A primitive ocean on Mars held more water than Earth’s Arctic Ocean, and covered a greater portion of the planet’s surface than the Atlantic Ocean does on Earth, according to new results published. An international team of scientists used ESO’s Very Large Telescope, along with instruments at the W. M. Keck Observatory and the NASA Infrared Telescope Facility, to monitor the atmosphere of the planet and map out the properties of the water in different parts of Mars’s atmosphere over a six-year period. These new maps are the first of their kind. The results appear online in the journal Science.
About four billion years ago, the young planet would have had enough water to cover its entire surface in a liquid layer about 140 metres deep, but it is more likely that the liquid would have pooled to form an ocean occupying almost half of Mars’s northern hemisphere, and in some regions reaching depths greater than 1.6 kilometres.
“Our study provides a solid estimate of how much water Mars once had, by determining how much water was lost to space,” said Geronimo Villanueva, a scientist working at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, USA, and lead author of the new paper. “With this work, we can better understand the history of water on Mars.”
The new estimate is based on detailed observations of two slightly different forms of water in Mars’s atmosphere. One is the familiar form of water, made with two hydrogen atoms and one oxygen, H2O. The other is HDO, or semi-heavy water, a naturally occurring variation in which one hydrogen atom is replaced by a heavier form, called deuterium.
As the deuterated form is heavier than normal water, it is less easily lost into space through evaporation. So, the greater the water loss from the planet, the greater the ratio of HDO to H2O in the water that remains.
The researchers distinguished the chemical signatures of the two types of water using ESO’s Very Large Telescope in Chile, along with instruments at the W. M. Keck Observatory and the NASA Infrared Telescope Facility in Hawaii. By comparing the ratio of HDO to H2O, scientists can measure by how much the fraction of HDO has increased and thus determine how much water has escaped into space. This in turn allows the amount of water on Mars at earlier times to be estimated.
In the study, the team mapped the distribution of H2O and HDO repeatedly over nearly six Earth years, equal to about three Mars years, producing global snapshots of each, as well as their ratio. The maps reveal seasonal changes and micro-climates, even though modern Mars is essentially a desert.
Ulli Kaeufl of ESO, who was responsible for building one of the instruments used in this study and is a co-author of the new paper, adds: “I am again overwhelmed by how much power there is in remote sensing on other planets using astronomical telescopes: we found an ancient ocean more than 100 million kilometres away!”
The team was especially interested in regions near the north and south poles, because the polar ice caps are the planet’s largest known reservoir of water. The water stored there is thought to document the evolution of Mars’s water from the wet Noachian period, which ended about 3.7 billion years ago, to the present.
The new results show that atmospheric water in the near-polar region was enriched in HDO by a factor of seven relative to Earth’s ocean water, implying that water in Mars’s permanent ice caps is enriched eight-fold. Mars must have lost a volume of water 6.5 times larger than the present polar caps to provide such a high level of enrichment. The volume of Mars’s early ocean must have been at least 20 million cubic kilometres.
Based on the surface of Mars today, a likely location for this water would be the Northern Plains, which have long been considered a good candidate because of their low-lying ground. An ancient ocean there would have covered 19% of the planet’s surface; by comparison, the Atlantic Ocean occupies 17% of the Earth’s surface.
“With Mars losing that much water, the planet was very likely wet for a longer period of time than previously thought, suggesting the planet might have been habitable for longer,” said Michael Mumma, a senior scientist at Goddard and the second author on the paper.
It is possible that Mars once had even more water, some of which may have been deposited below the surface. Because the new maps reveal micro-climates and changes in the atmospheric water content over time, they may also prove to be useful in the continuing search for underground water.
Mars has distinct polar ice caps, but Mars also has belts of glaciers at its central latitudes in both the southern and northern hemispheres. A thick layer of dust covers the glaciers, so they appear as surface of the ground, but radar measurements show that underneath the dust there are glaciers composed of frozen water. New studies have now calculated the size of the glaciers and thus the amount of water in the glaciers. It is the equivalent of all of Mars being covered by more than one meter of ice. The results are published in the scientific journal, Geophysical Research Letters.
Several satellites orbit Mars and on satellite images, researchers have been able to observe the shape of glaciers just below the surface. For a long time scientists did not know if the ice was made of frozen water (H2O) or of carbon dioxide (CO2) or whether it was mud.
Using radar measurements from the NASA satellite, Mars Reconnaissance Orbiter, researchers have been able to determine that it is water ice. But how thick was the ice and do they resemble glaciers on Earth?
A group of researchers at the Niels Bohr Institute have now calculated this using radar observations combined with ice flow modelling.
“We have looked at radar measurements spanning ten years back in time to see how thick the ice is and how it behaves. A glacier is after all a big chunk of ice and it flows and gets a form that tells us something about how soft it is. We then compared this with how glaciers on Earth behave and from that we have been able to make models for the ice flow,” explains Nanna Bjørnholt Karlsson, a postdoc at the Centre for Ice and Climate at the Niels Bohr Institute at the University of Copenhagen.
Nanna Bjørnholt Karlsson explains that earlier studies have identified thousands of glacier-like formations on the planet. The glaciers are located in belts around Mars between the latitudes 300-500, equivalent to just south of Denmark’s location on Earth. The glaciers are found on both the northern and southern hemispheres.
From some locations on Mars they have good detailed high-resolution data, while they only have more sparse data from other areas. But by supplementing the sparse data with information about the flow and form of the glaciers from the very well studied areas, they have been able to calculate how thick and voluminous the ice is across the glacier belts.
“We have calculated that the ice in the glaciers is equivalent to over 150 billion cubic meters of ice. That much ice could cover the entire surface of Mars with 1.1 meters of ice. The ice at the mid-latitudes is therefore an important part of Mars’ water reservoir,” explains Nanna Bjørnholt Karlsson.
That the ice has not evaporated out into space could actually mean that the thick layer of dust is protecting the ice. The atmospheric pressure on Mars is so low that water ice simply evaporates and becomes water vapour. But the glaciers are well protected under the thick layer of dust.