Martian Atmosphere

Some more bits and bobs that I didn't have time to write about yesterday. This time it is the Martian atmosphere. There have been lots of measurements by Mars Global Surveyor (MGS) and Mars Express using many instruments. The MGS had an accelerometer which took readings during the aerobraking phase as the spacecraft was inserted into its proper orbit. Aerobraking involves using the atmosphere to slow down the craft and basically lose some of the speed that it needed to get to Mars in the first place. The accelerometer measures the acceleration (deceleration in this case) and the amount of it tells you the density of the atmosphere. The first two phases of aerobraking on MGS took almost a year and it was possible to measure densities at heights between 110 and 160 km altitude in the atmosphere as it passed through. The Mars Odessy spacecraft did exactly the same thing and was able to measure densities at heights between 95 and 170 km. In fact, the Odessey spacecraft followed the 2001 dust storms.

The MiniTES instrument on board the Mars Rovers has been making its own atmospheric measurements by looking upwards - it can tell us stuff about the atmosphere at heights from 20m to 2km. It has seen an atmospheric boundary layer, some thermal structure in the atmosphere and evidence of aerosols. It also saw a strong inversion layer during the night. As Spirit is currently sitting on top of Husband Hill, they plane to compare the temperature of the air above the ridge with that over the plains. The hope is that they may detect a breeze or wind coming up the slopes of the hill.

ESA's Mars Express has also been making measurements of the atmosphere. The SPICAM instrument observes at ultra-violet (UV) wavelengths of light so, as you may realise if you think about why you wear sun cream, it can detect the presence of ozone (O3). It has found that there is 300 times less ozone in the Martian atmosphere than on Earth. They have also made the first observation of ozone during the Martian night. This is tricky because they rely on reflected sunlight normally and you don't have that at night. Instead they observed a star occultation. So, where is this ozone? Well it varies with season, latitude and longitude but there seem to be two layers; one near the ground and the other at heights of 40-50 km.

What about all that water that is 'discovered' every couple of years. Well, models suggest that the water ice is deposited during the night and then sublimates during the morning. This happens all year round in the northern hemisphere but only during the winter in the southern hemisphere.

Finally, SPICAM has seen plenty of clouds at mesospheric altitudes and even seen clouds as high as 100 km although they probably wouldn't be seen with your eye if you were there. They have been compared to noctilucent clouds on the Earth.

There was much more discussed but that should do for now.

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Posted in astro blog by Stuart on Wednesday 07th Sep 2005 (17:41 UTC) | Permalink
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