Comet C/2019 Y4 (ATLAS) glows green due to the presence of diatomic carbon, common in comets. Credit: Damian Peach
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Comet C/2019 Y4 (ATLAS) glows green due to the presence of diatomic carbon, common in comets. Credit: Damian Peach

A tale of two comets: ATLAS shrugs, Borisov is finally weird

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Apr 23, 2020, 9:00 AM EDT (Updated)

Last week I wrote about the first interstellar visitor to our solar system ever seen, called 'Oumuamua, and in that article I mentioned two comets: 2I/Borisov and C/2019 Y4 ATLAS. I have some updates about those comets.

First, Y4 ATLAS. Discovered in late 2019 in observations by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Hawaii, it promised to be a show-stopper. Although never getting closer than about 120 million kilometers from Earth, its position relative to us and the Sun and how rapidly it was brightening as it approached promised that by May 2020 it could be naked-eye visible and spectacular.

The orbit of the comet C/2019 Y4 ATLAS. Credit: NASA/JPL-Caltech

But then in early April, disaster: Astronomers reported it appeared to be breaking up, disintegrating. Comets do this sometimes; they are dust, gravel, and rocks held together by various ices, and as they approach the Sun that ice sublimates, turns directly to gas. While that is why they get so spectacular — the gas and dust leaving the comet can make a cloud thousands or tens of thousands of kilometers across, and when lit by the Sun can be quite bright — it can also mean a death sentence. If too much ice sublimates away the structural integrity of the comet gets compromised. It can calf (split into two big pieces) or it can fall apart.

The comet C/2019 Y4 (ATLAS) is disintegrating, as can be seen here in an image taken on 17 April 2020. The image has been processed to bring out details in the comet, which is why the stars look odd (they appear as streaks due to the long exposure tracking the comet). Credit: Damian Peach

Y4 ATLAS looks like it’s doing the latter. More observations have come in, and it's clear that there's a lot of debris around it; normally the tail near the head is smooth, but the lumps you can see in images are pieces of the comet that have fallen away from it.

A Hubble image taken just days ago on 20 April makes this case even more clearly:

Hubble observations taken on 20 April 2020 show comet Y4 ATAS has broken up into multiple fragments. Credit: Ye Quanzhi / NASA / ESA

This was taken by astronomer Ye Quanzhi, and was planned some time ago to determine the size of the comet's nucleus. He got more than he bargained for; now that the comet has fallen apart there are multiple nuclei, and it's hard to keep track of them:

There is still a bright nucleus there, so it's possible it will survive… but I have my doubts. It's still a bit farther from the Sun than Earth is, and still on approach to our star. In late May it will pass a mere 40 million kilometers from the Sun, well inside Mercury's orbit. If it's been disintegrating while still over 150 million km out, getting four times closer doesn't make me want to bet on it surviving the encounter*.

Virtual Telescope Project image of C/2019 Y4 ATLAS from 17 April 2020, shortly after breakup. Several small pieces can be seen. Credit: Gianluca Masi

That's too bad; I was looking forward to seeing another bright comet. I've caught quite a few over the years and they're so wonderful to see! But alas, probably not this time.

Artwork depicting comet 2I/Borisov as it passes through our solar system. Credit: NRAO/AUI/NSF, S. Dagnello

The second comet is 2I Borisov, the second interstellar visitor ever seen. Over the past couple of months I've hammered how completely non-weird it is (especially compared to 'Oumuamua which is nothing but weird). It's been behaving exactly like a regular locally sourced comet in pretty much every way except for the fact that it's moving through the solar system so rapidly it must come from another star. It even calved recently, which is something our own comets do.

But finally, something odd has come up about it. Astronomers used the wonderful ALMA telescope array to look at the chemical makeup of the comet, and, as expected, they saw a lot of two different molecules: hydrogen cyanide (HCN) and carbon monoxide (CO), both of which are common in our own comets. The amount of HCN they saw was the same as in local comets, too.

But it's the CO where things get weird: Borisov has somewhere between 9 and 26 times the amount of carbon monoxide as solar system comets!

ALMA observations of the interstellar comet 2I/Borisov revealed the presence of the molecules hydrogen cyanide (left) and carbon monoxide (right). The amount of CO is much higher than in solar system comets. Credit: ALMA (ESO/NAOJ/NRAO), M. Cordiner & S. Milam; NRAO/AUI/NSF, S. Dagnello

That's a big difference. Now, solar system comets vary in their CO content, but nothing like this has been seen from comets within about 300 million kilometers of the Sun. That's a lot of CO!

Carbon monoxide is a very common molecule in space. It's easy to form and has two very common atoms in it. It forms in very cold environments, roughly -250°C. Comets formed in the outer solar system back when the Sun and planets themselves were forming. It's very cold there, which is why we see CO in them.

But this much CO in Borisov indicates it must have formed under conditions different than found in our own system. We've seen disks of gas and dust around other stars where planets and comets form, and some of these disks are significantly bigger than the one where we formed. That means comets birthed around other stars could form in colder environments farther out, and get more CO in them. Or, it could mean it just formed around a cooler star than the Sun.

The problem here is we don’t really know, and we don’t have a lot of examples to look at. 'Oumuamua is just bizarre, and doesn't act much like a comet. Borisov does, but it's the only kinda sorta normal interstellar comet we've ever seen. Is this sort of CO content normal, or is Borisov an outlier? Do most systems form comets farther out than we did, or is our own recipe what's normally used by stars?

I suppose it's not unexpected that the first couple of galactic visitors we get raise more questions than answers. The obvious thing to do is find more of them. Given that we spotted two relatively close together in time, that implies we'll see lots more in the coming years, especially with bigger survey telescopes coming online. When that happens we’ll be able to start categorizing them, see how they are similar and/or different.

And once that happens comes the best part: Understanding them. The Universe is sending us samples of itself for free for us to study. I think it's wonderful we can accept such gifts.


*As usual when there's a bright comet, the conspiracy theorists get a jolt of adrenaline to their dumbosity. I've seen claims that Y4 ATLAS will hit Earth (it never gets closer to us than 120 million km!), or that it's a distraction from the government creating the COVID19 virus, or that the virus came from the comet, etc. These claims are really ridiculous, but I get a lot of email from people who don't know the science well enough to understand that, or from people who suffer from anxiety and are really concerned about this. Some of these conspiracy theorists are straight up con artists doing this for money or attention or giggles, and they are, in my opinion, flaming piles of human garbage.

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