In 2005 a new mission began orbiting Mars – Mars Reconnaissance Orbiter. It was equipped with the HiRise hi-resolution camera, CRISM hyperspectrometer and Sharad radar, among other things.
HiRise shoots the Martian surface below with the resolution of 26 cm per pixel. Such unprecedented resolution brought Mars studies to a new level, living up to the scientists’ expectations. It made the headlines when it photographed what can be called Martian ‘rivulets’, visible on crater slopes in mid latitudes.
The scientific community took years to figure out what it was they were seeing, but in the end they had to admit those were traces of water. Liquid water. At the same time low atmospheric pressure on Mars makes it impossible for liquid water to exist on the surface at 0 C or higher temperatures, with few exceptions. This issue was resolved when salt was added to the equation. Briny water can remain in the liquid state at subzero temperatures. When spring comes and day temperatures rise to -10 to -5 C, the brine thaws and gradually soaks the topsoil while moving down the slope, making it darker in colour. These wet streaks gradually stretch as far as the crater bottom, remaining there until dried by the summer sun. A similar process was observed in the Antarctic desert, which served as additional proof for the ‘water’ hypothesis.
MRO managed to find water where no one expected it would, in mid northern latitudes. The scientists have long wondered why there is such a pronounced difference in Mars reliefs. The planet’s Southern elevation is all mountaneous and cratered, whereas the Northern plains are very smooth.
The most cautious researchers suggested a lava sea in the north, much like the lunar maria. Optimists hoped that there could have once been an ocean in the northen parts of Mars, which levelled out the landscape, doubling as a ‘cushion’ against falling asteroids, keeping the surface from getting cratered. There is a lot of indirect evidence of a past ocean, but, as you probably remember, the Viking landers working in the northern plains didn’t find any traces of it.
Then the optimists literally got help from the skies. Observations from the MRO helped find fresh impact craters. There was something unusual about those recent traces of bombardment in the northern plains. Some bright white material was visible through the layer of red dust.
And this white stuff literally vanished into the air in a matter of weeks.
As far as we know, dust takes much longer to cover any traces on Mars, weeks or even years. So it could only be explained by water ice. The Viking landers couldn’t dig deep enough. If they had dug to the depth of about 20 cm, they might have reach this Martian ice.
Finally, in 2008, the Phoenix lander descended in the northern subpolar regions of the Red Planet, exactly where the subsurface layer was expected to consists of up to 60% water, according the Mars Odyssey data. Phoenix was sent specifically to search for water, it was equipped to search for water, and it did find some.
It was equipped with a special scoop, more powerful than those on the Viking landers, which managed to shove away a thin top layer of dust to find water ice underneath. The ice began to melt under direct sunlight and disappeared in just a few days, but it was enough for Phoenix to analyse both the topsoil and the water ice. Neither had traces of life or organics, but the matter of water on Mars was brought to a close, once and for all.
The research was far from complete, though. Further observations from the satellites revealed some regions on Mars a great distance away from the poles, which had nevertheless clear evidence of glaciers, as, for example, in the eastern part of Hellas Planitia.
It is believed that during certain periods in Mars’s history, the planet’s axis tilted to 45 degrees, which caused the south geographic pole to shift to as far as Hellas
. This axis inclination change led to dramatic climate change. One of the polar caps began to melt actively, resulting in snowfall on the new pole, which caused glaciers to accumulate.
But after a while the Martian poles returned to their original place and there were no more cataclysms. According to modern theory, this could have actually been happening relatively recently on the time scale, around 20 million years ago. Eventually, the atmosphere became too tenuous for an ocean or sea to exist on the surface. The warm and wet period ended about 1.5-2 billion years earlier.
The Curiosity rover, having landed in Gale Crater, located not far from the equator, found no ice there. But it found river pebbles for the first time.
The rover discovered that the topsoil layer contains 3 to 6% of chemically bound water. The Russian DAN instrument even managed to find layers in the soil. The topmost layer was ‘dry’ in some places with its 3% of water, whereas the lower layer (up to 60 cm in depth, visible for DAN) is ‘wet’ – up to 5%. In other places it was vice versa, with 5% on top and 3% at the bottom.
The SAM gas chromatograph showed that warming the soil to 400C allows to evaporate water and other gases which can be used by future human settlers, such as oxygen, nitrogen and carbon dioxide. If heated to higher tempretures, the spoil will yield sulphides and chlorides, both hazardous for life. In one sample SAM found 6% of water, as well as nitrates and organic compounds.
In 2015 the results
of two investigations of Martian water were reported. The first one was carried out from the Mars Express and MRO satellites using radar, which allowed to estimate water deposites in glaciers in mid latitudes.
Satellite sounding of mid latitude glaciers revealed that the amount of water they contain could theoretically create a one-meter-deep ocean on an ideally smooth Mars. Due to earlier radar investigations it was possible to calculate that if we thawed both the north and the south polar caps we could cover an ideal planet the size of Mars with a 22 meter layer of water. Now it only remained to estimate how much water must have been in the northern ocean.
Another investigation was carried out by the Curiosity rover. Scientists compared the results of the Rover Environmental Climate Station (REMS) with chemical composition data obtained by the Sample Analysis at Mars (SAM) instrument and with hydrogen concentration data obtained by the Dynamic Albedo of Neutrons tool (DAN). And they arrived at some interesting conclusions.
show that the relative humidity of the Martian atmosphere can reach 100% at night during the spring and autumn seasons. At the same time, perchlorates contained in the soil can absorb water from the atmosphere. In other words, at a certain moment salt in the soil will turn into liquid brine and currents will flow, like those registered by MRO from orbit. Curiosity has never reported anything like this, because it is relatively dry at the equator. However, higher levels of humidity might be detected in the mid latitudes.
Curiosity’s finds also helped to answer the question whether microbial life is possible on Mars today. It turns out that it actually is. By life we don’t simply mean sustaining organisms in comatosis, as is possible in open space, but life activities such as reproduction, metabolism and possibly even evolution. Of course, we are pretty unlikely to stumble on some Martian creatures, but we can already start selecting or breeding terrestrial extremophile species which we could populate Mars with in the future, this preparing it for terraforming.