Flamsteed Astronomy Society

Where did Mars’ water go?

Dr Andrew Coates — January 8, 2007

Andrew Coates is Head of the Planetary Science Group at UCL’s Mullard Space Science Lab.  His particular interest is about planetary magnetospheres and their interaction with the solar wind.  He is co-investigator for the ASPERA-3 device aboard ESA’s Mars Express mission.   ASPERA is looking at the interaction of the solar wind with Mars’ atmosphere and measuring how fast the atmospheric gases are carried away from the planet.


What was Mars like 3.8 billion years ago (3,800,000,000 years ago)?

Mars had active volcanoes.  Olympus Mons (now extinct, we think) is the largest volcano in the solar system.  It is 27 km high and 600 km in diameter.

Mars had a planetary magnetic field but doesn’t now.  Missions to Mars have mapped residual magnetic anomalies (local areas of magnetic field), but the planet-wide field has gone — we don’t know exactly why, but maybe Mars’ smaller core didn’t sustain the metallic flows necessary.

Mars was warmer and wetter than now.  There is all kinds of evidence for flowing and standing liquids on the surface in the past — channels and seepage, and evidence from rocks and soil formed in the presence of water.


Now Mars is volcanically inactive, there is only local remnant magnetism, the atmosphere is very thin (one-hundredth Earth pressure) CO2 rich, cold and dry. 



Dr Andrew Coates

(pic Mike Dryland)

So what happened to Mars’ water?


Some is in the ice caps.  There is evidence from the Mars missions of water ice in the polar caps and patches of ‘snow’ in craters.

Some is underground.  Ground penetrating Radar in the MARSIS device can ‘see’ 5km deep under Mars’ surface.  There is evidence also from interaction with cosmic rays which suggest water within 1m of the surface.

... and some has escaped.  Andrew’s particular interest is how the solar wind interacts with Mars’ atmosphere and the ASPERA experiment has supplied many surprising results.


ASPERA has shown that Mars atmosphere is loosing material to space at the rate of 100 to 500 grams a second, around 100 tons a day.  The solar wind penetrates down to 270 km of Mars’ surface, much lower than expected.  This is possible because Mars no longer has a planetary magnetic field to shield it from the solar wind.  3.8 billion years ago Mars may have had an atmosphere up to 10 times as dense as Earth’s.  But it has been carried away into space by the solar wind.


Is there life on Mars now?   Maybe, but evidence is still lacking.   Andrew describes data based on the meteorite ALH 84001 from 1996 as ‘unconvincing’.   Up to now missions to the planet have failed to provide proof — but equally they haven’t ruled it out.


There is evidence for Methane on Mars.  Mars can’t retain Methane in its atmosphere so there must be a source now.   Could be life (cows on Mars?) but could also be geological processes.


Mars has aurorae even though there’s no global magnetic field.  The local areas of remnant magnetic field can cause infra-red aurorae even in the equatorial regions.


We have much left to learn about Mars and there are many future missions planned including a sample return mission.    Maybe one day a manned mission?