First results from the ExoMars misión: absence of methane on Mars and variations in water vapor due to dust storms

The TGO orbiter of the ExoMars mission (ESA-Roscosmos) began in April 2018 its scientific mission from an orbit about four hundred kilometers above the surface of Mars. This distance allowed to study without risk a dust storm that covered the planet after a few months and to see how the increase of dust affected the water vapor in the atmosphere, essential data to understand the history of water on Mars. The journal Nature publishes these results today, as well as measures of trace gases that point to a lack of methane on Mars.
"The presence of two instruments such as NOMAD and ACS on board the Exomars-TGO mission is allowing a very precise knowledge of the Martian atmosphere, thanks to its specific design to measure the atmospheric composition and height distribution of each component, especially the minority compounds that play a fundamental role in the behavior of the atmosphere of Mars -says José Juan López Moreno, researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) and co-principal investigator of NOMAD-.

HOW DUST AFFECTS THE ATMOSPHERE

The first high-resolution measurements of NOMAD and ACS have made it possible to measure the vertical distribution of water vapor from near the Martian surface to more than eighty kilometers in height. By measuring during a global dust storm, it has been observed how these phenomena affect the water vapor profiles.
 

"En latitudes norte detectamos nubes de polvo a alturas de entre veinticinco y cuarenta kilómetros que no se encontraban allí antes, y en latitudes sur capas de polvo que se desplazaban a mayor altura –apunta Ann Carine Vandaele, científica del Real Instituto Belga de Aeronomía Espacial e investigadora principal del instrumento NOMAD–. El aumento de vapor de agua en la atmósfera ocurrió notablemente rápido, durante unos pocos días durante el inicio de la tormenta, lo que indica una reacción rápida de la atmósfera a la tormenta de polvo".

Las observaciones son consistentes con los modelos de circulación global: el polvo absorbe la radiación del sol, calienta el gas circundante y provoca que se expanda, lo que a su vez redistribuye otros ingredientes, como el agua, en un rango vertical más amplio. También se establece un mayor contraste de temperatura entre las regiones ecuatoriales y polares, lo que fortalece la circulación atmosférica. Al mismo tiempo, gracias a las temperaturas más altas, se forman menos nubes de hielo y agua, que normalmente limitarían el vapor de agua a altitudes más bajas.

Además, los equipos han estudiado por primera vez el agua “semipesada” (un tipo de agua con un átomo de hidrógeno reemplazado por un átomo de deuterio), simultáneamente con el vapor de agua. “Estas medidas son fundamentales para entender la evolución de Marte desde un clima cálido y húmedo en el pasado remoto hasta el actual clima seco y frío –señala Francisco González Galindo, investigador del Instituto de Astrofísica de Andalucía (IAA-CSIC) que participan en los trabajos–. Las abundancias de ambos compuestos en las capas altas de la atmósfera aumentan de manera significativa y muy rápida durante el desarrollo de la tormenta, lo que permite que el agua escape más fácilmente del planeta”.

"In the northern latitudes we saw features such as dust clouds at altitudes of around 25–40 km that were not there before, and in southern latitudes we saw dust layers moving to higher altitudes -says Ann Carine Vandaele, a scientist at the Royal Belgian Institute of Space Aeronomy. and principal investigator of the NOMAD instrument-. The enhancement of water vapor in the atmosphere happened remarkably quickly, over just a few days during the onset of the storm, indicating a swift reaction of the atmosphere to the dust storm".

The observations are consistent with global circulation models. Dust absorbs the Sun’s radiation, heating the surrounding gas and causing it to expand, in turn redistributing other ingredients – like water – over a wider vertical range. A higher temperature contrast between equatorial and polar regions is also set up, strengthening atmospheric circulation. At the same time, thanks to the higher temperatures, fewer water-ice clouds form – normally they would confine water vapor to lower altitudes.

In addition, the teams have studied for the first time the "semi-heavy" water (a type of water with one hydrogen atom replaced by a deuterium atom), simultaneously with water vapor. "These measures are fundamental to understanding the evolution of Mars from a hot and humid climate in the remote past to the current dry and cold climate," says Francisco González Galindo, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who participates in the studies-. The abundances of both compounds in the upper layers of the atmosphere increase significantly and very quickly during the development of the storm, which allows water to escape more easily from the planet".

METHANE MYSTERY PLOT THICKENS

The two complementary instruments also started their measurements of trace gases in the martian atmosphere. Trace gases occupy less than one percent of the atmosphere by volume, and require highly precise measurement techniques to determine their exact chemical fingerprints in the composition. The presence of trace gases is typically measured in ‘parts per billion by volume’ (ppbv), so for the example for Earth’s methane inventory measuring 1800 ppbv, for every billion molecules, 1800 are methane.

Methane is of particular interest for Mars scientists, because it can be a signature of life, as well as geological processes ­– on Earth, for example, 95% of methane in the atmosphere comes from biological processes. Because it can be destroyed by solar radiation on timescales of several hundred years, any detection of the molecule in present times implies it must have been released relatively recently – even if the methane itself was produced millions or billions of years ago and remained trapped in underground reservoirs until now. In addition, trace gases are mixed efficiently on a daily basis close to the planet’s surface, with global wind circulation models dictating that methane would be mixed evenly around the planet within a few months.

Reports of methane in the martian atmosphere have been intensely debated because detections have been very sporadic in time and location, and often fell at the limit of the instruments’ detection limits. ESA’s Mars Express contributed one of the first measurements from orbit in 2004, at that time indicating the presence of methane amounting to 10 ppbv.

Earth-based telescopes have also reported both non-detections and transient measurements up to about 45 ppbv, while NASA’s Curiosity rover, exploring Gale Crater since 2012, has suggested a background level of methane that varies with the seasons between about 0.2 and 0.7 ppbv – with some higher level spikes. More recently, Mars Express observed a methane spike one day after one of Curiosity’s highest-level readings.

The new results from TGO provide the most detailed global analysis yet, finding an upper limit of 0.05 ppbv, that is, 10–100 times less methane than all previous reported detections. The most precise detection limit of 0.012 ppbv was achieved at 3 km altitude.

As an upper limit, 0.05 ppbv still corresponds to up to 500 tons of methane emitted over a 300 year predicted lifetime of the molecule when considering atmospheric destruction processes alone, but dispersed over the entire atmosphere, this is extremely low.

“We have beautiful, high-accuracy data tracing signals of water within the range of where we would expect to see methane, but yet we can only report a modest upper limit that suggests a global absence of methane”, concludes ACS principal investigator Oleg Korablev from the Space Research Institute, Russian Academy of Sciences, Moscow.