The history of water on Mars is important for understanding the evolution of planets like Earth. Water escapes into space as atoms, but hydrogen (H) atoms escape faster than deuterium (D) (hydrogen atoms with a neutron in their nucleus), increasing the residual D/H ratio. The current ratio reflects the total amount of water Mars has lost.

There is ample evidence that Mars experienced an early wet period when liquid water flowed across the surface, leaving distinct erosion patterns and the presence of clay in the topsoil.

This wet climate period is thought to have ended over 3 billion years ago, and the fate of that water has attracted considerable interest.

As Mars cooled, some of the water remained trapped in the crust, some broke down into hydrogen and oxygen atoms, and many of the atoms escaped into space through the upper atmosphere.

“There are only two places water can go: it freezes to the ground, or the water molecules break down into atoms and those atoms escape through the top of the atmosphere into space,” said Dr John Clark, a researcher at Boston University.

“To understand how much water there was and what became of it, we need to understand how the atoms escaped into space.”

In the new study, Dr Clark and his colleagues combined data from NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) and the NASA/ESA Hubble Space Telescope to measure how many hydrogen atoms are escaping into space and the current rate of escape.

This information allowed the researchers to infer past rates of water escape and understand the history of water on Mars.

Specifically, the researchers measured hydrogen and its heavier isotope, deuterium.

Over time, more hydrogen than deuterium was lost, increasing the D/H ratio in the atmosphere.

Measuring this ratio today can give scientists clues about how much water may have been present on Mars during its warmer, wetter periods.

By studying how these atoms escape in the present, we can understand the processes that determined escape rates over the past 4 billion years and extrapolate back in time.

Most of the data comes from MAVEN, but the spacecraft is not sensitive enough to observe deuterium emissions throughout the entire Martian year.

Unlike Earth, Mars is farther from the Sun in its elliptical orbit during its long winters, making its deuterium emissions weaker.

The authors needed Hubble data to fill in the gaps and complete a three-Martian year (687 Earth days) annual cycle.

The Hubble Space Telescope also provided additional data going back to 1991, before MAVEN arrived at Mars in 2014.

Combining data from these missions provided the first complete picture of hydrogen atoms escaping Mars into space.

“In recent years, scientists have discovered that the annual cycle of Mars is much more dynamic than people would have expected 10 or 15 years ago,” Dr Clark said.

“The whole atmosphere is very turbulent, heating and cooling on short timescales of a few hours.”

“The brightness of the Sun on Mars varies by 40 percent over the course of a Martian year, causing the atmosphere to expand and contract.”

The team found that the rate at which hydrogen and deuterium are released changes dramatically as Mars gets closer to the Sun.

The classical view that scientists had until now was that these atoms would slowly diffuse upwards through the atmosphere until they reached a height where they could escape.

But that picture no longer accurately reflects the whole picture, because scientists now know that atmospheric conditions change very rapidly.

As Mars approaches the Sun, water molecules, the source of hydrogen and deuterium, rise rapidly through the atmosphere and release atoms at high altitudes.

The second discovery is that the transformation of hydrogen and deuterium is so rapid that the escape of the atoms requires additional energy to account for it.

At the temperatures of the upper atmosphere, very few atoms would be fast enough to escape Mars’ gravity.

When something gives atoms extra energy, faster (super hot) atoms are created.

These phenomena include the impact of solar wind protons entering the atmosphere and sunlight causing chemical reactions in the upper atmosphere.

of Survey results Published in the journal Scientific advances.

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John T. Clark others2024. Hydrogen and deuterium in the Martian atmosphere: seasonal changes and a paradigm for escape into space. Scientific advances 10(30);doi: 10.1126/sciadv.adm7499

This article is based on a NASA press release.