Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft observed that the ups and downs of hydrogen escape is the reason that water is lost on Mars.
After investigating the upper atmosphere of the red planet for a full martian year, NASA's MAVEN mission has determined that the escaping water does not always go gently into space. Sophisticated measurements made by a suite of instruments on the Mars Atmosphere and Volatile Evolution, or MAVEN, spacecraft showed the ups and downs of hydrogen escape - and therefore water loss.
The escape rate peaked when Mars was at its closest point to the Sun and dropped off when the planet was farthest from the Sun. The rate of loss varied dramatically overall, with 10 times more hydrogen escaping at the maximum. "MAVEN is giving us unprecedented detail about hydrogen escape from the upper atmosphere of Mars, and this is crucial for helping us figure out the total amount of water lost over billions of years," said Ali Rahmati, a MAVEN team member at the University of California at Berkeley who analysed data from two of the spacecraft's instruments. Hydrogen in Mars' upper atmosphere comes from water vapour in the lower atmosphere.
An atmospheric water molecule can be broken apart by sunlight, releasing the two hydrogen atoms from the oxygen atom that they had been bound to. Several processes at work in Mars' upper atmosphere may then act on the hydrogen, leading to its escape. This loss had long been assumed to be more-or-less constant, like a slow leak in a tire. However, previous observations made using NASA's Hubble Space Telescope and ESA's Mars Express orbiter found unexpected fluctuations.
Only a handful of these measurements have been made so far, and most were essentially snapshots, taken months or years apart. MAVEN has been tracking the hydrogen escape without interruption over the course of a Martian year, which lasts nearly two Earth years. "Now that we know such large changes occur, we think of hydrogen escape from Mars less as a slow and steady leak and more as an episodic flow - rising and falling with season and perhaps punctuated by strong bursts," said Michael Chaffin, a scientist at the University of Colorado at Boulder who is on the Imaging Ultraviolet Spectrograph (IUVS) team.
In the most detailed observations of hydrogen loss to date, four of MAVEN's instruments detected the factor-of-10 change in the rate of escape. Changes in the density of hydrogen in the upper atmosphere were inferred from the flux of hydrogen ions - electrically charged hydrogen atoms - measured by the Solar Wind Ion Analyzer and by the Suprathermal and Thermal Ion Composition instrument. IUVS observed a drop in the amount of sunlight scattered by hydrogen in the upper atmosphere. MAVEN's magnetometer found a decrease in the occurrence of electromagnetic waves excited by hydrogen ions, indicating a decrease in the amount of hydrogen present. By investigating hydrogen escape in multiple ways, the MAVEN team will be able to work out which factors drive the escape.
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