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ESA-Roscosmos ExoMars Trace Gas Orbiter has detected a new gas for the first time. Sea salt embedded in the dusty surface of Mars and lofted into the planet’s atmosphere has led to the discovery of hydrogen chloride. The spacecraft is also providing new information about how Mars is losing its water.

A major quest in Mars exploration is hunting for atmospheric gases linked to biological or geological activity, as well as understanding the past and present water inventory of the planet, to determine if Mars could ever have been habitable and if any water reservoirs could be accessible for future human exploration. Two new results from the ExoMars team published today in Science Advances unveil an entirely new class of chemistry and provide further insights into seasonal changes and surface-atmosphere interactions as driving forces behind the new observations.

A new chemistry

The Trace Gas Orbiter mission has been collecting atmospheric data on Mars since Spring 2018. Besides the search for methane, the detection of new gases was one of the main goals of the mission. The latest research on Mars has now led to the discovery, for the first time, of a new gas in the Martian atmosphere: hydrogen chloride. This is the first detection of a halogen gas in the atmosphere of Mars, and represents a new chemical cycle to understand.

Hydrogen chloride gas, or HCl, comprises a hydrogen and chlorine atom. Mars scientists were always on the look-out for chlorine- or sulphur-based gases because they are possible indicators of volcanic activity. But the nature of the observations – the fact that hydrogen chloride was detected in very distant locations at the same time, and the lack of other gases that would be expected from volcanic activity – points to a different source. That is, the discovery suggests an entirely new surface-atmosphere interaction driven by the dust seasons on Mars that had not previously been explored.

In a process very similar to that seen on Earth, salts in the form of sodium chloride (NaCl) – remnants of evaporated oceans and embedded in the dusty surface of Mars – are lifted into the atmosphere by winds. Sunlight warms the atmosphere, causing dust, together with water vapour (H2O) released from ice caps, to rise. The salty dust reacts with atmospheric water to release chlorine, which itself then reacts with molecules containing hydrogen to create hydrogen chloride. Further reactions could see the chlorine or hydrochloric acid-rich dust return to the surface, perhaps as perchlorates, a class of salt made of oxygen and chlorine.

HCl formation process
How hydrogen chloride may be created on Mars. Credit: ESA

Water seems to be critical in this chemistry: you need water vapour to free chlorine and you need the by-products of water – hydrogen – to form hydrogen chloride. But also the Martian dust plays an important role: more hydrogen chloride is observed when dust activity ramps up, a process linked to the seasonal heating of the southern hemisphere.

The team first spotted the gas during the global dust storm in 2018, appearing simultaneously in both northern and southern hemispheres, and witnessed its surprisingly quick disappearance again at the end of the seasonal dusty period. Since then, scientists have already been looking into the data collected during the following dust season and have seen the HCl rising again.

Extensive laboratory testing and new global atmospheric simulations will be needed to better understand the chlorine-based surface-atmosphere interaction, together with continued observations at Mars to confirm that the rise and fall of HCl is driven by the southern hemisphere summer.

Rising water vapour holds clues to climate evolution

As well as new gases, the Trace Gas Orbiter is refining our understanding of how Mars lost its water – a process that is also linked to seasonal changes.

Liquid water is thought to have flowed across the surface of Mars in the past, as evidenced in the numerous examples of ancient dried-out valleys and river channels. Today, it is mostly locked up in the ice caps and buried underground. Mars is still leaking water today, in the form of hydrogen and oxygen escaping from its atmosphere.

Understanding the interplay of potential water-bearing reservoirs and their seasonal and long-term behaviour is key to understanding the evolution of the climate of Mars. This can be done through the study of water vapour and ‘semi-heavy’ water (where one hydrogen atom is replaced by a deuterium atom, a form of hydrogen with an additional neutron).

The deuterium to hydrogen ratio, D/H, tells us about the history of water on Mars, and how water loss evolved over time.

With the Trace Gas Orbiter we can watch the path of the water isotopologues as they rise up into the atmosphere with a level of detail not possible before. Previous measurements only provided the average over the depth of the whole atmosphere. It is like we only had a 2D view before, now we can explore the atmosphere in 3D.

Says Ann Carine Vandaele from the Royal Belgian Institute for Space Aeronomy, principal investigator of the Nadir and Occultation for MArs Discovery (NOMAD) instrument that was used for this investigation.

Tracking water loss on Mars
ExoMars observing water in the martian atmosphere. Credit: ESA

The new measurements reveal dramatic variability in D/H with altitude and season as the water rises from its original location. The data show that once water is fully vaporised, it mostly displays a common large enrichment in semi-heavy water, and a D/H ratio six times greater than Earth’s across all reservoirs on Mars, confirming that large amounts of water have been lost over time.

ExoMars data collected between April 2018 and April 2019 also showed three instances that accelerated water loss from the atmosphere: the global dust storm of 2018, a short but intense regional storm in January 2019, and water release from the south polar ice cap during summer months linked to seasonal change. Of particular note is a plume of rising water vapour during southern summer that would potentially inject water into the upper atmosphere on a seasonal and yearly basis.

Future coordinated observations with other spacecraft including NASA’s MAVEN, which focuses on the upper atmosphere, will provide complementary insights to the evolution of water over the Martian year.

The changing seasons on Mars, and in particular the relatively hot summer in the southern hemisphere, seems to be the driving force behind our new observations such as the enhanced atmospheric water loss and the dust activity linked to the detection of hydrogen chloride, that we see in the two latest studies. Trace Gas Orbiter observations are enabling us to explore the Martian atmosphere like never before.

Seasonal variability of water in the martian atmosphere
Seasonal variability of water (left) and D/H (right) for the northern (top) and southern (bottom) hemispheres, as determined by the Nadir and Occultation for MArs Discovery (NOMAD) instrument onboard the ESA-Roscosmos ExoMars Trace Gas Orbiter.
Water is observed to reach high altitudes of greater than 80 km during regional and global dust storms, and at the onset of southern summer (labeled ‘aspirator’). Colder temperatures at the poles and in the middle atmosphere lead to fractionation of water and an apparent decrease of the D/H. Yet, when water is fully vapourised, it displays a strong enrichment of six times that of Earth’s oceans, confirming that large amounts of water have been lost to space over time.
Credit: Villanueva et al. (2021)



  • Dr. Ann Carine Vandaele, BIRA-IASB Research team “Planetary atmospheres”, ExoMars TGO NOMAD Principal Investigator.
    Email: a-c(dot)vandaele(at)aeronomie(dot)be

  • Dr. Karolien Lefever, BIRA-IASB Service “Communication and documentation”. Email: Karolien(dot)Lefever(at)aeronomie(dot)be

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Image of the Terra Sabaea and Arabia Terra regions of Mars, composed of data gathered by the High Resolution Stereo Camera on ESA's Mars Express spacecraft. Credit: ESA/DLR/FU Berlin (G. Neukum)
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Example data plot showing the hydrogen chloride (HCl) detection in spectra collected by the Atmospheric Chemistry Suite (ACS) onboard the ESA-Roscosmos ExoMars Trace Gas orbiter. The data was collected via the solar occultation method where the instrument points through the atmosphere toward the Sun and observes how different atmospheric ingredients absorb the Sun’s light. Since different chemicals have distinctive fingerprints, these observations provide a detailed inventory of the atmosphere’s composition. This plot details how HCl varies with altitude for a particular set of measurements.
Credit: Korablev et al (2021)
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Data plot showing measurements of hydrogen chloride in the atmosphere of Mars, as collected by the Atmospheric Chemistry Suite (ACS) onboard the ESA-Roscosmos ExoMars Trace Gas orbiter. The detections were also confirmed by the complementary instrument, Nadir and Occultation for MArs Discovery (NOMAD). The global dust storm of 2018 is indicated by the brown/orange gradient. The plot shows the locations of the measurements over time (solar longitude) and planetary latitude.
Credit: Korablev et al (2021)