Each new investigation reveals that Mars has large amounts of water in the subsurface, and now we are just learning that its interior is hot and molten.
We have several proofs of the importance of water in its evolution obtained in the last decades, thanks to comprehensive monitoring with various devices in orbit and rovers on the surface of any small change made on the planet. A recent study led by Eva L. Scheller of the California Institute of Technology (Caltech) indicates that the change in water in many of its minerals has made it possible to sequester a good part of this water in the crust, in quantities that can vary between 30. and 99% of the water initially present.
The evidence is accumulating: the discovery of the flow of brine on Mars, the existence of permafrost regions underground, and even the possible existence of subterranean lakes point to the possibility that it may or has been home to microorganisms. All this is revealed as an interesting excuse to attack human exploration of the red planet very soon.
From its superficial exploration we could deduce that water can only be in the polar caps. However, much of it remains in the form of permafrost in the Martian subsurface, as recently revealed by craters excavated by the impact of small asteroids in certain regions. Some regions may even harbor large bodies of liquid water.
Since 2018, an Italian team, using the MARSIS radar in the Mars Express survey of the European Space Agency (ESA), has discovered several subglacial lakes under the subsurface near the south polar cap. Among them a deep lake about 20 km wide.
Origin of water on Mars
The red planet was formed, to a large extent, from the aggregation of hydrated planetesimals. So it should not be surprising that it ended up hosting a lot of water. Not without reason, he recently agreed to call philosophical in this first era covering the first 500 million years of the planet’s evolution, based on the presence of phyllosilicates (clay) in these ancient terrains.
In fact, some Martian meteorites that have reached Earth, such as the famous Allan Hills 84001, reveal that water flowed over many regions of Mars. We studied precisely this meteorite at the Institute of Space Sciences (CSIC-IEEC) and it has carbonates in its fractures that grew in different phases, supporting the existence of various periods of hydration while they were part of the Martian surface. Perhaps these phases were a consequence of volcanic activity in the successive episodes of flooding that occurred in the rock environment from which this meteorite came.
But after this golden stage, everything indicates that Mars suffered a gradual cooling, losing its magnetic field and its dense primordial atmosphere. So liquid water on the surface ceases to be stable, as atmospheric pressure and temperature fall. The flow of degrading UV radiation that, since then, inexorably reaches the surface of the planet also increases.
The loss of its dense atmosphere
Some Martian meteorites, the only samples we currently have from the red planet, support that early Mars had appreciable igneous activity that led to significant degassing on a global scale, giving rise to a dense atmosphere in the past read them. This atmosphere gradually disappeared because as it cooled, it lost its magnetic field and, with it, the ability to retain and shield ultraviolet radiation. This probably happened during the Teicican era so called to refer to the sulfate minerals that formed then, between 3.5 and 4.0 billion years ago.
As the planet lost this atmosphere and volcanic activity decreased, the greenhouse effect ceased and the temperature decreased. The water is gradually removed underground, where it is present in the form of permafrost and hydrated minerals. This can be seen in recent impacts that have exposed the subsurface to freezing in the coldest regions and the poles.
The mysterious liver is leaking
A decade ago, the Mars Global Surveyor orbiter discovered a type of outflow moving along some crater slopes and slopes of the planet Mars. These black flows seem to undergo seasonal changes. During the Martian summer, they grow, and they withdraw or disappear in winter.
Since their discovery, these flows have been monitored from space, and are known as renewable slope lines (RSL). The best explanation available so far for these observations is that they are salt flows mixed with sand.
Brine flows and water is retained in the subsoil
NASA’s Mars Reconnaissance Orbiter (MRO) has spent years monitoring subtle variations in these dark flows that preferentially propagate during warm seasons. Equipped with high-resolution cameras and spectrometers, the MRO revealed that these long streams are several meters wide and tens of meters long. Its extent and albedo vary during warm periods.
Saline flows appear to be formed by hydrated salts in which magnesium and sodium chlorate and perchlorate are abundant. In the right proportions, these salts could lower the freezing point of water to 80 K (-193 ºC) and, therefore, would give these hydrated salts the possibility to flow at very low temperatures on Mars today.
The interest in understanding them is such that the rovers are guided in these regions to characterize their specific nature on a microscopic scale.
The most interesting thing is that the discovery supports that this water stored in the subsoil can flow in episodes of hydrothermalism and, therefore, an underground hydrological cycle can occur, at least locally. Not only that, detailed studies by the Perseverance rover of the Jezero crater environment revealed the presence of aromatic organic compounds that would appear at the stage when the crater was a hydrothermal environment.
Possible Martian nest for extreme bacteria
We look at these environments changed by water with a magnifying glass because, if life also arose on Mars, extremophile bacteria could appear, microorganisms characterized by a level of adaptability higher than conventional ones, able to move to the last nests that inhabit them.
We certainly do not know if these niches existed. But if knowledge is the result of curiosity, these hypotheses arouse interest in future manned missions and make us focus our efforts on finding the most promising places for future landings.
Brine, evaporites and the origin of life… also on Mars?
Could the evidence of water flowing in the interior of Mars, even present today thanks to the remaining internal heat, have implications for the search for Martian life? Definitely.
Decades ago, Caltech’s Joseph Kirschvink suggested that life on Earth may have originated from ribose formed on Mars and transported to Earth via meteorites. Kirschvink’s hypothesis is generally considered as bold as it is improbable, because the transport mechanism on Earth would have been aboard meteorites that would have taken millions of years to reach our planet. However, certain areas of the surface of Mars appear to have seen the formation of evaporites and other aqueous mineral changes.
Perhaps future studies suggest that Kirschvink was not so wrong in proposing evaporites as potential catalysts for life.
Despite our current ignorance of the exact scenario necessary for the origin of life, the discovery of these aqueous environments on Mars is an exciting reason to further explore the red planet.
Josep M. Trigo Rodríguez, Principal Investigator in the Group of Meteorites, Minor Bodies and Planetary Sciences, Institute of Space Sciences (ICE – CSIC)