How water fled from the red planet

Mars once had oceans but is now dry, leaving many to wonder how the water was lost. Researchers at the University of Arizona have discovered a surprisingly large amount of water in Mars’ upper atmosphere, where it is quickly destroyed, explaining part of this Martian mystery.

Shane Stone, graduate student of the UArizione Lunar and Planetary Laboratory and lead author of a new paper published today in Science, describes himself as a planetary chemist. Once a laboratory chemist who helped develop polymers that could be used to wrap and deliver therapeutic drugs more effectively, he is now studying the chemistry of planetary atmospheres.

Since 2014, he has been working on the NASA MAVEN mission, short for Mars Atmosphere and Volatile EvolutioN. The MAVEN spacecraft began orbiting Mars in 2014 and has since recorded the composition of the upper atmosphere of Earth’s planetary neighbor.

“We know that billions of years ago there was liquid water on the surface of Mars,” Stone said. “There had to be a thicker atmosphere, so we know that Mars lost the majority of its atmosphere in space. MAVEN is trying to characterize the processes responsible for this loss, and part of that is understanding exactly how Mars got there. lost its water. “

Study co-authors include Roger Yelle, Arizona professor of planetary science and Stone’s research advisor, as well as researchers from NASA’s Goddard Space Flight Center and the Center for Research and Exploration in Space Science. and Technology in Maryland.

Monitor the water

As MAVEN orbits Mars, it dives into the planet’s atmosphere every 4.5 hours. The on-board NGIMS instrument, short for Neutral Gas and Ion Mass Spectrometer, measures 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 deduce how much water is in the atmosphere.

Past observations using MAVEN and the Hubble Space Telescope have shown that water loss from the upper Martian atmosphere varies with the seasons. Compared to Earth, Mars takes a more oval path around the sun and comes closest to it in summer in the southern Martian hemisphere.

Stone and his team discovered that when Mars is closest to the sun, the planet heats up and more water – found on the surface as ice – moves from the surface to the upper atmosphere where it is lost in the ‘space. This happens once in every Martian year or about every two Earth years. Regional dust storms that occur on Mars each Martian year and global dust storms that occur on the planet about once every 10 years cause further warming of the atmosphere and increased upward movement of water. .

The processes that make this cyclical movement possible contradict the classic image of water escaping from Mars, showing that it is incomplete, Stone said. In the classical process, water ice is converted into gas and destroyed by the rays of the sun in the lower atmosphere. This process, however, would play out like a slow, steady trickle, unaffected by seasons or dust storms, which does not match current observations.

“This is important because we did not expect to see water in the upper atmosphere of Mars at all,” Stone said. “If we compare Mars to Earth, water on Earth is confined near the surface because of something called hygropause. It’s just a layer in the atmosphere that is cold enough to condense (and therefore stop) any rising water vapor. “

The team argues that the water is passing past what is expected to be the Mars hygropause, which is likely too hot to stop water vapor. Once in the upper atmosphere, water molecules are broken down by ions very quickly – within four hours, they calculate – and the by-products are then lost into space.

“The loss of its atmosphere and water in space is one of the main reasons Mars is cold and dry compared to the hot and humid Earth. This new data from MAVEN reveals a process by which this loss still occurs today, ”Stone said.

A dry and dusty world

When the team extrapolated their findings back a billion years, they found that this process could explain the loss of a global ocean about 17 inches deep.

“If we took water and spread it evenly over the entire surface of Mars, this ocean of water lost in space due to the new process we are describing would be over 17 inches deep,” Stone said. . “A further 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 carried into the upper atmosphere. For example, a 45-day global dust storm releases the same amount of water into space that Mars would lose in a calm Martian year – 687 Earth days.

And while Stone and his team can’t extrapolate more than a billion years ago, he believes this process probably didn’t work the same way before that, as Mars could have had a stronger hygropause ago. long time.

“Before the process we are describing starts to work, there must have already been a significant atmospheric leak to space,” Stone said. “We have yet to determine the impact of this process and when it started to work.”

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 an important role,” Stone said. “As a former synthetic organic chemist, I look forward to studying these processes.”