[meteorite-list] Sunshine on Comets: Part I (Deep Impact)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Oct 3 12:39:57 2005
Message-ID: <200510031638.j93GcZ018125_at_zagami.jpl.nasa.gov>

http://www.astrobio.net/news/article1731.html

Sunshine on Comets: Part I
Astrobiology Magazine
October 3, 2005

Summary (Oct 03, 2005): Jessica Sunshine is the Deep Impact mission
scientist responsible for the onboard infrared spectrometer. In the
first half of this two-part interview, she discusses what the
comet's nucleus looked like before and after impact, and explains
why it's so difficult to piece together the spectroscopic data.


Sunshine on Comets
Part 1

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?

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 <http://deepimpact.jpl.nasa.gov/images/spectrometer2.jpg>
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.

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?

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.

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.

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.
Received on Mon 03 Oct 2005 12:38:34 PM PDT