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How Mars lost its air

Electricity and magnetism · level 8, lesson 5 of 6

A question to start

Mars once had rivers and lakes, so its air must once have been much thicker. Where did that air go?

The idea

Much of it was lost to space, and it is still going. NASA's MAVEN orbiter, at Mars since September 2014, measured how.

High in the air, sunlight knocks electrons off atoms, making ions. The solar wind carries a magnetic field, and as it flows past Mars it sets up an electric field that accelerates those ions away. With no global magnetic field to hold the wind off, this happens just above the upper air. MAVEN found gas escaping:

  • down the tail, behind Mars, where the solar wind flows past;
  • above the poles, in a polar plume;
  • from an extended cloud of gas round the planet.

Some ions are also swept round and slammed back into the upper air, knocking other atoms out into space: sputtering.

MAVEN's numbers:

  • today, about 100 grams a second is stripped away, and much more during solar storms;
  • about 65% of the argon Mars ever had has been lost to space;
  • sputtering removed most of the planet's carbon dioxide.
solar windtailpolar plumecloud round the planet

Worked examples

1. A year at today's rate. \(100 \text{ g/s} \times 3.16\times10^7 \text{ s} = 3.2\times10^6\) kg, about 3,200 tonnes a year.

2. The whole air. Mars's air has a mass of about \(2.5\times10^{16}\) kg. At today's rate, it would take \(2.5\times10^{16}/3.2\times10^6 = 7.9\times10^9\) years to lose it all.

3. So the loss was faster then. In 4.5 billion years at today's rate, Mars would lose \(3.2\times10^6 \times 4.5\times10^9 = 1.4\times10^{16}\) kg, about half the air it has now. Yet it once had air thick enough for rivers and lakes, so it lost far more than that. The young Sun was more active, and the loss rate was much higher billions of years ago.

Try it

  1. Why does a planet without a global field lose its air more easily?
  2. Name the three places MAVEN saw gas escaping.
  3. How much gas does Mars lose in a day at 100 g/s?
  4. Why do scientists think most of the loss happened long ago?
Answers
  1. The solar wind reaches its upper air, and its electric field carries ions away.
  2. The tail, a plume over the poles, and a cloud round the planet.
  3. \(100 \times 86{,}400 = 8.6\times10^6\) g, about 8.6 tonnes.
  4. Today's rate is far too slow to remove that much air; the young Sun was more active, so the loss was much faster.

Common mistake

"Mars lost its air because its gravity is weak." Weaker gravity helps the gas escape, but the solar wind, acting on a planet with no global field, did much of the work.

Where it leads