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When Mars had an ocean

These are the latest findings from an international team of scientists used the European Southern Observatory’s (ESO) Very Large Telescope based in the Atacama Desert in Chile with the aid of instrumentation at the W. M. Keck Observatory and NASA’s Infrared Telescope Facility.

The researchers monitored Mars’ atmosphere over a six-year period. The data collected enabled them to map the properties of the water in different parts of Mars’ atmosphere over a six-year period. The maps, revealed in the journal [i]Science[/i] Mar. 5, are the first of their kind to be published.

Once a water-world

From their observations, the researchers estimate that, roughly 4 billion years, ago a youthful Mars possessed enough water to cover its entire surface to a depth of 140 meters (about 460 feet). But they suggest a more likely scenario is that the liquid water pooled in depressions on the planet’s surface to form an ocean. Such an ocean would have covered almost 50 percent of the Red Planet’s northern hemisphere, reaching depths of more than 1.6 kilometers (one mile) in places.

Last December, NASA reported its Mars Curiosity Rover had found evidence of sediments deposited in what was once a size-able lake-bed on the planet near Mount Sharp at Gale Crater. NASA suggested that a primitive Mars could have maintained a climate conducive to producing long-lasting lakes dispersed across the Red Planet.

The latest ESO observations estimating that a substantial body of water once existed on Mars add to the body of evidence suggesting that the planet once harboured conditions much more suitable for life to exist than it does today.

Geronimo Villanueva, a scientist working at NASA’s Goddard Space Flight Center, and lead author of the research paper commented, “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.”

The new evaluation of Martian water emanates from detailed observations of two slightly different forms of water in Mars’ atmosphere.

One is the everyday form of water — H20 — comprising two hydrogen atoms coupled with one of oxygen. But the other, much less common form of water is HDO. HDO, otherwise known as semi-heavy water, is a water variant whereby one hydrogen atom is replaced by a heavier form of hydrogen called deuterium.

Since water in deuterated form is heavier than normal water, it’s less likely to be lost into space through evaporation. It follows from that that the more the planet loses water, the higher the ratio of HDO to H2O in the remaining water. On Earth the ratio of H2O to HDO is about 3200:1.

Using ESO’s Very Large Telescope in Chile together with instruments at the W. M. Keck Observatory and the NASA Infrared Telescope Facility in Hawaii the researchers isolated the chemical signatures of the two types of water.

Although Mars surface rovers, such as Curiosity, and Mars orbiters are capable of providing significantly more detailed “on-site” data, these robotic explorers aren’t suitable when it comes to monitoring the Martian atmosphere as a whole. Planet-wide data on Mars’ atmosphere is best obtained by taking advantage of infrared spectrographs on large telescopes based back on Earth.

These spectrographs provided information from which the ratio of HDO to H20 in the Martian atmosphere could be measured. By looking at that ratio, scientists can gauge by how much the fraction of HDO has increased and determine how much water has escaped into space. That then allows them to estimate the amount of water that existed on Mars at a much earlier era.

The distribution of H2O and HDO was repeatedly mapped over a period of six Earth years — approximately three Martian years — producing global snapshots of each, as well as the ratio of the two water variants. The maps produced show seasonal changes and micro-climates in contrast with the Mars of today which is, to all intents, a desert planet.

Of particular interest were Mars’ polar regions since these contain the planet’s largest known reservoirs of water. Scientists believe the water locked away in these Martian ice-caps provides an archive of how water evolved on Mars dating from the wet Noachian period to the present day. The Noachian period, which ended about 3.7 billion years ago, was a period in Mars’ history when the planet may have contained water in abundance but during which time the planet suffered planetary bombardment from asteroids and meteors.

Noachian-aged terrains on Mars are prime targets for robotic Mars explorers searching for fossil evidence of life. During the Noachian period, the atmosphere of Mars was denser than it is today. That atmospheric insulation blanket may have meant that the Martian climate was warm enough for rainfall to occur.

The ESO investigations reveal that atmospheric water in the near-polar region was enriched in HDO by a factor of seven when compared to ocean water on Earth. That implies that water in Mars’ permanent ice caps is HDO-enriched eight-fold. Such a high level of enrichment suggests that Mars must therefore have lost a volume of water 6.5 times larger than the present polar caps. Since the present volume of Mars’ polar caps is a known quantity, it can be deduced that the volume of Mars’ early ocean must have been at least 20 million cubic kilometers.

An artist's impression showing how Mars may have looked about 4 billion years ago. The young plane...

An artist’s impression showing how Mars may have looked about 4 billion years ago. The young planet would have had enough water to cover its entire surface in a liquid layer about 140 metres deep. More likely, however, the liquid would have pooled forming an ocean spanning almost half of Mars’ northern hemisphere, in some regions reaching depths greater than 1.6 kilometers.
ESO/M. Kornmesser

Based on Mars’ topography as observed today, the likely location for this vanished Martian ocean would be the planet’s Northern Plains, known as the Vastitas Borealis, literally the “northern waste”. The Vastitas Borealis is the most likely candidate since they comprise an expanse of low-lying ground, the largest lowland region on the planet. If the ESO researchers’ analysis is correct, an ocean covering Mars’ Northern Plains would once have submerged 19 percent of the planet’s surface. That compares to the Atlantic Ocean covering 17 percent of 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,” explained Michael Mumma, a senior scientist at Goddard Space Flight Center MD and the second author on the paper.

Apart from once possessing an ocean, it’s possible Mars once had even more water, with some of it deposited underground. Since the new ESO maps of Mars also show micro-climates and changes in the atmospheric water content over time, they could help with future searches for the existence of water underground on the Red Planet.

Such a find would have important implications for any future manned mission to Mars since the first explorers to set foot on the Red Planet would not need to carry so much water supplies with them but could mine for water after reaching Mars.

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