Amazing astronomy

Mars is known for its thin atmosphere, where CO2 dominates and provides most of the atmospheric mass and pressure. In fact, the pressure is similar to that in the Earth’s stratosphere, which is a layer of the atmosphere, at more than 30km above the surface.

But what about water? Water on Mars is currently found on the surface as a layer of ice – several kilometres thick – at the north pole. It also appears as seasonal frost at the coldest times of the year, and in the atmosphere as vapour and ice. Nevertheless, the Martian atmosphere is extremely dry compared to Earth’s, with about 100 times less water. While precipitation on Earth results in water layers several centimetres thick, water that would precipitate on Mars would only form a thin film of less than a millimetre.

Water escapes from the Martian atmosphere?

The evidence suggests that Mars was not always the cold, arid planet we observe today. There is plenty of evidence of water on Mars’ surface in the distant past – about four billion years ago. At that time, liquid water flowed in great streams and stagnated in the form of pools or lakes, such as in the Jezero crater explored by the Perseverance rover, in search of traces of past life.

For liquid water to circulate and reside on the surface long enough to leave these marks, there must have been a radically different climate than the one we see today. Mars, Earth and Venus probably formed from the gradual accumulation of the same basic materials, which means that they must have had great similarities early in their history. But while Earth and Venus have retained most of their thick atmosphere, Mars, because of its small size and low gravity, has lost most of its atmosphere.

It is indeed this “loss of gas to space” that helps explain the current tenuousness of Mars’ atmosphere. This loss occurs very high in the atmosphere, above 200km, where molecules have already broken down into atoms and where the lightest ones, such as hydrogen, can be torn away from the weak gravity of Mars. Exposed to the energetic particles of the solar wind, Mars’ exosphere (the upper layer of the atmosphere) has allowed the equivalent of hundreds of present-day atmospheres to be lost to space.