The surface features of Mars suggest that it was once a wet planet. Scientists know some water went underground, but what happened to the rest?
NASA‘s Hubble Space Telescope and MAVEN mission are working to solve this mystery. Mars’s water can freeze into the ground or break into atoms, which then escape into space. We need to understand how these atoms leave the planet to determine how much water Mars had and what happened to it.
Using data from Hubble and MAVEN, scientists measured the number and current escape rate of the hydrogen atoms escaping into space. This information helped scientists estimate how fast water was escaping over time, allowing them to understand the history of water on Mars.
Water molecules in Mars’ atmosphere break apart in sunlight into hydrogen and oxygen atoms. Scientists measure hydrogen and deuterium, a heavier form of hydrogen with an extra neutron. Because deuterium is heavier, it escapes into space more slowly than regular hydrogen. As more hydrogen is lost, the ratio of deuterium to hydrogen increases.
By measuring this ratio, scientists can estimate how much water Mars had in its warmer, wetter past. They use current escape rates to understand these processes over the last four billion years.
Reflected sunlight from Mars at these wavelengths shows scattering by atmospheric molecules and haze, while the polar ice caps and some surface features are also visible. Hubble and MAVEN showed that Martian atmospheric conditions change very quickly. When Mars is close to the Sun, water molecules rise very rapidly through the atmosphere, breaking apart and releasing atoms at high altitudes.
Credits
Science
NASA, ESA, STScI, John T. Clarke (Boston University)
While MAVEN provides most of the data, it can’t always detect deuterium because of seasonal changes on Mars. To fill in these gaps and cover a full Martian year (687 Earth days), Clarke and his team used additional data from the Hubble Space Telescope, which also provided information from before MAVEN arrived in 2014.
This combined data offers the first holistic view of hydrogen atoms escaping Mars into space.
Study leader John Clarke of the Center for Space Physics at Boston University in Massachusetts said, “In recent years, scientists have found that Mars has an annual cycle that is much more dynamic than people expected 10 or 15 years ago. The atmosphere is very turbulent, heating up and cooling down on short timescales, even hours. The atmosphere expands and contracts as the brightness of the Sun at Mars varies by 40 percent over a Martian year.”
The team found that the rates at which hydrogen and deuterium escape from Mars change quickly when the planet is close to the Sun. Previously, scientists thought these atoms slowly moved upward through the atmosphere before escaping.
However, new findings show that water molecules rise rapidly when Mars is near the Sun, releasing hydrogen and deuterium at higher altitudes. The escape rates are so fast that atoms need extra energy to break free. In the upper atmosphere, most atoms don’t have enough speed to escape Mars’ gravity. Extra energy from sources like solar wind or sunlight-driven chemical reactions speeds up these atoms, allowing them to escape.
Studying Mars’ water history is key to understanding our solar system and Earth-sized planets around other stars. As astronomers discover more of these planets, detailed study is challenging. Mars, along with Earth and Venus, is in or near the habitable zone of our solar system, where liquid water could exist. Despite their similar location, these planets have very different conditions today. Scientists can better understand the nature of distant planets throughout our galaxy by studying Mars.
Journal Reference:
- John Clarke, Majd Mayyasi et al. Martian atmospheric hydrogen and deuterium: Seasonal changes and paradigm for escape to space. Science Advances. DOI: 10.1126/sciadv.adm7499