Surprising discovery in the atmosphere of the red planet helps explain the Martian mystery


This artist’s concept represents the early Martian environment (right) – believed to contain liquid water and a thicker atmosphere – compared to the cold, dry environment seen on Mars today (left). Credit: NASA Goddard Space Flight Center

New research from the University of Arizona updates our understanding of how water escapes Mars – not like a leaky faucet but with a sudden splash.

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, a graduate student of the UAri’s Lunar and Planetary Laboratory and lead author of a new forthcoming article 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 NASAThe MAVEN mission, abbreviation of 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 must have been a thicker atmosphere, so we know that Mars lost most 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 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 – measured 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 The Hubble Space Telescope has 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 found 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, the ice formed from water 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, the water on Earth is confined near the surface because of what is called hygropause. It’s just a layer in the atmosphere that’s cold enough to condense (and therefore stop) any rising water vapor.

The team argues that the water is moving 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 byproducts 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 Earth’s hot and humid. 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 across the 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. . “An additional 6.7 inches would be lost solely due to the effects of global dust storms.”

During global dust storms, 20 times more water can be carried to 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 over a billion years ago, he believes this process probably didn’t work the same way before that, as Mars may have had a stronger hygropause ago. long time.

“Before the process we are describing begins 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 SaturnIt is the Titan moon.

“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.”

Reference: “Hydrogen Escape from Mars Due to Seasonal Transport and Dust Storm” by Shane W. Stone, Roger V. Yelle, Mehdi Benna, Daniel Y. Lo, Meredith K. Elrod and Paul R Mahaffy, November 13, 2020, Science.
DOI: 10.1126 / science.aba5229


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