Mars leaks faster when it is closer to the sun

Seasonal changes can have a dramatic impact on how quickly Mars loses its water to space, a joint study by Hubble Space Telescope and NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission.

Over three billion years ago, Mars was warm and humid, with large amounts of water on its surface and a thicker atmosphere. Today, however, Mars is desolate, cold and dry. So what happened to all that water?

“There are only two places the water can go,” said John Clarke of Boston University in a opinion. “It can freeze in the ground, or the water molecules can break down into atoms and the atoms can escape from the atmosphere into space.”

Much Water on Mars is still on the Red Planet. Huge reservoirs seem to be sealed deep underground at depths between 11.5 and 20 kilometers (7.1 and 12.4 miles). There is enough water inside Mars for a global equivalent layer (GEL, which essentially indicates how deep a planet-wide ocean would be) between 1 and 2 kilometers (0.62 and 1.24 miles).

Related: Mars rock samples show traces of water in the Jezero crater – could there once have been life here?

Relatively small amounts of water ice are also trapped in the shallow permafrost and polar caps of Mars. During the Martian summer, this ice can sublimate and release water vapor into the atmosphere. Most of this water vapor circulates from pole to pole and freezes in the hemisphere where it is winter, but some finds itself in the upper atmosphere, where ultraviolet sunlight can photodissociate H2O water molecules, breaking them down into their constituent atoms. The oxygen in the water either oxidizes materials on the surface (which is why Mars appears rust-red) or combines with carbon to form carbon dioxide. Meanwhile, the hydrogen atoms (or their heavier isotopic counterpart, deuterium) can escape into space (if they are energetic enough to reach escape velocity) and travel with the Solar wind.

MAVEN, which reached Mars in 2014, is tasked with measuring this hydrogen leak.

Compare the thickness of Mars' atmosphere and its water loss from perihelion to aphelion in these Hubble Space Telescope images of the Red Planet. (Image credit: NASA/ESA/STScI/John T. Clarke (Boston University).)

Because deuterium, a heavy form of hydrogen, does not easily escape the Martian atmosphere, the ratio of deuterium to hydrogen (D/H) in the Martian atmosphere is critical. The proportion of deuterium to hydrogen increases over time because Mars loses hydrogen at a faster rate. Earth and Mars probably have acquired their water According to the same sources, the original D/H ratio of water on Mars 3 to 4 billion years ago would have been the same as on Earth today. The current D/H ratio on Mars is about 8 to 10 times higher than on Earth. The measurements are not entirely conclusive, but by comparing this original water ratio on Mars with today's ratio and taking into account the rate of hydrogen and deuterium loss to space, it is possible to extrapolate backwards and calculate how much water Mars has probably lost over its history.

Based on MAVEN's previous observations, Mars has lost enough water to space to form a GEL ten to one hundred meters deep. Combined with the huge amount of water recently found inside Mars, this suggests that the Red Planet was rich in water in its distant past.

However, MAVEN, with the help of the Hubble Space Telescope, has now discovered unexpected details of the history of water loss on Mars. Together, the instruments have shown that the rate of hydrogen loss is seasonal, with large increases in the escape rate at perihelion, the closest point to Mars in its orbit around the sunThis coincides with a strong rise of water vapor into the middle atmosphere caused by seasonal warming. At perihelion, Mars' southern hemisphere is tilted toward the Sun and the Red Planet is decelerated by its annual Dust storm Season; airborne dust can contribute to atmospheric warming and water vapor content.

At perihelion, MAVEN measured densities of deuterium and hydrogen in the upper atmosphere that are about 5 and 20 times higher, respectively, than at aphelion, Mars' farthest point from the Sun in its elliptical (elongated, not circular) orbit. At aphelion, deuterium loss is so small that MAVEN is not even sensitive enough to detect it. This is where the Hubble Space Telescope has to step in and fill in the gaps. The observations also showed that escape rates for deuterium and hydrogen are 10 to 100 times higher at perihelion than at aphelion. In fact, both deuterium and hydrogen escape so quickly at perihelion that they are limited only by the amount of water vapor present in the atmosphere.

“In recent years, scientists have discovered that Mars' annual cycle is much more dynamic than anyone expected 10 or 15 years ago,” Clarke said. “The whole atmosphere is very turbulent, heating up and cooling down over short periods of time, even hours. The atmosphere expands and contracts as the brightness of the Sun on Mars varies by 40% over the course of a Martian year.”

This poses a puzzle when it comes to explaining the deuterium loss, which appears to be greater than what would be expected from normal thermal escape alone, where a deuterium atom is warm enough to have the energy needed to escape into space. To increase the deuterium loss rate to match the observed D/H ratio on Mars, extra energy must be being introduced into the atmosphere somewhere. This could come from protons from the solar wind entering the atmosphere and colliding with deuterium atoms, or from chemical reactions caused by ultraviolet sunlight, which can give the deuterium an extra boost.

The results were published on July 26 in the journal Science Advances.

Mars leaks faster when it is closer to the sun

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