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SYFY WIRE Bad Astronomy

A dead comet that doesn't chafe

By Phil Plait
The nucleus of comet P/2016 BA14 as seen by NASA’s Goldstone radar facility when the comet passed close to Earth in March 2016. Credit: NASA/JPL-Caltech/GSSR

In space, no one can hear you chafe.

You didn't know that? Well then, why else would dead comets be covered in talcum powder?

Or at least it appears that one dead comet may have a layer of talcum-powder-like grains on its surface, presumably so it can stay comfortable despite the sticky heat of being out in the Sun all the time.

The comet in question is called P/2016 BA14 (PANSTARRS)*. Discovered in 2016, it was initially thought to be an asteroid, but then very weak cometary activity was seen — comets and asteroids are similar in many ways, but comets tend to have more ice that turns to gas when they get near the Sun, creating the characteristically fuzzy coma and tail.

BA14, however, doesn't behave that flamboyantly. That is likely due to it being what's called a dead (or nearly dead) comet: One that has passed close to the Sun so many times over so many orbits that all the ice has sublimated away, leaving behind just the solid nucleus. While that may not make it as photogenic as other comets, that does make it very interesting scientifically: We don't know what happens to comet nuclei once all the ice is gone. What happens to their surface? What are they like chemically?

Also, some comets come from deep space, well past the orbit of Neptune, and represent pristine examples of what the solar system was like when it was young. That makes them like 4.6 billion year old time capsules! But if we want to understand what they're like when they're fresh, it helps to know what they're like when they're old, too, to disentangle aging effects from things that happen to a comet early on.

Hubble images of the comet 252P/LINEAR from 2016, showing changes in its activity over the span of a few hours. Credit: NASA, ESA, and J.-Y. Li (Planetary Science Institute)

BA14 passed just a few million kilometers from Earth in early 2016, and was intensely studied. A team of astronomers observed it with a camera on the 8.2-meter Subaru telescope amusingly called the Cooled Mid-Infrared Camera and Spectrometer, or COMICS, getting images and spectra of it. It only appeared as an unresolved dot, which means they saw no coma, no gas escaping from it. That means their observations show what's going on right on the comet's solid surface.

The spectrum shows weak absorption features. Different atoms and molecules absorb different wavelengths (colors) of light, and these features in the spectrum can be used to ID them. Many comet nuclei, for example, show absorption from minerals like olivine and pyroxene, but those don't match what was seen on BA14.

Instead, what matches the features best are what are called phyllosilicates, which include things like chlorite, clinochlore, serpentine, and clays. The scientists compared the comet spectrum to various meteorites, and found a decent match to one called Orgueil, which fell in France in 1864. It's known to have phyllosilicates in it. Given the nature of the spectrum, BA14 matches best for coarse grains of material, ones bigger than about 100 microns in width (0.1 millimeters, or about the width of a human hair).

And here's something I didn't know: Talcum powder, made of ground up talc, also belongs to the phyllosilicate family.

The orbit of the nearly dead comet P/2106 BA14 takes it very close to Earth and as far out as Jupiter; the giant planet has no doubt affected the comet’s orbit over time. Credit: NASA/JPL-Caltech

So is the surface of BA14 covered in talcum powder? Well, kinda. That would fit the spectrum, though typically the talcum powder you buy in stores has a grain size somewhat smaller than 100 microns. It's not inconsistent, though.

I read a lot about comets, and it's getting tough to surprise me. This surprised me.

So are all dead comets covered in a powder like this? It's a good question, but one that can't be answered by looking at just one. We need to study a lot more to see if any others have this feature. It would be helpful to look for it in active comets, too, since it's not clear if this powder accumulates as the comet ages or if it's been there all along.

The nucleus of comet P/2016 BA14 as seen by NASA’s Goldstone radar facility when the comet passed close to Earth in March 2016. Credit: NASA/JPL-Caltech/GSSR

I'll note that BA14 is on an orbit extremely similar to that of the comet 252P/LINEAR 12, and may be a piece that calved off. BA14 is about 800 meters wide, while 252P is likely half that. Interestingly 252P is still active, and it's not clear why the two are so different if indeed they were once the same body.

There is a class of asteroid (called the D-type) that may include dead comets (like 3552 Don Quixote). Asteroids and comets, in some cases, may be more than just similar. The two broad categories of objects may actually overlap, with ones like BA14 right on that intersection. These can be among the most interesting of objects, since they can tell us about both groups, and even, perhaps, how they change over time.

They also force us to think about what it means to be in one group or another, and if the lines between them are fuzzier than you might have thought at first. I find that sort of thing refreshing. Hard and bright boundaries make our thinking too rigid, and nature is rarely that obvious. BA14 is a great reminder of that.

*Despite the BA designation, there's no relation. That's just due to the way comets are named.