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Comet encounter sparked meteor storm over Mars

Jenny Winder, News Writer
Nov 8, 2014, 20:31 UTC

Sen—Comet C/2013 A1 Siding Spring's close brush with Mars last month produced a meteor storm over the Red Planet, orbiting space probes have discovered.

The  NASA and European spacecraft did not witness the event itself, as they had been steered to shelter on the other side of Mars during the encounter on 19 October.

However, they later detected a new layer of ionized magnesium in the martian atmosphere, showing that tons of comet dust had fallen into it from the comet. The probes were also able to gather new information about the basic properties of the comet’s nucleus.

The comet is of special interest to planetary scientists because it is on its first visit to the inner Solar System from a swarm of icy debris that surrounds it, called the Oort Cloud. 

Dust from the comet impacted Mars and was vaporized in the atmosphere, resulting in significant temporary changes to the planet’s upper atmosphere and possible longer-term perturbations. 

Siding Spring made a close approach within about 87,000 miles (139,500 kilometres) of the Red Planet. This is less than half the distance between Earth and our Moon and less than one-tenth the distance of any known comet flyby of Earth.

Observations by NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission, NASA’s Mars Reconnaissance Orbiter (MRO), and 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.

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Images of comet Siding Spring taken as it passed Mars acquired by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. Image Credit: NASA/JPL-Caltech/University of Arizona

For the first time on any planet, including Earth, scientists were able to make a direct connection from the input of debris from a specific meteor shower to the formation of this kind of transient layer in response.

“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.

“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.”

The remote-sensing Imaging Ultraviolet Spectrograph aboard MAVEN 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.

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Spectrograms from MARSIS on Mars Express show the intensity of radar echo in Mars' far-northern ionosphere at three times on Oct. 19 and 20, 2014. The middle plot reveals effects attributed to dust from the comet. Image credit: ASI/NASA/ESA/JPL/University of Rome/University of Iowa

MAVEN also directly sampled 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.

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS), 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 spike occurred at a substantially lower altitude than the normal density peak in the Martian ionosphere. The increased ionization appears to be the result of fine particles from the comet burning up in the atmosphere.

Images from MRO’s Shallow Subsurface Radar (SHARAD) determined that the electron density of the ionosphere on the planet's night side was five to 10 times higher than usual.

MRO's High Resolution Imaging Science Experiment (HiRISE) camera revealed the nucleus is smaller than the expected 1.2 miles (2 kilometres), with a rotation period of eight hours, which is consistent with recent observations by the Hubble Space Telescope.

How NASA's Mars Orbiter observed Comet Siding Spring. Credit: JPLraw