Deep Impact

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Deep Impact A Story That May Never End Here
comes a brief summary of what happened so
far Hermann Boehnhardt (MPS Katlenburg-Lindau)
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Mission Spacecraft

• Event
• impact time 4/7/2005 055202 UT
• target Comet 9P/Tempel 1
  (Jupiter Family Comet
• maybe just 200 years as SPC)
• impact speed 10 km/s (19GJ)
• impact site at southern limb
• obliquity 20-40deg to local
  horizon
• Mission spacecraft
• mission fly-by impactor S/C
• impactor 360kg (50 Cu)
• instrumentation
• flyby S/C - 2 vis.1 IR camera,
• - IR spectrometer
• impactor - vis. camera
• Mission profile
• launch 12 Jan. 2005
• arrival few days before
  perihelion

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Encounter Schematics
Impactor Release E-24 hours
AutoNav Enabled E-2 hr
ITM-1 Start E-88 min
ITM-2 E-48 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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DI The Sequence of Events

• oblique impact 20-30deg to local surface
• hot gas at impact site ? detector saturation
• poof ejection (1 ton of hot gas at 5km/s)
• ejecta cloud of dust (1000-10000tons at few
  hundred m/s
• maximum speed also very low velocities of order
  m/s seen)
• subsequent gas sublimation due to solar
  illumination
• Conclusion for cratering experiment
• - crater formation dominated by gravity (not
  strength)
• - crater formation lasted for several minuntes
  (at least)
• - crater and crater formation not imaged because
  of image
• saturation (first phase) and high opacity due
  to ejecta

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Impact Phenomena
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DI Movies from the Event
? impactor approach
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(No Transcript)
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Nucleus Properties

• Dimensions
• irregular shape 7.6x4.9km
• (between ellipsoid and pyramid)
• mean radius 3.0/-0.1km
• Rotation
• period 40.7h
• axis RA/DEC 294/73deg
• (almost perpendicular to orbital plane)
• Albedocolor
• albedo 0.04
• homogenous across nucleus with variations of
  0.02 color variations lt 2
• Dust-to-Gas Ratio
• 10 0/-0.5

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Gravity Density

• Gravity (local)
• 0.05 cm/s2 _at_ impact site
• (width of cone base)
• Escape velocity
• 1.3 m/s
• Density
• 0.35/-0.12 g/cm3
• (assuming uniform density and applying shape
  model)
• ? 2 1013kg mass
• Strength
• 200 Pa for impact site
• excavated layers
• high porosity of nucleus material
• sample return easier than thought before ( 10N
  may be enough for coring a sample)

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Surface Structure

• smooth rough terrain
• flows?, scarps, layers (from cometesimals?)
• several dozen circular features (very non-uniform
  distribution)
• ? impact craters !?
• size distribution consistent with impact crater
  populations (Gaspra, not Wild2/Borelly)

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Surface Temperature

• 260-329K on sunlit side equilibrium with sunlight
  
• consistent with STM plus roughness to warm areas
  near terminator Ilt20 W K-1 m2 s0.5
• no locations as cold as sublimation temperature
  of H2O ice
• ice must be below the surface but not far below
• diurnal skin depth 3 cm, annual skin depth 0.9m
• ? primordial/unmodified material within few dm
  below surface
• ? surface renewed during every apparition,
  deep surface layering may not exists

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Normal Activity Outbursts

• short duration (explosive 200 m/s)
• outbursts common - typically 2 per
• week
• outbursts correlated with rotational
• phase
• ? outbursts are endogenic and
• related to surface insolation

• 109kg per perihelion (10cm radius)
• peak of dust production 50-60 days
• before perihelion, peak of water
• production 30 days before perihelion
• factor 2 below previous apparitions
• gt4 active regions on nucleus

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Surface Activity
P positive rotational pole E Ecliptic north S
Sunward

• Dust is better correlated
• with CO2 than with H2O, but
• not perfectly with either
• CO2 activity _at_ south pole in
• darkness
• ? chemically heterogenous
• nucleus

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Water Ice Patches Activity I

• nominal (non-ice) nucleus laboratory water ice
• 3-6 water ice 30 10 µm size particles (large
  grains for
• undisturbed surface)
• ? solids dominate surface layers even in icy
  patches
• not enough surface to be significant in overall
  outgassing (sub-surface outgassing)
• frost from source of outbursts on shoulder?

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Gas Properties

• S/C CO2, H2O, HCN, CH3CN, NH3, C2H2,SO2?, H3O,
  CH-X organics
• - organics ? strong increase
• after impact,
• - vaporizing in initial ejecta cloud?
• Earth H2O, CO2, C2H6, HCN, C2H2, CH4, CO, H2CO,
  CH3OH,
• OH, CN, C2, C3, NH2, NH, (CS,
• CH)
• - moderate (1.2-1.5) increase of H2O
• others after impact
• - cloud enriched in ethane 3
• - similar production rate ratios
• as for OC comets!

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Onboard Spectroscopy
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Isotopes
Isotopic ratio best fit for an isotopic mixture
12C/13C 95 15 (Earth 89)
14N/15N 145 20 (Earth 229)
mean observed spectrum synthetic spectrum of
12C14N, 12C15N and 13C14N
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Dust Characteristics

• increased N band flux
• post-impact until 8 July 2005
• ? normal activityrotation
• amorphous crystalline
• silicates found with
• crystalline enhanced post-
• impact
• ? crystalline silicate requires processing T
  gt 1000K
• impact energy not sufficient to heat amount
  of dust ejecta to this T
• ? ejecta pristine material

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Dust Composition from Spitzer
phyllosilicates (clay?) present in 9P dust? ?
implications for dust and comet formation scenario
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Ejecta Dust Grains

• pre-impact grains mostly gt 10 µm
• dominated by amorphous olivine-like
• mineralogy
• post-impact (sub-)micron grains
• abundant (? crater obscuration)
• peaked size distribution with
• large grain slope of 3-3.2?
• ? impact energy not enough to shatter
• grains
• ? ejecta grains are pristine
• ? surface dust is loose aggregate of
• small grains of very low strength