Mars was once wet, with an ocean's worth of water on its surface.
Today, most of Mars is as dry as a desert except for ice deposits in its polar regions. Where did the rest of the water go?
Some of it disappeared into space. Water molecules, pummelled by particles of solar wind, broke apart into hydrogen and oxygen atoms, and those, especially the lighter hydrogen atoms, sped out of the atmosphere, lost to outer space.
But most of the water, a new study concludes, went down, sucked into the red planet's rocks. And there it remains, trapped within minerals and salts. Indeed, as much as 99 per cent of the water that once flowed on Mars could still be there, the researchers estimated in a paper published this week in the journal Science.
Data from the past two decades of robotic missions to Mars, including Nasa's Curiosity rover and the Mars Reconnaissance Orbiter, showed a wide distribution of what geologists call hydrated minerals.
"It became very, very clear that it was common and not rare to find evidence of water alteration," said Bethany Ehlmann, a professor of planetary science at the California Institute of Technology and one of the authors of the paper.
Ehlmann, speaking at a news briefing on Tuesday (Wednesday NZT) at the Lunar and Planetary Science conference, said that as the rocks are altered by liquid water, water molecules become incorporated into minerals like clays. "Water is effectively trapped into the crust," she said.
To get a sense of the amount of water, planetary scientists talk about a "global equivalent layer" — that is, if Mars were smoothed out into a uniform, featureless ball, how deep would the water have been?
The scientists estimated that the depth would have been 100 to 1500m.
The most likely depth was about 610m, they said, or roughly one-fourth as much water as is in the Atlantic Ocean.
The data and simulations also indicated that the water was almost all gone by 3 billion years ago, around the time on Earth when life consisted of single-cell microbes in the oceans.
"This means that Mars has been dry for quite a long time," said Eva Scheller, a Caltech graduate student who was the lead author of the Science paper.
Today, there is still water equivalent to a global ocean 20-40m deep, but that is mostly frozen in the polar ice caps.
Planetary scientists have long marvelled at ancient evidence of flowing water carved in the Martian surface — gigantic canyons, tendrils of winding river channels and deltas where the rivers disgorged sediments into lakes.
Nasa's latest robotic Mars explorer, Perseverance, which landed last month in the Jezero crater, will be headed to a river delta at its edge in hopes of finding signs of past life.
Without a time machine, there is no way to observe directly how much water was on a younger Mars more than 3 billion years ago. But the hydrogen atoms floating today in the atmosphere of Mars preserve a ghostly hint of the ancient ocean.
On Earth, about 1 in 5000 hydrogen atoms is a version known as deuterium that is twice as heavy because its nucleus contains both a neutron and a proton. (The nucleus of a common-variety hydrogen atom has only a proton, no neutrons.)
But on Mars, the concentration of deuterium is markedly higher, about 1 in 700. Scientists at the Nasa Goddard Space Flight Centre who reported this finding in 2015 said this could be used to calculate the amount of water Mars once had.
Mars probably started with a similar ratio of deuterium to hydrogen as Earth, but the fraction of deuterium increased over time as the water evaporated and hydrogen was lost to space, because the heavier deuterium is less likely to escape the atmosphere.
The problem with that story, said Renyu Hu, a scientist at Nasa's jet propulsion laboratory and another author of the current Science paper, is that Mars has not been losing hydrogen fast enough.
Measurements by Nasa's Mars Atmosphere and Volatile Evolution orbiter, or MAVEN, have showed that the current rate, extrapolated over 4 billion years, "can only account for a small fraction of the water loss", Hu said. "This is not enough to explain the great drying of Mars."
That led to the new research concluding that a great majority of water went into the rocks.
"This is a very interesting new study in which many processes are combined to provide alternative scenarios for the fate of water on Mars," said Geronimo Villanueva, one of the Nasa scientists who performed the earlier deuterium measurements.
"This opens the possibility for an even wetter past, and that rocks on Mars now hold more water than we initially thought."
Written by: Kenneth Chang
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