ExoMars NOMAD reveals new insights into the atmosphere of Mars

NOMAD, an instrument onboard the ExoMars Trace Gas Orbiter, was launched in 2016 and has been measuring the atmosphere of Mars since 2018. Recently, scientists in the NOMAD team have published 16 papers, presenting new results on a wide range of topics: atmospheric temperature and density; measurements of dust and aerosols; measurements of CO2 ice clouds; water vapour concentration and isotope ratios; ozone concentrations; carbon monoxide concentrations; measurements of airglow; and observations of surface frost and ice.

NOMAD is a suite of three spectrometers, measuring in the ultraviolet, visible and infrared regions of the spectrum. In these spectral regions, many molecules either absorb or emit radiation. These distinctive patterns allow us to determine which gases are present and in what concentrations.

The presence (or not) of absorption bands can tell us a lot about the atmosphere of Mars:

  • The shape of the measured carbon dioxide (CO2) absorptions gives us the temperature and the density of the observed atmosphere, allowing us to model the atmosphere.
     
  • Different isotopes of the same molecules absorb sunlight differently, allowing us to measure the isotope ratios of gases and to determine, for example, how much water has been lost from the surface of Mars over time.
     
  • Clouds and surface ice also have distinctive spectral patterns - reflecting or scattering sunlight - as do dust and aerosol particles, which block different wavelengths of sunlight depending on the size of the particles in the air. Therefore, by measuring in different wavelengths, we can detect and measure dust and ice, either in clouds or on the planet’s surface.
     
  • By measuring the limb of Mars, we can measure the strength of red and green light emitted by the atmosphere, which is an indicator of the temperature and density of the atmosphere at high altitudes.
     

By making tens of millions of measurements across many years, locations, altitudes and times of day, we can map seasonal and geographical trends, which allow us to understand more about the processes that are occurring in the atmosphere of Mars. Then, by correlating the observed concentrations of different molecules with each other, with clouds, and with dust particle sizes, we can understand the atmospheric cycles and chemistry better than ever before.

This in turn leads to improvements in our simulations and circulation models of Mars, as the discoveries increase our understanding of the ongoing atmospheric processes.

A full list of publications is available on the NOMAD website

Graph five NOMAD measurements temperature atmosphere
Five NOMAD measurements of the temperature of the atmosphere, from 60 to 95 km above the surface of Mars (coloured lines). On the left is the temperature: in places where the temperature drops below the CO2 condensation line (black), clouds of CO2 can form. On the right is the transmittance, directly measured by NOMAD: the presence of a cloud can cause the transmittance to drop below 1, such as is observed in the purple, blue and green lines.

The altitude on the y-axis is the minimum observable altitude along NOMAD’s line of sight; therefore the clouds can actually be at a higher altitude than is given here, depending on the position of the cloud along the line of sight.
Graph seasonal evolution ozone abundance
Seasonal evolution of ozone abundance observed by the ultraviolet-visible spectrometer, for six different latitude ranges up to 70 km above the surface. The left panels show the ozone retrievals without any filtering; in the middle panels we applied a detection limit (DL) filter, to remove points where the signal of ozone was too low to give a meaningful detection; and in the right panels we applied both the DL filter and an additional Δχ2 filter to remove spurious detections.