UI research contributing to latest findings
Wednesday, October 19, 2016

University of Iowa scientists have contributed to the latest findings that shed new light on what may have happened to the water 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 revealed 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 decreased when the planet was at its farthest. The rate of loss varied dramatically overall, with 10 times more hydrogen escaping at the maximum than the minimum.

Jasper Halekas, a MAVEN team member from the UI and the instrument lead for the Solar Wind Ion Analyzer (SWIA), headed one of four studies that revealed the pronounced variability in hydrogen escape.

“The new MAVEN measurements are painting a remarkably comprehensive picture of the hydrogen cycle at Mars, which we had only caught a few brief glimpses of before MAVEN’s arrival,” says Halekas, corresponding author on one study forthcoming from the Journal of Geophysical Research.

SWIA measures the byproducts from reactions occurring when the solar wind strikes hydrogen atoms in Mars’ upper atmosphere. The instrument was designed primarily to measure the incoming solar wind, but due to its high sensitivity and resolution, it also can measure Martian hydrogen.

“I’m personally very gratified, and I’m proud of the team that made the MAVEN mission and the SWIA instrument a reality,” says Halekas, associate professor in the UI Department of Physics and Astronomy. “It just goes to show that if you design and build the best scientific experiments you can, you maximize your opportunity to make unexpected and exciting discoveries.”

Hydrogen in Mars’ upper atmosphere comes from water vapor in the lower atmosphere. An atmospheric water molecule can be broken apart by sunlight, releasing the two hydrogen atoms from the oxygen atom 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. But 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.

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. Researchers then inferred changes in the density of hydrogen in the upper atmosphere 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. The Imaging Ultraviolet Spectrograph (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 that escape. Scientists already know that Mars’ elliptical orbit causes the intensity of the sunlight reaching the planet to vary by 40 percent during a Martian year. A seasonal effect also controls how much water vapor is present in the lower atmosphere, as well as variations in how much water makes it into the upper atmosphere. The 11-year cycle of the sun’s activity is another likely factor.

By making observations for a second Martian year and during different parts of the solar cycle, the scientists will be better able to distinguish among these effects. MAVEN is continuing these observations in its extended mission, which has been approved until at least September 2018.