The successive discoveries on Martian environment, and remarkably those achieved by means of rover Curiosity’s investigations throughout the last five years, are unveiling a whole new world, significantly different from that depicted just before its exploration began. Together with long-term observations from orbiters (Mars Global Surveyor, Mars Reconnaissance Orbiter, Mars Odyssey and Mars Express), Curiosity’s inquiries have changed the understanding of the planet in almost every aspect which can be regarded (geological, geochemical, topographic, atmospheric, historic etc). Importantly, many of these new insights are centred around what can be considered the most remarkable and transcendental of them all: the existence of an intense and widespread water activity in a daily cycle involving the surface and the atmospheric boundary layer, which includes liquid state as one of its phases. Together, these findings pose major implications for the better knowledge of the environmental conditions on the planet with relation to the assessment of its habitability, one central issue of its exploration efforts, making out of Mars a preferential study target regarding the search for signs of extra-terrestrial life, one of the biggest scientific challenges of our time.
The possibility that water is present in a transient liquid state on Martian soil is not a new idea. It was advanced in 1966 (Robert B. Leighton and Bruce C. Murray), but it has been a highly controversial issue until a study conducted using data gathered by Curiosity rover established the occurrence of a water exchange cycle between the ground and the atmospheric boundary layer on a diurnal basis, along which it is very likely that a liquid phase, in the form of salty solutions or brines, takes place. Besides, native organic compounds and fixed forms of nitrogen were also found in the exploration area, completing the set of basic requirements for life to thrive, and reinforcing this way the interest for the central inquiry guiding Mars exploration: its habitability conditions, both past and present. Moreover, Curiosity confirmed the presence of methane in the Martian atmosphere and its mysterious behaviour, which had been observed with ground-based telescopes for the first time, and then detected by some of the aforementioned orbiters, although these detections were widely attributed to artefacts caused by the observation instruments themselves.
On the other hand, the analysis of rocks along the rover’s traverse has cleared out that crater Gale, the Curiosity’s landing site, was in the past a hydrological basin during periods of time long enough to allow the formation of some of the found minerals, such as hematite, which generates within an aqueous medium over geological timescales.
One by one, none of these factors themselves would mean but a refinement of the knowledge about a particular aspect of Martian environment but, all together, pose new clues to reinterpret the past and present habitability on the planet, being the presence of liquid water the core issue of the matter.
These important advances in the exploration of Mars have been achieved largely thanks to the cornerstone of the mission, the Sample Analysis at Mars (SAM) instrument, which more than an instrument, is a complex system of analysis that justifies itself the name of the mission: Mars Science Laboratory (Curiosity is the name that was assigned specifically to the robotic vehicle by popular consultation). SAM is a sophisticated set of devices that includes three analytical tools, namely, a Quadrupole Mass Spectrometer (QMS), a Tunable Laser Spectrometer (TLS), and a Gas Chromatograph (GC), calibrated according to the "follow the carbon" criteria, and with which it is possible to find a greater variety of organic compounds than with any other instrument previously sent. With them, SAM analyses atmospheric and soil samples, which in this latter case are fragments of pulverized rock obtained with the powerful hammer drill it carries in the turret of instruments of his robotic arm. Once the soil samples are obtained in powder form, and sieved to obtain particles smaller than 150 μm, they are poured into the Sample Handling System, which includes among its elements furnaces, pumps, pipes, manometers, valves and so on. This system distributes the samples in different containers, among the 74 it counts on, for the different processes which they are subjected to.
SAM can apply three different chemical analysis procedures to the samples. The first is the Evolved Gas Analysis (EGA), which is carried out directly on the volatiles extracted from the solid sample in a pyrolysis furnace that heats it to a temperature of 1,000° C. In the first instance, these volatiles are ionized by electronic impact and monitored with the quadrupole mass spectrometer, and a part is isolated for subsequent analysis by gas chromatography and/or by laser spectrometry. A second procedure is combustion, in which part of the solid sample is heated in the presence of pure molecular oxygen to analyse the products of its oxidation. In this procedure, the laser spectrometer allows the additional and indirect identification of organic compounds by quantifying the carbon evolved in the process. Finally, in the wet chemical analysis procedure, a portion of the solid sample is subjected to a low temperature derivatization process using N-methyl-N (tert-butyldimethylsilyl) -trifluoroacetamide (MTBSTFA) as reagent, or to thermal desorption using in this case tetramethylammonium hydroxide, and then analysing the resulting products.
Thanks to all this intricate system of devices and operations, Curiosity verified a reliable detection of organic compounds in the bottom of the Gale Crater. The study was based on 19 measurements made on three different samples taken at the point called Sheepbed, located in the Yellowknife Bay stratigraphic formation, an ancient fluvio-lacustrine environment where there is a relatively high amount of clays from the smectite group. In these measurements, SAM found chlorobenzene in amounts ranging from 150 to 300 ppbm (parts per billion by weight) and several dichloroalkanes with two to four carbon atoms (dichloroethane, dichloropropane and dichlorobutane) in amounts up to 70 ppbm, which are considered to be a product of the reaction between Martian organics and oxychlorine compounds, which appear to be ubiquitous throughout the surface of Mars, and which may have played a crucial role in the preservation of the former ones despite the strong ionizing radiation and abundance of oxidizing species they are exposed to. The study led to the conclusion that there is organic carbon in the soil of Mars, probably in the form of aromatic and aliphatic compounds, whose origin may be volcanic, hydrothermal, atmospheric, or biological processes, or could have been contributed by meteorites, comets or interplanetary dust particles; this is one of the many questions that the discoveries of Curiosity leave pending.
Reaching this conclusion, however, was not easy, because the high complexity of the analyses and the precision of the instruments made necessary a thorough work of previous calibration and the elaboration of an inventory of all possible reactions, with identification and quantification of the resulting products which would be possible to obtain from those between the components of the samples and the materials and reagents used in the analysis systems, which could lead to false detections. Upon this initial difficulty, one of the containers of the MTBSTFA reagent for the wet chemistry analyses was damaged at some point and poured its contents into the sample handling system. Once confirmed the accident, it was verified that this compound could be oxidized until CO2 contributing to the increase of the amounts of Carbon monoxide (CO) and Nitric oxide (NO) detected and, eventually, their derived organic compounds. This accident imposed the necessity of controlling carefully the foreseen repercussion in this sense, and to determine to what extent it could accelerate the inevitable degradation of the materials used as hydrocarbon separators, which in turn could produce unwanted by-products. All these spurious factors were adequately considered before reaching an unequivocal conclusion: at least in the bottom of the Gale crater, the soil of Mars preserves organic compounds whatever their origin.
As for the “fixed” nitrogen, the presence of indigenous nitrogen-bearing compounds in Martian soil means the existence of a source of biochemically accessible nitrogen, and suggests that it could be a nitrogen cycle sometime along the evolution of Mars as a planet.
The detection was verified again by means of SAM instruments suite, in samples taken at three different points. Two of them come from drillings performed at Sheepbed mudstone, and the other was a sample from an Aeolian deposit which was thought to be representative of the global Martian dust. SAM measured different amounts of NO in the three samples, ranging from 20 to 250 ppm. This NO is a result of the decomposition of indigenous nitrates during the analysis processes, giving an equivalent quantity of nitrates in Martian soil, once the amount of nitrogen from contamination from SAM itself was subtracted, of 70 to 1,100 ppm for the different samples.
It has been established that the nitrates came from fixation of atmospheric diatomic nitrogen in the atmosphere (N2, a quite inert compound that is not useful for life), during meteoritic impacts. Another possible cause of N2 fixation, according to Mars Express data, could be the photodissociation by UV radiation in the upper atmosphere, but it is not known how much of fixed nitrogen produced by this mechanism can reach the low atmosphere and surface. Anyway, the calculations of the amounts of nitrates produced by impacts and their further chemical evolution under Martian conditions fit with the data obtained by SAM, so this can be considered the main source of fixed nitrogen on Mars.
The availability of biochemically useful nitrogen, together with the inferred ancient conditions of the Martian environment and with the existence of organic compounds in the soil, completes a scenario of high habitability potential on ancient Mars. Regarding present Mars, fixed nitrogen is as well an important element to be taken in account for the assessment of the actual habitability potential that Mars surface and subsurface can offer, given that known life needs this kind of nitrogen compounds to synthesize biomolecules as important as proteins, RNA and DNA.
Since the detection of methane in Mars’ atmosphere with Canada-France-Hawaii Telescope in Mauna Kea was announced for the first time, there has been several measurements of the gas over the last years with different instruments, both ground based telescopes and orbiters (Mars Express and Mars Global Surveyor).
These observations were apparently in disagreement with each other, and some of them suggested a distribution pattern of the methane demarcated both spatial (from some source in the northern hemisphere) and temporarily (with a concentration peak during summer after which methane disappeared in a few months). Both facts are unexplainable by means of the available general circulation and photochemical models, which summarise the current understanding of Mars atmosphere. According to them, if there is really methane in its composition, it would remain for 300 years on average, being homogeneously distributed for the whole atmosphere during that time. Given the lack of a model able to explain methane generation, location, and quick loss, the detections began to be brought into question, and were attributed to artefacts of the measurements, taking into account that they were obtained in the limit of the used instruments’ capability, and that the values registered were in the order of ppbv (parts per billion in volume).
In this context, and when it seemed more and more convincing that data gathered so far were rough to say the least, if not invalid, expectations to resolve the question lied on the ability of the instrument SAM to obtain accurate measurements. By means of its TLS unit, SAM was detecting background values in the methane concentration of around 0.7 ppbv, and then registered an episodic increase of up to ten times this value, that is to say, of about 7 ppbv over a period of 60 sols. These data based on observations during nearly a whole Martian year (almost two Earth years), the period foreseen as the duration of the nominal mission, during which Curiosity traversed some 8 kilometres within Gale crater. In this time, that spans the four Martian seasons, the reference to data from the meteorological station REMS (Rover Environmental Monitoring Station), allowed the establishment of possible correlations with some environmental parameters this instrument monitors, namely, relative humidity, air and ground temperature, pressure and atmospheric opacity, this latter measured both by REMS UV sensor and by MastCam, a camera used to give support to certain atmospheric studies. The possible existence of a seasonal variability in the methane concentration related to these environmental variables, in any case, only can be confirmed through continuous measurements hereafter, specifically pointed to discern what factors can be determinant for the sporadic emission and later degradation of the methane. Regarding the spatial location of methane plumes, it has been concluded that they are generated in short and weak events.
TLS has two channels to analyse in the infrared range, specifically in 2,7 μm wavelength (first channel), and 3,27 μm wavelength (second channel), this latter purposely dedicated to methane detection. It has a resolution of 0.0002 cm-1, that allows it to identify methane through its spectrographic fingerprint of three well resolved lines. The procedure followed (absorption of laser light through a sample in a closed cell) is “simple, non-invasive and sensitive”, as has been highlighted in the paper in which the study was published . The cell can be filled with Mars atmosphere or evacuated to vacuum to take contrasted measurements, which are as well compared with measurements from a methane enriched sample, so the confidence interval reaches 95%, and new data can be considered as definitive.
Being methane a remarkable by-product of biological activity (most of that in our atmosphere comes from this source), great expectations for the possibility this was as well the case of Mars arose, so the preliminary list of sources of the gas to be inquired on Mars must necessarily include the biological activity as one of its possible origins.
However, the unknowns raised by this study outnumber the answers brought. It is a discovery that close definitively the question of whether is methane on Mars atmosphere or not, but it opens some others more transcendental. First, the explanation of its origin, that is believed to be in one or more additional sources apart from those already known, among which biological methanogenesis is not discarded yet, and second, its strange later evolution. Both will have to be solved through further research. NASA’s orbiter MAVEN (Mars Atmosphere and Volatile Evolution), operating at Mars, will hopefully provide new data, and ESA/Roscosmos’ Trace Gas Orbiter (TGO), within ExoMars Mission, will measure methane concentrations in wider scales, providing a context for the obtained results so far, and allowing to go into depth in the understanding of Mars methane dynamics.
The presence of water ice in the Martian polar caps and in different reservoirs (permafrost) all over the surface (mainly located in midlatitudes) has relatively long been known. There are even some assessments of the total amount of water on the planet: it estimated that a global ocean 30 m deep could formed if all the ice detected so far melted.
However, the water vapour content in the atmosphere is very low (by comparison, only a layer of a few microns over the planet could form from it), and the remarkable discovery of Curiosity regarding the Martian water was the existence of a daily water exchange cycle between the ground and the overlying atmosphere, which is mediated by perchlorates, a highly hygroscopic kind of chlorine salts which seem to be ubiquitous throughout the planet’s regolith. The presence of these salts determines a phase along the cycle in which water stay in a metastable and transient liquid state, as it was stated in the study which brought the fact to light.
The study at issue consisted in a joint analysis of data from three different instruments on board the rover, namely, the already described SAM, which provided data on the composition of the soil, DAN (Dynamic Albedo of Neutrons), which measure the water equivalent H up to 60 cm deep in the ground and hence the amount of water available, and REMS, which provided the environmental frame of the process in terms of temperature (of the ground and air), pressure and relative humidity. Perchlorates, besides considerably reducing the freezing point of the water, are highly hygroscopic, that is to say, they have the capacity to capture water vapor from the environment in sufficient quantity to produce the phenomenon known as deliquescence, for which the salts themselves dissolved in the absorbed water forming highly concentrated solutions or brines. These could remain in a metastable liquid state at certain periods of the Martian day during which the relative humidity is above the relative humidity of deliquescence, and the temperature is higher than the eutectic temperature of the solution. The daily rate of REMS measurement (5 minutes per hour plus extended measurements of between 1 and 3 hours each day) has made it possible to establish accurately that liquid brines would be stable in the upper 5 cm of the Gale soil from sunset to dawn at throughout the winter. Crater Gale is located near the Martian equator, the driest stripe of the planet, so it is expected that in higher latitudes the phenomenon occurs during longer periods of time.
In the subsurface, from 15 cm deep, the conditions would allow the perchlorates to remain constantly hydrated throughout the day in any season. It is believed that perchlorates, probably of calcium or magnesium, are the product of photochemical processes occurring in the atmosphere, which would explain their uniform distribution throughout the planet's surface, but also, according to the data obtained by the DAN instrument it has been observed that its abundance grows with the depth in the 60 cm of the ground that the apparatus can penetrate. Given its high solubility, this circumstance would also be determined by the presence of liquid water formed on the surface, which upon draining entrains the perchlorates it carries dissolved.
The likely formation of liquid brines in the regolith provided a satisfactory mechanism to explain one dynamic surface feature described thanks to the images gathered by the Resolution Imaging Science Experiment (HiRISE) instrument (arrived to Mars in 2006 on board the Mars Reconnaissance Orbiter, -MRO-), the so-called Recurrent Slope Lineae (RSL), which are only the marks left by landslides of loose material deposited on topographic slopes. Traditionally, they have been attributed to the formation of CO2 ice (dry ice) within these materials, which lost their support when it sublimated causing them to fall off downhill. But the fact of having observed them in numerous locations around the equatorial stripe, where it is not possible to freeze CO2, led to think of the formation and subsequent drying of perchlorate brines as the most plausible cause of the phenomenon, a hypothesis that this study has come to support eventually.
Recently, a new type of Martian linear surface features, the Slope Streaks, have been identified (Bhardwaj et al 2017) as a consequence of the occurrence of transient liquid water, again basing on the analysis of images gathered by HiRISE and ConTeXt (CTX) imager (also a MRO’s instrument). They are formed at low latitudes on terrains with low thermal inertia, high albedo and covered by a relative abundant amount of fine dust, being the proposed cause a sudden and short-term process of deliquescence prompted by the presence of perchlorates during favourable environmental conditions of temperature and relative humidity. In this case, no downslope displacement of material is involved, and they are the brines themselves which outline the feature while they spread through the dust by capillarity.
Hence, it seems that liquid water is a common phenomenon on present day Mars, what poses major implications with regard to the habitability of the planet’s surface, whose assessment is one of the main scientific goals for the missions to come, as it was for almost every other since the Viking landers. In particular, they are three of them already scheduled, namely, NASA’s InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport), which will provide important information about the processes that shaped the rocky planets of the inner solar system, NASA’s 2020 rover, a twin of Curiosity which will mean a step beyond the investigations performed by this latter, and ESA/Roscosmos’ ExoMars 2020 mission, consisting of a Surface Platform and a rover equipped with a drill able to take underground samples up to a depth of 2 meters, where it is more likely that some biosignatures from the past can be preserved.
In the search for signs of life, the places in which liquid water is more likely to be found are obviously the most interesting to perform the related explorations, but they are at the same time those in which terrestrial organisms hidden in nooks of the spacecrafts could thrive. Actually, the risk of causing biocontamination of those sites, given the practical impossibility of achieving a total sterilization of the spacecrafts to be sent, is becoming a more and more worrying concern among the space agencies involved, whose Planetary Protection offices are working in the implementation of the necessary protocols to minimize the risk. Needless to say, such contamination would be a catastrophic event which would ruin any further astrobiological investigation, so it is a worthy aim, even though it implies constraints to the search for life itself.
Nevertheless, the clues pointing to the presence of liquid water on present day Mars are indirect, and a univocal confirmation of its occurrence under the deduced determinant conditions should be verified for the definitive and general acceptance by the scientific community. And that is what Habitability, Brine Irradiation, and Temperature (HABIT) experiment will do.
The HABIT instrument will be part of the scientific payload of the Russian Surface Platform in the ExoMars 2020 mission. It is composed of two units, BOTTLE (Brine Observation Transition to Liquid Experiment) and ENVPACK (Environmental Package), which will monitor the formation of brines and the conditions in which it takes place respectively.
BOTTLE can be considered as a first attempt to develop a device to measure the amount of liquid water presumably produced daily on Mars surface through deliquescence of salts. Basically, it consists of a container of salts divided in six different vessels in which deliquescence process will be monitored as it occurs, providing a first in situ direct observation of liquid water. It includes a set of hygroscopic salts whose presence in the Martian regolith has been reported so far, such as magnesium, sodium and calcium perchlorates, as well as sodium chloride.
They will be exposed to Martian environment mimicking the process that is thought to occur within the soil, so that the water is captured and the change in the conductivity the process causes in the salts is measured by three sensors connected to two pins mounted in each vessel at different heights. These parameters indicate the state of hydration (dry solute, hydrated solute and briny solution), as well as the state change of the brines themselves once formed (liquid to frozen brines). The composition of the mixture of salts on the vessels will be stablished in accordance to the thermal regime of the particular landing site chosen.
The vessels are covered with High Efficiency Particulate Air (HEPA) filters to avoid the deposition of dust over the salts while allowing the air flow through them. Two of them are empty and openly exposed to Martian atmosphere, so that they will be filled little by little with dust along the mission. The conductivity of this dust under the influence of atmospheric water vapour will provide a valuable reference to evaluate the measurements taken from the salt-filled vessels. Each of them mounts a sensor to monitor the salt/brine temperature, and is equipped with a heating system to evaporate the water captured during the night if necessary. This possibility will be very useful in order to achieve a quantification of the total amount of water that could be gathered, regarding the estimation of its availability as a resource to be exploited. This procedure also permits the regeneration of the salts to their initial dehydrated state, assuring their indefinite operational lifespan.
The other unit that forms HABIT is an environmental set of sensors including an ultraviolet sensor (UVS) composed of 6 photodiodes to measure the UV irradiance, a ground temperature sensor (GTS) with an infrared thermopile in the range 8-14 μm, and three air temperature sensors (ATS) to measure the ambient temperature as well as the wind regime in three different directions. All these sensors derive from those already mounted in Curiosity’s REMS, and its operational regime will be the same. Moreover, the operational regime of HABIT will be also the same as REMS one, that is to say, it will take measurements autonomously with each one of its sensors during 5 min per hour, at 1 Hz frequency, plus extended continuous acquisitions of between 1 and 4 hours long that will be scheduled when convenient.
In sum, studies carried out by the analysis of the voluminous data provided by the different instruments of Curiosity have revealed interesting aspects of Mars, and have allowed to deepen the understanding of their present characteristics and of the processes that, from a very planet similar to the Earth in its earliest times, have transformed it into the cold body it is today; a planet that is not as dry nor as inhospitable as it seemed. Curiosity will keep doing so as it ascends Mount Sharp, a peak that rises in the center of the basin and is made up of sedimentary materials accumulated over hundreds of millions of years. As it goes up, Curiosity will have access to a rich geological record in which the keys of the evolution of the planet have been recorded throughout its history.
The new panorama shows a planet that was quite similar to the Earth in the first phases of the Solar System: a carbonaceous one, provided with a huge supply of water from beyond the ice line (defined in every planetary system by the distance from the central star at which water gets frozen and can aggregate) such as the Earth was, and submitted to an active geological dynamic boosted by internal heat. Mars counted then on a thicker atmosphere warming the surface, which made possible the water to stay in the liquid state in large bulks for long periods of time in geological timescales.
By leaning on this schematic basis, maybe we could give rise to a kind of naïve speculation, but always trying to keep the due consistence. First, we must assume that life is a common phenomenon in the universe; a dissipative structure (according to the definition by Ilya Prigogine) which would generate wherever and whenever the favourable conditions converge. Some scientists at the forefront of research on the origin of life (Nick Lane, Bill Martin), point to the particular disequilibrium life would have come to alleviate on Earth: that existing between H and CO2, two chemical species which were present in the womb of antique Martian oceans like they were here, immersed in a similar flux of energy which kept the system far from the thermodynamic equilibrium. The geochemical processes which eventually gave rise to life, always following the aforementioned authors, took place in particular structures (hydrothermal vents), formed by the interaction of materials from the mantle and the oceans through the crust. These vents’ formation was linked to the process known as serpentinization, which is also expectable to have been produced in the Martian oceans.
From these postulates, it could be thought that there is high likelihood that these processes would have coursed on Mars in a similar way as they did on Earth, and therefore, the existence of Martian life in the planet’s past appears as a plausible possibility. And if life once arose there, it could have persisted in a “deep hot biosphere”, provided there is some remaining liquid water reservoirs as it seems to be the case.
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