A "once-in-a-lifetime" chance to watch a comet flying close to Mars gave a unique insight into the effect of such a near miss on a planet's atmosphere, according to a University of Leeds academic.
Professor John Plane, a member of the Atmospheric and Planetary Chemistry group in the University's Faculty of Mathematics and Physical Sciences, collaborated in the analysis and interpretation of observations made with NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft of Comet C/2013 A1 Siding Spring’s flyby on October 19.
The comet travelled within about 87,000 miles of Mars, less than half the distance between Earth and the Moon and more than ten times closer than any comet is known to have come to our planet.
Professor Plane said: “The chance to see a comet from the Oort Cloud, the most distant region of our Solar System, brush past a planet in the inner Solar System is incredibly small, so this was a once-in-a-lifetime opportunity. What’s more, NASA’s MAVEN spacecraft happened by coincidence to arrive at Mars a month before the comet. It was amazingly good luck.”
Data from observations carried out by the MAVEN, NASA’s Mars Reconnaissance Orbiter (MRO), and a radar instrument on the European Space Agency's (ESA’s) Mars Express spacecraft revealed that debris from the comet added a temporary and very strong layer of ions to the ionosphere, the electrically charged layer high above Mars.
Scientists were for the first time able to make a direct connection between the input of debris from a specific meteor shower to the formation of this kind of transient layer.
Professor Plane said: “The comet caused a huge perturbation in the atmosphere of Mars above 100 km. This is where the dust particles from the comet, travelling at 120,000 mph, would have flash heated and boiled off metal atoms.”
Professor Plane was asked to advise the MAVEN mission on what the likely effects of the flyby would be and what the orbiter’s Imaging UV Spectrometer (IUVs) should be looking for. He modelled the amount of metals such as iron and magnesium that would be injected into the Red Planet’s upper atmosphere and what emissions could be expected.
“Our model of the Martian atmosphere correctly predicted where metallic ions would be injected in the atmosphere and how long they would persist, but there were also a number of big surprises that warrant further study,” he said. “It will be fascinating to see how the small particles formed by this flyby affect the chemistry and climate of the planet as they descend through the atmosphere.”
The MAVEN spacecraft detected the comet encounter in two ways. The remote-sensing imaging ultraviolet spectrograph observed intense ultraviolet emission from magnesium and iron ions high in the atmosphere in the aftermath of the meteor shower. Not even the most intense meteor storms on Earth have produced as strong a response as this one. The emission dominated Mars' ultraviolet spectrum for several hours after the encounter and then dissipated over the next two days.
MAVEN was also able to directly sample and determine the composition of some of the comet dust in Mars’ atmosphere. Analysis of these samples by the spacecraft’s neutral gas and ion mass spectrometer detected eight different types of metal ions, including sodium, magnesium and iron. These are the first direct measurements of the composition of dust from an Oort Cloud comet.
Elsewhere above Mars, a joint U.S. and Italian instrument on Mars Express observed a huge increase in the density of electrons following the comet's close approach. This instrument, the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS), saw a huge jump in the electron density in the ionosphere a few hours after the comet rendezvous. This spike occurred at a substantially lower altitude than the normal density peak in the Martian ionosphere. The increased ionisation, like the effects observed by MAVEN, appears to be the result of fine particles from the comet burning up in the atmosphere.
MRO’s Shallow Subsurface Radar (SHARAD) also detected the enhanced ionosphere. Images from the instrument were smeared by their passage through the temporary ion layer created by the comet's dust. SHARAD scientists used this smearing to determine that the electron density of the ionosphere on the planet's night side, where the observations were made, was five to 10 times higher than usual.
Studies of the comet itself, made with MRO's High Resolution Imaging Science Experiment (HiRISE) camera, revealed that the nucleus is smaller than the expected 1.2 miles (2 kilometers). The HiRISE images also indicate a rotation period for the nucleus of either eight or 16 hours, which is consistent with recent with preliminary observations by NASA’s Hubble Space Telescope.
MRO’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) also observed the comet to see if signs of any particular chemical constituents stood out in its spectrum. Team members said the spectrum appears to show a dusty comet with no strong emission lines.
In addition to these immediate effects, MAVEN and the other missions will continue to look for long-term perturbations to Mars’ atmosphere.
MAVEN's principal investigator is based at the University of Colorado's Laboratory for Atmospheric and Space Physics in Boulder, and NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the mission. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, manages the Mars Reconnaissance Orbiter. Mars Express is a project of the European Space Agency; NASA and the Italian Space Agency jointly funded the MARSIS instrument.
Image credit: NASA/JPL-Caltech
Further information
Professor Plane is available for interview.
Contact: Sarah Reed, Press Officer, University of Leeds; phone: 0113 343 4196 or email s.j.reed@leeds.ac.uk.
For information about the NASA mission contact:
Dwayne Brown
Headquarters, Washington
202-358-1726
dwayne.c.brown@nasa.gov
Guy Webster
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6278
guy.webster@jpl.nasa.gov
Nancy Neal Jones/Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Md.
301-286-0039 / 301-614-5438
nancy.n.jones@nasa.gov / elizabeth.a.zubritsky@nasa.gov
For more information about NASA's Mars missions, visit: www.nasa.gov/mars