Understanding more about Mars

19 January 2021

Artist's impression of the ExoMars Trace Gas Orbiter (TGO) analysing teh maritan atmosphere

Dr Kevin Olsen from Oxford’s Department of Physics, working with international colleagues, has gathered new insights into Mars’ atmosphere thanks to data gathered from the ExoMars Trace Gas Orbiter.

The on-board spectrometers – the Atmospheric Chemistry Suite – aim to investigate the detailed composition of the Martian atmosphere and help us better understand the cycles of water and carbon in the atmosphere of Mars. The results reported in Nature Geoscience provide the first insights into the vertical structure of carbon monoxide. They demonstrate stable abundances for the first 90 km in altitude, above which the conversion of CO2 into carbon monoxide (CO) by sunlight is revealed by a dramatic increase in the number of CO molecules.

Analysing the Martian atmosphere

Dr Olsen comments: ‘The Martian atmosphere is mostly made of CO2 (95%) which breaks down in sunlight, and turns into carbon monoxide. A long-standing problem in understanding the history of Mars is why the atmosphere is so stable in its composition – why doesn't all of the CO2 get destroyed over time?’

A partial answer is in the so-called oxidising power of the Martian atmosphere; this is the ability of reactive molecules related to water to convert CO back into CO2. But, this has been difficult to reproduce in models, and is highly variable (since the amount of water in the atmosphere changes greatly with the seasons).

Comparing data

Dr Olsen, in collaboration with colleagues from Oxford as well as from the Laboratoire Atmosphères, Observations Spatiales (LATMOS) in France and the Space Research Institute of the Russian Academy of Sciences, was able to compare data from near the equator to data from near the polar regions, which reveal a direct look at the global circulation. An important circulation pattern on Mars is called the Hadley cell circulation, and this occurs on Earth too. Air is warmed near the equator, and rises, which increases the pressure in the air above the equator; this, in turn, expands and pushes outward, towards the poles, where it then cools and descends. The group’s observations reveal the descent of the air from the upper atmosphere, which contains many more CO molecules, over the polar regions.

Dr Olsen continues: ‘The observations cover the start of a global dust storm in 2018 which highlighted the phenomenon: the dust lifted into the atmosphere absorbs sunlight and warms the atmosphere, which, in turn, causes water vapour to rise. The extra water vapour increases the amount of reactive gases, which can interact with CO. What we therefore observe is a rapid decline in the abundance of CO, which we interpret as an increase in the oxidising power of the Martian atmosphere – more CO is being converted back into CO2.

Unprecedented accuracy

‘While CO and its abundance were already well known in the Martian atmosphere, the ability to probe its vertical structure, and with unprecedented accuracy, is a new capability provided by the spectrometers on the ExoMars TGO. These early observations are among the first direct looks at the global circulation and oxidising potential on Mars and future work will be to look at long-term trends and seasonal changes, especially over the poles.’

The vertical structure of CO in the Martian atmosphere from the ExoMars Trace Gas Orbiter, K Olsen et al, Nature Geoscience, 18 January 2021

Image caption: Artist's impression of the ExoMars Trace Gas Orbiter (TGO) analysing the martian atmosphere; it depicts the observation technique the group use where, during every orbit of Mars, they wait until the TGO was moving into the shadow of Mars (or coming out of it), and looked across the edge of the planet directly at the sun, with the atmosphere between the satellite and the sun. The group can tell what gases are in the atmosphere by analysing how much sunlight is absorbed by them (and then how much of a gas there is by how much light is absorbed).
Image ©ESA/ATG medialab