Core of the problem: Cometary IR

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  • Published: Aug 1, 2017
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Core of the problem: Cometary IR

It's good to be WISE

A new study suggests that distant

NASA's Wide-field Infrared Survey Explorer (WISE) spacecraft has delivered new insights into the properties of long-period comets. In work led by James Bauer of the University of Maryland it has been shown that there are seven times as many long-period comets at least 1 kilometre in diameter than previously predicted.

It is difficult to study comets that take more than a couple of centuries to orbit the sun, not least because astronomers, while a patient bunch, are not necessarily around long enough to watch the object's comings and goings. After all, known, "long-period comets" may never approach the Sun in a person's lifetime and others that travel inward from the Oort Cloud, some 300 billion kilometres from the sun may have periods measured in thousands if not millions of years. Nevertheless, we must study such objects if we are to understand our Solar System and make predictions about distant stellar systems. Moreover, there is the inherent risk to life on Earth of such objects which might cross our planet's orbit.

The team has now only found that there are many more long-period comets than were previously thought but many are, on average, almost twice as large as the "Jupiter family” comets, whose orbits are shifted and shaped by the gravitational pull of Jupiter and travel around the sun in less than 20 years. Details are reported in The Astronomical Journal

Cometary audit

"The number of comets speaks to the amount of material left over from the solar system's formation," Bauer explains. "We now know that there are more relatively large chunks of ancient material coming from the Oort Cloud than we thought."

The Oort Cloud itself is far too distant to be seen with current telescopes. It is thought to be a spherical distribution of small icy bodies beyond what we usually think of as the planetary outer limits of the solar system. The density of comets within the Oort Cloud it actually low, so the probability of any of those distant comets colliding with each other and spitting off chunks of rock and ice that might head towards the inner planets is rather low. Astronomers believe that the long-period comets observed by WISE were probably kicked out of the Oort Cloud millions of years ago and sent into the highly elliptical orbits. The observations reported in this paper were made in 2010 during the spacecraft's primary mission. It has since been renamed NEOWISE and its mission has been to observe near-Earth objects (NEOs) since 2013.

"Our study is a rare look at objects perturbed out of the Oort Cloud," adds Amy Mainzer of the Jet Propulsion Laboratory in Pasadena, California. Mainzer is principal investigator on the NEOWISE mission. "They are the most pristine examples of what the solar system was like when it formed."

Coma threat

We already had broad estimates of how many long-period comets and how many Jupiter family comets are present in the solar system. But, until the WISE data was analysed there was no good estimate of the physical size of these long-period comets. Fundamentally, the cloud of gas and dust that surrounds a comet, its coma, makes any image of a distant comet appear as nothing but a blurry haze within which the comet's nucleus lies behind the veil.

The team used the WISE data to extract the infrared glow from the coma and to "subtract" the coma from each comet and so give them a view of the inner nucleus that could be interpreted as the size of said comet. The WISE observations were of 164 cometary bodies, including 95 Jupiter family comets and 56 long-period comets, which could be subtracted and analysed in this way.

The researchers explain that the new results from the WISE data reinforce the idea that comets that pass by the sun more frequently tend to be smaller than those spending much more time away from the sun. At the heart of this phenomenon is the fact that Jupiter family comets are more frequently heat treated by the sun, which leads to the sublimation and evaporation of volatile substances, including water, from the comet’s surface, which also takes with it other material.

"Our results mean there's an evolutionary difference between Jupiter family and long-period comets," Bauer adds. The existence of so many more long-period comets than predicted suggests that more of them have likely impacted planets, delivering icy materials from the outer reaches of the solar system. Such an insight could have implications for our understanding of the source of water on our own planet and perhaps even the seeds of life in the form of organic matter thought to be present in comets.

But, if the study of comets has implications for the dawn of life on Earth, it may also have implications for the end of life on our planet. "Comets travel much faster than asteroids, and some of them are very big," Mainzer adds. "Studies like this will help us define what kind of hazard long-period comets may pose."

Related Links

Astronom J 2017, online: "Debiasing the NEOWISE Cryogenic Mission Comet Populations"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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