When Comet Siding Spring made its historic close approach with Mars last month, the icy interloper showered the Martian upper atmosphere with meteors. Throughout Siding Spring's close encounter, three robotic orbiters were able to directly link the impact of these tiny particles of ice and dust with the comet that delivered it - the first time in history that we have been able to do so on any planet, including Earth.
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"This historic event allowed us to observe the details of this fast-moving Oort Cloud comet in a way never before possible using our existing Mars missions," said Jim Green, director of NASA's Planetary Science Division at the agency's Headquarters in Washington. "Observing the effects on Mars of the comet's dust slamming into the upper atmosphere makes me very happy that we decided to put our spacecraft on the other side of Mars at the peak of the dust tail passage and out of harm's way."
During the Mars flyby on Oct. 19, NASA's Mars Atmosphere and Volatile Evolution (MAVEN) satellite, NASA's Mars Reconnaissance Orbiter (MRO) and the European Space Agency's Mars Express detected a very strong ionization in Mars' ionosphere. This ionization was caused by cometary particles raining through the upper atmosphere, generating a layer of highly charged particles.
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The cometary dust that created this ionization likely generated an "impressive meteor shower," according to a NASA news release, but there has yet to be any reported sightings of bright meteors by NASA's Curiosity and Opportunity rovers.
But the orbital results are profound. The cometary flyby without doubt generated a short-lived (transient) event in the Martian ionosphere and likely caused longer-term perturbations.
Using its Imaging Ultraviolet Spectrograph, MAVEN detected strong ultraviolet emissions from magnesium and iron ions in the upper atmosphere for several hours after the encounter. As noted by NASA, even the most intense meteor storms on Earth do not generate such a strong UV signal. Mars Express' Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument also detected a surge of electrons in the Red Planet's ionosphere - a sure sign of meteor activity.
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The MRO's Shallow Subsurface Radar (SHARAD) corroborated these results, also detecting an enhancement in ionization in Mars' upper atmosphere.
Through the use of MAVEN's Neutral Gas and Ion Mass Spectrometer, which is designed to directly analyze samples from the uppermost reaches of the atmosphere, 8 different types of metal ions (including sodium, magnesium and iron) were detected. But these metals did not come from Mars, they came from Siding Spring.
Siding Spring is a ‘virgin' comet from the Oort Cloud, a hypothetical region surrounding the sun around 1 light-year away. The Oort Cloud is the outermost reaches of the solar system where the sun's gravity holds billions of icy bodies - leftovers from our solar system's formation - in a gravitational embrace. Occasionally these Oort Cloud objects become destabilized and fall toward the sun as a comet. Siding Spring is one of those comets, making its first voyage through the inner solar system, helpfully passing Mars on its way.
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MAVEN has therefore already done something amazing early in its mission - it has tasted particles from the dawn of our solar system's history, the first time any mission has ever directly sampled an Oort Cloud comet.
Now scientists are continuing to monitor MAVEN's observations of the Martian atmosphere so they can better understand the long-term impacts of a comet coming within 87,000 miles of a planetary atmosphere, a historic event that happened just above the second-most populated planet in the solar system - albeit populated by robots.