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?
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Some of it disappeared into space. Water molecules, pummeled
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 percent 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 Tuesday 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 100m to 1,500m.
The most likely depth was about 609m, 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 20m to 40m deep, but that is mostly frozen in the polar ice caps.
Planetary scientists have long marveled 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 5,000 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 Center 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,” Geronimo Villanueva, one of the NASA scientists who performed the
earlier deuterium measurements, wrote in an email. “This opens the possibility
for an even wetter past, and that rocks on Mars now hold more water than we
initially thought.”
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