October 7, 2005
Sunshine on Comets: Part 2
Jessica Sunshine is the Deep Impact mission scientist responsible for the onboard infrared spectrometer. In the second half of this two-part interview, she discusses whether Deep Impact has altered our ideas of how comets are formed and how important they've been in Earth's history.
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 what the mission has taught us so far about how comets are constructed, and the role they have played in Earth's history.
Astrobiology Magazine (AM): I've always heard comets referred to as "dirty snowballs." But lately I've been hearing them referred to as "icy dirtballs." That seems to suggest there is less water ice in comets than we thought. Is that because of Deep Impact?
Jessica Sunshine (JS): Deep Impact showed us that there appears to be, at least in the upper tens of meters, at least equal if not more dust to water. So I guess it's more of an icy dirtball than a dirty snowball. Although if you've ever played with snowballs, you'll know that it doesn't take very much dirt to darken the snow.
AM: The nucleus had dust but not rocky material?
JS: The nucleus is partly made of microscopic silicate grains, and is actually very fluffy. It's like talcum powder at best. And given that it's got a lot of ice in it, I think it's more like high altitude snow, that sort of powdery good skiing.
Another thing that is worth noting is there is layering. That's consistent with the photo interpretation of the images, and with what we're seeing from the spectrometer. So on some level, there's an onion flavor to this snowball.
AM: You know there's layering because each layer is a different constituent material?
JS: I wouldn't say that it's a simple stratigraphy of a layer of organics, a layer of water ice, and a layer of silicates. I think it's too early to say exactly what the layers are.
AM: Do you know how many layers the nucleus consists of?
JS: I couldn't give you a number at this point.
AM: At least two.
JS: Well, at least three I would say. The images show scarps and you can see layers. There's no way to create this topography on the surface unless you've had differential melting. We probably impacted a relatively old part of the comet, or one that had been stripped.
AM: So the nucleus isn't generally uniform in composition? Is there a harder clump here and a fluffier part there"¦
JS: It's hard to imagine why the nucleus would be made of fundamentally different materials. The impact material was extremely fine, and we know the impact process didn't pulverize the stuff. It was already that way, and it's probably always been that fine. Therefore it suggests the nucleus is globally that way.
I think it's more likely that the nucleus started out as a uniform body, and there were subsequent processing events that were triggered by it coming in closer to the sun. Different parts have experienced different amounts of heating.
We know that comets formed in the early solar system 4.6 billion years ago. At some point comet Tempel 1 was kicked out of the outer solar system beyond Neptune, where it formed, and evolved into its current short period orbit near Jupiter, and it's come around the sun many times. But the surface may have different ages. This is the first comet that unquestionably has impact craters - we saw large, 200- to 300-meter impact craters right between where we landed - and that says the surface has been around for a long time. But there are younger, smooth areas that were resurfaced at some point, somehow.
It's also worth noting the large natural outburst that the comet experienced on June 22. The spectrometer captured that, and we clearly saw differences between what happened during that natural outburst and the impact. We haven't had a chance to really think about that, except to note it. The spacecraft was much further away from the nucleus at that point, and the signal-to-noise isn't as good, but still, there's something to be learned there. We've learned by watching Tempel 1 that outbursts happen very frequently. We weren't surprised by that, but we didn't know that.
AM: Do you think the natural outbursts are caused by pockets of ice getting warmed by the sun as the comet rotated?
JS: We can tie them to specific points in the light curve, which then go back to specific parts of the nucleus, so they do happen when certain things are coming into the sun. We know there's not water ice on the surface, because the surface temperature is too high for there to be ice. The minimum temperature that we saw was about 225 Kelvin, and ice sublimes - turns directly from a solid to a gas - at 200 K. So ice can't be on the surface unless it's covered in other things that provide enough thermal insulation, like organics or sooty stuff. The outbursts could be due to ice in the subsurface that gets heated.
AM: Comets are thought to have played an important role in the origin of life, through the delivery of water and organics. Much of the Earth's oceans are believed to have been delivered by comets. Based on what we've learned from Deep Impact, do we have to rethink any of that or is it still a valid theory?
JS: I think it's perfectly valid. Nothing we saw is inconsistent with that. I think what Deep Impact showed us is that the materials that were inside the comet, if anything, have more organics than we thought. Tempel 1 could have been rock on the inside. There could have been no organics, or the organics were just a minor component sitting on the surface. That's clearly not the case. The building blocks are there.
So then it becomes a theoretical question of how do you get them here. Certainly most of Earth's water should have disappeared in the early time period when we had heavy bombardment and all that volcanism. We know comets hit us, we know they hit us a lot and often, and we know they have water and organics.
We're seeing the same ingredients elsewhere - the Saturnian system is full of this stuff. So, stepping back from Deep Impact a little bit, spectroscopy is showing us there are more places in the solar system where you have the basic building materials for life as we understand it. So you have to think of the solar system as the source of everything we see on Earth.
I think one of the fascinating results from the Mars rover mission was that they found a meteorite. That wasn't surprising, but it still shows you that it's an interconnected system, and you really do have to understand the solar system in order to understand us.
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