Mars once had an ocean that covered nearly one-fifth of its surface to a depth of maybe a mile, according to new research using Mauna Kea telescopes.
The findings, to be published Friday in the journal Science, add to the body of evidence that the red planet could have harbored life in its ancient past.
Scientists using the Keck Observatory and NASA Infrared Telescope Facility report that the water could have formed an ocean covering almost half of Mars’ northern hemisphere.
All this was some 3.7 billion years ago, toward the end of the planet’s wet Noachian period.
Nearly all of the water — 87 percent — has been lost to space, but some remains frozen at the planet’s poles. There could also be water reservoirs underground, scientists speculate.
"Our study provides a solid estimate of how much water Mars once had, by determining how much water was lost to space," Geronimo Villanueva, a scientist at NASA’s Goddard Space Flight Center and first author of the paper, said in a statement Thursday. "With this work, we can better understand the history of water on Mars."
Based on the surface of Mars today, a likely location for the water would be in the low-lying northern plains. An ancient ocean there would have covered 19 percent of the planet’s surface, the scientists said.
By comparison, the Atlantic Ocean takes up 17 percent of Earth’s surface. Because Mars is smaller than Earth, the ancient ocean would have been slightly larger than the Arctic Ocean on Earth.
"With Mars losing that much water, the planet was very likely wet for a longer period of time than was previously thought, suggesting the planet might have been habitable for longer," said Michael Mumma, a senior scientist at Goddard, also an author of the paper.
University of Hawaii geophysicist Peter Mouginis-Mark, who was not involved with the study, said, "It sounds quite plausible to me, and the conclusion is supported by a lot of observational data that has been collected from Mars orbit and from the Curiosity rover."
The research team used the Near-Infrared Spectrograph on the Keck Observatory to study signatures of water in the planet’s atmosphere.
Key to the calculations is the ratio between regular water molecules, which have one oxygen and two hydrogen atoms, and a variant that has one oxygen atom, one hydrogen atom and one atom of deuterium, a heavier cousin of hydrogen that has a neutron and proton in its nucleus.
Nearly all deuterium was created during the big bang, so its presence in relation to regular hydrogen is relatively constant: 156 atoms of deuterium per million hydrogen on Earth and slightly higher in some comets.
The process by which water was lost to space would have favored the lighter hydrogen atoms, leaving an enhanced concentration of heavy water behind.
The amount of heavy water, chemical symbol HDO, or deterium detected indicated that there was once eight times more water there than there is now, or at least 4.8 million cubic miles, the scientists calculated.
"The actual technique that they used (mapping H2O and HDO) is outside my field of expertise, but I would trust investigators at NASA Goddard with this type of analysis," said Mouginis-Mark, who took issue with the illustration (on A19).
"One would not have an ocean on Mars and still retain the North Polar Cap at its present size, but that is a minor point," he said.
In addition to the Mauna Kea facilities, researchers used the European Southern Observatory’s Very Large Telescope in Cerro Paranal, Chile.
The team mapped water and heavy water repeatedly over nearly six Earth years — about three Mars years — producing global snapshots of each as well as their ratio of abundance. The unprecedented maps reveal seasonal changes and areas of small climate variability, even though modern Mars is essentially a desert.
Meanwhile, an instrument aboard the Curiosity rover has found unequivocal evidence of occasional spikes of methane gas in the Martian atmosphere.
The conclusion was based on "an exhaustive analysis of data obtained during 605 soles or Martian days," researchers reported separately in Science.
Mumma and colleagues first reported trace amounts of methane in the Martian atmosphere in 2003, based on studies from the NASA Infrared Telescope Facility. That was backed up a year later by the Canada-France-Hawaii Telescope and the Mars Express satellite.
But such findings have been controversial.
Practically all the existing methane in Earth’s atmosphere originates from biological activity, which has raised expectations that Martian methane could have a similar origin.
