Sunshine on Comets: Part I

Bulls-eye. Smashing success with impact. Deep Impact ejecta from interceptor which probed the interior of comet Tempel 1. Credit: NASA/JPL

On July 4th of this year, NASA sent a spacecraft into the path of the comet Tempel 1. When the comet hit the spacecraft, the resulting explosion sent out a huge cloud of dust and gas. Meanwhile, the Deep Impact mothership, which had flown safely past the comet nucleus, turned back around to watch the fireworks. The onboard spectrometer focused on the impact debris cloud in order to determine what materials were blown out by the blast.

Jessica Sunshine, of the Science Applications International Corporation, is the member of the Deep Impact science team responsible for the spacecraft’s infrared spectrometer. She recently sat down with Astrobiology Magazine’s Leslie Mullen to discuss why it’s so difficult to untangle the spectroscopic data, and what the mission has taught us so far about how comets are constructed.

Astrobiology Magazine (AM): What was the spectral signature of the comet before impact?

Comet ingredients.
Credit: Spitzer/JPL

Jessica Sunshine (JS): The nucleus is darker than charcoal, so it has very low reflectivity. What we mostly saw was a little bit of reflected sunlight, plus emission in the thermal. The spectrum of the nucleus is relatively smooth, but you expect the features to be subtle because the nucleus is so dark. The only real feature that stands out is the ambient coma between us and the nucleus. So we saw trace amounts of water and carbon dioxide, and also faint amounts of organics.

The smooth spectral curve of the nucleus provided an incredibly dramatic contrast, because it instantaneously changed from the impact. It was amazing when it happened, and it still amazes me.

AM: But weren’t you expecting a lot of material to come out of the comet’s nucleus from the impact?

JS: Well, expecting and seeing are two different things. We also didn’t know where the spectrometer was going to be relative to the impact. The pointing was always an issue. We had biased it to be downrange, but you never know, and in fact it took us a while to figure out where the spectrometer was. We could’ve been up-range of the impact, and it would not have been as spectacular.

Arrows a and b point to large, smooth regions. The impact site is indicated by the third large arrow. Small arrows highlight a scarp that is bright due to illumination angle, which shows the smooth area to be elevated above the extremely rough terrain. The scale bar is 1 km and the two arrows above the nucleus point to the sun and the rotational axis of the nucleus. Celestial north is near the rotational pole.
Credit: NASA/JPL/UMd

AM: Why did you want the spectrometer to be downrange of the impact, rather than up-range or dead-center?

JS: We wanted to be downrange so that we could see changes in the material flowing out from the impact ejecta. If you’re dead-on, it would be very hard to see changes. We would’ve loved to have been dead-on afterwards, when the spacecraft had passed the nucleus and then turned back around to look at it. But we would’ve had to have incredible luck for the spectrometer to have gone over the impact site at that point, because we were moving so quickly and the spectrometer’s field of view is very small.

AM: So after the impact, the spectrometer detected more water and carbon dioxide…

JS: What’s really exciting is it’s not just more, it literally became glowing. If you were in a darkened room, the hot gases would’ve lit up the room. The vapor cloud was just phenomenal – it saturated some of our pixels. But the cloud also was moving very quickly, because by the next integration it was gone.

AM: How long is an integration?

JS: 720 milliseconds, at that point. The other thing was the organics. We knew organics were going to be vaporized, and we were expecting to see each organic material have a peak at a specific wavelength, depending on what it was. But what we saw was that anything that had a carbon-hydrogen bond was vaporized, and we got a conglomerate peak. That means there were a lot of organics, and a lot of different kinds, but we couldn’t tell what any of it was.

AM: Are you now working on separating out all those lines?

Tempel 1 nucleus shortly before crater-rendering impact. Scientists wondered whether the whitish material was icy rocks or some other surface feature previously unseen on other comets because of lack of image resolution.
Credit: U.Md/NASA/JPL

JS: I don’t think there is a way to separate it out. But if we stand back and use the ground-based observations of the coma before and after impact, we can see specific features, and we’re still working on trying to interpret what they are. The difference between 10 minutes before and 5 to 10 minutes after impact is that both the water and the carbon dioxide went up by factors of 10, and the organics went up by factors of 20.

AM: What kinds of organics were seen before impact?

JS: The best you can say is they’re probably consistent with what we expect to see in a coma – formaldehyde, methanol.

AM: And the organics that were seen after impact – probably more of the same, plus some others?

JS: There were things we hadn’t seen before. The near-infrared folks who have a high resolution ground-based spectrometer asked me what’s going on beyond 3.6 microns, because that’s where they cut off. Clearly there is stuff between 3.7 and 4 microns, but we don’t know what it is yet. In the past, it’s taken decades to figure out what these things are.

AM: Why is it so hard to figure it out?

JS: Because even in a normal comet situation, you’re dealing with a changing environment. Interaction with the solar wind changes the chemistry, causing the components to photo-dissociate and then recombine into other things. When you add this impact event, it complicates the picture even more. When we hit the nucleus, the components started reacting immediately with each other.

The cometary crater left behind as simulated in digital rendering prior to the July 4th encounter.
Credit: Maas Digital for NASA/JPL

Our spacecraft had very high spatial and temporal resolution, but relatively low spectral resolution. The guys on the ground have very good spectral resolution over a limited range, but they’re looking at a completely different time scale. They’re also looking at the bulk coma, and so they’re going to see many more of the secondary components, while we might be seeing the primary materials. Putting all this data together is what’s fun. If it were easy, it wouldn’t be as interesting.

AM: So after putting your spacecraft data together with their ground-based data…

JS: And putting the timescales back together…

AM: Then you might be able to figure out exactly what organics were released by the impact?

JS: Yes. I think the ground-based data already shows clear evidence of some. They saw a tremendous change in ethane before and after; they saw a lot more ethane after. They’re also seeing formaldehyde and methanol, but not large changes. But we still have a lot to do.

Coma gas from Comet 9P/Tempel 1 seen with the ESA OGS telescope (narrowband filter)
Credit: ESA

AM: I heard it said that rather than organics, some of the spectral bands from the impact could be due to hot water. Because I guess hot water has different spectral bands.

JS: That was for one particular identification that we were calling H3O+. These things are controversial, and that’s why we say they are preliminary identifications. But on the other hand, different ground groups have said, "Wow, if H3O+ is really there, it makes sense because otherwise I don’t understand what we’re seeing." There’s just no question that we are seeing a dramatic increase in the organic hydrocarbons, and that they are different from what was seen pre-impact.

AM: Based on what we thought was the water content of comets, I would have assumed water would be the first thing you’d think of.

JS: Well, it is. But "hot water" is actually a technical term. When you have a large enough amount of water, it has to release heat somewhere, and it does that through fluorescing. It’s not hot like the impact vapor that was thousands of degrees. It’s just that, if you have enough of it, water can take different paths to release its energy, and some bands which are normally not very active can become active. It’s probably more accurate to call them minor bands than hot bands.

Read Part 2 of this interview.