|Artist impression of Mars ocean|
Excerpt from news.discovery.com
But how much water did Mars possess? According to research published in the journal Science, the Martian northern hemisphere was likely covered in an ocean, covering a region of the approximate area as Earth’s Atlantic Ocean, plunging, in some places, to 1.6 kilometers (1 mile) deep.
“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, of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the new paper, in an ESO news release. “With this work, we can better understand the history of water on Mars.”
Over a 6-year period, Villanueva and his team used the ESO’s Very Large Telescope (in Chile) and instruments at the W. M. Keck Observatory and the NASA Infrared Telescope Facility (both on Mauna Kea in Hawaii) to study the distribution of water molecules in the Martian atmosphere. By building a comprehensive map of water distribution and seasonal changes, they were able to arrive at this startling conclusion.
It is becoming clear that, over the aeons, Mars lost the majority of its atmosphere to space. That also goes for its water. Though large quantities of water were likely frozen below the surface as the atmosphere thinned and cooled, the water contained in an ocean of this size must have gone elsewhere — it must have also been lost to space.
The water in Earth’s oceans contains molecules of H2O, the familiar oxygen atom bound with 2 hydrogen atoms, and, in smaller quantities, the not-so-familiar HDO molecule. HDO is a type of water molecule that contains 1 hydrogen atom, 1 oxygen atom and 1 deuterium atom. The deuterium atom is an isotope of hydrogen; whereas hydrogen consists of 1 proton and an electron, deuterium consists of 1 proton, 1 neutron and 1 electron. Therefore, due to the extra neutron the deuterium contains, HDO molecules are slightly heavier than the regular H2O molecules.
Also known as “semi-heavy water,” HDO is less susceptible to being evaporated away and being lost to space, so logic dictates that if water is boiled (or sublimated) away on Mars, the H2O molecules will be preferentially lost to space whereas a higher proportion of HDO will be left behind.
By using powerful ground-based observatories, the researchers were able to determine the distribution of HDO molecules and the H2O molecules and compare their ratios to liquid water that is found in its natural state.
Of particular interest is Mars’ north and south poles where icecaps containing water and carbon dioxide ice persist to modern times. The water those icecaps contain is thought to document the evolution of water since the red planet’s wet Noachian period (approximately 3.7 billion years ago) to today. It turns out that the water measured in these polar regions is enriched with HDO by a factor of 7 when compared with water in Earth’s oceans. This, according to the study, indicates that Mars has lost a volume of water 6.5 times larger than the water currently contained within the modern-day icecaps.
Therefore, the volume of Mars’ early ocean must have been at least 20 million cubic kilometers, writes the news release.
Taking into account the Martian global terrain, most of the water would have been concentrated around the northern plains, a region dominated by low-lying land. An ancient ocean, with this estimate volume of water, would have covered 19 percent of the Martian globe, a significant area considering the Atlantic Ocean covers 17 percent 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, also of NASA’s Goddard Space Flight Center.
This estimate is likely on the low-side as Mars is thought to contain significant quantities of water ice below its surface — a fact that surveys such as this can be useful for pinpointing exactly where the remaining water may be hiding.
Ulli Kaeufl, of the European Southern Observatory and co-author of the paper, added: “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 kilometers away!”