"There are only two places water can go. It can freeze into the ground, or the water molecule can break into atoms, and the atoms can escape from the top of the atmosphere into space," said John Clarke, study leader from the Center for Space Physics at Boston University. "To understand how much water there was and what happened to it, we need to understand how the atoms escape into space."
Clarke's team used data from Hubble and MAVEN to assess the current escape rate of hydrogen atoms into space, which enabled them to trace the escape process back through time, offering insights into Mars' water history.
Hydrogen and Deuterium: Clues to Water Loss
Water molecules in Mars' atmosphere break apart due to sunlight, producing hydrogen and oxygen atoms. The team measured hydrogen and deuterium, a form of hydrogen with an additional neutron, giving it double the mass of regular hydrogen. This added mass causes deuterium to escape more slowly into space than hydrogen.
Over time, Mars has lost more hydrogen than deuterium, resulting in a higher deuterium-to-hydrogen ratio. By measuring this ratio, scientists can estimate how much water existed during Mars' warmer, wetter periods. By examining current escape rates, researchers can infer the processes that influenced water loss over the past 4 billion years.
Although MAVEN provided most of the data, it was not always able to detect deuterium emissions during the Martian winter due to Mars' elliptical orbit, which takes it far from the Sun. Clarke and his team turned to Hubble to fill in gaps in the data and complete a picture of hydrogen and deuterium escape over a full Martian year - 687 Earth days. Hubble also provided historical data stretching back to 1991, prior to MAVEN's arrival at Mars in 2014.
Together, Hubble and MAVEN offered the first comprehensive view of hydrogen atoms escaping Mars into space.
Unraveling the Martian Atmosphere
"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," explained Clarke. "The whole atmosphere is very turbulent, heating up and cooling down on short timescales, even down to hours. The atmosphere expands and contracts as the brightness of the Sun at Mars varies by 40 percent over the course of a Martian year."
The team observed that escape rates for hydrogen and deuterium increase dramatically when Mars nears the Sun. Previously, scientists thought these atoms drifted slowly upward through the atmosphere before escaping. However, this new data reveals that the atmospheric conditions fluctuate rapidly. As Mars approaches the Sun, water molecules rise quickly, releasing hydrogen and deuterium atoms at higher altitudes.
Another critical finding was that the rapid changes in hydrogen and deuterium escape rates require additional energy to explain. At the upper atmosphere's temperature, only a small fraction of atoms move fast enough to escape Mars' gravity. These super-thermal atoms are created when they receive an extra energy boost - either through collisions with solar wind protons or chemical reactions triggered by sunlight.
Implications Beyond Mars
Understanding Mars' water loss is key to comprehending not only the evolution of planets within our solar system but also those orbiting other stars. As astronomers discover more Earth-sized planets, Mars offers a crucial analog for understanding conditions on distant worlds. Mars, Earth, and Venus, all within or near our solar system's habitable zone, have vastly different atmospheres today. Studying Mars and its atmospheric history offers vital insights into planetary evolution across the galaxy.
Research Report:Martian atmospheric hydrogen and deuterium: Seasonal changes and paradigm for escape to space
Related Links
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