HAbitability, Brines, Irradiation and Temperature (HABIT) is a novel instrument never used before in the exploration of Mars, nor even on any other place. Therefore, it is having to be developed from scratch, at least with regard to its Brine Observation Transition To Liquid Experiment (BOTTLE) unit. The Field Site Prototype (FSP-HABIT) has been the first embryo of the device, and has made possible to identify the very basic problems its design and functioning pose, as well as serving as the Master Thesis subject for Johannes Güttler, the master student who built it and tested it, pointing the ways to follow in the subsequent development of HABIT.
HABIT will be mounted on the Russian module of the ExoMars 2020 mission, the Surface Platform (SP), and it is composed of two units: ENVironmental PACKage (ENVPACK) and BOTTLE. ENVPACK is a set of environmental sensors, namely, Air Temperature Sensors (three of them), Ground Temperature Sensor (GTS), and UltraViolet sensor (UVS), and BOTTLE, is intended to monitor the formation of perchlorate brines by deliquescence and their change of state (including a liquid phase) throughout every Martian day, a particular phenomenon which seems to occur on the soil all over the planet’s surface. This latter device will mimic the hydration process of the salts it carries, namely, magnesium, sodium, and calcium perchlorates and calcium chloride, by measuring the conductivity through them, while ENVPACK records the environmental conditions in which the process takes place.
The direct in situ observation of liquid water presumably produced on a daily basis on present day Mars will provide information to go into depth on the knowledge of Martian environment in several aspects, which can be summarized in two main goals of the mission: to assess for the first time the habitability conditions of the upper layer of the Martian regolith, and to determine the water cycle between the ground and the atmospheric boundary layer. In addition, HABIT is expected to provide the data which will allow to make an estimation of the total amount of liquid water which could be gathered for its use as an in-situ resource for an eventual crew exploring the planet in the future.
The basic idea is simple; BOTTLE is a container divided in several vessels which will be filled with the same kind of hygroscopic salts found to be widespread over the Martian regolith. However, whilst the sensors that compose ENVPACK have been inherited from those currently in operation on board Curiosity rover/MSL mission as part of the Rover Environmental Monitoring Station (REMS) instrument, and hence they count on a high readiness level and maturity, the conductivity sensors in BOTTLE and the procedures to analyse their measurements have never been applied before, so they are absolutely new and the physical design of the instrument and the sensors themselves, their calibration, and the procedures to interpret appropriately the obtained data must be implemented from scratch. The starting point of this long and barely begun process was the building of FSP-HABIT, and its testing along the performance of three field site campaigns, which have marked a milestone in the way of the instrument’s development. These campaigns were carried out in Abisko National Park, Sweden, Iceland and Ladakh, India, and the prototype itself evolved significantly throughout the works.
The goal of the FSP-HABIT project was the design, building and testing of a portable instrument composed of the two units which was able to provide measurements and to identify the problems (or as many of them as possible) which were to be faced for the development of HABIT afterwards. This is especially important in the case of BOTTLE as explained before, though the sensors of ENVPACK were added to count on an environmental reference for the analysis of the obtained measurements and to try and discern the optimal arrangement for them in the ultimate instrument. However, some commercial support instruments were taken to the campaigns in order to get reliable data of the correspondent environmental parameters (i.e. pressure, air and ground temperature, UV irradiance and relative humidity).
Previous to the first works at Abisko, it was decided to adopt a “contact” method to measure the conductivity, that is to say, with electrodes in direct contact with the salts instead of a contactless induction system, which is bulkier, less sensitive, and is subjected to electromagnetic interferences. The “direct contact” solution entails the risk of corrosion of the electrodes and therefore an unwanted factor affecting the measurements, but it is a controllable matter just by choosing an appropriate material and by using alternating current. Another previous idea was to implement measuring electrodes at three different heights in each vessel, so that additional information about the hydration state of the salt, the hydration process, and the total amount of water captured by the salts could be gained. Different laboratory tests were previously performed in order to start gathering the first insights in the functioning of the system in real conditions, so that the corrosion process for different materials and current conditions was thoroughly characterized, and the voltage divider which permits to get detailed measurements from different hydration state of the salts was checked.
Nevertheless, during Abisko campaign only one rudimentary vessel was tested, and it was not filled with any salt. Preliminary measurements were taken with the vessel empty, filled with ice, and with a mixture of ice and water, showing the very first differences in the conductivity, from whose values it was possible to distinguish the three conditions. This campaign can be considered the first rung on the ladder to mount a proper prototype for the next field site trials to be performed in Iceland and India, and the outcomes of the works carried out during the campaign provided the guides to proceed with the development of the instrument. The next step was to bring into being a physical device to be carried to the following campaigns.
In Iceland campaign, FSP-HABIT acquired its corporal entity including the environmental sensors, whose integration in the instrument should be enquired apart from its utility as a reference to assess the results of the conductivity measurements (a reference supported by the use of the mentioned commercial instruments, such as a HOBO H21 weather station with UV sensor, and a PeakTech 5040 3-in-1 IR temperature and relative humidity meter). It is worth remarking that the ATS’s, in addition to the monitoring of the air temperature, are going to be used to characterize the wind regime at ExoMars 2020 landing site, which implies the implementation of a whole new analysis procedure of the data collected by this particular set of sensors; here too, the FSP-HABIT prototype has provided crucial information to proceed with this novel project. As for the BOTTLE unit, the method of measuring the conductivity through the hydrated salts at 3 different measurement heights was validated, though neither in this case the salts to be used in HABIT were available for the expedition. Instead, the vessels were filled with Calcium chloride (CaCl2), potassium hydroxide (KOH), and sodium hydroxide (NaOH), which show a high similarity with the perchlorates in terms of corrosiveness and hygroscopic capability. All in all, measurements of the hydration process and deliquescence were finally obtained in several places, selected for showing some Martian like conditions, while an environmental context to the phenomenon was defined by monitoring the correspondent parameters in which they occurred.
The campaign yielded valuable results in several aspects, not only by providing data to start performing the correspondent analysis, but by revealing different clues on the improvements that should be implemented further on, and about some possible issues that had to be considered, such as the most appropriate form of the salts (dust, grains, pellets…), the effect of the arrangement of the salts within the vessels, (which can be changed due to vibrations affecting the measurements), or the behaviour of the environmental sensors in different orientations an conditions, to give just a few examples. In addition, the campaign was intended to perform habitability studies in extreme environments, and hence included the taking of microbiological samples from ground, water, ice and air, which is in accordance with the broad goal of ExoMars 2020 mission: to search for signs of past and present life on Mars. The assessment of the habitability potential of present day Mars with relation to the presence of liquid water is a key question within this investigation, and this campaign allowed to relate the studies carried out from FSP-HABIT data with the habitability of the environment at the field sites chosen, which is evaluated from the microbial populations living there.
Only two days separate the end of Iceland campaign from the beginning of that in Ladakh, India, which gave time just to fix some minor damages suffered by the prototype as a consequence of the handling and functioning during the first one. However, this time the “Martian salts” could be used. In particular, the vessels were filled with sodium perchlorate, calcium perchlorate, and calcium chloride, and this time two vessels were always containing the same salt in order to identify new factors that could determine differences in the measurements. Indeed, this arrangement served to realize that there are different results for the same salt depending on whether the vessel is covered or not (one of the six vessels is going to remain open to the atmosphere while the others will be covered by a HEPA filter and a lid so that only air enter in them).
Overall, a good assessment of the behaviour of the salts and of the performance of the instrument have been reached from the three campaigns, which has meant a big step towards the refinement of HABIT in its capability to determining accurately the state of hydration of the salts it will be filled with once on Mars. On the other hand, the works carried out during the campaigns have raised a number of questions, issues and possibilities that must be investigated in depth, and which would not have been noticed otherwise, affecting not only to the conductivity measurements, but as well to the design of the instrument in terms of shape, arrangement of its elements, electronics, materials etcetera. So, the work to be done is still enormous, but the outcomes from the field site campaigns have open a promising perspective and ways to keep on advancing along.