Icarus 187 (2007) 4–15www.elsevier.com/locate/icarus

The shape, topography, and geology of Tempel 1 fromDeep Impact observations

Peter C. Thomas a,∗, J. Veverka a, Michael J.S. Belton b, Alan Hidy c, Michael F. A’Hearn d,T.L. Farnham d, Olivier Groussin d, Jian-Yang Li d, Lucy A. McFadden d, Jessica Sunshine d,

Dennis Wellnitz d, Carey Lisse e, Peter Schultz f, Karen J. Meech g, W. Alan Delamere h

a Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853, USAb Belton Space Exploration Initiatives, 430 S. Randolf Way, LLC, Tucson, AZ 85716, USA

c Department of Geology, Utah State University, Logan, UT 84322, USAd Department of Astronomy, University of Maryland, College Park, MD 20742, USA

e Applied Physics Laboratory, 11000 Johns Hopkins Rd., Laurel, MD 20723, USAf Department of Geological Sciences, Brown University, Providence, RI 02912, USA

g Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USAh Delamere Support Services, 525 Mapleton Avenue, Boulder, CO 80306, USA

Received 7 August 2006; revised 18 December 2006

Available online 8 January 2007

Abstract

Deep Impact images of the nucleus of Comet Tempel 1 reveal pervasive layering, possible impact craters, flows with smooth upper surfaces,and erosional stripping of material. There are at least 3 layers 50–200 m thick that appear to extend deep into the nucleus, and several layers1–20 m thick that parallel the surface and are being eroded laterally. Circular depressions show geographical variation in their forms and suggestdifferences in erosion rates or style over scales >1 km. The stratigraphic arrangement of these features suggests that the comet experiencedsubstantial periods of little erosion. Smooth surfaces trending downslope suggest some form of eruption of materials from this highly porousobject. The Deep Impact images show that the nucleus of Tempel 1 cannot be modeled simply as either an onion-layer or rubble pile structure.© 2007 Elsevier Inc. All rights reserved.

Keywords: Comets, nucleus; Comet Tempel-1

1. Introduction

Deep Impact returned the highest resolution images yet ob-tained of a cometary nucleus. About one-quarter the surfacewas imaged at well under 10 m/pixel, and small areas wereimaged at less than 1 m/pixel. A’Hearn et al. (2005a) identifiedsome of the most important aspects of the surface character-istics of Tempel 1, including the diversity and complexity ofthe surface, the occurrence of crater-like depressions, layering,evidence of extensive surface erosion, and the unexpected oc-

* Corresponding author. Address for correspondence: 422 Space Sciences,Cornell University, Ithaca, NY 14850, USA.

E-mail address: pct2@cornell.edu (P.C. Thomas).

0019-1035/$ – see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.icarus.2006.12.013

currence of very smooth terrains. Another major result fromDeep Impact which affects the interpretation of surface featuresis the very low average density of the nucleus, ∼0.3 gm/cm3(Richardson and Melosh, 2006a, 2006b).

In this paper we present a more comprehensive survey ofsurface features on Tempel 1 and use an improved shape modelto derive local slopes and gravitational gradients over the well-imaged portion of the nucleus. Such information makes it pos-sible in many cases to refine the geological interpretation.

Conventions established by the International AstronomicalUnion (IAU) (Seidelmann et al., 2005) for coordinate systemsof comets and asteroids are based on positive and negative lati-tudes with respect to right-hand rule for spin, and positive longi-tudes. We also apply the terms north, south, east, and west hereto facilitate local directional descriptions. North is the positive

http://www.elsevier.com/locate/icarusmailto:pct2@cornell.eduhttp://dx.doi.org/10.1016/j.icarus.2006.12.013

Geology of Tempel 1 5

Fig. 1. Control points and shape model on MRI image MV173728441. Negative (south) pole is to lower left. Sub-spacecraft point −44◦ , 303◦; range 1238 km.

pole, positive longitudes run east on Tempel 1. Surface coordi-nates are body-centric.

2. Shape of the nucleus of Tempel 1

Before the arrival of Deep Impact, ground-based studies(summarized in Fernandez et al., 2003 and Belton et al., 2005)indicated a nucleus with a mean radius of about 3 km, and anirregular shape. These studies suggested axial ratios of at least1.4 and possibly as high as 3. The spin period was determinedto be 41.85 h (Belton et al., 2005). The orientation of the spinvector was poorly constrained before the flyby, but subsequentanalyses of spacecraft and ground-based data have shown con-vergence of solutions (A’Hearn et al., 2005a).

Given the flyby speed, the comet’s slow rotation, and the ap-proach phase angle, less than half the object was well-resolved,and shape and spin vector determination required lightcurvecomparisons and some assumptions about the homogeneity ofthe object.

An approximate shape model was first developed with limband terminator positions in the inbound and outbound images(Figs. 1 and 2) obtained with the Medium Resolution Instru-ment (MRI). The instruments on Deep Impact are described inHampton et al. (2005). The sense of rotation was visible in thelow-resolution inbound images, but images that revealed morethan a few surface features spanned only a few degrees of rota-tion of the nucleus. With this preliminary model, control pointswere obtained for the region seen at pixel scales of 50 m or bet-ter (Fig. 1). Solution of a control point requires measurement inat least three images, and convergence angles greater than 10◦.

Fig. 2. Outbound image 173730609 showing ejecta plume slightly over the hori-zon and full disk silhouette. View from +22◦ , 104◦; range 20,930 km.

Control points used are circular forms, bright spots, and otherdistinctive small features. Our solution has 189 points, using1520 measurements in 70 images, with an average residual of0.4 pixels. The pixel residuals are slightly larger than for ob-jects with well-known spin vectors and sharp crater rims thatare easily centroided, but this solution provides useful topogra-phy with an average radial uncertainty of approximately 20 mat the control point locations. Some of the control points are inan area near the south pole of the nucleus that is illuminatedonly by light scattered from the impact ejecta.

6 P.C. Thomas et al. / Icarus 187 (2007) 4–15

Fig. 3. Image map of Tempel 1, simple cylindrical projection. ITS and MRI images used.

With 30% of the object’s shape well-constrained and therest very approximately estimated, a comparison of the shapemodel’s maximum moment orientation (assuming a homo-geneous object) and the assumed spin vector showed a dif-ference of more than 15◦. The lightcurve observed on ap-proach (Belton et al., 2006) also suggested errors in eitherthe shape or spin vector. We then used a new spin vectoralong the direction of the calculated maximum moment vec-tor, and set the coordinate origin at a model center of mass,again assuming a homogeneous interior. A small change toone part of the model not seen at high resolution, but fac-ing the spacecraft in Fig. 2, resulted in some improvementin the lightcurve match, but further modification may be war-ranted. Some additional adjustments of the limb and terminatormatches were made for the current model, which involved an-other iteration of checking moment orientation and center ofmass.

The current spin model is a positive pole at RA = 294◦, Dec73◦, with an uncertainty of approximately 5◦ (this vector differsfrom the preliminary value in A’Hearn et al., 2005a). The primemeridian is set at the center of a 350-m crater west of the impactsite (Fig. 3), such that W = 252.63◦ + 212.064◦ d, where d isthe number of days since the standard epoch (JD = 2451545.0).MRI and Impactor Targeting Sensor (ITS) images have beenprojected to produce an image map of part of the surface ofTempel 1 (Fig. 3). In Fig. 4 we display the image map projectedon the shape model as viewed from four different directions,a presentation that shows which portions of the nucleus wereimaged well by Deep Impact.

The shape model (Table 1) has a mean radius of 3.0 ±0.1 km. This is the radius of a sphere of equivalent volume.The uncertainty in the volume is estimated by the amountof additional volume that can be added or subtracted with-

out violating either the limb views (look-back images suchas Fig. 2 are crucial here), or generating physically implau-sible shapes, such as holes to the center of the object in the“unseen” longitudes. The mean radius compares well withground-based measurements, but the elongation is less thanmost pre-encounter estimates. For comparison, the nucleus ofWild 2 has a mean radius of ∼2 km (Duxbury et al., 2004;Kirk et al., 2005), and that of Borrelly is about 3 km but ismore elongate than Tempel 1 (Britt et al., 2004).

Richardson and Melosh (2006b) have used observations ofthe evolution of the solid ejecta plume to model local gravi-tational acceleration, and thereby derive the mean density ofa homogeneous nucleus. Their result, ρ = 0.35 ± 0.25 g cm−3(2σ uncertainty), suggests an extremely underdense object.

The data make it possible to derive gravitational topogra-phy and slopes (Thomas, 1993) on the well-imaged portion ofthe nucleus (Fig. 5). The slopes on this object, in the well-controlled regions, are slightly greater than those measured onwell-imaged asteroids and small satellites (Fig. 6). The higherslopes may result from the lack of a smoothing effect from theredistribution of regolith by impacts, seismic effects, and otherdownslope transport. Slopes of individual features and local ar-eas are discussed below. Modeled gravitational heights for thenucleus as a whole are shown in Fig. 7. These assume a homo-geneous interior. Uncertainties in the shape in poorly imagedareas create uncertainties in the local gravity vectors, but theseare small compared to the overall pattern of topography andslopes. Uncertainties in the mean density have little effect onthe slope calculations because the very slow rotation adds onlyvery small accelerations. Much of the relief in the well-imagedpart of this object derives from the somewhat faceted appear-ance of this region, with ridges bounding flatter areas being thegravitationally high areas.

Geology of Tempel 1 7

Fig. 4. Four views of the shape model with image mosaic (Fig. 3) projected on surface.

Table 1Shape model of Tempel 1

Mean radius 3.0 ± 0.1 kmDiameter range 5.0–7.5 kmGravity 0.024–0.030 cm s−2Area 119 km2

Range of gravitational heights 0.73 km

3. Surface features and geology

We begin this discussion by classifying the various mor-phologic forms and albedo features visible on the nucleus ofTempel 1. Keys to surface features are shown in Fig. 8; panel Bshows regions of occurrence of those features that can beclassed as morphologic units, distinct from those that are spe-cific forms that may occur on wider units. The other panels ofFig. 8 give selected examples of specific morphologies. Theclassification below is arranged in pseudo-stratigraphic order,oldest first, with the caveat that because interpretation of theforms is uncertain, the sequence is also.

a: Linear outcrops. Darker and brighter banding runs approxi-mately NNE–SSW for >3.5 km, exposed over widths as smallas a few 10’s of m to over 200 m. The scarp bounding part ofunit h parallels some of these bands.

b: Intermediate roughness pitted surface. Much of the westernfacet appears less rough and perhaps less eroded than unit c.

c: Pitted and rough surface. This unit is perhaps the roughestpart of Tempel 1, and has what may be many merged depres-sions 10’s to 100 m across. It appears somewhat darker thanmuch of unit h and the smooth areas. Its boundary with parts ofunit h are poorly defined.

d: Isolated, rimless depressions. These occur in the westernfacet, in unit b. They range from 100 to 400 m in diameter, haveno raised rims, usually have flat floors, and display a variety ofroughly concentric albedo markings. Depths are probably wellunder 50 m.

e: Rim remnants. These occur in unit h and reach diameters of350 m. They appear to be nearly circular raised rims of varyingwidth, darker than their surroundings, with interior fill similarto material exposed outside.

f: Close-packed depressions. Two zones, that may be withinunit c, have round depressions and arcuate scarps indicatingclosely packed depressions 100–200 m in diameter, and

8 P.C. Thomas et al. / Icarus 187 (2007) 4–15

Fig. 5. Slopes and topography with two smooth areas and major layers indicated. View is from 31◦ S, 334◦ E.

Fig. 6. Slopes on small objects. Slopes are the angle between the local acceleration vector and the surface normal.

a bounding scarp up to 40 m in height as shown by shadowlengths. The western depression (g2) appears to cut g1, and thuspost-date it, if g1 was ever a complete circular form. Other ar-cuate scarps north of the eastern depression suggest remnantsof other such forms now largely degraded.

h: Stripped surface. The surface surrounding the impact site is aflat unit, interrupted by several scarps, that suggest removal oflayers ∼1–10 m thick.i: Smooth units. There at least two examples of smooth surfacesthat are distinct from their surroundings and suggest flows, i1and i2 in Fig. 8. A third elongate example, i3, may connect to i1,and possibly to a wider area of smooth material, but is difficultto map because it is only illuminated by light scattered from theejecta cloud.

j: Exposures of ice. Sunshine et al. (2006) found three very lo-calized areas that have exposure of water ice (3–6%). Two of

these are within the large depressions, the third is near a scarppossibly defining a small remnant of a similar form.

Panel B of Fig. 8 provides a broader classification of terrainareas to emphasize the scale at which terrains may be grouped.Some of the boundaries, such as between scarped/pitted andthin layers are subject to a range of interpretations, but ourobjectives are broad grouping and global scales. The two sec-tions of thin layers mapped in Fig. 8B are covered by images ofvery different resolutions, and might, in detail, be quite differ-ent from each other.

Photometric variations in color and roughness have beenmapped by Li et al. (2007). These variations are modest, anddo not easily differentiate the morphologic provinces.

4. Layering

One dramatic aspect of the geology of Tempel 1 is pervasiveevidence of layering. Suggestive evidence of layering can be

Geology of Tempel 1 9

Fig. 7. Tempel 1 shape model with gravitational heights color-coded. The dark-est blue region is the poorly controlled area of the shape model and is uniformlyset to the lowest value; only those areas outside this region have valid values.

discerned in the earlier images of the nuclei of Borrelly andWild 2 obtained by Deep Space 1 and Stardust. However, itis only in the images of Tempel 1 that evidence of layering isobvious and dramatic.

Two distinct, but possibly related manifestations of layer-ing are observed. First, there are extensive exposures of layerssome one hundred meters thick in the central portions of theDeep Impact coverage. Second, there is pervasive evidence ofmuch thinner layering or stratification of material characteris-tics in the manner in which the surface has eroded in region h,especially.

Linear markings (unit a), most likely due to changes inroughness as well as possibly albedo, extend for ∼3.5 kmacross the well-imaged section of Tempel 1, on one side ofthe ridge formed by the two visible facets of the global shape(Fig. 8). Viewed in stereo, Fig. 9, these features show their in-termittent exposure and lack of strong topographic association.Their three-dimensional forms are shown in Fig. 10. The bestinterpretation is that these markings show the presence of layersextending deep into the body. These features appear to nearlyparallel the west facet. Their subtlety of expression indicatesonly minor differences in their response to erosion. Thicknessesrange from 10’s to ∼200 m. If they extend through the ob-ject with these thicknesses, they would have areas in excess of6 km2, implying a volume of the thickest layer of ∼1 km3.

Close inspection of unit h—the stripped surface—showsclear evidence that the stripping or erosion has exploited thinlayers (perhaps 1–10 m in thickness) that are generally parallelto the present surface in this region of Tempel 1. The small areasof ITS coverage at

10 P.C. Thomas et al. / Icarus 187 (2007) 4–15

Fig. 8. Key to examples of surface features on Tempel 1. (A) Composite of ITS frames with examples of types of features; see text for details and descriptions.(B) Topographic regions of Tempel 1. Unit names from (A) with generalized physical descriptions. (C) Portion of MRI frame MV173728497 showing smooth areasnear terminator and associated complex topography. (D) Definition of “facets.” (E) Strongly stretched portion of the ITS composite image; arrows denote scarpbounding smooth unit.

sions, and appearance at edges of scarps strongly suggeststhey are distinct from the other depressions and that theyhave no connection to impacts. The stereo view in Fig. 9shows some of these as merging clusters of arcuate scarps.

Easily identified circular depressions on Tempel 1, apartfrom the two large arcuate depressions, are identified in Fig. 11.In what follows we assume that only the first two categories(shown in yellow and green in Fig. 11) are plausible impactcrater remnants.

The area density of these depressions is shown in Fig. 12as an R-plot (see Crater Analysis Technique Working Group,1979). The density (or R-value) is about 0.05–0.1 those of Eros,Ida, and Gaspra. Ida has near-equilibrium values with a hori-

zontal slope and Gaspra has high density but a negative slope)(Chapman et al., 2002). Cratering age estimates for the sur-face of Gaspra center around 108 yr (Carr et al., 1994). Whilethere is no reliable way of predicting the flux of impactors thathas interacted with Tempel 1, the surface density of circularfeatures suggests, if these are indeed impact remnants and as-suming that cratering at Gaspra is similar to that of short-periodcomets, minimal sublimation over periods in excess of 106 yr.Stability implies residence in long-period orbits, and such sta-bility for many cometary nuclei has been predicted in dynam-ical models (Levison and Duncan, 1994) that predict cometsswitch several times between long and short periods in 107 yr,but are stable for less than 106 yr in any short-period configu-ration.

Geology of Tempel 1 11

Fig. 9. Stereo view of ridge between the east and west facets. ITS frame from −23.1◦ , 291.9◦; range 2114 km. MRI frame from −44.0◦ , 302.6◦; range 1238 km.

Fig. 10. Stereo view of shape model, keyed for gravitational heights, with traces of linear outcrops.

For the well-resolved diameters, above 700 m, the R-plotslope, nearly level or slightly positive, is not the negative slopeexpected for a primary production population of impact craters.However, the data are consistent with a population of highlyeroded craters in which smaller craters are removed preferen-tially.

The two different types of round depressions correlate withthe different terrain units b, h, and c in Fig. 8A. These regionaldifferences may imply either a different response of the sub-strate to the impact process, or different styles of erosion ofimpact forms and the local substrate materials. Different re-

sponses to impacts implies different physical properties suchas porosity or composition, which might be recording differentregions or processes of accretion. Alternatively, different unithistories, such as one unit being covered by ejecta or other ma-terial, for some period of time while the other unit was sufferingerosion, might also account for a difference in the appearanceof the depressions.

It is possible that units b and c identified in Fig. 8 owetheir characteristics to whatever process was responsible for theformation and modification of the circular depressions. Boththe “intermediate roughness pitted surface” and the “pitted and

12 P.C. Thomas et al. / Icarus 187 (2007) 4–15

Fig. 11. Depressions on Tempel 1. Yellow denotes rimless depressions; green the rimmed features, and red the grouped depressions and arcuate scarps. The largearcuate scarps (unit g, Fig. 8), 250◦–290◦ E, are not included in this classification. The mismatch of some of the green symbols with image map features resultsfrom reprojecting the ITS composite image as one image, thereby introducing some registration errors. The features were mapped from individual ITS frames thatare fully registered on control points.

Fig. 12. R-plot of density of round depressions on Tempel 1. The two curvesdiffer in that “All” includes those of unit f, the grouped depressions. The areaused, 43 km2, corresponds to the whole well-imaged region.

rough surface” display topography which could be the result ofthe gradual merging of highly eroded depressions. Today bothregions show somewhat mottled albedo patterns and variablerandom roughness in combination with the different mappedrounded depressions. Unit c appears to be rougher than unit b.In both cases the surfaces may be changing during perihe-lion passages. The difference in appearance may indicate thatin recent times erosion has proceeded faster in unit c than in

unit b. Whether this difference is due to differences in mate-rial properties or in insolation history cannot be determinedbecause we cannot trace the spin state of the nucleus back intime.

These depressions are largely distinct from those observedon Wild 2 (Brownlee et al., 2004). The close-packed depres-sions (unit f) appear as a group somewhat analogous to groupsof the “flat floor” depressions that characterize much of the sur-face of Wild 2 (Brownlee et al., 2004), but the latter are muchlarger. Perhaps the most general analogy between Tempel 1 andWild 2 would be between the large arcuate depressions, unit g,and the most common landforms, the “flat floor” depressionson Wild 2. Certainly most of Tempel 1 is far smoother than thesurface of Wild 2, which displays very sharp peaks and evenoverhangs (Brownlee et al., 2004).

6. Smooth units

There are two, and possibly three, areas of remarkablysmooth terrain marked “i1,” “i2,” and “i3” in Fig. 8A. Thesesurfaces are completely devoid of craters and must be smoothat scales of less than 5 m. Two of these (i1 and i2) show def-inite evidence of emplacement by flow. Both i1 and i2 occupygravitational lows (Fig. 5). The better imaged of the two (i1) isan elongated flow-like tongue about 3 km long, 1 km wide, andat least 20 m thick, terminating on the northern side in a steepscarp (Fig. 8E). High-pass filtered versions of the best DeepImpact images (Fig. 13) show digitate markings suggestive ofa spreading flow at its distal end which is marked by an abrupt

Geology of Tempel 1 13

Fig. 13. High-pass filtered image of smooth unit i1. ITS frame IV173727787,from −23.1◦ , 291.6◦; range 879 km.

Fig. 14. Topographic profiles of smooth areas i1 and i2. Derived from the 2 × 2degree shape model; VE is vertical exaggeration.

scarp some 10–20 m high. This unit is topographically boundedto the west (Fig. 5, upper right direction). To the east it is nottopographically bounded to the precision available from controlpoints, but the eroded topography might be remnants of a moreextensive unit, or i1 might need only 5–10 m of relief to be lat-erally confined. Along the long axis of the flow there is initiallya “downhill” slope (determined by control points at the edge ofthe unit) which levels out in the mid-portions of the flow andreverses slightly near the terminal end (Fig. 14).

The lack of sharp markings prevents stereogrammetric gen-eration of profiles transverse to the flow. The high-pass filtered

Fig. 15. Upslope area of flow i1; high-pass filtered portion of MRI frameMV0173728459; center of image is −58◦ , 321◦; range 1077 km.

image shows not only the finger-like markings suggestive ofspreading at the end of the flow, but in the transverse directionacross the flow there is a clear pattern of a brighter central bandsuggesting that the flow has either different roughness or albedoin the center, or possibly a slight transverse curvature.

The probable source region for this flow is illuminated bylight scattered from the crater ejecta (Fig. 8C, Fig. 15). No spe-cific vent or construct is obvious. The flow may originate frommore smooth layered materials that have been partly eroded,forming the highlighted scarp midway between the i1 and i3 la-bels in Fig. 8C. Thus this flow is of indeterminate original widthat this position, and could be an extending tongue from a muchlarger feature.

The other extensive area of smooth terrain i2 is not imaged aswell. It appears to fill part of a large arcuate depression which isan actual topographic low and trends downslope for some 2 km(Fig. 14). There is no evidence of flow-like markings in imagesof i2, but the viewing geometry is less favorable (more oblique)than in the case of i1. Smooth flow i2 appears to be connectedwith the scarp that bounds arcuate depression g1 and which isthe locale of the exposures of water ice identified by Sunshineet al. (2006).

It is difficult to escape the conclusion that smooth areas rep-resent materials that have been erupted or otherwise releasedonto the surface. In the case of flow i1 it is extremely unlikelygiven the volume of the flow and the topography of the likelysource region that the flow consists solely of very mobile mass-wasted material flowing downhill. Even in the case of flow i2volumetric considerations make it unlikely that all of the mate-rial present derives from the retreat of the scarp which boundsthe containing arcuate depression g1. The characteristics of theflows, especially of i1, indicate that the materials involved werevery fluid and able to flow considerable distances “downhill” inthis very low-gravity environment. The extreme smoothness ofthe flows suggest that they consist of a very homogeneous, andprobably fine-textured, material. The abrupt scarp at the north-ern terminus of i1 indicates that since emplacement the materialin i2 has lost some of its fluidity and is now able to support rel-atively steep slopes.

14 P.C. Thomas et al. / Icarus 187 (2007) 4–15

Fig. 16. Comparison of Tempel 1, Wild 2, and Borrelly. ITS composite imageis better than 10 m/pixel; Stardust image N2075WE01 of Wild 2 at upper left.Borrelly image, bottom, was obtained at about 60 m/pixel.

Possible eruption of materials onto the surface of this cometmust be viewed in the context of known properties and activity.The nucleus is highly porous, certainly on average over 50%.It does release material in jets and to the coma. Most of therelease of material is currently from distributed areas, ratherthan from obvious isolated vents or from the localized areasof water ice exposure (Sunshine et al., 2006). The flows arenot very distinct photometrically (Li et al., 2007), which sug-gests they might be the more typical “comet material” entrainedby volatile flow rather than some form of direct condensate,a conclusion supported by the lack of any distinctive spectralsignature (Sunshine et al., 2006). The very low gravity impliesthis flow could have occupied a long time, perhaps many hours,and is likely to have undergone very different compaction andother modifications from what we are familiar with in 1-g envi-ronments.

The simplest interpretation of the smoothness of the flows isthat they are the very freshest features exposed on the surface ofTempel 1. At least in the case of i1 enough time has passed sinceemplacement for erosional scarps to develop along the bound-aries of the flow. It is not clear how far the scarp has retreatedacross unit h, however.

Views of Borrelly (Fig. 16) suggest a smooth region, appar-ently an isolated mesa top of km scale (Britt et al., 2004), andSoderblom (personal communication, 2005) emphasized thispossible comparison to Tempel 1. Unfortunately, the Borrellyimages (Soderblom et al., 2004; Britt et al., 2004) have lowerresolution than the Deep Impact images, and the similarity can-not be established conclusively, although the view in Fig. 16,

and especially the stereo views presented by Britt et al. (2004),Fig. 3, strongly suggest erosion of a smooth-surfaced layeredunit. The smoothest areas on Wild 2 are the floors of the “flatfloor” depressions. The very different settings of the smoothareas on the two comets makes any definitive comparison diffi-cult.

7. Summary

The Deep Impact images show that the nucleus of Tempel 1cannot be modeled as either onion-layer or rubble pile struc-ture. Layers of a great range of thicknesses, and probably of atleast two formation mechanisms provide much variegation tothe structure. Erosion that exploits layers is only part of thesublimation occurring on the object. The nucleus has appar-ently gone through at least one period of minimal sublimationsuch that impact features could form and be preserved. Periodsof low sublimation have been predicted by dynamical models(Levison and Duncan, 1994). While layers may be crucial ele-ments of many cometary nuclei, the two well-imaged cometarynuclei, Tempel 1 and Wild 2 have experienced very differentsurface, and possibly internal, histories. Wild 2 has been in theinner Solar System only a few decades (Sekanina and Yeo-mans, 1985). Such evolutionary complexities emphasize thatorigin models (e.g., Weissman, 1986; Weidenschilling, 1997)may face more severe observational limits than thought.

Erosion and associated slope retreat have been importantsurface processes on Tempel 1. Given that sublimation is thedominant current erosion process, the absence of talus depositsalong scarps is not surprising. What is interesting is that therates and style of erosion have varied considerably over the sur-face, strongly hinting at heterogeneity of near-surface proper-ties. Unlike the case of Wild 2 where steep slopes predominate,Tempel 1 displays a wide range of scarp slopes and morpholo-gies. It is difficult to make a case that these differences resultsimply from differences in exposure ages, older scarps beingmore subdued. Rather the variety again suggests differences inmaterial or compositional properties.

A significant observation is that while Tempel 1 displayedabundant jet and outburst activity (A’Hearn et al., 2005b;Farnham et al., 2007) around the time of the DI encounter, ithas proven difficult to identify specific landforms that can beidentified as the “vents” discussed for many decades in classi-cal comet literature, as it is difficult to locate them on Borrelly(Soderblom et al., 2004) and Wild 2 (Brownlee et al., 2004).

Acknowledgments

We thank K. Consroe, B. Carcich, and T. McCarthy for tech-nical assistance. Dan Britt and Bill McKinnon provided helpfulreviews. Funded in part by the Deep Impact project.

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http://dx.doi.org/10.1029/2004JE002316http://dx.doi.org/10.1029/2004JE002316

The shape, topography, and geology of Tempel 1 from Deep Impact observationsIntroductionShape of the nucleus of Tempel 1Surface features and geologyLayeringEvidence of cratering?Smooth unitsSummaryAcknowledgmentsReferences