Mars once had oceans but is now bone-dry, leaving many to wonder how the water
was lost. University of Arizona researchers have discovered a surprisingly
large amount of water in the upper atmosphere of Mars, where it is rapidly
destroyed, explaining part of this Martian mystery.
Shane Stone, a graduate student in the UArizona Lunar and Planetary
Laboratory and lead author of a new paper to be published in Science,
describes himself as a planetary chemist. Once a laboratory chemist who
helped to develop polymers that could be used to wrap and deliver
therapeutic drugs more efficiently, he now studies the chemistry of
planetary atmospheres.
Since 2014, he has worked on NASA's MAVEN mission, short for Mars Atmosphere
and Volatile Evolution. The MAVEN spacecraft began orbiting Mars in 2014 and
has been recording the composition of the upper atmosphere of Earth's
planetary neighbor ever since.
"We know that billions of years ago, there was liquid water on the surface
of Mars," Stone said. "There must have been a thicker atmosphere, so we know
that Mars somehow lost the majority of its atmosphere to space. MAVEN is
trying to characterize the processes responsible for this loss, and one
portion of that is understanding exactly how Mars lost its water."
Co-authors of the study include Roger Yelle, a UArizona planetary sciences
professor and Stone's research adviser, as well as researchers from NASA
Goddard Space Flight Center and the Center for Research and Exploration in
Space Science and Technology in Maryland.
Watching for Water
As MAVEN orbits Mars, it dips into the planet's atmosphere every 4 1/2
hours. The onboard NGIMS instrument – short for Neutral Gas and Ion Mass
Spectrometer – has been measuring the abundance of charged water molecules
called ions in the upper Martian atmosphere, about 100 miles from the
planet's surface. From this information, scientists can infer how much water
is present in the atmosphere.
Past observations using MAVEN and the Hubble Space Telescope showed that
loss of water from the Martian upper atmosphere varies with the seasons.
Compared to Earth, Mars takes a more oval-shaped path around the sun and is
closest to it during summer in the Martian southern hemisphere.
Stone and his team found that when Mars is nearest the sun, the planet
warms, and more water – found on the surface in the form of ice – moves from
the surface to the upper atmosphere where it is lost to space. This happens
once every Martian year or about every two Earth years. The regional dust
storms that occur on Mars every Martian year and the global dust storms that
occur across the planet about once every 10 years lead to further heating of
the atmosphere and a surge in the upward movement of water.
The processes that make this cyclical movement possible contradict the
classical picture of water escape from Mars, showing it is incomplete, Stone
said. According to the classical process, ice formed from water is converted
to a gas and is destroyed by the sun's rays in the lower atmosphere. This
process, however, would play out as a slow, steady trickle, unaffected by
the seasons or dust storms, which doesn't mesh with current observations.
"This is important because we didn't expect to see any water in the upper
atmosphere of Mars at all," Stone said. "If we compare Mars to Earth, water
on Earth is confined close to the surface because of something called the
hygropause. It's just a layer in the atmosphere that's cold enough to
condense (and therefore stop) any water vapor traveling upward."
The team argues that water is moving past what should be Mars' hygropause,
which is likely too warm to stop the water vapor. Once in the upper
atmosphere, water molecules are broken apart by ions very quickly – within
four hours, they calculate – and the byproducts are then lost to space.
"The loss of its atmosphere and water to space is a major reason Mars is
cold and dry compared to warm and wet Earth. This new data from MAVEN
reveals one process by which this loss is still occurring today," Stone
said.
A Dry and Dusty World
When the team extrapolated their findings back 1 billion years, they found
that this process can account for the loss of a global ocean about 17 inches
deep.
"If we took water and spread it evenly over the entire surface of Mars, that
ocean of water lost to space due to the new process we describe would be
over 17 inches deep," Stone said. "An additional 6.7 inches would be lost
due solely to the effects of global dust storms."
During global dust storms, 20 times more water can be transported to the
upper atmosphere. For example, one global dust storm lasting 45 days
releases the same amount of water to space as Mars would lose during a calm
Martian year, or 687 Earth days.
And while Stone and his team can't extrapolate farther back than 1 billion
years, he thinks that this process likely didn't work the same before that,
because Mars might have had a stronger hygropause long ago.
"Before the process we describe began to operate, there must have been a
significant amount of atmospheric escape to space already," Stone said. "We
still need to nail down the impact of this process and when it began to
operate."
In the future, Stone would like to study the atmosphere of Saturn's moon
Titan.
"Titan has an interesting atmosphere in which organic chemistry plays a
significant role," Stone said. "As a former synthetic organic chemist, I'm
eager to investigate these processes."
Reference:
Shane W. Stone, Roger V. Yelle, Mehdi Benna, Daniel Y. Lo, Meredith K.
Elrod, Paul R. Mahaffy. Hydrogen escape from Mars is driven by seasonal and
dust storm transport of water. Science, 2020 DOI:
10.1126/science.aba5229