Mars – the blue and red planet
At least 20 percent of Mars was once covered with water
Four and a half billion years ago Mars was a watery planet. Traces found in the planet’s rock and sand suggest that around 23 cubic kilometres of water once flowed there. In a current publication in the specialist journal Science, a team involving participants from the Max Planck Institute for Solar System Research comes to a slightly different conclusion: according to them the planet contained at least 20 million cubic kilometres of water. This means that around 20 percent of the surface of Mars was under the cover of water. The two results are not necessarily at odds with each other. They both point to the fact that hidden water reserves still exist deep beneath the surface of Mars today.
Water is something of a rarity on Mars. Large volumes are only found at the Red Planet’s distinctive polar caps. Exactly how much frozen water exists in the soil of the desert-like landscapes at lower geographical latitudes is unclear. It cannot be very much. However, 4.5 million years ago, our cosmic neighbour must have presented a completely different vista. Geological tests reveal that large watercourses once flowed there. But exactly how much water did they contain?
Researchers working with Geronimo L. Villaneuva from the US space agency NASA’s Goddard Space Flight Center have adopted what would appeaser to be an unusual approach to this prehistoric water inventory: instead of scrutinising the soil on Mars for information like many of their colleagues, they look to its atmosphere. Although it contains less than one percent water vapour, the specific composition of this atmospheric water is highly informative.
“From this we can reconstruct how the planet’s water regime developed and how much water was able to escape into space,” says Paul Hartogh from the Max Planck Institute for Solar System Research, a co-author of the study. Through the influence of the solar radiation, the water in the upper atmosphere decomposes into its individual and lighter components – some of which can then leave the planet’s field of gravity.
But water is not simply water. In addition to ‘normal’ water, which consists of two hydrogen atoms and one oxygen atom, other types exist, for example that known as semi-heavy water. In semi-heavy water, one hydrogen atom is replaced by the heavier isotope deuterium, which has an extra neutron in its nucleus.
“Frozen semi-heavy water transforms into the gaseous state at a somewhat higher temperature than normal water,” explains Hartogh. When the soil warms up in the cold permafrost regions of Mars in spring, normal water tends to evaporate while ice with a high deuterium content remains in the soil.
As opposed to this, in autumn, the semi-heavy water from the atmosphere condenses. “In this way, deuterium accumulates in the soil within one annual cycle,” says Hartogh. Furthermore, because normal hydrogen is more likely to escape from the planet’s atmosphere, the deuterium content of Mars’s entire water reservoir has been increasing over the course of time.
To understand the ratio of semi-heavy to normal water vapour in Mars’s atmosphere, the researchers did not determine a global mean value, as would otherwise be the case, but examined the planet bit by bit. In this way they were able to create the first two-dimensional map of the ratio between the two substances with a spatial resolution of just 500 kilometres. The key factors that enabled them to create this water map were not only powerful telescopes, which were able to detect the typical fingerprints of semi-heavy and normal water in Mars’s infrared radiation, but also, and above all, the carefully selected observation times.
The difficulty with this approach however, is that the Earth’s atmosphere also contains water vapour. To separate the infrared radiation of Earthly water from that of Mars water, the scientists only looked in the direction of Mars through the Very Large Telescope of the European Southern Observatory in Chile, the W. M. Keck Observatory and the NASA Infrared Telescope Facility on Mauna Kea in Hawaii when it was particularly close to the Earth.
“The Earth rotates faster around the sun than Mars,” says Paul Hartogh. Occasionally it laps Mars and overtakes it from behind on the inside orbit. “In this situation, the relative difference in speed between the two planets is greatest in the direction of observation,” explains the scientist. The wavelengths of the radiation originating from the Mars water are particularly strongly displaced compared with those of the Earthly radiation. This so-called Doppler effect can also be observed in ambulances travelling at high speeds. The frequency of the sirens in such vehicles differs from that of stationary ones.
The analysis, which took from March 2008 to January 2014, also tested the scientists’ patience. The necessary observation conditions only arose for a few months around every two years. Nevertheless, this enabled the researchers to experience the different seasons on the Red Planet: from late winter to spring in the northern hemisphere. “Our maps show strong spatial and seasonal variations,” says Hartogh. Hence, in some locations, the ratio of deuterium to hydrogen can be similar to that found in Earthly water while in others the values are up to nine times higher.
The regions near the equator and at low geographical latitudes where it is very hot in summer are of particular interest to the researchers. It gets so warm there that all of the frozen water – both normal and semi-heavy – evaporates from the soil. For every single molecule of semi-heavy water that reaches the atmosphere there, around 900 molecules of normal water do the same. The corresponding ratio on Earth is seven times more.
“In contrast, the ice in the polar caps never completely evaporates. For this reason, deuterium accumulates there over the course of a year,” explains the Max Planck researcher. Accordingly, the ratio of deuterium to hydrogen must exceed the value of 1:900; model calculations give a value of around 1:800.
This is the crucial unknown when it comes to determining how much water Mars lost over the course of its evolution. It describes the current state of the largest water reservoir found on the planet today, the polar cap. Direct measurements are not available from this location as all of the landing missions carried out up to now have focused on far lower geographical latitudes.
The scientists took the ancient reference value from measurements of Mars meteorites which detached from their mother planet billions of years ago and fell to Earth. Information about the original conditions on the Red Planet is preserved in the water they contain.
“According to our calculations, at least 20 percent of Mars was covered by water 4.5 billion years ago,” says Hartogh. Traces left by this water on the surface geology of the planet point to ancient water cover on a similar scale: “This discrepancy could mean that the current inventory of water on Mars is not yet complete.”
The missing water may lie hidden deep beneath the surface of the planet. It would not interact whatsoever with the atmosphere and would, therefore, be excluded from this atmospheric perspective.
BK / HOR