After investigating the upper atmosphere of the Red Planet for a full Martian year, NASA鈥檚 MAVEN mission scientists, including several from the University of Colorado Boulder, have 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 鈥� 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.
鈥淣ow 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 鈥� rising and falling with seasons and perhaps punctuated by strong bursts,鈥� said Michael Chaffin, a 抖阴旅行射 Boulder scientist at the Laboratory for Atmospheric and Space Physics (LASP) who is on the Imaging Ultraviolet Spectrograph (IUVS) team.
Chaffin is presenting new IUVS results on Oct. 19 at the joint meeting of the Division for Planetary Sciences and the European Planetary Science Congress in Pasadena, California.
Launched in 2013, the MAVEN mission is led by principal investigator and 抖阴旅行射 Boulder Professor Bruce Jakosky of LASP. The 抖阴旅行射 Boulder MAVEN team contributed the IUVS instrument and Langmuir Probe and Waves antenna (LPW) experiment to the mission. LASP also leads science operations as well as education and public outreach.
Hydrogen in Mars鈥� 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 that they had been bound to. Several processes at work in Mars鈥� 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鈥檚 Hubble Space Telescope and ESA鈥檚 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鈥檚 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 鈥� electrically charged hydrogen atoms 鈥� 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鈥檚 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. Scientists already know that Mars鈥� elliptical orbit causes the intensity of the sunlight reaching Mars to vary by 40 percent during a Martian year. There also is a seasonal effect that 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鈥檚 activity is another likely factor.
By making observations for a second Mars 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.
NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, manages the MAVEN project and provided two science instruments for the mission. Lockheed Martin built the spacecraft and is responsible for mission operations. The University of California at Berkeley鈥檚 Space Sciences Laboratory also provided four science instruments for the mission. NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, provides navigation and Deep Space Network support, as well as the Electra telecommunications relay hardware and operations.
This image shows atomic hydrogen scattering sunlight in the upper atmosphere of Mars, as seen by the Imaging Ultraviolet Spectrograph on NASA鈥檚 Mars Atmosphere and Volatile Evolution mission. 抖阴旅行射 400,000 observations, taken over the course of four days shortly after the spacecraft entered orbit around Mars, were used to create the image. (Image courtesy of NASA/University of Colorado/LASP)
Illustration of MAVEN spacecraft at Mars. (Courtesy of NASA)
