What we learned about Mars in the 35th Martian year
10/20/2021 14:01
Tuesday, October 19, 2021, marks 5 years since the Martian interplanetary station Trace Gas Orbiter (TGO) of the Russian-European mission ExoMars 2016 entered orbit around Mars. There are four scientific instruments on board the TGO: two Russian and two European. If we summarize all the results that were obtained during their work, then we can say that with the arrival of TGO in the study of Mars, the paradigm was broken.
This primarily concerns the Martian climate and weather. It is possible that even in the relatively recent past, there was more water on Mars than hitherto assumed. However, the question of life on the Red Planet is still being resolved, rather, in a negative direction, although here there is still room for some mysteries.
The scientific complex of the TGO apparatus includes two spectrometric complexes: the Russian ACS and the Belgian NOMAD. Their main task is to search for small gaseous constituents of the Martian atmosphere, that is, substances whose share does not exceed 1%, as well as to study aerosol particles.
The Russian ACS complex includes three spectrometers operating in different parts of the infrared spectrum. They are distinguished by record spectral resolution and good sensitivity - the ACS complex is capable of registering gas components, the concentration of which does not exceed several tens of particles per trillion per unit volume.
The composition of the Martian atmosphere seems simple. 95% is carbon dioxide CO2, then nitrogen (about 3% percent), argon (less than 2%) and the so-called minor components (total share less than 1%). These include water vapor, oxygen, ozone and other substances. They are of particular interest, since it is hoped that among them biomarkers can also be found - gases that may indicate the presence of life.
Although the question of life on Mars remains on the agenda, in recent months the attention of researchers has turned to the problems of the chemistry of the Martian atmosphere. As it turned out, the models available today do not explain the picture of what is happening, which is actually observed on Mars. We are collecting the details of this picture now.
Methane, ethane, ethylene, phosphine ...
What unites all the named compounds? The answer is that they are all currently considered possible signs of biological activity. In March, May and June 2021, three articles were published in scientific journals on the search for these gases on Mars. The first place, of course, is still methane.
Searching for methane is one of the main tasks of the ACS spectrometer. Already in 2018, almost immediately after the start of regular work, it became clear that the methane in the atmosphere of Mars is orders of magnitude less than it was assumed on the basis of ground-based observations - no more than 50 particles per trillion per unit volume (or 0.05 particles per billion, parts per billon by volume, ppbv). This result was released to the public in 2019 after extensive reviews.
In a new article published in June 2021 in Astronomy & Astrophysics, these estimates have been tightened even more. Researchers Frank Montmessin (LATMOS laboratory, France) and Oleg Korablev (IKI RAS, Russia) and their co-authors processed ACS data for one and a half Martian years (approximately 2.7 Earth years) - 34th and 35th in Martian reckoning (MY34 and MY35). As before (in the article by Korablev et al., Published in the journal Nature in 2019), no traces of methane were found in the TGO spectra. Moreover, the upper limit was improved, i.e., it was found that the concentration of methane in the atmosphere does not exceed 0.02 ppbv with a probability of 99%.
These measurements again contradict the fact that the American Mars rover Curiosity registers methane in concentrations that are orders of magnitude higher - up to 19 ppbv, and on average no less than 0.2-0.5 ppbv. However, if Curiosity operates close to the surface, then ACS observes the atmosphere from several kilometers above it. Thus, it is possible to "reconcile" the results of the two devices, if we assume that the methane in the lower atmosphere is immediately destroyed or falls into some kind of "trap" and does not rise to the upper layers. However, there are no suggestions yet about what process could destroy methane so quickly or "isolate" it from the rest of the atmosphere.
Roughly the same can be said about phosphine, which has recently come to be considered a potential biomarker. ACS did not detect phosphine absorption bands. Its content in the atmosphere does not exceed 0.1–0.6 ppbv. Let us clarify that we are not talking about real observable "traces" of matter, but about what sensitivity is needed for the instruments of the following devices in order to capture these gases - if, of course, they are.
An article with this result was published in Astronomy & Astrophysics in May 2021; its first authors were Kevin Olsen (Kevin Olsen, Oxford University, Great Britain) and Alexander Trokhimovsky (IKI RAS).
Hydrochloric acid from Martian salt
If the presence of methane on Mars before the start of TGO's work was almost certain and expected only inevitable confirmation, then there was no such conviction that there is hydrogen chloride (HCl, in fact, hydrochloric acid) in the planet's atmosphere. It was assumed to exist, but it was not possible to detect it experimentally before ACS. Again, it was a low concentration - as the first estimates showed, its concentration should not exceed 0.2–0.3 ppbv.
In February of this year, Science Advances published an article (first by Oleg Korablev) on the discovery of hydrogen chloride (HCl) in the atmosphere of Mars. The discovery was made by the Russian spectrometer ACS. According to measurements, hydrogen chloride appeared in the atmosphere during a global dust storm that occurs on Mars every few years, and gradually disappeared after its end. Observations refer to the 34th Martian year (MY34). Its content, according to these data, ranged from 1–4 ppbv. And, unlike phosphine, this is the real content: in the spectra that ACS receives, the absorption bands of HCl have been detected with high reliability.
A reasonable assumption arose that the formation of HCl is associated precisely with the presence of a large number of aerosols lifted by winds from the surface. However, the ACS team of researchers decided to clarify this hypothesis: in a March article in Astronomy & Astrophysics, an article by Kevin Olsen, Alexander Trokhimovsky and their colleagues was published on the appearance of hydrogen chloride in the next 35th Martian year (MY35) - although there was no global dust storm that year.
The absolute values of the HCl content for both years are very close - 0.1–6 ppbv. Thus, the authors suggest that not a dust storm, but rather a "dust season" (the time when the amount of dust in the atmosphere is maximum, in this case summer in the southern hemisphere) is the cause of the formation of hydrogen chloride.
At the same time, the mechanism of the appearance and rapid disappearance of this gas is not yet completely clear. One can speculate, and both articles consider the hypothesis that HCl, or at least part of it, enters the atmosphere from the interior of the planet, as a result of volcanic processes. The ACS has detected this gas several times during the quiet summer season in the northern hemisphere, when there is almost no dust in the atmosphere.
Another article by Alexander Trokhimovsky and his colleagues, published in the same journal in July, is devoted to the study of the isotopic composition of chlorine in atmospheric hydrogen chloride: H35Cl and H37Cl.
Most of the Martian gases are significantly enriched in heavy isotopes due to the long-term loss of its atmosphere by Mars. However, it was precisely for hydrogen chloride that it was determined that its isotopic ratio almost corresponds to the terrestrial one. Most likely, this means that the observed hydrogen chloride, and in particular chlorine in its composition, does not participate in long-term exchange processes between the surface and the atmosphere - in other words, chlorine is more or less "locked" in the lower layers of the Mars atmosphere.
The water flies away and remains, but heavy
Unlike chlorine, the isotopic composition of hydrogen (H) in the Martian atmosphere differs from that on Earth. Mars has five times more deuterium (D) - "heavy" hydrogen, the nucleus of which, in addition to one proton, contains one more neutron - than on Earth.
Knowing this fact helps to estimate the amount of water that Mars has lost during its history. The main "supplier" of hydrogen to the atmosphere is water molecules H2O. Since, unlike Earth, water on Mars is rather actively leaving through the atmosphere into space, then if you know the rate of losses, you can restore the chain "back" and understand how much water was in the beginning.
Roughly speaking, we need to understand how quickly a water molecule, once in the upper atmosphere, decays into ions, which either leave, in one way or another, the atmosphere, or bind in some other substance and remain on the planet.
It can be assumed that initially, at the time of planet formation, the Martian D / H ratio was similar to that of the Earth. But "light" hydrogen evaporates faster than heavy hydrogen, so over hundreds of millions of years this ratio has changed to the level that we observe today.
It was assumed that this process is determined by two mechanisms. The first is condensation, that is, the transition of atmospheric water from a gaseous state to an ice state, the formation of "snow" clouds. The second is photolysis, that is, the decay of molecules into a hydrogen ion H and an OH radical under the action of solar ultraviolet radiation. The problem that researchers are working on right now is how these mechanisms work, what is the result of their "action", if measured in terms of the amount of "lost" water and or changes in the D / H index.
In June 2021, the journal Nature Astronomy published an article presenting the results of measurements of the concentration of water (H20) and heavy water (HDO, where one of the hydrogen atoms is replaced by a deuterium atom) on Mars, depending on the height above the surface.
Its authors Juan Alday (University of Oxford, Great Britain), Alexander Trokhimovsky (IKI RAS) and their colleagues, including those from the LATMOS laboratory, compared the data obtained by ACS with the expected rate of photolysis, and came to the conclusion that for the formation of hydrogen ions in the atmosphere, it is the photolysis mechanism that is most important. In addition, it turned out that in its course hydrogen atoms are formed in larger quantities than deuterium atoms (it was previously believed that for the "separation" of hydrogen and deuterium in the atmosphere, condensation is more important).
The second ACS result, described in an article by Denis Belyaev (IKI RAS) and his colleagues (published in May 2021 in Geophysical Research Letters), is based on observations of the concentration of water vapor at altitudes of 100–120 km above the surface. Previously, this layer (mesosphere and upper mesosphere) was not studied in the study of water distribution.
According to the new results, the maximum concentration of water vapor molecules H2O was 10-50 parts per million per unit volume (ppmv) during the global dust storm of 34 Martian years, as well as during the two summer solstices in the southern hemisphere - MY34 and MY35. (In other periods, the maximum values at these heights were significantly lower and did not reach 2 ppmv).
As mentioned above, there was no global dust storm in the 35th Martian year. But water molecules nevertheless reached such heights where they could already be easily destroyed by solar ultraviolet light. Thus, it is very likely that it is the change of seasons - the onset of the southern summer, and not just dust storms - that increases the rate of water loss.
This factor was previously underestimated: it was believed that “bursts” of losses occur precisely during global dust storms, while at other times the water “evaporates” at a more or less even rate. But the Martian atmosphere turns out to be much more dynamic. In addition to global dust storms, regional ones also play an important role. The article, published in August in the journal Nature Astronomy, collected data from three artificial satellites of Mars, including the ACS / TGO experiment, on the amount of dust, temperature, concentration of ice, water vapor and hydrogen in the atmosphere of Mars during a regional dust storm ( southern hemisphere summer MY34, January - February 2019 on Earth). As their analysis showed, the rate of hydrogen loss at this time can increase by 5–10 times. Since regional dust storms occur almost every year, their contribution to the evolution of the Martian atmosphere can be very significant.
Martian years are counted from the terrestrial 11 April 1955 - Ls 0, the moment of the vernal equinox in the Northern Hemisphere. At the same time, since the orbit of Mars is quite elongated, the seasons differ in length - the northern spring and summer are shorter than the southern ones.
You can convert Earth to Martian time using, for example, The Mars Climate Database Projects web server. By the way, now (October 2021) in the southern hemisphere of Mars there is a deep winter of the 36th Martian year.
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The ExoMars project is a joint project of the Roscosmos State Corporation and the European Space Agency. It is implemented in two stages. The first mission, launched in 2016, includes two spacecraft: the Trace Gas Orbiter (TGO) for observing the atmosphere and surface of the planet and the Schiaparelli lander for testing landing technologies.
The scientific tasks of the TGO apparatus are registration of small components of the Martian atmosphere, including methane, mapping the abundance of water in the upper layer of the soil with a high spatial resolution of the order of tens of kilometers, and stereo imagery of the surface. The apparatus is equipped with two instruments created in Russia: the ACS spectrometric complex (ACS - Atmospheric Chemistry Suit) and the FREND high-resolution neutron telescope (FREND, Fine-Resolution Epithermal Neutron Detector). Russia also provides a Proton booster rocket with a Briz-M upper stage for launching.
The second phase of the project (launch in 2022) provides for the delivery of the Russian landing platform Kazachok to the surface of Mars with the European rover Rosalind Franklin on board. Russia provides a Proton-M booster rocket with a Briz-M upper stage for launch.
Within the framework of both stages, a ground-based scientific complex of the ExoMars project, combined with ESA, is being created in Russia for receiving, archiving and processing scientific information.
Source: IKI RAN
https://www.roscosmos.ru/33053/