An infrared NASA space telescope has been keeping tabs on dozens of comets, particularly the gases they release.
Since its prime mission began in 2009, the Wide-field Infrared Survey Explorer (WISE) has been detecting a multitude of comets zip through the solar system, tracking the quantities of carbon dioxide (CO2) and carbon monoxide (CO) being belched into space, greenhouse gases that are typically hard to spot from ground-based observatories.
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The NEOWISE mission was born from WISE, a spacecraft designed to seek out the faint infrared signals of distant galaxies and other mysterious phenomena - but it also had the knack of detecting infrared objects within our own solar system. After it ran low of cryogenic fuel to maintain its instruments in a cool state, NASA scientists rebooted the spacecraft in 2013 to seek out near-Earth objects (NEOs) - or asteroids and comets that could pose a threat to Earth.
But the WISE/NEOWISE spacecraft does far more than simply being an early warning system, it has been collecting valuable science from the asteroids and comets it tracks.
In research published in the Astrophysical Journal, NEOWISE astronomers have found that, although water ice drives sublimation processes when comets heat up as they approach the sun, at more distant and therefore cooler orbits, molecules like CO and CO2 dominate sublimation. CO and CO2 were very common molecules during our solar system's formation and these primordial ices are frozen in with cometary materials that can be studied when comets are discovered making their plunge through the inner solar system.
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Both CO and CO2 are abundant in Earth's atmosphere - CO2 is a potent greenhouse gas, whereas CO has strong secondary impacts on the formation of greenhouse gases - so detecting these molecules in space from the ground is troublesome. But space-based observatories are perfectly placed to measure these gases subliming into space from comets.
"This is the first time we've seen such large statistical evidence of carbon monoxide taking over as a comet's gas of choice when they are farther out from the sun," said James Bauer, deputy principal investigator of the NEOWISE mission from NASA's Jet Propulsion Laboratory in Pasadena, Calif. "By emitting what is likely mostly carbon monoxide beyond four astronomical units (4 times the Earth-sun distance; about 370 million miles, 600 million kilometers) it shows us that comets may have stored most of the gases when they formed, and secured them over billions of years. Most of the comets that we observed as active beyond 4 AU are long-period comets, comets with orbital periods greater than 200 years that spend most of their time beyond Neptune's orbit."
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By keeping tabs on the gases expelled by comets at various distances from the sun, a picture is forming as to the ratios of the types of molecules being expelled. As the quantity of carbon dioxide, carbon monoxide and dust particles expelled increases as a comet heats up, the closer it comes to the sun, a higher proportion of gases from volatiles, such as water, overtake as the prime driver of sublimation. But further out, these volatiles remain inert (in a frozen state), while CO and CO2 become the prime drivers of sublimation.
"As they get closer to the sun, these comets seem to produce a prodigious amount of carbon dioxide," said Bauer. "Your average comet sampled by NEOWISE would expel enough carbon dioxide to provide the bubble power for thousands of cans of soda per second."
This NEOWISE study can be used to supplement findings by the European Rosetta mission that is currently orbiting Comet 67P/Churyumov-Gerasimenko. Rosetta has been with 67P since 2014 and recently experienced perihelion (the point of closest approach to the sun), keeping track of the cocktail of chemicals being outgassed by the frozen mass.
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Not only has Rosetta been tracking CO2 and water vapor quantities in 67P's coma (the comet's atmosphere surrounding the solid nucleus), it's detected a "stinky" concoction of ammonia, methane, hydrogen cyanide and formaldehyde. All of these chemicals were locked into the comet's ices during formation when our sun was just sparking to life billions of years ago.
Therefore, astronomical studies by NEOWISE and up-close investigations by Rosetta are not only critical for us to understand the nature of comets, they are also an "archaeological dig" into our solar system's formative years.