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

26 Apr 2006Fermilabmfa 2 Anatomy of a Comet Ion Tail Dust Tail Coma Nucleus (1-10 km) 80,000 km 33,000,000 km To Sun

26 Apr 2006Fermilabmfa 3 Un-numbered Periodic Comets Numbered Periodic Comets Un-numbered Centaurs Numbered Centaurs

26 Apr 2006Fermilabmfa 4 Oort Cloud and Kuiper Belt

26 Apr 2006Fermilabmfa 5 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

26 Apr 2006Fermilabmfa 6 Interplanetary Trajectory Launch Jan. 12, 2005 S/C Earth Orbit X Earth at Encounter Impact! July 4, 2005 Tempel 1 Orbit (5.5 yr Period) Sun

26 Apr 2006Fermilabmfa 7 Encounter Schematic Tempel-1 Nucleus Shield Mode Attitude through Inner Coma Science and Autonav Imaging to Impact 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

26 Apr 2006Fermilabmfa 8 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 CO 2 and organics are enhanced relative to water below the surface

26 Apr 2006Fermilabmfa 9 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).

26 Apr 2006Fermilabmfa 10 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?

26 Apr 2006Fermilabmfa 11 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.

26 Apr 2006Fermilabmfa 12 Differences Among Nuclei Stardust Team L. Soderblom Stardust Web Site

26 Apr 2006Fermilabmfa 13 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)

26 Apr 2006Fermilabmfa 14 MRI MRI , 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.

26 Apr 2006Fermilabmfa 15 Approach Photometry

26 Apr 2006Fermilabmfa 16 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 Sun EclN CelN

26 Apr 2006Fermilabmfa 17 Shape – rendered with Hapke scattering model North Pole Sun EclN CelN 1 km HRI Image at encounter, 50° rotation behind left

26 Apr 2006Fermilabmfa 18 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 ImpactHST

26 Apr 2006Fermilabmfa 19 Thermal Map of Nucleus First real thermal map of a nucleus Consistent with STM No locations as cold as sublimation temperature of H 2 O ice Therefore ice must be below the surface but “not far” below

26 Apr 2006Fermilabmfa 20 Anomalously Colored Regions Deconvolved High Resolution Color Images

26 Apr 2006Fermilabmfa 21 Modeling Surface Water Ice Nominal (non-ice) nucleus + laboratory water ice –3-6% water ice –30 ± 10 µm size particles

26 Apr 2006Fermilabmfa 22 Activity off Limb

26 Apr 2006Fermilabmfa 23 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 Sun Ecliptic North

26 Apr 2006Fermilabmfa 24 CO 2 : H 2 O CO 2 N Sun H2OH2O 1 st Distribution Map of CO 2 for a Comet!

26 Apr 2006Fermilabmfa 25 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.: v 2 /2 > maximum energy per mass of any chemical explosive D. K. Yeomans CSR page 1-12

26 Apr 2006Fermilabmfa 26 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

26 Apr 2006Fermilabmfa 27 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

26 Apr 2006Fermilabmfa 28 Impact Flash in MRI MRI Brightness 0 - 1s (62 ms between images)

26 Apr 2006Fermilabmfa 29 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/s 2 ) Shape model yields mass, assuming uniform density. Density = 0.4±0.1 g/cc (if uniform; was 0.6±0.3 in Science paper)

26 Apr 2006Fermilabmfa 30 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

26 Apr 2006Fermilabmfa 31 Volatile Composition Radiance (W/[m 2 sr  m]) Wavelength (  m) H2OH2O CO 2 H3O+H3O+ SO 2 ? NH 3 HCN C 2 H 2 CH 3 CN? { CH-X First 0.2 secI+4 min

26 Apr 2006Fermilabmfa 32 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 Radiance (W/[m 2 sr  m]) Wavelength (  m) H2OH2O CO 2 CH-X Pre-Impact Post-Impact CO 2 CH-X

26 Apr 2006Fermilabmfa 33 Thermal Solar Continuum Removed Average Early Ejecta 3 µm Absorption –Present even without thermal component –Consistent with microscopic crystalline H 2 O ice particles Near-Surface Water Ice Ejecta are cold, thus unprocessed!!

26 Apr 2006Fermilabmfa 34 Ejecta over Time Dust (2 µm reflectance) H 2 O Ice Absorption H 2 O Gas Emission Time Impact

26 Apr 2006Fermilabmfa 35 Monitoring OH K ü ppers et al., 2005 Nature Observations with OSIRIS on Rosetta Enables determination of total water released in impact ~ 4000 tons

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 Amorph Carbon Water Ice Smectite (clay) Fitting Spitzer Data with Small Particles

26 Apr 2006Fermilabmfa 37 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 CO 2 and organics are enhanced relative to water below the surface

26 Apr 2006Fermilabmfa 38 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 (CO 2 /H 2 O) radial or cometesimal? When and where were the impact craters produced?

26 Apr 2006Fermilabmfa 39 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