Figure 8.42: Methane measurements vs. martial solar day (sol) at Gale Crater, with error bars. The graph covers a span of time from August 2012 to September 2014, labelled on the horizontal axis by the number of sols. The mean value of ‘low methane’ results averages 0.7 ppbv, meaning 0.7 methane molecules per billion molecules of Martian atmosphere. During short periods, the methane concentration climbed to several ppbv, with mean value of high methane averaging about 7 ppbv.


0.7 parts per billion – significantly lower than earlier claims. Occasionally, however, the rover recorded atmospheric levels of methane as high as roughly 7 parts per billion – a tenfold rise (Figure 8.42). Tus, Mars seems to be actively both producing and releasing methane. However, the source, or even whether it is biological or


geological, is unknown. For example, a possible biological origin could be microbes living in the groundwater below a perma frost zone, whose waste methane could percolate up and leak out. Alternatively, a geological source for the methane could be buried volcanic rocks rich in the mineral olivine, chemically reacting with water to produce methane. Another possible origin could be that the methane is escaping from possible ancient clathrates, buried deposits of methane ice formed long ago by one of the other two mechanisms.


8.7.2 CH4 Hydrate on Mars? In the subsurface of Mars CO2 hydrate, in addition to CH4 , is


predicted to be stable if the guest molecule gas is present in requisite concentrations. CO2


(commonly known as ‘dry ice’) is an abundant volatile


on Mars, comprising 90% of the planet’s atmosphere, the remaining 10% being composed of water vapour, nitrogen, and other gases. It is possible for CO2


hydrates to exist just below


the surface of Mars because of the very cold temperatures, and it has been estimated that the gas hydrate stability zone could extend more than a kilometre beneath the surface. Te surface temperature of Mars has seasonal changes


which may cause continuous variations of atmospheric composition due to hydrate formation and dissociation. CO2 hydrates may exist on the Martian surface in the impressive polar caps and also in the atmosphere in the form of clouds. If they are present in the former, then the cap will not melt as readily as it would if it consisted only of water ice, because of the clathrate’s lower thermal conductivity, higher stability under pressure, and higher strength, when compared to pure frozen water.


286


Figure 8.43: This set of images was taken by the High Resolution Imaging Science Experiment (HiRISE) camera in 2010 and 2013. A new channel is shown forming on the Martian slope. New research reveals that sand propelled on a cushion of CO2


8.7.3 Surface Features on Mars It has been proposed that the decomposition of CO2


gas could be responsible for slicing into the red planet’s surface.


hydrate


could play a prominent role in the ‘terraforming’ processes on Mars, and many of the observed surface features may be partly attributed to it. Martian gullies (Figure 8.43) seen on the surface of the


planet, for instance, have long been a mystery. Te gullies are small, incised networks of narrow channels and their associated downslope sediment deposits. It has been argued that these were formed not by liquid water but by CO2


, since the present


Martian climate does not allow liquid water to exist on the surface in general and recent observations now indicate the gullies are indeed primarily formed by the seasonal freezing of CO2


, not liquid water. Te polar surface, which is covered by layers of sand and dust, is made up of frozen CO2


and water. In


the spring, when the sun warms up the polar slopes, the frozen gas and water do not melt but instead the gases sublimate (i.e. change from solid directly to gas, without pausing to form a liquid). Te vapour lifts the sediment off the surface, reducing the friction and allowing the sand to move more easily. Tis sand is moved down steep slopes, gashing the surface like water running downhill. Te temperature at which CO2


solid to gas is -78.5°C. Te possibility of extracting water from hydrates could


help stimulate attempts at human colonisation of Mars. But to ‘terraform’ Mars, or turn the planet into a smaller version of Earth, unfortunately the first settlers will have to live below ground to protect themselves from cosmic rays.


8.7.4 Hydrates on Planets and Moons?


At the moment, the existence of hydrates on planets, moons and other solar bodies is based on our knowledge of their pressure and temperature conditions and the gases present


changes from


NASA/JPL-Caltech


NASA/JPL-Caltech/Univ. of Arizona


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