[extropy-chat] Deep Impact on comet Tempel 1, DPS presentations

Amara Graps amara at amara.com
Wed Sep 14 23:31:48 UTC 2005



I saw someone recently post a link to an article about Deep Impact
findings on comet Tempel 1. Since I was at that meeting of the
article: the American Astronomical  Society Division of Planetary
Sciences (DPS), Cambridge, England:
http://www-outreach.phy.cam.ac.uk/dps2005/ I can a say a little bit
more.

The meeting gave many Deep Impact posters and presentations last
Wednesday, so I will first summarize by using the article in New
Scientist this week: Tales of the Unexpected" by Stuart Clark. The
article begins:

"The Deep Impact team had hoped that when the impactor spacecraft
hit Tempel 1, it would kick up a relatively small cloud of dust,
expose an area of pristine icy material underneath, and instigate
some spectacular jet activity. This is exactly what didn't happen.
The dust cloud was more than 10 times larger than expected, and the
effect on Tempel 1's activity was almost nil."

The article proceeds to compare the four close encounter of comets,
which are each very different from the other.


Halley:

S/C: Giotto
Encounter date: March 1986
Nucleus: 16x8x8 km
Distinguising features: Intense activity, no large craters

Borrelly:

S/C: Deep Space One
Encounter date: September 2001
Nucleus: 8x3x3 km
Distinguising features: Smooth plains, no large craters

Wild 2:

S/C: Stardust
Encounter date: January 2004
Nucleus: 5x5x5 km
Distinguishing features: Large surface depressions, nightside activity

Tempel 1:

S/C: Deep Impact
Encounter date: July 2005
Nucleus: 7x5x5 km
Distinguishing features: two impact craters, smooth plains

Brownlee of University of Washington says: "It's a mystery to me how
comets work at all. I think that some process is allowing heat to
get down below the surface of a comet and drive the activity from
the inside out, " he says.


The article says:

"The NASA scientists hoped their impactor would not only eject
material from them to analyse but also kick-start a new area of
research by exposing an area of pristine, icy material inside the
comet, and maybe that would provide a few clues to what drives comet
activity. Unfortunately, things didn't quite go according to plan.
The Deep Impact team thought their 370-kilogram impactor would
liberate about a month's worth of dust, based on normal emission
rates, but it now seems more likely that a whole year's worth
escaped the comet. One of the project scientists: Peter Schultz
said: "If I had to choose just one surprising result from this
encounter, it would be the amount of material thrown up," says
Schultz.

The article also describes the material. Michael A'Hearn says: "The
surface material can have no more strength than lightly packed snow,
otherwise we would not have seen that amount of dust." Another
surprise is the two craters. Although craters seem to be ubiquitous
on every other solid surface in the solar system, craters have never
before been seen on a comet.

The article goes on to say that they might not be impact craters,
however. The depressions have flat floors and their walls appear
like staircases, suggesting that they were caused by an explosion
within the comet. Because of the porous structure, light might
penetrate beneath the surface and heat the interior (Brownlee
thinks). The dark layers stop heat escaping, and pressure builds up,
eventually resulting in an explosion, and an unusually shaped
crater.

Unfortunately, the amount of dust released, combined with a focusing
fault on Deep Impact's high-resolution camera means that the images
the team hoped to take of the newly formed crater may now elude
them. There might be no way to confirm what happened.


Now from the talks and posters: some stated observations.

(O'Hearn)

800 s window to observe everything

Evidence of layers
Large smooth surfaces
Scarps
Stripped terrain
Outburst seen before from ground-based, jets not yet associated with
specific surface features
First thermal map of a nucleus, consistent with standard thermal model.
Primarily small particles in ejecta <<10 microns
Total ejected mass: ~1-2x10^7 kg, consistent with observed dust
Total mass of volatiles: ~3-6x10^6 kg
Composition: mix of silicates and volatiles, H20, CO2, CH-X, HCN
First ice from comet detected IN-SITU
Ejecta showed a large increase in organics relative to water

(Belton)

Spin period: 1.7 day
Outbursts are frequent, no correlation with any particular region

(Schultz)

The calculated crater size (not seen yet) is diameter 150 to 200 m,
but also could be 100 m, but deep.
Thirteen min after impact: ejecta plume 300-500 m across,
composition: H20, Ch-X, HCN, shadow on comet nucleus, which
indicates an attached curtain.
Density of material: 0.62 +/- 0.47 gr/cm^3
Temperature of nucleus: 285K
Near-surface water ice detected (near surface because it was the
first thing seen from the ejecta.
Tempel 1 is layered

(Groussin)

Temperature Map of Tempel 1 nucleus: 260-330K, temperature matches
the topography
No active areas have been detected.

(Meech)

Earth-based Observations:

At time of Deep Impact: 40 observatories, 130 observers
Prior to 2005, 229 nights of observations.
Dust expanded outward to ~30,000 km, 0.2 km/sec acceleration
Chemistry consistent with Oort cloud
Almost no radio activity
Dramatic change in mid-IR dust: pre to post impact
Dust size distribution peaked near 0.5-1.0 micron
Ejected dust consistent with small dust: submicron to micron
Mass of ejected material 10^6-10^7 kg

(Keller) "Deep Impact Observations from OSIRIS of Rosetta

Water production: 4.6 x 10^6 kg
Variability up to one week -> Large particles

(Sugita) poster 44.12
"Mid-IR Observations of Dust Plume induced by Deep Impact Collision (SUBURU)

DI excavated fresh cometary material containing fine silicate grains
with high crystallinity, very similar to active Oort cloud comets.
It probably reached a volatile-rich layer.
Dust size distribution peaked around sub-micron to micron size.
Total dust mass in plume ~10^6 kg


-- 

Amara Graps, PhD      www.amara.com
Istituto di Fisica dello Spazio Interplanetario (IFSI)
Istituto Nazionale di Astrofisica (INAF),
Adjunct Assistant Professor Astronomy, AUR,
Roma, ITALIA     Amara.Graps at ifsi.rm.cnr.it



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