The planets Venus and Mars have revealed a little of their watery past. Meanwhile, Saturn’s moon Titan has been revealed as containing a complex system of lakes, rivers and rain, not of water but probably of the hydrocarbon methane.
Evidence for the watery past of Mars has come from PhD student, M Ramy El Maary, of the Max Planck Institute for Solar System Research. He analysed the systems of cracks seen at the bottom of some Martian craters, which look similar to those that appear on dried up lake beds on Earth. Other suggestions as to their origin include contraction of the CO2 permafrost in the area as it got colder. He simulated the effect of permafrost contraction and found that method could create polygons up to 65 metres in size. In contrast the polygons on Mars are on average 70-140 metres in size, extending up to 250 metres. El Maary also believes that some of the polygons could’ve been formed more recently than the 3.8 billion years ago that Mars is believed to have last held water. Transient lakes created from melt water released by impacts even today could last for thousands of years before being fully evaporated, creating dessication patterns such as the ones seen.
Meanwhile results from the Mars Odyssey spacecraft have shown that up to a quarter of the Martian atmosphere finds itself locked in the Martian ice caps for part of the year. Thomas Prettyman of the Planetary Science Institute in Tucson, Arizona and colleagues used the neutron spectrometer on the probe to map dry ice levels in the Martian north polar region. As dry ice (CO2) builds up at one pole, the other is depleted, altering the surface composition of the poles and keeping the atmospheric CO2 levels up, though altering concentrations.
The observations also showed an area of the outer cap where water ice absorbed and held sufficient heat to preclude CO2 ice from forming until late in the winter. Additionally, the instrument was able to measure argon and nitrogen concentrations in the atmosphere. These elements remain in the atmosphere at all times and so are good tracers of atmospheric circulation.
Meanwhile, the Venusian atmosphere has been probed for its circulation and moisture secrets by two instruments aboard Venus Express. SPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus) and VIRTIS (Visible and Infrared Thermal Imaging Spectrometer) have been looking at wind speeds at different altitudes and characterising water vapour.
The atmosphere of Venus super-rotates – that is it spins round once every four days, whereas the planet itself turns at a slower rate – once every 243 days. How this is kept up with both the planet and the solar wind dragging on the atmosphere is a bit of a mystery, and taking a look at the wind speeds didn’t quite help on this occasion. SPICAM looked and saw no winds running north-south between the poles and tropics on Venus. What was seen was an occasional lower atmospheric cloud structure moving at 40 km per hour. Upper atmospheric winds were present at great speeds.
Measurements of water vapour in the Venusian atmosphere by the two instruments found something interesting – twice the heavy to normal water ratio is found in the upper atmosphere compared to the lower atmosphere. Heavy water – in which a heavier isotope of hydrogen called Deuterium is found as part of the molecule – is bound to the planet more strongly than normal water by gravity, meaning its rate of escape at the top of the atmosphere is less. Additionally, if the molecule is hit by solar wind particles, it requires more energy to reach escape velocity, meaning less can be taken out this way.
What the results also mean is that there was quite a bit of water down there to begin with. Like the Martian ice caps, this means surface material has become atmospheric material – this time, however, the change is permanent as Venus is in the grip of a runaway greenhouse effect. As a result, a wetter, more humid Venus has been dried up – the water has left the surface for the atmosphere, then left the atmosphere.
Meanwhile, the Cassini space probe has been looking down onto the surface of Saturn’s moon Titan. Here, the cycle of material from the surface to the atmosphere is in full swing. The material this time isn’t water or CO2 but most probably methane or ethane. Radar mapping of the surface has shown dark smooth regions in the northern hemisphere of the satellite, which is presently in its ‘summer’. During this season, the atmosphere and surface are warm enough for methane to become liquid and rain out of the atmosphere as torrential rain. In winter bound southern Titan, the landscape is arid, with a desert like landscape of ice crystals forming dunes and the temperature too low for rain or snow to form. The equator sees sporadic tropical storms, which form additional dark spots, which are probably short lived lakes.