped?
  3. What is the heavy kind of hydrogen in Mars's water called?
  4. About what share of the argon Mars once had is still there?
Answers
  1. Light atoms are a little easier to knock away, so they leave faster.
  2. It never joins with anything, so it cannot be locked in rocks; the only way to lose it is to space.
  3. Deuterium.
  4. About 35%, a third.

Common mistake

"Mars's water all soaked into the ground." Some did, and some is frozen at the poles and under the surface. But the deuterium shows that much of it escaped to space.

Where it leads