The Schiaparelli space probe has been in the news quite a lot this last week or so. It was due to land on the surface of Mars last Wednesday (19 October), but lost contact about one minute before this. On Friday (21 October) NASA released images taken by its Mars Reconnaissance Orbiter which have led ESA to conclude that Schiaparelli exploded on impact, probably due to a failure of the thruster rockets which were meant to guide it gently down over its last few kilometres of descent. For more on that story, see here. This separate story suggests that the failure of the thruster rockets to burn correctly was due to a computer glitch, and that they only burned for 3 seconds instead of the intended 29 seconds.
What has received far less attention than Schiaparelli is the larger spacecraft which transported it to Mars – the Trace Gas Orbiter (TGO). The TGO was successfully put into orbit about Mars after it and Schiaparelli separated. Whilst ESA scientists worried about the silence of Schiaparelli, they were nevertheless jubilant that the TGO had successfully manoeuvred into orbit about the red planet.
The TGO’s primary scientific mission is to look for traces of methane emanating from Mars. This is of great scientific interest, because methane could be due to life on Mars. Many bacteria on Earth, in particular those that respire anaerobically, emit methane. The best known example are the bacteria which help digest food in the stomachs of many animals, including us. This is why cows are one of the primary sources of methane emission, the gas is coming from the bacteria in their stomachs.
Methane was first detected in the Martian atmosphere in 2003 by NASA scientists. The following year NASA’s Mars Express Orbiter and some ground-based observations detected methane at the level of about 10 parts per billion. Large temporal and positional variations in the methane concentration were measured between 2003 and 2006, which suggests that the methane is both seasonal and local.
The other possible source of methane is geological activity. Any methane in the Martian atmosphere is quickly broken down by ultraviolet light from the Sun (there is no ozone layer to protect the molecules from UV light, as there is on Earth). This means that any methane present in the Martian atmosphere but have been recently produced. So, how can we tell the difference between methane due to bacteria and methane due to geological activity?
The key is to look for the presence of other gases along with the methane. If the methane is geological in origin it will be accompanied by sulphur dioxide. If, however, it is due to bacteria it will be accompanied by ethane and other similar molecules. The TGO will be able to measure both the methane and these other gases, and so hopefully will help us determine the origin of the methane. In addition, it will be able to measure and image other things, including sub-surface hydrogen down to a depth of a metre. This will help us better map out the amount and extent of subsurface water ice on Mars.
In all, the TGO has four scientific instruments on it, namely
- The Nadir and Occultation for Mars Discovery (NOMAD). This instrument has two infrared and one ultraviolet spectrometer channels.
- The Atmospheric Chemistry Suite (ACS) has three infrared spectrometer channels.
- The Colour and Stereo Surface Imaging System (CaSSIS) is a high-resolution colour stereo camera which will be able to resolve down to a resolution of 4.5 metres on the Martian surface. Being stereo, it will be able to create an accurate elevation map of the Martian surface.
- The Fine-Resolution Epithermal Neutron Detector (FREND), a neutron detector which can indicate the presence of hydrogen in the form of water or hydrated minerals. FREND can detect hydrogen down to a depth of 1 metre in the Martian surface.
NOMAD and ACS are the two instruments which will measure the methane and other trace molecules in the atmosphere. Twice each orbit, when the Sun is both rising and setting as seen from the TGO, it will use the passage of the Sun’s light through the Martian atmosphere to detect and measure the presence of trace molecules, down to a few parts per billion (ppb).
The TGO will orbit Mars at an altitude of 400 km, in a circular orbit taking only 2 hours to orbit once. The orbit will be inclined at 74 degrees to the Martian equator. It was launched on the 14 March, so took just over 6 months to get to Mars. In 2021 ESA plans to land a rover on the Martian surface, but whether this schedule is delayed due to the failure to successfully land Schiaparelli remains to be seen.