Transcript Deep Impact: Excavating Comet Tempel 1 Michael F. A’Hearn and The Deep Impact Team.

Deep Impact: Excavating Comet Tempel 1
Michael F. A’Hearn
and
The Deep Impact Team
Outline
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Review of Mission and Summary of Data
Density, porosity, & strength (energy balance)
Layering
Where is the ice?
Natural Outgassing
Activity & Outbursts
Dust/Ice Ratio
Craters
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mfa - 2
Deep Impact
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Mating of flyby with impactor,
April 2004
Last step prior to system
environmental testing
Impactor
– 1/3 ton
– 50% copper
– Impactor Camera
• 10 rad/pixel
• White light
•
Flyby
– 2/3 ton
– Medium Res camera
• 10 rad/pixel
• 8 filters
– High Res Camera
• 2 rad/pixel
• 8 filters
– Near-IR Spectrometer
• 10 rad/pixels & slit
• 1.05 <  < 4.8 m
• 230 <  < 700
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Interplanetary Trajectory
Launch
Jan. 12, 2005
Sun
S/C
X
Earth
Orbit
Earth at
Encounter
Tempel 1 Orbit
(5.5 yr Period)
Impact!
July 4, 2005
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Encounter Schematic
Impactor Release
E-24 hours
AutoNav Enabled
E-2 hr
ITM-2
E-48 min
ITM-1 Start
E-88 min
ITM-3
E-15 min
Tempel-1
Nucleus
64
kbps
2-way
S-band
Crosslink
500 km
Flyby S/C
Deflection Maneuver
E-23.5 hr
Science and
Autonav Imaging to
Impact + 800 sec
Flyby S/C Science
And Impactor Data
at 175 kbps*
Shield Mode
Attitude through
Inner Coma
Flyby Science
Realtime Data
at 175 kbps*
TCA +
TBD sec
Flyby S/C Science
Data Playback at 175 kbps*
to 70-meter DSS
Look-back
Imaging
* data rates without Reed-Solomon encoding
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Key Results
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Approach held as many surprises as the impact event
Results from approach
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Dust has normal size distribution, overall similar to but slightly less than predicted contrast with dust after impact
Topographic features are very puzzling
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Remarkably frequent natural outbursts - rule out exogenic theories of origin
Ice on surface, but not responsible for ambient outgassing, more likely frost from
subsurface sublimation during “night”
Nucleus is chemically heterogeneous - evolutionary or primordial?
Must rethink layering and active areas
Must rethink concept of active areas
Results from impact
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Ultra low strength (200 Pa) and very low gravity => high porosity
Very different size distribution
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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 and others)
Ice is very near the surface (H2O <~ 20 cm; CO2 <~ 2 m)
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Smooth areas that look like flows, but with puzzling stratigraphy
Many layers, possibly related to origin from cometesimals
Very non uniform distribution of “craters” in different stratigraphic units
Only two features are different in photometric behavior
CO2 and organics are enhanced relative to water below the surface
Recall Harwit’s book, Cosmic Discovery
Data are all public at PDS-SBN (as of January 2006)
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26 July 2007
Vis images reasonably well calibrated
IR spectra not well calibrated
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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 on oblique
surface (~35° from
horizontal). Ecliptic north is
roughly near the bottom
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QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
mfa - 7
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
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
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MRI Movie
• Frames every 62 msec
• Initial stages of excavation
only
• Small “poof” that goes
rapidly to left at onset is
– hot, self-luminous plume
– vapor + liquid or solid
particles
– Moves @ 5 km/s
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
• 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
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Volatile Composition
2.00
Pre-Impact Nucleus
Model
H2O
2
Radiance (W/[m sr µm])
Impact
1.50
1.00
CO2
CH-X
0.50
HCN
0.00
2.0
2.5
3.0
3.5
4.0
4.5
Wavelength (µm)
First 0.2 sec
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ITS Sequence
• ITS images - impact site indicated by arrows (now right side
up - ecliptic north in upper right quadrant, sun to right)
• Sense of rotation - top is approaching (P ~ 40 hrs)
• Oblique impact - 36° from horizontal by shape model but 20
to 35° from assuming circular craters
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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-orless planar areas, i.e. the
shape is as close to
pyramidal as to ellipsoidal
• Impact site is between two
craters near bottom of
image.
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Shape Model
Volume well determined
from back-illuminated limb
even though topography is
determined only on one
side
g is a global measurement
Our g is consistent with
non-grav acceleration
models
Uncertainties
Gas acceleration
early stage only!
Angle of ejection
Neither likely to be a
very large effect
If dust/ice by mass ~1
porosity > 80%
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Why Are Shapes So Different?
• Nuclei are obviously different
– Overall shape, topographic features, amount of activity
– Nothing is obviously correlated with dynamics or age except that
the only Oort cloud comet is more active than the others
• What is common?
– Source of activity? Depth of ice? Heterogeneity? Cratering?
Other topographic features? Basic physics of how comets work?
Origin?
• What is the pattern that will tell us about origin of solar
system?
Basilevsky & Keller 2007 Sol.Sys.Res.41, 109
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Fallback of Ejecta Yields g
• Assuming ballistic
trajectories and gravity
scaling!!!
• Limit of lift-off of cone gives
upper limit to strength
– Orig <= 200±100 Pa
– now < 12 kPa
– Could --> 0
• Measure width of base of
plume
• Yields local gravity about 50
mgal at impact site (g =
0.05 cm/s2) & escape
velocity ~1.3 m/sec
• Shape model yields mass,
2x1016 g, assuming uniform
density.
• Density = 0.4 g/cc (if
uniform; was 0.6±0.3 in
Science paper
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Richardson et al. 2007, Icarus, submitted
mfa - 15
Does Gas Acceleration Matter?
• Our model assumes ballistic trajectories
– Adding gas acceleration adds many free parameters
• Total gas ~4x1032 molecules, mostly H2O
• Much H2O is still ice at I+45 minutes
– No quantitative model yet for amount of ice
– Pure ice grains should last ~5 hours at 1.5 AU (Patashnick
& Rupprecht 1977)
– Grains come out in a hollow cone while subsurface
volatiles (immediate sublimation) come out in center of
cone dragging extra grains (Schultz experiments)
– Cone still optically thick up to ~7 km at I+45 (Holsapple &
Housen, in press)
• Assume total gas over 5 hours from sphere of R~3 km
– Average flux ~< 0.1 sub-solar free sublimation at 1.5 AU,
i.e., smaller drag than normal outgassing
• Gas drag matters but probably not a major change
– Ignoring lateral acceleration probably overestimates g
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Strength
• Lack of detachment of ejecta means upper limit only!!!
Strength itself not measurable
• What kind of strength
– Tensile mostly, rather than shear or compressive
(compressive >> shear ~ tensile)
– Dynamic, post-shock strength rather than static
• How Measured?
– Max height of detachment (few 100m) yields upper limit
on minimum ejection velocity
– Equate upper limit on KE/m to upper limit on dynamic
strength (using bulk density) --> 200 Pa (talcum powder)
• Issues
– Static strength > dynamic strength
– Data consistent with 12 kPa static strength (Holsapple &
Housen)
– Implies less mass in ejecta, still strength << ice or rock
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Energy & Momentum
• K.E. of impactor: 19 GJ
• Orbital Change
– < 1 GJ from change in orbital energy
– Momentum transfer efficiency perhaps 2x-3x (model
dependent)
– Depends on obliquity of impact (ejecta momentum not
anti-parallel to impactor momentum)
• Hot Plume (~100 ton)
– K.E. of plume has most of the impact energy
• May need additional internal energy source
– Sublimation and melting has 10% or less of impact energy
• Excavated material (~104 ton)
– K.E. << 1% of impact energy, but momentum exceeds
input momentum
– Sublimation of water MUST be due to sunlight evaporating
excavated ice; total energy of sublimation >> impact
energy
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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-orless planar areas, i.e. the
shape is as close to
pyramidal as to ellipsoidal
• Impact site is between two
craters near bottom of
image.
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Layers
Deep layers wrap around,
roughly parallel to “top”
facet.
Regional differences in
erosion and array of
topographic forms
Why is the boundary
between regions parallel to
body layers?
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Other Features
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Thomas et al. 2007 Icarus 187, 4
mfa - 21
Layering on east facet
• Red-blue anaglyph for 3-D
view shows that layer wraps
around to dark side
• Fringes on right are an
artifcact of deconvolution of
the HRI image responding
to the boundary of the
image
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TALPS Model
• Observationally, layering is
not onion skin, but rather
randomly oriented but
extending over large areas
• Accretion regime allows
flattening and ejecta
blanket rather than
fragmentation during
collisions
• Based on ideas of Donn
(interpenetration +
compaction of fractals) and
Sirono & Greenberg
(deformation + compaction
of fractals)
• Talps are primordial, not
due to geologic evolution
• Expect compositional
differences in talps
26 July 2007
Belton et al. 2007 Icarus, in press
COSPAR Workshop
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Thermal Map of Nucleus
• First real thermal map of a
nucleus
• Consistent with STM plus
roughness to warm areas
near terminator; I~<20 W
K-1 m2 s0.5
• No locations as cold as
sublimation temperature of
H2O ice
• Therefore ice must be below
the surface but “not far”
below
• Diurnal skin depth 3 cm,
annual skin depth 0.9m for
plausible separation of
components of I
Groussin et al. 2006 Icarus, submitted
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Where Is the Ice?
• No bulk ice on surface to explain active areas - all the
major sources of water are below the surface
• There is a lot of H2O ice <10 cm below the surface over
large areas
• There is a lot of CO2 within 1 m of surface in some
reasonably large areas
• There is considerable heterogeneity in chemical
composition as well as surface topography
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Anomalously Colored Regions
Deconvolved
High Resolution
Color Images
Sunshine et al.
2006, Science
311, 1453
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Modeling Surface Water Ice
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Nominal (non-ice) nucleus + laboratory water ice
– 3-6% water ice
– 30 ± 10 µm size particles
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Not enough surface to be significant in overall outgassing
Frost from source of outbursts on shoulder?
1.6
1.6
Water Ice Absorptions
Water Ice Absorptions
1.4
1.2
1.0
Ice-Rich Nucleus
3% 70 µm Ice +
97% Non-Ice Nucleus
9% 5 µm Ice +
91% Non-Ice Nucleus
0.8
Reltaive Reflectance
Reltaive Reflectance
1.2
0.8
Ice-Rich Nucleus
4% 20 µm Ice +
96% Non-Ice Nucleus
Non-Ice Nucleus (offset)
0.4
0.6
1.2
1.6
2.0
2.4
Wavelength (in µm)
2.8
3.2
1.2
1.6
2.0
2.4
Wavelength (in µm)
2.8
3.2
Sunshine et al. 2006 Science 311, 1453
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Activity off Limb
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Ice in Early Ejecta
• Each row is a spectrum with
slit fixed in position
• Early rows are 0.7-sec
exposures
• Later rows (longer slit
lengths) are 1.4- and 2.8sec exposures
• Ice is present by fourth
spectrum, i.e., by <3
seconds after impact
• Ice is concentrated in the
down-range direction
Sunshine et al. 2007. Icarus, in press.
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Ejecta - Icy and not
Sunshine et al. 2007.
Icarus, in press
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Structure Summary
• Fine-grained material
– No boulders
– No hard crust
• Grains are fragile
aggregates
– fragment during excavation
– Fragements ~1-3 m
• Layers within 1 impactor
diameter of surface at
impact site
– Topmost layer (few cm?)
devoid of ice (see later
slides)
• Layers are ubiquitous
– Varying thickness
– Some may be primordial
– Smooth layers not yet
explained
26 July 2007
Schultz et al 2007, Icarus, submitted
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Detection of Asymmetric Inner Coma
Ecliptic North
Sun
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26 July 2007
1 hour before impact
~440 m/pixel resolution
Northern and southern regions examined
Spectra show comparable H2O but
factor of 2 increase in CO2 relative to
H2O in the south
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Spatial Distributions Vary by Species
P = positive
rotational pole
E = Ecliptic
north
S = Sunward
Dust is better correlated with CO2 than
with H2O, but not perfectly with either
Feaga et al. 2007. Icarus, in press
Farnham et al. 2007. Icarus, 187, 26
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Results from Spectral Maps
• Optically thin at edges of field
• Ambient Q(H2O) ~ 4x1027 s-1
– Good agreement with ground-based measurement in FOV
1000x larger
• Q(CO2) ~ .07 Q(H2O)
– Few x variation with azimuth
• Dust not well correlated with either H2O or CO2
– Better with CO2
– How does dust get lifted?
– Models for dust ejection need to be rethought
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Approach Photometry
Outbursts common - typically 2 per week
Outbursts correlated with rotational phase (2 phases with at least 3 outbursts each)
Thus, outbursts are endogenic and related to surface insolation
Probably super-volatiles close below surface but
Dina Prialnik can make them with water and
Stochastic nature of outbursts not understood
08:51
09:52
11:57
10:47
A’Hearn et al. 2005 Science 310, 258
2 July Outburst by D. Lindler
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Monitoring OH
Küppers et al., 2005 Nature 437, 987
Observations with OSIRIS on Rosetta
Enables determination of total water released in
impact ~ 4000 tons original estimate, revised
upward (4500 - 9000 tons) with better
calibration (Keller et al. 2007 Icarus 187, 87)
Many other observers (incl. ODIN & groundbased OH) find 4 to 10x103 tons of water
Other species (CO & CO2 best determined) of
order 5-10% of water
Total volatiles - est 8x103 tons ±3×103
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Refractory Ejecta
I - preimpact
Schleicher et al.
2006 AJ 131, 1130
Above - a/
Model - a/
• Schleicher et al. need predominantly 0.5 to 2.5 m grains
– Avoid estimating total mass, but no particles with  < .13
– Similar analysis of OSIRIS data by Jorda et al. finds no need for
an unusual size distribution
– Optical data are not sensitive to particles >> few microns
• Most distributions have total mass sensitive to largest particle
• Very hard to estimate total mass of refractories!!!
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Dust/Ice Ratio
• Fallback
– 90% of solid ejecta fall back to surface in first hour
– Does the ice get buried deeply enough to not vaporize?
– Or does the H2O correspond to all ejecta while refractory
mass from remote sensing corresponds only to escaping
fraction?
• Size cutoff
– Radiation pressure effects do not require an unusual
distribution to explain optical data (e.g., Jorda et al.) but
optical data not sensitive to large particles
– There is IR evidence for changes in the size distribution
after impact compared to before (Lisse et al. Science,
Subaru & Gemini data - Diane Wooden’s talk)
• Quoted dust masses vary dramatically
– Probable range 103 to 104 tons
– I.e. Dust/Ice ratio 0.1 to 1
– If fallback of ice grains still leads to excess sublimation,
Dust/Ice ratio could go up to 10
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Dust Before and After
4 Dust hits measured by ADCS correlated with image
pointing between -21 and -3 s
Last hit was at -3 s, after last transmitted image and had
largest mass
Expectation values of mass
3 between 1 - 10 mg, 1 (last hit) 100-1000 mg
Impactor cross-section about 1.1 m2
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26 July 2007
Size distribution needed to successfully model
SST spectra
Distribution of surface area per unit mass before (ambient release) and after
(mechanically excavated grains) impact
While largest particles still dominate total
mass, they no longer dominate total crosssection
Interpretation is that surface materials are all
weak, large aggregates of smaller pieces with
typical size of a few m
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(Pre-Post)/Pre = Ejecta/Pre-Impact Coma
Best-Fit Model

Spitzer IRS I+45 Min
(95% C.L. = 1.13)
Carbonates
PAHs
Amorph
Carbon
Water Ice
Smectite (clay)
FeMgS2
Pyroxenes
Olivines
Lisse et al. 2006 Science in press
An Aside on Craters
• There are ~ 60 circular features on Tempel 1
• Size distribution matches impact craters on Ida, Gaspra,
etc. (Thomas et al. in press)
• Size distribution on Wild 2 is bimodal and nothing like
the size distribution other places in the solar system
(Thomas, priv. comm.)
• Circular features appear in many different layers on
Tempel 1
• The two largest “craters” appear to be exhumed - when
did they form & how did they get filled?
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The Bottom Line
DI will produce new results for a long time
Only need is scientists to use the data
Comets are still fascinating and full of surprises
More missions to comets are needed
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More Missions
• EPOXI
– Uses the DI flyby spacecraft
– Flies to 85P/Boethin in Dec
2008
– On the way observes
transits of extrasolar
planets
• Stardust NExT
– Uses the Stardust
spacecraft
– Flies to 9P/Tempel 1 on 14
Feb 2011
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Backup Slides
Approach Photometry
Outbursts common - typically 2 per week
Outbursts correlated with rotational phase (2 phases with at least 3 outbursts each)
Thus, outbursts are endogenic and related to surface insolation
Probably super-volatiles close below surface but stochastic nature of outbursts not understood
A’Hearn et al. 2005 Science 310, 258
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Ejecta over Time
Time
Impact
Dust
(2 µm reflectance)
H2O Ice Absorption
H2O Gas Emission
Sunshine et al. 2007 Icarus, in prep.
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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
D. K. Yeomans
CSR page 1-12
• 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
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Deconvolved HRI Image
• IR + green + violet
• Forced to average gray
• Note very localized “bluish”
areas
• Note curvature of ejecta in
up-range direction
– Consistent with lab
experiments
– Later (I+195s) detachment
of these rays from crater
suggests layering
– Layering also suggested by
hot plume in previous
movie
–
Schultz et al., in prep.
• Note smoothness of ejecta
in radial direction
– Primarily small particles
– Rays from initial conditions
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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
•
•
26 July 2007
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
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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
26 July 2007
Radiance (W/[m2 sr m])
Spectra off Southern Limb
H2O
CO2
CO2
CH-X
CH-X
Pre-Impact
Post-Impact
Wavelength (m)
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Average Early Ejecta
2.0
1.0
0.9
0.8
Continuum Removed Reflectance
2
Radiance (W/[m sr µm])
1.4
Thermal
0.7
0.6
0.5
0.4
0.3
Solar
Continuum Removed
1.2
1.0
0.8
0.6
0.2
2.0
2.5
3.0
3.5
4.0
4.5
2.0
2.5
3.0
3.5
4.0
Wavelength (µm)
Wavelength (µm)
3 µm Absorption
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•
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Present even without thermal component
Consistent with microscopic crystalline H2O ice particles
Near-Surface Water Ice
Ejecta are cold, thus unprocessed!!
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Dust on Impactor
4 Dust hits measured by ADCS correlated with image pointing between -21 and -3 s
Last hit was at -3 s, after last transmitted image and had largest mass
Expectation values of mass
3 between 1 - 10 mg, 1 (last hit) 100-1000 mg
Impactor cross-section about 1.1 m2
26 July 2007
COSPAR Workshop
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Size Distribution at E+45m
Lisse et al. 2006. Science, in press
26 July 2007
• Size distribution needed to
successfully model SST
spectra
• Distribution of surface area
per unit mass - before
(ambient release) and after
(mechanically excavated
grains) impact
• While largest particles still
dominate total mass, they
no longer dominate total
cross-section
• Interpretation is that
surface materials are all
weak, large aggregates of
smaller pieces with typical
size of a few m
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Nature of Ejecta
•
•
•
•
•
Total ejecta from scaling laws - 1-2x107 kg
Total volatiles (Küppers H2O, Biver H20, Feldman CO) - 5x106 kg, but with
Schleicher’s H2O, 15x106 kg
Mass of refractory dust sensitive to large end of size distribution ~104 kg
Dust/Ice ratio 100±0.5 from considering various sources
Particle size - mostly < 10 m
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•
•
Superheat
Mass per cross-section
Radiation pressure model (Schleicher)
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•
Strong mid-IR emission features (Lisse et al.)
Implies pre-existing 1m grains or very weak aggregates thereof to depths of tens of meters
–
•
Particle composition
–
Mix of silicates, organics, and volatile solids
•
–
•
But see possible contrary view by Keller
Ice detected in situ in ejecta
Additional refractories seen with Spitzer (carbonates?)
Gaseous Ejecta
• In situ large increase in CO2/H2O), larger increase in organics/H2O
• Remote sensing - less variation relative to water because water came
out as ice & then sublimed
• Some species large variation
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Key Results
•
•
Approach held as many surprises as the impact event
Results from approach
–
–
Dust has normal size distribution, overall similar to but slightly less than predicted - contrast with dust
after impact
Topographic features are very puzzling
•
•
•
•
–
–
–
–
–
•
Remarkably frequent natural outbursts - rule out exogenic theories of origin
Ice on surface, but not responsible for ambient outgassing, more likely frost from subsurface sublimation
during “night”
Nucleus is chemically heterogeneous - evolutionary or primordial?
Must rethink layering and active areas
Must rethink concept of active areas
Results from impact
–
–
Ultra low strength (200 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
and others)
Ice is very near the surface (H2O <~ 20 cm; CO2 <~ 2 m)
•
•
Smooth areas that look like flows, but with puzzling stratigraphy
Many layers, possibly related to origin from cometesimals
Very non uniform distribution of “craters” in different stratigraphic units
Only two features are different in photometric behavior
CO2 and organics are enhanced relative to water below the surface
Recall Harwit’s book, Cosmic Discovery
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Bigger Implications & Issues
Comets formed gently to preserve bulk porosity
– How does this happen?
– TALPS scenario proposed by Belton et al. submitted to Icarus
– Does scenario also require radial mixing in protoplanetary disk?
• Sample return easier than previously thought
– Penetrating to the water ice is easy if one can hold the spacecraft
down
– E.g., 10N thruster may be enough for coring a sample
– Large (1 gram) particles are fragile
• “Active areas” are not evident on the surface
– Bulk of H2O sublimation is from sub-surface but close enough to
surface to see diurnal variation (~< 10 cm)
– CO2 remembers seasonal thermal wave at negative pole (depth
~1m)
– Dust jets better correlated with CO2 than with H2O, but not well
with either
– Possible (probable?) primordial chemical heterogeneity in volatiles
would require radial mixing in proto-planetary disk
– Outbursts are endogenous & deposit frost of ice very locally
between outbursts
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Shape Model
Volume well determined
from back-illuminated limb
even though topography is
determined only on one
side
g is a global measurement
Our g is consistent with
non-grav acceleration
models
Uncertainties
Gas acceleration
early stage only!
Angle of ejection
Neither likely to be a
very large effect
If dust/ice by mass ~1
porosity > 80%
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The darker area with close round depressions is the
part of Tempel 1 that most closely resembles Wild-2.
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Fallback of Ejecta Yields g
• Simulation assumes 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) & escape
velocity ~1.3 m/sec
• Shape model yields mass,
2x1016 g, assuming uniform
density.
• Density = 0.35±0.12 g/cc
(if uniform; was 0.6±0.3 in
Science paper; this error 1 Icarus paper will quote 2 errors)
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Richardson et al. 2006, Icarus, submitted
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Thermal Map of Nucleus
• First real thermal map of a
nucleus
• Consistent with STM plus
roughness to warm areas
near terminator; I~<20 W
K-1 m2 s0.5
• No locations as cold as
sublimation temperature of
H2O ice
• Therefore ice must be below
the surface but “not far”
below
• Diurnal skin depth 3 cm,
annual skin depth 0.9m for
plausible separation of
components of I
Groussin et al. 2007 Icarus, 187, 16
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The darker area with close round depressions is the
part of Tempel 1 that most closely resembles Wild-2.
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Where is ice?
• Uprange rays have no ice
and come from a layer
much less than size of
impactor (<few 10s of cm)
• Downrange rays have ice
and come from a layer
comparable to impactor size
(~ 1m)
• Ice appears downrange
within 2 seconds
Dust
Ice
Sunshine et al. 2006 LPSC
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EPOXI -
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