The tricky development of HABIT

HABIT is a quite simple instrument in its very basic idea, whose main goal is to demonstrate and to monitor the formation of liquid aqueous solutions by deliquescence of highly hygroscopic chlorine salts present in the Martian soil. In order to accomplish this core objective, it contains some of these salts which are supposed to be widespread all over the Martian surface in the correspondent vessels of its BOTTLE (Brine Observation Transition To Liquid Experiment) unit. However, there is a wide gap between the HABIT conception, very clear, simple, promising and nice, and the final embodiment of the working device, which will require a tricky, complex, challenging and audacious investment of efforts, not only for the development of the instrument itself, but for the implementation of the ways (calibrations, analytical methods and procedures…) which will permit to make worthy science out of its measurements.

Scheme of the daily water exchange cycle between the ground and the atmosphere on Mars
. Credits: Martín-Torres-Zorzano

According to the environmental data registered by NASA’s Curiosity rover, the formation of brines process, which is part of a constant water exchange between the atmosphere and the ground, is taking place on a daily basis at Gale crater, and they could remain in a liquid phase for some periods of every diurnal cycle under the registered environmental conditions (temperature, pressure and relative humidity). HABIT is intended to reproduce the water absorption by chlorine salts, its presumed deliquescence, and the later dehydration that closes the cycle, while monitoring the process by measuring the changing conductivity through them as their hydration state varies. So far so good.
Yet, this general depiction has been deduced from indirect information, and there are very few and partial certainties about this transcendental issue, which can mean a milestone in Martian exploration if HABIT, as it is intended to do, provides a confirmation eventually.
The work sessions held during the First HABIT Science Team Meeting at Luleå have brought to light the real challenges the development of the instrument will suppose, which go beyond the design and making of the physical device. Apart from the technical and engineering difficulties the configuration of the instrument itself will pose, the physics supporting its scientific goals will have to be revisited to find specific references which give sense to the sets of data their sensors will (hopefully) provide, with relation to the monitoring of the deliquescence process, the assessment of habitability, and the environmental studies to be performed.

The salts

The presence of the salts has been inferred from data gathered by earlier missions (mainly the chemical analysis performed by means of Phoenix), and confirmed by the Curiosity’s instrument SAM (Sample Analysis at Mars), which have reported the detection of chlorine in the analysed samples from the regolith and soil. Together with the detection of other elements such as magnesium, calcium, sodium, potassium and oxygen, it led to the conclusion that different types of oxychlorine species must be there, including perchlorates of the mentioned elements, though it is uncertain what amount of each one could be found in the eventual landing site of the ExoMars 2020 mission. The distribution of chlorine, albeit widespread, has shown to be irregular in terms of amount available in different locations, so counting on an accurate assessment of the quantity which could be expected to be found on the landing site would be extremely useful. It will be possible by performing specific and previous studies on remote sensing data from satellites and geological imaging-based characterization of the area. Of course, the availability of water on the ground is a primordial factor which can be also estimated by means of the same mentioned tools.
It must be taken into account that not all the oxychlorine salts to be included in HABIT (calcium chloride in addition to the magnesium, sodium and calcium perchlorates) show the same hygroscopic behaviour, which impose the convenience of implementing an optimal mixture of the chosen salts within BOTTLE’s vessels with relation to the particular conditions of the final landing site selected, as well as a good assessment of their foreseeable hydration response once on Mars.

Deliquescence and how to measure it

Illustration of deliquescence

Occurrence of deliquescence is another matter. Regarding the eutectic temperature of the different perchlorates solutions and applying the correspondent phase diagrams, it was deduced that, under Martian conditions, brines are formed by deliquescence, and they could be in a liquid state for a while during the aforementioned exchange of water between the soil and the atmosphere [figure]. Curiosity is operating near the equator, the driest strip of the planet, so it is expected that, in higher latitudes on the northern hemisphere, the phenomenon is more intensive in terms of the amount of water absorbed by the salts, and more extensive in terms of the periods of permanence of the brines in the liquid phase. At first, the calcium perchlorate seems to be the more likely responsible for the formation of brines, and it has been proven to deliquesce under simulated Martian environment, forming metastable solutions. Nevertheless, the deliquescence of the other element’s perchlorates could be determined by the convergence of other factors including differences of irradiation on some topographic features, seasonality, depth, or local environmental particularities.
Then, the “timing” of the deliquescence process has to be carefully ascertained. BOTTLE is intended to measure the electric conductivity through the salts as it varies depending on their hydration state, so it is necessary to have information about the pace at which the process proceeds, since it would pose major implications for the configuration of the instrument. It has been proposed that maybe deliquescence is a sudden event which occurs almost instantly. If so, a completely different scenario from that taken into account so far should have to be considered, to define the operations of HABIT and the ways to interpret its data afterward.
Finally, the influence of other circumstances such as the presence of materials of the regolith, with which the salts are mixed, or the role played by the harsh Martian irradiation have to be carefully inquired. An intense work must be invested for the ascertainment of these conundrums, not only in the testing schedule they will require (to be performed both in laboratory and during field site dedicated campaigns in Martian-like locations), but in theoretical studies on the basic physics of the hygroscopy phenomenon which will be essential for guiding the development of HABIT.
Furthermore, the BOTTLE unit is an absolutely new device never used before in a space exploration mission or, for that matter, in any other place, so it is necessary to develop from scratch a whole protocol to precisely establish a correspondence between every conductivity measurement taken with each state of hydration of the set of salts within the vessels, so a thorough process of calibration is being developed by performing the correspondent tests which, as far as it is possible, will be carried out under Martian conditions in some of the available Mars chambers (which are not abundant). Hopefully, Luleå University of Technology will count with his own Martian chamber (MarsLab), which will be devoted to support this and other aspects of HABIT project advance.
The other unit of HABIT, ENVPACK (Environmental Package), presents less problems, since the sensors it is composed of are inherited from the instrument REMS (Rover Environmental Monitoring Station), currently operating on Mars as part of the aforementioned Curiosity rover’s scientific payload, and its functioning is exactly the same. Actually, ENVPACK can be considered a temporal and geographical extension of the studies carried out by REMS instrument so far. However, ENVPACK will introduce some novelties with respect to the exploitations of the data it is going to obtain. On one hand, their three air Temperature Sensors measurements are going to be used to characterize the wind regime in the landing site. Although REMS counts on two proper wind sensors to measure wind speed and direction, one of them was broken during the landing, so the other one has only been providing partial data. The knowledge about the winds on Mars is poor because of the lack of observational data, which in turns derives from the difficulty that the thinness of the atmosphere entails when acquiring measurements.
A complete method to translate the air temperature records into air movement around the spacecraft carrying HABIT (the Surface Platform) is being developed. The information eventually provided will enrich the knowledge of the Martian atmosphere dynamics and will boost the improvement of the available circulation models, contributing in the end a better understanding of the Martian environment.
On the other hand, new studies on dust dynamics by means of the Ultraviolet sensor (UV) are going to be scheduled, needing as well novel analytical procedures.

Drawing of HABIT's BOTTLE unit

Astrobiological investigation vs. planetary protection

The likely presence of transient liquid water on Mars, and in particular on the pre-selected landing sites for ExoMars 2020 poses a curious contradiction regarding one of the main scientific objectives of the programme, namely, the search for signs of past and present life on Mars. The pursuing of this goal has determined in a good measure the selection of the landing sites, since they are presumed to show optimal conditions of habitability and for the preservation of hypothetical biosignatures from the distant past, (considering “optimal conditions” in the frame of Martian environment of course). These conditions, and in particular the alleged presence of liquid water, have enhanced the worry about a possible biological contamination of the planet with terrestrial organisms, which would be catastrophic because, among other things, it would ruin further astrobiological investigation on the planet.
Every team involved in the mission is concerned about this matter, which is rather critical in the case of HABIT, since it is going to contain liquid water. Therefore, the application of Planetary Protection policies is being reinforced with studies on the types of organisms which could survive or even thrive on Mars to prevent any chance for them to be carried on the spacecraft. In passing, some clues about what kind of Martian life, if ever life arose on the planet, could be still found in the studied environments.
All the aforementioned is just a hurried glance to the main challenges that the development of an innovative space exploration instrument such as HABIT means. But there are many others, related to engineering, mechanical, theoretical and operational aspects (to mention just a few), which will have to be faced and solved from now on in a daily and hard work until the launch of the mission, without taking for granted that it is going to be successful.
Notwithstanding, and as Prof. María Paz Zorzano remarked, every inquiry, ascertainment or conclusion reached along the way will mean a noteworthy contribution to Mars exploration and to science and thus a success in themselves.