Last month, an exotic deep space comet flew by Mars and unleashed an unexpectedly strong meteor shower that briefly changed the chemistry of the Red Planet's upper atmosphere, NASA said Friday.

The comet, Siding Spring, came from a distant region of the solar system known as the Oort Cloud.

Its rare shave past Earth's neighboring planet on October 19 at a speed of 35 miles (56 kilometers) per second was closely monitored by a host of human-made spacecraft.

"We believe this type of event occurs once every eight million years," said Jim Green, director of NASA's Planetary Science Division.

The comet released far more dust than NASA anticipated -- thousands of kilograms by preliminary estimates -- as it passed 87,000 miles from Mars.

"The comet's dust slammed into the upper atmosphere, creating a massive and dense ionospheric layer, and literally changed the chemistry of the upper atmosphere," Green told reporters.

This additional layer of ions, in an electrically charged layer high above the planet, was temporary.

NASA said it was the first time scientists have ever connected debris from a meteor shower to such a significant change in the atmosphere.

The US space agency hopes further study will reveal if there are any long-term effects.

- Meteor shower on Mars -

The resulting meteor shower likely lasted an hour or more, according to data from an orbiting NASA spacecraft called the Mars Atmosphere and Volatile Evolution Mission (MAVEN).

Its scientific instruments also detected eight different types of metal ions, including sodium, magnesium and iron, marking the first such measurements of an Oort Cloud comet, which some experts likened to a snowball in space.

MAVEN "observed intense ultraviolet emission from magnesium and iron ions high in the atmosphere in the aftermath of the meteor shower," said the US space agency.

"Not even the most intense meteor storms on Earth have produced as strong a response as this one."

Had humans been on Mars, they might have seen a yellow glow in the sky, said Nick Schneider, instrument lead for MAVEN's Imaging Ultraviolet Spectrograph at the University of Colorado, Boulder.

"It would have been truly stunning to the human eye."

There were "probably thousands of shooting stars per hour," Schneider told reporters.

A layer of sodium was likely left behind, high in the Martian atmosphere, leading to a yellow afterglow, he added.

Orbiters including MAVEN, NASA's Mars Reconnaissance Orbiter (MRO) and the European Space Agency's Mars Express were maneuvered to the other side of Mars to avoid the debris from the comet.

It's a good thing they did, because they might have suffered irreparable damage otherwise.

"I think it is pretty obvious they wouldn't have (survived), based on the tremendous response of Mars's atmosphere to the comet tail," said Green.

An instrument made by the United States and Italy aboard the Mars Express orbiter also observed a big jump in the density of electrons in the few hours following the comet's pass.

NASA's Opportunity rover, a solar-powered robotic vehicle on the Mars surface, captured a grainy image of the flyby. Scientists are still poring over data from the Curiosity rover, which landed in 2012.