A comet whizzes closely by Mars, and the tail of cosmic dust in its wake penetrates the Martian upper atmosphere and produces a new dense layer of ionization in its wake, essentially a new ionosphere.

That's what an instrument built at the University of Iowa observed when the comet Siding Spring flew past Mars on Oct. 19, 2014. NASA's Jet Propulsion Laboratory released the findings in a press conference.

"It is a big deal, because it's the first time we've ever directly measured an interaction of a comet with a terrestrial planet," says UI physics and astronomy professor Don Gurnett. A comparable, very close cometary flyby of a planet like Earth or Mars is a very rare thing, thought to occur only about once every 10 million years.

Gurnett is the lead investigator for the ionospheric radar sounder on the Mars Express spacecraft, a European Space Agency mission in orbit since 2003.

The instrument is a radar that can remotely sound the Martian ionosphere with its 130-foot antenna.

As Mars Express swept in close to the Red Planet a few hours after the comet's rendezvous, the radar saw that the density of electrons in Mars' ionosphere had jumped by a huge factor. The jump in density occurred in a layer at an altitude substantially lower than the normal density peak in the Martian ionosphere.

He and his research team concluded that the increased ionization was produced by the impact of fine particles from the comet-essentially tiny meteors-burning up in Mars' atmosphere, a finding that was confirmed by instruments on the recently launched NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, which is now in orbit around Mars.

The Iowa researchers detected the new ionosphere layer about seven hours after the comet passed by Mars, when MARSIS was at the closest point to the planet in its orbit. Mars Express then continued on its seven-hour elliptical orbit, and by the time the instrument was back at that closest point again, the newly formed ionospheric layer caused by the comet had dissipated.

The University of Iowa played a considerable role in the observation process. Years before Mars Express was launched, Gurnett suggested that an ionospheric sounding instrument be incorporated into the mission, and won a contract to build a major part of the instrument at Iowa. As Gurnett says, without the initiative to build the instrument at the UI, Mars Express may have never observed the ionosphere created by the comet.