Billions of years ago, the Red Planet was far more blue; according to
evidence still found on the surface, abundant water flowed across Mars and
forming pools, lakes, and deep oceans. The question, then, is where did all
that water go?
The answer: nowhere. According to new research from Caltech and JPL, a
significant portion of Mars's water—between 30 and 99 percent—is trapped
within minerals in the planet's crust. The research challenges the current
theory that the Red Planet's water escaped into space.
The Caltech/JPL team found that around four billion years ago, Mars was home
to enough water to have covered the whole planet in an ocean about 100 to
1,500 meters deep; a volume roughly equivalent to half of Earth's Atlantic
Ocean. But, by a billion years later, the planet was as dry as it is today.
Previously, scientists seeking to explain what happened to the flowing water
on Mars had suggested that it escaped into space, victim of Mars's low
gravity. Though some water did indeed leave Mars this way, it now appears
that such an escape cannot account for most of the water loss.
"Atmospheric escape doesn't fully explain the data that we have for how much
water actually once existed on Mars," says Caltech PhD candidate Eva
Scheller (MS '20), lead author of a paper on the research that was published
by the journal Science on March 16 and presented the same day at the Lunar
and Planetary Science Conference (LPSC). Scheller's co-authors are Bethany
Ehlmann, professor of planetary science and associate director for the Keck
Institute for Space Studies; Yuk Yung, professor of planetary science and
JPL senior research scientist; Caltech graduate student Danica Adams; and
Renyu Hu, JPL research scientist. Caltech manages JPL for NASA.
The team studied the quantity of water on Mars over time in all its forms
(vapor, liquid, and ice) and the chemical composition of the planet's
current atmosphere and crust through the analysis of meteorites as well as
using data provided by Mars rovers and orbiters, looking in particular at
the ratio of deuterium to hydrogen (D/H).
Water is made up of hydrogen and oxygen: H2O. Not all hydrogen atoms are
created equal, however. There are two stable isotopes of hydrogen. The vast
majority of hydrogen atoms have just one proton within the atomic nucleus,
while a tiny fraction (about 0.02 percent) exist as deuterium, or so-called
"heavy" hydrogen, which has a proton and a neutron in the nucleus.
The lighter-weight hydrogen (also known as protium) has an easier time
escaping the planet's gravity into space than its heavier counterpart.
Because of this, the escape of a planet's water via the upper atmosphere
would leave a telltale signature on the ratio of deuterium to hydrogen in
the planet's atmosphere: there would be an outsized portion of deuterium
left behind.
However, the loss of water solely through the atmosphere cannot explain both
the observed deuterium to hydrogen signal in the Martian atmosphere and
large amounts of water in the past. Instead, the study proposes that a
combination of two mechanisms—the trapping of water in minerals in the
planet's crust and the loss of water to the atmosphere—can explain the
observed deuterium-to-hydrogen signal within the Martian atmosphere.
When water interacts with rock, chemical weathering forms clays and other
hydrous minerals that contain water as part of their mineral structure. This
process occurs on Earth as well as on Mars. Because Earth is tectonically
active, old crust continually melts into the mantle and forms new crust at
plate boundaries, recycling water and other molecules back into the
atmosphere through volcanism. Mars, however, is mostly tectonically
inactive, and so the "drying" of the surface, once it occurs, is permanent.
"Atmospheric escape clearly had a role in water loss, but findings from the
last decade of Mars missions have pointed to the fact that there was this
huge reservoir of ancient hydrated minerals whose formation certainly
decreased water availability over time," says Ehlmann.
"All of this water was sequestered fairly early on, and then never cycled
back out," Scheller says. The research, which relied on data from
meteorites, telescopes, satellite observations, and samples analyzed by
rovers on Mars, illustrates the importance of having multiple ways of
probing the Red Planet, she says.
Reference:
Long-term drying of Mars by sequestration of ocean-scale volumes of water in
the crust. E. L. Scheller et al., Science DOI:
10.1126/science.abc7717
No comments:
Post a comment