The discovery on Mars of environmental conditions that could be considered as “habitable” with relation to the requirements of certain terrestrial organisms, as well as the increasing interest of different space agencies to search for signs of life in the planet (three rovers and as many landers are scheduled to be sent within the next years), have triggered concern about the risk of biological contamination. This possibility, whose occurrence would mean not only an intolerable aggression to a pristine environment, but a severe compromise for the further and proper study of the planet, has impose the establishment of planetary protection policies that have the status of international legal obligation, whose prescriptions must be implemented during the design and execution of any exploration mission.
The worry is not new; the first measures to avoid a possible contamination of celestial bodies by spacecrafts were discussed in the Committee on Contamination by Extraterrestrial Exploration (CETEX) created in 1958 in the frame of the International Council of Scientific Union, with the aim of assessing the risk of compromising the future exploration of the Moon and other bodies of the Solar System in the case of becoming contaminated. Afterwards, the competency regarding this matter was transferred to the Committee on Space Research (COSPAR), integrated the International Council of Science, which established a first Planetary Protection Policy (PPP) in 1964 as a standard set of requirements to prevent biological contamination during the planetary exploration missions. This regulation was taken into account by the United Nations to enact its 1967 Space Treaty, for whose accomplishment the Planetary Protection Policy serves in turns as a main reference.
The search for signs of extraterrestrial life is nowadays a remarkable endeavour that strongly determines scientific activity within the planetary exploration field. Indeed, it is an express aim to be faced in a direct way for a number of the foreseen missions to Mars, starting with ESA’s ExoMars Programme whose first phase, ExoMars 2016, is already underway. Therefore, at first, the worries about biological contamination are focused mainly on Mars, since it is the planet on which the search for signs of life is going to be performed imminently. Furthermore, conditions have been found that configure habitable environments in which some known organisms could thrive according to the ultimate concept of “habitability” (of course formulation of this policy has been changed through the years according to the advance in the knowledge of the limits of life and extraterrestrial environments).
Curiosity rover’s investigations have unveiled that Martian ground contains organic compounds, fixed nitrogen and, importantly, liquid water (though in a transient way), offering the full set of requirements that some organisms need to survive. A few centimeters under the surface, protected from the harsh radiation that hits the planet, they could find the suitable habitability conditions for them to develop metabolic activity and to reproduce. These discoveries have led to the redefinition and accurate delimitation of the so-called Special Regions on Mars; zones in which the cautions must be maximised since they could offer suitable circumstances for life to prosper. At the same time, these areas are the most favourable to host indigenous life if it exists at all, which poses an obvious problem when trying to get access to it for its study. This would imply a direct contact of a robot (or eventually an astronaut) with the local population to get samples, a scenario in which the risk of contamination is extreme since the total sterilization of the latter cannot be granted even after having applied the most thorough cleaning procedures. The other way round, when eventually the samples can be returned to Earth for their analysis, the risk of bringing potentially dangerous organisms for terrestrial life would exist, a possibility also considered by the PPP in advance.
Regarding the scientific objectives of the missions, COSPAR has established five different categories, from those that are sent to hardly habitable locations in which no specific biological investigation is scheduled, to those designed to return samples. Mars and its Special Regions are expressly treated with the correspondent subcategories. In this case, even orbiters must accomplish some requirements, for a contamination issue could supervene in case of a crash. As an example of the validity of PPP, the ExoMars 2016 mission, which is right now travelling to Mars, could be mentioned. It is composed of an orbiter, the Trace Gas Orbiter (TGO), and an Entry, Descent and Landing module (Schiaparelli EDM), which will reach the surface of the planet. The requirements for the former have consisted basically of an accurate calculation of the possibilities of a crash in several periods after its entrance in the operational orbit, which have been found to be acceptable. As for the landing module, its assembly was carried out in a new cleanroom built to reach the PPP prescriptions, and then subjected to numerous cleaning cycles, both chemical and thermal, as well as to some three thousand microbiological tests.
Atmospheric Science Group, as part of the ExoMars programme, has entirely made the worry about the dramatic consequences of a possible biological contamination of the Martian environment their own. Given that HABIT instrument (the main project the Group is working on, and which will be part of ExoMars 2018 Surface Platform scientific payload) is designed among other things to investigate the formation of aqueous liquid solutions on the ground, it necessarily will operate in an environment in which, because of the expected presence of liquid water, the possibilities for organisms to survive are high, so a special care must be applied. This is the reason why Dr. Petra Rettberg, an expert in habitability and the limits of known life, has joined the HABIT science team. She will give advice about the best options and the measures to be taken into account in order to avoid any contamination risk during the design phase of the instrument, as well as the cautions to be applied during the operations once on Mars. The MarsLab facilities at Rymdcampus, Kiruna, which are under construction, have been designed to pay careful attention to this matter. They include a Chemical Chamber, a Microbiology Section and a Biosafety Cabinet in which, apart from performing experiments implemented for the calibration and development of the HABIT instrument (as well in the phase of making), valuable information to assess the risk of contamination can be obtained.
Furthermore, one of the work lines that have been posed within the Group is the development of models to assess the habitability potential of certain locations from data gathered by drones equipped with remote sensing instruments a few meters above the surface. At first, the utility of this strategy is to get access to points that cannot be reached by robotic vehicles, but it could be very useful to study areas in which a certain possibility to be habitable or, as the case might be, to be inhabited, is appreciated, avoiding a direct contact.