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Comet Metal Rained Down on Mars

Magnesium and iron measurements suggest the hourly meteor rate overhead on Mars must have been tens of thousands of shooting stars per hour for hours.

A rain of metallic stardust lit up the skies of Mars in the wake of the close passage of Comet Siding Spring, which roared past the Red Planet last October.

This is according to the instruments aboard NASA's MAVEN spacecraft, which made the first direct detection of sodium, magnesium, aluminum, chromium, nickel, copper, zinc, iron and other metals high in the Martian atmosphere that can be linked directly to the material sloughing off the comet.

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"This must have been a mind-blowing meteor shower," said Nick Schneider of the Laboratory of Atmospheric and Space Physics at the University of Colorado.

MAVEN's instruments were put to use as soon as the spacecraft arrived -- even before the instruments were fully commissioned -- to measure the effects of the comet on the Martian atmosphere.

Based on the strong signal of magnesium and iron measurements seen by MAVEN's Imaging Ultraviolet Spectrograph, Schneider says the hourly meteor rate overhead on Mars must have been tens of thousands of "shooting stars" per hour for hours.

"I'm not sure anyone alive has ever seen that," said Schneider, "and the closest thing in human history might the the 1833 Leonid shower."

The metal ions were the remains of pebbles and other pieces shed from the comet that burned up, or "ablated" into individual atoms -- when they hit the Martian atmosphere at 56 kilometers per second (125,000 miles per hour). It's the same thing that happens on Earth, except that not even on Earth had a spacecraft been in the right place at the right time to detect so many ions fresh off a comet.

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"This is the first time that we detect the full panel of metal ions from sodium to zinc," said Mehdi Benna, a MAVEN researcher at the University of Maryland and NASA's Goddard Space Flight Center who works on the team that runs the Neutral Gas and Ion Mass Spectrometer (NGIMS).

That instrument captures, identifies and counts the charged metal atoms high in the ionosphere of the red planet. Only, the first measurements NGIMS did were under unprecedented and unexpected circumstances, he said.

"The nice thing about Comet Siding Spring is that we know the source of the dust particles," said Benna. "We know the source and the speed." These are numbers they can put into the models they use to sort out the details of such events and glean information about Mars' ionosphere, the comet's composition, and even the workings of Earth's ionosphere when it's hit by comet or asteroid debris. "Here we really have a controlled experiment. We measured ions shortly after they has deposited. If we had waited a week or even a few days we wouldn't have such a clear detection."

Benna and Schneider are lead authors on separate papers about their respective instruments' measurements recently published online by the journal Geophysical Research Letters. One of the things that can not be found those papers, however, is the role serendipity played in the work. MAVEN was not launched with Comet Siding Spring in mind. It was just luck that the spacecraft arrived in time for the flyby with instruments that could study the event.

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"If someone had said 'Go measure the elemental and isotopic composition of dust from a first-time Oort cloud comet,' any sane person would have said 'can't be done'," said Schneider of Benna's work. "Yet that's exactly what they did, and it should keep the cosmochemists occupied for a while!"

"The funny thing is that actually we did not plan for the spacecraft and the instrument to get that data," said Benna. "A few months before we were told about Siding Spring and so we looked to see if we could do the measurements." As it turned out, the NGIMS is the close brother to the instrument that was onboard the CONTOUR spacecraft which was launched in 2002 to visit and study comets. But that spacecraft never made it to any comets.

"So a very similar instrument finally got comet measurements on MAVEN," said Benna. It's a great lesson, he said: "Never lose hope."

Note: Independent science writer Larry O'Hanlon is also the blog manager and social media coordinator for the American Geophysical Union, which publishes Geophysical Research Letters.

This computer-generated view depicts part of Mars at the boundary between darkness and daylight, with an area including Gale Crater beginning to catch morning light.

On Aug. 6, 2014, the European Space Agency's Rosetta spacecraft completed its decade-long journey to reach Comet 67P/Churyumov-Gerasimenko, becoming the first spacecraft to ever orbit a comet. The mission will reach its epic climax when it releases a small robotic lander, called Philae, onto the cometary surface in November. The lander will drill into the surface while Rosetta tags along with the comet's orbit as Churyumov-Gerasimenko makes close approach of the sun. Although Rosetta is unprecedented in that no other mission has achieved orbital insertion around a comet, it's certainly not the first robotic probe to make an intimate cometary encounter. So here's a rundown of 7 encounter of 6 comets by 5 spacecraft since the first close encounter with Halley's Comet in 1986.

Unquestionably the most famous comet in history, Halley's Comet was a prime target for space agencies in 1986 during its 75- to 76-year orbit through the inner solar system. Comet science is still a developing field, but in 1986, very little was known about the composition of these interplanetary vagabonds. In October of that year, the 15-kilometer-long Halley's Comet was visited by the European Space Agency's Giotto mission. The half-ton probe came within 600 kilometers (373 miles) of the comet's nucleus, taking the first photographs of the outgassing vapor from discrete areas of the surface producing its tail and coma (the gas surrounding the nucleus). It was this mission that confirmed the "dirty snowball" theory of cometary composition: a mix of volatile ices and dust. However, Giotto was only able to get so close to the famous comet with the help of the "Halley Armada," a number of international spacecraft all tasked with observing this rare event. Giotto captured the closest imagery, but two Russia/France probes (Vega 1 and 2) and two Japanese craft (Suisei and Sakigake) observed from afar.

At roughly half the size of Halley's comet, Comet Borrelly was found to have similar attributes to its famous cousin. The nucleus was also potato-shaped and blackened. Outgassing vapor was also observed coming from cracks in the nucleus crust where volatiles were exposed to sunlight, sublimating ices into space. NASA's Deep Space 1 probe flew past the comet with a close approach of 3,417 kilometers on Sept. 22, 2001.

Comet Wild 2 -- pronounced "Vilt" after its Swiss discoverer Paul Wild who spotted it in 1978 -- underwent a dramatic alteration in 1974. It is calculated that due to a close pass of Jupiter in 1974, the 5 kilometer-wide comet now orbits the sun every 6 years as opposed to its leisurely 43 years before the gas giant bullied it. The orbital modification meant that Wild 2 was an ideal target for NASA's Stardust mission to lock onto. On Jan. 4, 2004, the Stardust probe gave chase, getting so close to the comet that it was able to collect particles from Wild 2's coma. This image was taken at a distance of less than 240 kilometers (149 miles). The Stardust sample return canister came back to Earth safely, landing in Utah on Jan. 15, 2006. The microscopic particles captured from the comet continue to provide a valuable insight into the organic compounds comets contain. Interestingly, the Stardust spacecraft was granted a mission extension (dubbed New Exploration of Tempel 1 -- NExT). In 2011 it rendezvoused with its second comet, Tempel 1 -- the scene of NASA's 2005 Deep Impact mission -- to analyze the crater that Deep Impact's impactor left behind on the cometary surface.

NASA's Deep Impact mission reached the eight-kilometer-wide (five-mile-wide) comet Tempel 1 in 2005. On July 4, the probe deliberately smashed its impactor into the comet's nucleus, producing a cloud of fine material. A crater -- 100 meters wide (328 feet) by 30 meters (98 feet) deep -- was left behind. A treasure trove of compounds were spotted by the Deep Impact spacecraft and the explosion could be observed from Earth. In 2011, the recycled Stardust-NExT mission visited comet Tempel 1 for the second time.

The fifth space probe encounter with a comet happened on Nov. 4, 2010. NASA's recycled Deep Impact probe -- now the EPOXI mission -- visited comet Hartley 2, examining its strange-shaped nucleus. Described as a "peanut" or "chicken drumstick," this comet is an oddity. During its close approach of under 700 kilometers (435 miles), EPOXI photographed the comet's irregular topography: two rough lobes connected by a smooth center. Jets of gas could be seen being ejected from discrete locations. During the Hartley 2 flyby press conference at NASA's Jet Propulsion Laboratory (JPL), mission scientists expressed their surprise that these jets of vapor are being emitted from sun-facing and shaded regions on the comet surface. Needless to say, analysis of the Hartley 2 flyby data will keep scientists busy for some time to come. "This is an exploration moment," remarked Ed Weiler, NASA's Associate Administrator for the Science Mission Directorate, during the conference.

On Feb. 14, 2011, the veteran Stardust-NExT (New Exploration of Tempel) mission made history by visiting a comet for the second time. Comet Tempel 1 was first encountered by NASA's Deep Impact mission in 2005 after smashing the cometary nucleus with an impactor. This second encounter provided scientists with an unprecedented opportunity to study the same comet after six years of orbiting the sun. Preliminary findings suggested Tempel 1 has undergone some erosion during those six years in deep space. Also, the impact crater left behind by Deep Impact was imaged during the Stardust-NExT flyby and it appeared to match the size and shape predicted after the 2005 impact. However, the crater appeared smoother than expected, so work is ongoing to analyze the 72 photographs taken by the flyby to understand the processes shaping the comet's nucleus.

At 5:29 a.m. EDT (9:29 a.m. GMT) on Aug. 6, 2014, the European Rosetta spacecraft completed a 6.5 minute-long engine burn to insert itself into orbit around Comet 67P/Churyumov-Gerasimenko. Once under the influence of the comet's weak gravity, the spacecraft began to carry out a series of triangular loops, taking several days to complete. The long-duration mission is the first of its kind, where the spacecraft will study the comet from orbit, watching for surface changes as it approaches the sun, making perihelion (the point of closest solar approach). In November, a small lander called Philae will touch down on the surface to drill into the comet's material, revealing its small-scale composition. This photograph from Rosetta was captured on Aug. 3 when the probe was fast approaching the comet at a distance of less than 300 kilometers.

READ MORE: Rosetta Probe Makes Historic Comet Rendezvous