An ancient ocean on the Red Planet covered a greater portion of the surface than the Atlantic Ocean does on our planet and held more water than Arctic Ocean, says a group of scientists writing in the journal Science.
This artist’s impression shows how Mars may have looked about 4 billion years ago. Image credit: M. Kornmesser / ESO.
About 4.5 billion years ago, the young Mars would have had enough water to cover its entire surface in a liquid layer about 140 m deep, but it is more likely that the liquid would have pooled to form an ocean occupying almost half of the planet’s northern hemisphere, and in some regions reaching depths greater than 1,600 m.
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, H2O. The other is HDO, or semi-heavy water, a naturally occurring variation in which one hydrogen atom is replaced by deuterium.
As HDO 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 scientists distinguished the chemical signatures of the two types of water using ESO’s Very Large Telescope, along with instruments at the W. M. Keck Observatory and NASA’s Infrared Telescope Facility.
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.
“Our study provides a solid estimate of how much water Mars once had, by determining how much water was lost to space. With this work, we can better understand the history of water on Mars,” said Dr Geronimo Villanueva of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who is the first author of the Science paper.
Dr Villanueva and his colleagues mapped H2O and HDO levels several times over nearly 3 Martian years.
The resulting data produced global snapshots of each compound, as well as their ratio. These first-of-their-kind maps reveal regional variations called microclimates and seasonal changes, even though modern Mars is essentially a desert.
The scientists were interested in regions near Red Planet’s north and south poles, because the polar ice caps hold the planet’s largest known water reservoir.
The water stored there is thought to capture the evolution of Mars’ water during the wet Noachian period, which ended about 3.7 billion years ago, to the present.
From the measurements of atmospheric water in the near-polar region, they determined the enrichment, or relative amounts of the two types of water, in the planet’s permanent ice caps.
The enrichment of the ice caps told them how much water Mars must have lost – a volume 6.5 times larger than the volume in the polar caps now. That means the volume of Mars’ early ocean must have been at least 20 million cubic km.
The young Mars would have had enough water to cover its entire surface in a liquid layer about 140 m 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 km. Image credit: M. Kornmesser / ESO / N. Risinger, skysurvey.org.
Based on the surface of Mars today, a likely location for this water would be in the Northern Plains, considered a good candidate because of the low-lying ground.
An ancient ocean there would have covered 19 percent of the planet’s surface. By comparison, the Atlantic Ocean occupies 17 percent of Earth’s surface.
“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 km away,” said study co-author Dr Ulli Kaeufl of ESO.
“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,” added co-author Dr Michael Mumma of NASA’s Goddard Space Flight Center.
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G. L. Villanueva et al. Strong water isotopic anomalies in the Martian atmosphere: Probing current and ancient reservoirs. Science, published online March 05, 2015; doi: 10.1126/science.aaa3630
