The thorough selection of ExoMars 2020 landing site

To land a spacecraft on Mars is not an easy goal. Apart from the technological challenges it entails, there is another question which could seem prosaic but which require a painstaking and intense process of elucidation: the selection of the landing site. Professor Javier Martín-Torres and Professor María Paz Zorzano Mier, Principal Investigator and Co-Principal Investigator of the ExoMars 2020’s instrument HABIT respectively, are playing a remarkable role in the course of this mission’s landing site selection process.

Foreseen landing sequence of ExoMars 2020 mission. Credits: ESA

The choice of a location for sending a spacecraft to Mars (or, for that matter, to any other body of the Solar System) is subjected to a wide variety of criteria which must be considered in a complex net of interrelations. Geographical (or Areographical in this case, as it was used to say in the 19th century with reference to Mars topographic studies), scientific, environmental and operational aspects, among others, must be taken into account to select an optimal area in which the spacecraft at hand can land safely, and where the goals of the correspondent mission can be plentifully fulfilled.
As Prof. Martín Torres clears out, “Potential landing sites are selected by consensus within the scientific community, and are screened based on scientific potential, engineering factors and security. For a scientific mission, it matters the interest of the landing site, which depends on the state of the knowledge and the interpretation of the previous observations from orbiters. Once this is clear, then safety comes first: landing safely, and, if the spacecraft needs solar cells, making sure that the rover or lander will have energy during its designed operational lifetime. For a safe landing, it matters that the ground surface is even and does not show inclined surfaces or large rocks, but it also matters that local winds are predictable, ideally soft, such that they cannot deviate the trajectory during the Entry Descent and Landing (EDL) phase. It also helps if the site is deep and there is a larger mass of air (total atmospheric pressure), such that the atmosphere can efficiently slow down the entry, for example a deep crater, like the crater Gale is (the landing site of Curiosity). Mars is a very cold planet, and sites with extremely low temperatures induce an additional stress to the hardware and moving parts, so this must be considered when designing the nominal duration of the mission. And finally, depending on where the spacecraft is going to land and what are the scientific goals of the mission, there are certain Planetary Protection protocols that must be applied to the hardware, to sterilize it and avoid bio-burden contamination of the site with Earth microscopic life forms and organics”.
Regarding the matter of the standing Planetary Protection Policy, and with relation to the selection of the ExoMars 2020 landing site, Prof. Martín-Torres and Prof. Zorzano were invited to be part of a small group of experts, convened by the European Space Foundation (www.esf.org), to contribute insights into the conditions of the pre-selected landing sites. This request departed from an initiative of the European Space Agency Planetary Protection Officer, Gerhard Kminek, intended to frame the mission into a Planetary Protection perspective, because, as Prof. Martín-Torres and Prof. Zorzano explain, “The ExoMars 2020 mission will carry an ambitious payload to search for signatures of life (bio-signatures) on Mars. Since present day habitability of Mars is being investigated, if we are looking for potential remnants of life forms or organics, or if we pretend to distinguish local life forms from external life forms, then we should make sure, as in any other experiment we do in the laboratory, that we do not contaminate the object of study with our “tools”. In the case of Mars, our tool is the lander or rover. Earth life forms can survive the traverse, they can generate spores, and they may produce new generations when cultivated with sufficiently high temperatures, nutrients and liquid water. That is why, for instance, when we go to Martian sites where there is frozen water in the surface or sub-surface, we should make sure that our hardware is absolutely clean from Earth bio-burden. In other words, we avoid contamination to make clear, unquestionable measurements. These missions are a true marathon of scientific and engineering efforts, and the object of study should not be destroyed by our own investigation.
On the other hand, there is the debatable fact about our right to contaminate another planet, with the consequences that this may have for any hypothetical life forms inhabiting the planet.
To minimize the risks of Earth-lifeforms contamination of an explored site there are three factors that are relevant:

  1. Cleaning the platforms.
  2. Looking for new “records” of habitability extremes on Earth-life forms (those extreme conditions are met by the so-called extremophiles).
  3. Analysing the “Earth-like habitability” of the Martian sites (namely a thorough investigation about how are the conditions of the proposed sites with respect to the conditions that are defined as the limiting conditions for Earth microbial life forms to metabolize and reproduce).
Location of Oxia Planum on Mars. Credit: NASA

The cleaning procedures and criteria for testing are defined by the agencies. Then there are different specialists involved in the process: For example, biologists investigate the limiting records of life. Our contribution is to do the analysis of habitability from the environmental and atmospheric point of view. One basic question for us is for instance: can liquid water be stable at this site at all? Because we know that all known life forms need liquid water. And this is precisely one of the research goals of HABIT. Another basic question that we try to solve is: do the Martian surface temperatures allow for reproduction and metabolism of the potential Earth-life forms? Since Mars is so cold, this is generally not the case, at least during the windows of time when liquid water is stable as brine”.
In this sense, as representatives of the Luleå Tekniska Universitet’s Atmospheric Science Group, they both feel “extremely happy to contribute to this international effort. Our advice is taken into account to asses or predict what the environmental conditions may be at the surface and to evaluate if the conditions of the so-called “Special Regions” are met and thus if “extra-cleaning” needs to be put in place”.
At the moment, the most likely site to be the landing one for ExoMars 2020 mission seems to be Oxia Planum, but next 27th-28th of March, it will be held the 4th ExoMars Landing Site Selection (LSSW) workshop, with the participation of Prof. Martín-Torres, in which additional locations will be proposed to accompany this site as the definitive destination of the mission. The intention is to certify two sites and, at a later stage, to identify one as primary and the other as backup.
An important part of the workshop will be voting, though the final responsibility for the recommendation lies on the Landing Site Selection Working Group (LSSWG), who may decide to deviate (for good reasons) from the voting results.
However, it will be the consortium ESA/Roscosmos who, quoting Prof. Martín-Torres, “will make the final decision. But there are multiple panels, including the scientists and engineers that give advice on various subjects. In particular, for the ExoMars mission, there is an additional issue to be taken into account. This final factor is the potential variety, impact and feasibility of scientific goals, that can be achieved within the ExoMars rover traverse, and of course the technical risk of the rover traverse itself. The selection of the final candidate may be influenced by this too, and will have to take into account technical factors like the mobility and autonomy of the rover and the energy budget for this”.
As for the particular opinion of Prof. Martín-Torres and Prof. Zorzano, they assure that “we are equally interested on all the proposed landing sites. However, for one of the experiments of HABIT it would be interesting to be at a site where during the duration of the nominal lifetime of ExoMars (1 Martian year) the relative humidity at night-time is high, because this will allow for the salts of HABIT to deliquescence and form a transient liquid brine within the HABIT container units. With our present knowledge, and bearing in mind that we have never landed at the proposed landing sites, and we do not know what the environmental conditions will be, we can only extrapolate from our models and orbiter measurements. According to this, all the proposed sites behave almost similarly with respect to this condition”.