# Presentation: 509856

## Deep Impact: Excavating Comet Tempel 1

- Michael F. A’Hearn
- and
- The Deep Impact Team

## Anatomy of a Comet

**Ion Tail**

**Dust Tail**

**Coma**

**Nucleus**
**(1-10 km)**

**80,000 km**

**33,000,000 km**

**To Sun**

- Fermilab

- 26 Apr 2006

## Un-numbered Periodic Comets

**Un-numbered Periodic Comets**

**Numbered Periodic Comets**

**Un-numbered Centaurs**

**Numbered Centaurs**

- Fermilab

- 26 Apr 2006

## Oort Cloud and Kuiper Belt

- Fermilab

- 26 Apr 2006

## Scientific Objectives

- Primary Scientific Theme
- Understand the differences between interior and surface
- Determine basic cometary properties
- Search for pristine material below surface
- Secondary Scientific Theme
- Distinguish extinction from dormancy
- Additional Science Addressed
- Address terrestrial hazard from cometary impacts
- Search for heterogeneity at scale of cometesimals
- Calibration of cratering record

- Fermilab

- 26 Apr 2006

## Interplanetary Trajectory

- Fermilab

- 26 Apr 2006

## Encounter Schematic

**Tempel-1**
**Nucleus**

**Shield Mode**
**Attitude through**
**Inner Coma**

**Science and **
**Autonav Imaging to**
**Impact + 800 sec**

**ITM-1 Start**
**E-88 min**

**ITM-2**
**E-48 min**

**ITM-3**
**E-15 min**

**Impactor Release**
**E-24 hours**

**TCA  +**
**TBD sec**

**AutoNav Enabled**
**E-2 hr**

**Flyby S/C**
**Deflection Maneuver**
**E-23.5 hr**

**2-way**
**S-band **
**Crosslink**

**Look-back**
**Imaging**

**500 km**

**Flyby S/C Science **
**Data Playback at 175 kbps***
**to 70-meter DSS**

**Flyby Science **
**Realtime Data**
**at 175 kbps***

*** data rates without Reed-Solomon encoding**

**Flyby S/C Science **
**And Impactor Data**
**at 175 kbps***

**64**
**kbps**

- Fermilab

- 26 Apr 2006

## Key Results

- Approach held as many surprises as the impact event
- Results from approach
- Dust has normal size distribution, overall similar to but somewhat less than predicted - contrast with dust after impact
- Topographic features are very puzzling
- Smooth areas that look like flows
- Many layers, possibly related to origin from cometesimals
- Only two features are different in photometric behavior
- Remarkably frequent natural outbursts - rule out exogenic theories of origin
- Ice on surface, but not responsible for ambient outgassing
- Nucleus is chemically heterogeneous
- Results from impact
- Ultra low strength (65 Pa) and very low gravity => high porosity
- Very different size distribution
- Peaked at few-micron sizes
- Led to obscuration of final crater
- Implies surface particles, both ice and silicate, are weak aggregate particles (as predicted by Mayo Greenberg)
- Ice is very near the surface
- CO2 and organics are enhanced relative to water below the surface

- Fermilab

- 26 Apr 2006

## Impactor Approach

- Original movie (not registered) to show pointing jitter
- Note one big jitter early due to ITCM.  Note big jitters in last 30 seconds due presumably to dust hits
- Orientation is “upside down” mirror image of “sky” to visualize landing.  Ecliptic north is roughly near the bottom
- Angle of incidence is oblique (~30° from horizontal).

- Fermilab

- 26 Apr 2006

## HRI Color Composite

- Red, green, and blue
- Note effects of deconvolution on HRI images
- Color constant over nearly all surface except a couple of bright areas near “top” brighter in UV than most of surface.
- Brightness variations 2x
- Large smooth areas (no craters), scarps, ridges, old cratered terrain lower than high smooth terrain; large areas have low curvature.  Impact craters?

- Fermilab

- 26 Apr 2006

## ITS Composite Image

- Note geological features
- Large, smooth surfaces
- Round features = craters? (size-freq plot consistent)
- Stripped terrain (old)
- Scarps
- Evidence of layers
- Overall Shape
- Effective radius 3.0±0.1 km
- Max-min diameters 7.6 and 4.9 km but still uncertain
- Well-mapped surface is mostly in 3 large, more-or-less flat areas
- Impact site is between two craters near bottom of image.

- Fermilab

- 26 Apr 2006

## Differences Among Nuclei

*Stardust Team*

*L. Soderblom*

*Stardust Web Site*

- Fermilab

- 26 Apr 2006

## Highly Stretched ITS

- Different stretch on different parts of image (not as much dynamic range as Stardust composite on previous slide)
- Jets not yet associated with specific areas on surface (except for impact jet)

- Fermilab

- 26 Apr 2006

## MRI 9001050

- MRI 9001050, 9.8 m/pixel, stretched and displayed to emphasize the indirectly illuminated area beyond the terminator.
- Due to the highly variable intensity of the indirect illumination, the boundary between directly and indirectly illuminated areas is no longer defined by a single brightness contour.
- The spacecraft is now flying even further “under” the nucleus.
- Rough and smooth terrains are now seen beyond the terminator by sunlight scattered by the plume.
- Note that there is a diffuse bright area located in the same area as seen in the ITS images, relative to the previously noted surface features.
- Note that this diffuse bright area is located at the boundary of what appears to be another smooth area, not previously seen.

- Fermilab

- 26 Apr 2006

## Approach Photometry

- Fermilab

- 26 Apr 2006

## Outburst Differences 2 Jul

- Clockwise from upper left
- 08:51, 09:52, 10:47, 11:57
- Outburst was at 08:34
- All differenced with 07:47
- Deconvolution after difference
- Scale  ~3.3 km/pixel
- All are clear filter
- All ejecta are in NE quadrant with peak in ENE

- Fermilab

- 26 Apr 2006

## Shape – rendered with Hapke scattering model

- 1 km

- HRI Image at encounter, 50° rotation behind left

- Fermilab

- 26 Apr 2006

## June 14 Outburst - 2 Perspectives

- Celestial north up

- Outburst near peak B of light curve,
-    *i.e.,* when “hidden” side is sub-solar
- Structure implies short-duration
- Best time of outburst by extrapolating back the HST velocity

- UT 15:19/UT 14:12
- v = 144 m/s

- Ecliptic north up

- UT 13:18 - UT 09:18
- v ~ 0.2 km/s

**Deep Impact**

**HST**

- Fermilab

- 26 Apr 2006

## Thermal Map of Nucleus

- First real thermal map of a nucleus
- Consistent with STM
- No locations as cold as sublimation temperature of H2O ice
- Therefore ice must be below the surface but “not far” below

- Fermilab

- 26 Apr 2006

## Anomalously Colored Regions

**Deconvolved**** High Resolution Color Images**

- Fermilab

- 26 Apr 2006

## Modeling Surface Water Ice

- Nominal (non-ice) nucleus + laboratory water ice
- 3-6% water ice
- 30 ± 10 μm size particles

- Fermilab

- 26 Apr 2006

## Activity off Limb

- Fermilab

- 26 Apr 2006

## Detection of Asymmetric Inner Coma

- 1 hour before impact
- ~440 m/pixel resolution
- Northern and southern regions examined
**Spectra show comparable H****2****O but factor of 2 increase in CO****2**** relative to H****2****O in the south**

- 1 hour before impact
- ~440 m/pixel resolution
- Northern and southern regions examined

- Fermilab

- 26 Apr 2006

## 1st Distribution Map of CO2 for a Comet!

- 1st Distribution Map of CO2 for a Comet!

- Fermilab

- 26 Apr 2006

## Possible Scenarios

- Crater formation on an intact nucleus
- Gravity controlled crater
- Compression controlled crater
- Split nucleus
- Crater formation on an intact nucleus
- Strength controlled crater
- Aerogel-like capture of the impactor
- Shattered nucleus
- Transit through the nucleus
- Above are roughly in order of decreasing probability (as guessed by the PI)
- N.B.:  K.E. of Impactor << Gravitational Binding Energy of Cometary Nucleus
- N.B.:  v2/2 > maximum energy per mass of any chemical explosive

*D. K. Yeomans*
*CSR page 1-12*

- Fermilab

- 26 Apr 2006

## HRI Movie

- Much slower frame speed than with MRI
- Longer period included in movie
- “Vertical bar” immediately after impact is bleeding of the saturated CCD, not real ejecta
- Note shadow cast by optically thick ejecta

- Fermilab

- 26 Apr 2006

## MRI Movie

- Frames every 62 msec
- Initial stages of excavation only
- Small “poof” that goes rapidly to left at onset is hot, self-luminous
- Later ejecta are cold
- Water ice survives the ejection
- Speeds start at few x 100 m/s and drop to below escape velocity as excavation continues

- Fermilab

- 26 Apr 2006

## Impact Flash in MRI

- MRI Brightness
- 0 - 1s
- (62 ms between images)

- Fermilab

- 26 Apr 2006

## Fallback of Ejecta Yields g

- Assuming ballistic trajectories and gravity scaling!!!
- Measure width of base of plume
- Yields local gravity about 50 mgal at impact site (g = 0.05 cm/s2)
- Shape model yields mass, assuming uniform density.
- Density = 0.4±0.1 g/cc (if uniform; was 0.6±0.3 in Science paper)

- Fermilab

- 26 Apr 2006

## Spectral Mapping

- Slit runs right -to-left in normally displayed images
- One end of slit points to sun when solar panels are normal to sun direction
- Spatial scans are made in orthogonal direction
- Get a spectrum for each point in slit at each point in scan.
- Timing is complicated
- Slit read out one end to the other (r to l in this image)
- Spatial scans continuous, not step & integrate
- Net projected slit is tilted depending on scan rate
- Spectral resolving power >200 everywhere, ~450 at 1.8 m, ~700 at 1.05 m
- Currently believe calibration only 2.0 to 4.5 m

- Fermilab

- 26 Apr 2006

## Volatile Composition

- First 0.2 sec

- I+4 min

- Fermilab

- 26 Apr 2006

## Spectra off Southern Limb

- Post-impact spectrum obviously different from pre-impact spectrum
- Initial ejecta are hot but after few seconds ejecta leave nucleus cool
- At end of observations, composition seems to be asymptotic suggesting we excavated “deep enough”!
- CO (4.7 m) and minerals in reflection (1.05 to 2 m) waiting on improved reduction of spectra at ends
- Most species have optically thick lines so that column densities are not easy to determine

**CO****2**

**CH-X**

- Fermilab

- 26 Apr 2006

## Average Early Ejecta

**Thermal**

**Solar**

**Continuum Removed**

- 3 μm Absorption
- Present even without thermal component
- Consistent with microscopic crystalline H2O ice particles
- Near-Surface Water Ice
- Ejecta are cold, thus unprocessed!!

- Fermilab

- 26 Apr 2006

## Ejecta over Time

- Dust
- (2 μm reflectance)
- H2O Ice Absorption
- H2O Gas Emission

**Time**

**Impact**

- Fermilab

- 26 Apr 2006

## Monitoring OH

- Küppers et al., 2005 Nature
- Observations with OSIRIS on Rosetta
- Enables determination of total water released in impact ~ 4000 tons

- Fermilab

- 26 Apr 2006

## (Pre-Post)/Pre = Ejecta/Pre-Impact Coma 

**Best-Fit Model**

**PAHs **

**Pyroxenes**

**Spitzer IRS I+45 Min **

**FeMgS****2**** **

**Olivines**

**Carbonates**

- (Pre-Post)/Pre = Ejecta/Pre-Impact Coma

****************
**(95% C.L. = 1.13)**

- Lisse *et al.* 2006

**Amorph **
**Carbon**

- Water Ice

**Smectite (clay)**

- Fitting Spitzer Data with Small Particles

## Key Results

- Approach held as many surprises as the impact event
- Results from approach
- Dust has normal size distribution, overall similar to but somewhat less than predicted - contrast with dust after impact
- Topographic features are very puzzling
- Smooth areas that look like flows
- Many layers, possibly related to origin from cometesimals
- Only two features are different in photometric behavior
- Remarkably frequent natural outbursts - rule out exogenic theories of origin
- Ice on surface, but not responsible for ambient outgassing
- Nucleus is chemically heterogeneous
- Results from impact
- Ultra low strength (65 Pa) and very low gravity => high porosity
- Very different size distribution
- Peaked at few-micron sizes
- Led to obscuration of final crater
- Implies surface particles, both ice and silicate, are weak aggregate particles (as predicted by Mayo Greenberg)
- Ice is very near the surface
- CO2 and organics are enhanced relative to water below the surface

- Fermilab

- 26 Apr 2006

## Implications & Issues

- Comets formed gently to preserve bulk porosity
- Strength is remarkably low, at least in outer tens of m
- Can Rosetta hold itself down?
- Penetrating to the water ice is easy if one can hold the spacecraft down
- Thrusting into the surface may be enough to penetrate
- Large (1 gram) particles are fragile
- Lots of organic material below the surface
- Still need to determine the molecular composition
- Heterogeneity -
- Obvious topographic regimes: are they same structure?
- Differences among comets: are they similar in structure?
- Is the chemical heterogeneity (CO2/H2O) radial or cometesimal?
- When and where were the impact craters produced?

- Fermilab

- 26 Apr 2006

## The Bottom Line

- DI will continue to produce new results for a long time
- Comets are still fascinating and full of surprises
- More missions to comets are needed

- Fermilab

- 26 Apr 2006