How water escaped from the red planet

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

Shane Stone, a graduate student at the Arizona Lunar and Planetary Laboratory and lead author of a new article 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 administer therapeutic drugs more efficiently, he now studies the chemistry of planetary atmospheres.

Since 2014 he has worked 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 planetarium neighbor.

“We know that billions of years ago, there was liquid water on the surface of Mars,” Stone said. “There must have been a denser atmosphere, so we know that Mars somehow 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. he lost his water. “

Co-authors of the study include Roger Yelle, an Arizona professor of planetary sciences and Stone’s research consultant, as well as NASA researchers Goddard Space Flight Center and the Center for Research and Exploration in Space Science and Technology in Maryland.

Looking for 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 present in the atmosphere.

Past observations using MAVEN and the Hubble Space Telescope have shown that water loss 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 the summer in the southern Martian hemisphere.

Stone and his team found that as Mars is closer to the sun, the planet warms up and more water, which is on the surface in the form of ice, moves from the surface to the upper atmosphere where it is lost in space. This happens once every Martian year or roughly every two Earth years. Regional dust storms that occur on Mars every Martian year and global dust storms that occur around the planet about once every 10 years lead to further warming of the atmosphere and a surge in upward movement of water.

The processes that make this cyclical motion possible contradict the classic picture of water leaking out of Mars, showing that it is incomplete, Stone said. According to the classical process, frozen water is converted into a gas and is destroyed by the sun’s rays in the lower atmosphere. This process, however, would manifest as a slow and steady dripping, unaffected by the seasons or sandstorms, which does not fit current observations.

“This is important because we didn’t 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 close to the surface due to something called a hygropause. It is just a layer in the atmosphere that is cold enough to condense (and thus stop) the water vapor traveling to. the top “.

The team argues that the water is moving beyond what is supposed to be Mars’ hygro pause, which is probably too hot to stop water vapor. Once in the upper atmosphere, the water molecules are destroyed by the ions very quickly – within four hours, they calculate – and the by-products are then lost to space.

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

An arid and dusty world

When the team extrapolated their findings back 1 billion years, they found that this process may explain the loss of a global ocean about 17 inches deep.

“If we took the water and distributed it evenly across the surface of Mars, that ocean of water lost to space due to the new process we describe would be more than 17 inches deep,” Stone said. “Another 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 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, or 687 Earth days.

And while Stone and his team can’t extrapolate more than 1 billion years back, he thinks that this process probably didn’t work the same way before, because Mars may have had a stronger hygrouse a long time ago.

“Before the process we describe began to work, there must have already been a significant amount of atmospheric escape into space,” Stone said. “We still need to define 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 a significant role,” Stone said. “As a former synthetic organic chemist, I am eager to investigate these processes.”