Comet Tempel 1: Overview of Stardust-NExT results

Icarus 222 (2013) 424–435
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Icarus
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Return to Comet Tempel 1: Overview of Stardust-NExT results
J. Veverka a,⇑, K. Klaasen b, M. A’Hearn c, M. Belton d, D. Brownlee e, S. Chesley b, B. Clark f, T. Economou g,
R. Farquhar h, S.F. Green i, O. Groussin j, A. Harris k, J. Kissel l, J.-Y. Li c, K. Meech m, J. Melosh n,
J. Richardson n, P. Schultz o, J. Silen p, J. Sunshine c, P. Thomas a, S. Bhaskaran b, D. Bodewits c, B. Carcich a,
A. Cheuvront q, T. Farnham c, S. Sackett a, D. Wellnitz c, A. Wolf b
a
Cornell University, Ithaca, NY 14853, USA
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
University of Maryland, College Park, MD 20742, USA
d
Belton Space Exploration Initiative, Tucson, AZ 85716, USA
e
University of Washington, Seattle, WA 98195, USA
f
Space Science Institute, Boulder, CO 80301, USA
g
University of Chicago, Chicago, IL 60637, USA
h
Kinetx, Tempe, AZ 85084, USA
i
The Open University, Milton Keynes, MK7 6AA, UK
j
Laboratoire d’Astrophysique de Marseille, 13388 Marseille Cedex 13, France
k
Space Science Institute, La Canada, CA 91011, USA
l
Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany
m
University of Hawaii, Honolulu, HI 96822, USA
n
Purdue University, Lafayette, IN 47907, USA
o
Brown University, Providence, RI 02412, USA
p
Finnish Meteorological Institute, Helsinki 00560, Finland
q
Lockheed Martin, Littleton, CO 80127, USA
b
c
a r t i c l e
i n f o
Article history:
Available online 12 April 2012
Keywords:
Comets, dust
Comets, nucleus
Comet Tempel-1
Comet Wild-2
a b s t r a c t
On February 14, 2011 Stardust-NExT (SN) flew by Comet Tempel 1, the target of the Deep Impact (DI)
mission in 2005, obtaining dust measurements and high-resolution images of areas surrounding the
2005 impact site, and extending image coverage to almost two thirds of the nucleus surface. The nucleus
has an average radius of 2.83 ± 0.1 km and a uniform geometric albedo of about 6% at visible wavelengths.
Local elevation differences on the nucleus reach up to 830 m. At the time of encounter the spin rate was
213° per day (period = 40.6 h) and the comet was producing some 130 kg of dust per second. Some 30% of
the nucleus is covered by smooth flow-like deposits and related materials, restricted to gravitational
lows. This distribution is consistent with the view that the smooth areas represent material erupted from
the subsurface and date from a time after the nucleus achieved its current shape. It is possible that some
of these eruptions occurred after 1609 when the comet’s perihelion distance decreased from 3.5 AU to the
current 1.5 AU. Much of the surface displays evidence of layering: some related to the smooth flows and
some possibly dating back to the accretion of the nucleus. Pitted terrain covers approximately half the
nucleus surface. The pits range up to 850 m in diameter. Due to their large number, they are unlikely
to be impact scars: rather they probably result from volatile outbursts and sublimational erosion. The
DI impact site shows a subdued depression some 50 m in diameter implying surface properties similar
to those of dry, loose snow. It is possible that the 50-m depression is all that remains of an initially larger
crater. In the region of overlapping DI and SN coverage most of the surface remained unchanged between
2005 and 2011 in albedo, photometric properties and morphology. Significant changes took place only
along the edges of a prominent smooth flow estimated to be 10–15 m thick, the margins of which receded
in places by up to 50 m. Coma and jet activity were lower in 2011 than in 2005. Most of the jets observed
during the SN flyby can be traced back to an apparently eroding terraced scarp. The dust instruments
detected bursts of impacts consistent with a process by which larger aggregates of material emitted from
the nucleus subsequently fragment into smaller particles within the coma.
Ó 2012 Elsevier Inc. All rights reserved.
⇑ Corresponding author.
E-mail address: [email protected] (J. Veverka).
0019-1035/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.icarus.2012.03.034
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J. Veverka et al. / Icarus 222 (2013) 424–435
Table 1b
Encounter circumstances.
1. Introduction
In 2007 NASA approved an extended mission for the Stardust
spacecraft, which had successfully completed its mission to Comet
Wild 2 by collecting and returning dust samples from the comet’s
coma to Earth (Brownlee et al., 2004). The extended mission, Stardust-NExT (for New Exploration of Tempel) flew by Comet Tempel
1, the target of the Deep Impact (DI) mission in 2005 (A’Hearn
et al., 2005), on February 14, 2011. Key parameters describing
Tempel 1, its orbit, and the circumstances of the Deep Impact
and Stardust-NExT encounters are summarized in Table 1.
The primary goal of Stardust-NExT (SN) was to use the spacecraft camera, NAVCAM, to obtain high-resolution images of the nucleus to
(a) Look for changes on the comet’s surface that might have
occurred between the 2005 perihelion passage and that in
January 2011.
(b) To extend coverage of the surface to regions not imaged by
DI.
(c) To image the impact site where Deep Impact’s impactor hit
the surface.
All three of these goals were achieved. The 2005 impact threw
up so much ejecta that the cameras on the main DI spacecraft
could not see the surface in the impact area, and therefore the size
of the crater that had been excavated could not be determined
(A’Hearn et al., 2005). In addition to the NAVCAM camera, the
spacecraft carried two experiments to study the comet’s dust environment: the Dust Flux Monitoring Instrument (DFMI) designed to
determine the fluence and size distribution of dust particles
(Tuzzolino et al., 2003) and the Comet and Interstellar Dust Analyzer (CIDA) designed to measure the elemental composition of
individual dust grains (Kissel et al., 2004). Results obtained by
the three SN instruments are reported in this special issue of Icarus.
To reach Tempel 1 the spacecraft performed an Earth flyby on
January 14, 2009 and some dozen subsequent TCM’s or trajectory
correction maneuvers (Fig. 1). The targeted flyby distance was
200 km. The actual flyby occurred at 178 km at a speed of
10.9 km/s. In addition to distant images on approach and departure, 72 images were obtained within ±4 min of closest approach:
the best images obtained were at 11 m/pixel.
The solar phase angle varied from 81° on approach through 15°
at closest approach, reaching 98° on departure. DFMI data were
collected from 22 min before to 8 min after closest approach
Table 1a
Tempel 1 characteristics.
Orbit semi-major axis
Orbit eccentricity
Orbital period
Perihelion date
Mean radius
Surface area
Volume
Densitya
Range of radii
Spin period (at encounter)
Spin poleb
Surface acceleration
Direction of maximum momentc
Average geometric albedo
a
b
c
Richardson et al. (2007).
Observations on July 2005 and February 2011.
Assuming uniform density.
3.12 AU
0.52
5.52 years
January 12, 2011
2.83 ± 0.1 km
108 km2
95 km3
400 kg m 3
2.21–4.00 km
40.6 h
RA = 255, DEC = 64.5
0.025–0.032 cm s 2
200.5 W, +82.0
0.059 (Hapke model)
0.045 (Minnaert model)
Encounter time from perihelion
Flyby distance
Flyby speed
Activity (H20)
Deep Impact
Stardust-NExT
1 day
500 km
10.2 km/s
5 1027 mol/s
+34 day
178 km
10.9 km/s
3 1027 mol/s
( 14,000 km to +5200 km from the nucleus). CIDA collected spectra for 2 h around closest approach (±78,000 km).
An essential aspect of the mission was to arrive at the comet at
a time when the region previously imaged by Deep Impact would
be visible and well illuminated. This requirement meant that the
comet’s spin state and spin rate had to be predicted with high
accuracy well before the encounter. Dynamically the optimum
time to make a time-of-arrival adjustment (TOA) was about 1 year
before the scheduled February 2011 flyby (Fig. 1). Accordingly, a
TCM was executed on February 17, 2010 to delay the arrival at
the comet by about 8.5 h.
The magnitude of the correction was based on extensive analyses of the comet’s rotation behavior reaching back to 2000, based
on observations of the comet’s light curve by a worldwide network
of astronomers organized to support the Deep Impact and Stardust-NExT missions (Meech et al., 2005 and Meech et al., 2011).
The light curve observations were analyzed by two independent
groups to predict the comet’s rotation state at the time of encounter (Belton et al., 2011).
2. Imaging
The primary science instrument on Stardust-NExT is the navigation camera (NAVCAM) the performance and calibration of which
is described in detail by Klaasen et al. (2013). NAVCAM uses a
1024 1024 pixel CCD detector and optics that provide a field of
view (FOV) of about 3.5° (60 lrad/pixel) (Newburn et al., 2003).
The camera’s filter wheel failed early in the primary mission
(Brownlee et al., 2004), so all NExT images are acquired through
a broadband filter (510–760 nm). Imaging within ±4 min of
encounter (E) was restricted by available spacecraft memory to
72 full-frame, compressed images. Images were taken on 8-s centers outside of E ± 144 s and on 6-s centers inside this interval. The
latter provided excellent stereo coverage of the nucleus near closest approach. Image data can be returned fully encoded to 14 bits
or compressed to 8 bits per pixel using an onboard look up table.
The data system can support a maximum imaging rate of one full
frame of compressed data every 6 s. NAVCAM performance was
monitored throughout the Stardust-NExT mission using standard
calibration sequences that involved imaging a variety of stars,
acquiring dark current frames, etc. (Klaasen et al., 2013). A problem
with recurring camera contamination was controlled successfully
by periodic heating of the instrument using its internal electrical
heaters and placing direct sunlight on the camera radiator. No
evidence of optical contamination is observed in any of the
encounter images.
Repetitive imaging of the comet was carried out before and
after closest approach to monitor the light curve and coma behavior. NAVCAM imaging of Tempel 1 was initiated at E 60 days
using exposures of 10 and 20 s (the maximum commandable),
but the comet was too faint to be detectable until E 27 days.
Images with good signal to noise ratios became available at
E 7 days. Imaging sets were obtained every 2 h from this point
until E 2 days when approach imaging was halted to prepare
the spacecraft for encounter.
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Fig. 1. Summary trajectory of the Stardust-NExT mission.
The pixel scale ranged from 158 m/pixel to 11 m/pixel at closest
approach. Four of the 72 images (first, last, and those at E ± 72 s)
were overexposed intentionally to allow better detection of nearnucleus jets (Farnham et al., in review). Slight saturation of the
bright limb occurred in two frames (E 33 s and E 15 s) due to
the actual arrival time being 15 s earlier, which caused some
images to be taken at lower phase angles than planned. A sample
of nine close encounter images is shown in Fig. 2.
Departure imaging began at E + 1 day and continued through
E + 10 days to support high-time-resolution monitoring of coma
activity. The sampling rate was one image every 5 min at the
beginning of the monitoring period, and was decreased to once
every 11 min at E + 7 days.
were active at the time of the 2005 perihelion: most of the angular
acceleration of the nucleus appears to have occurred well before
perihelion. They also find that the rotational impulse during the
2011 perihelion was weaker than in 2005.
No evidence was found in the densely sampled departure photometry of the pseudo-periodic mini-outbursts seen in the Deep
Impact approach photometry in 2005 (Belton et al., 2008), perhaps
because the SN encounter occurred 34 days post-perihelion while
the DI encounter took place 1 day before perihelion. The comet’s
activity is known to peak some 60 days pre-perihelion (Schleicher,
2007).
3. Light curve observations
The spin period of Tempel 1 changes with time in a complex
fashion (Belton et al., 2011) but is approximately 40 h. Combining
imaging data from the Deep Impact and Stardust-NExT flybys
yields an improved estimate of the spin axis orientation (Thomas
et al., 2013). Approximately 480 manually selected control points
were used to solve for both the pole orientation and for the
body-centered coordinates of a shape model. The solution employed data from four cameras on three spacecraft (Deep Impact
Flyby and Impactor craft and Stardust-NExT). The effective spin
pole orientation is found to be RA = 255°, DEC = 64.5°, a solution
nearly 16° different from the Deep Impact estimation of 293.8°,
72.6° (Thomas et al., 2007). Some of this displacement is undoubtedly due to the larger uncertainty in the original Deep Impact solution (±5°) and some could be due to precession. Belton et al.
(2011) estimated that precession should, at most, amount to
1°/perihelion passage. Ambiguities of limb coordinates were
The signature of the comet’s rotation was detected in the photometry from E 3 days to E + 5 days. The predictions in Belton
et al. (2011) compare well with the NExT results. The time-averaged photometry over the 8-day observing period yields a spin rate
of 210 ± 3°/day. With the additional constraint that the sub-solar
longitude at encounter was 321.7°W, the spin rate is
213.3 ± 0.8°/day. The predicted spin rate at encounter was
213.5 ± 0.2°/day (Belton et al., 2011). The observed sub-solar longitude at encounter of 321.7°W differs from the predicted value of
342 ± 29°W by only 21°. A post-encounter reanalysis of the problem of predicting the expected spin state at the time of encounter
is provided by Chesley et al. (2013). Chesley et al. update the
conclusions of Belton et al. (2011) most of which remain unchanged. However, they provide new insights into the torques that
4. Pole orientation and shape
J. Veverka et al. / Icarus 222 (2013) 424–435
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Fig. 2. A sample of Stardust-NExT near-encounter images. Inbound images at upper left to outbound at lower right (n30033–n30041). Range/phase angle: (top row) 352 km/
52°; 297, 46°; 249, 38°; (middle row) 209/26°; 185/15°; 182/22°; (bottom row) 202/38°; 239/51°; 286/60°.
probably the major cause of the poor spin pole solution in Thomas
et al. (2007). The new solution is free of such defects.
The shape model is well constrained by the combined Deep Impact and Stardust-NExT images, which cover two-thirds of the surface, and by limb silhouettes from the 2005 data. The mean radius
derived (2.83 ± 0.1) km is slightly smaller than the value derived by
Thomas et al. (2007) from DI data alone, but the resulting change in
estimated volume is insignificant (Table 1). The value for the comet’s mean density estimated by Richardson et al. (2007) at 400
(+600, 200) kg m 3 remains unchanged. No estimate of the comet’s mass could be obtained from the SN flyby.
Chesley et al. (2013) show that the improved shape model and
revised pole orientation provide significantly improved fits to
accumulated light curve data and that precession must be negligibly small. Thomas et al. (2013) note that the pole direction is
within a few degrees of the maximum moment axis based on the
shape model assuming a homogeneous density distribution within
the nucleus.
5. Nucleus photometry
Stardust-NExT images of Tempel 1 cover phase angles from 81°
on approach, down to 15° at closest approach, increasing to 98° on
departure. The photometric properties of the nucleus modeled
from NExT images agree closely with those derived by Li et al.
(2007) from Deep Impact data. Comparison of the two image sets
obtained one-comet year apart reveals no significant photometric
changes. Other than small variations (at the 10% level) in albedo,
the surface is photometrically homogeneous. The average clear-filter geometric albedo is 0.059 ± 0.009; the error bar quoted includes
the uncertainty in extrapolating to zero phase. Outbound images
show isolated brighter spots that have albedos only 25% greater
than surrounding areas. Significantly, no ‘‘bright albedo patches’’
similar to those reported by Sunshine et al. (2006) and shown to
be associated with exposed water ice are seen in the areas imaged
by NExT for the first time. The bright patches reported by Sunshine
et al. had albedoes up to four times higher than surrounding areas.
NExT images do not cover the region where these patches were
observed in 2005.
6. Surface morphology
The approximately one third of the comet’s surface imaged at
high resolution by Deep Impact in 2005 revealed a geologically diverse surface consisting of three major units (Thomas et al., 2007):
extensive regions of layered terrains displaying varying degrees of
erosion; smooth areas with preserved flow-like characteristics suggestive of down-slope movement of material that had erupted onto
the surface; and regions characterized by generally rimless, craterlike pits.
Fig. 3 shows the expanded coverage of the surface obtained by
Stardust-NExT added to the Deep Impact observations. Approximately two thirds of the comet’s surface has been imaged between
the two missions. Morphologically, Tempel 1’s surface can be divided broadly into two types of terrain: pitted terrains and smooth
terrains. The expanded coverage reveals additional areas of smooth
terrain and demonstrates that this terrain occurs preferentially in
gravitational lows on the nucleus (see below).
6.1. Pitted terrain
A significant fraction of the newly imaged portion of Tempel 1’s
surface is covered by a relatively rough pitted terrain somewhat
similar to that on Wild 2 (Brownlee et al., 2004), and unlike
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Fig. 3. Maps of Tempel 1. (A) Global mosaic of combined DI and SN coverage. The region around 270°E was imaged only by DI. Common coverage (DI and SN) extends over
southern and equatorial latitudes between about 200 and 40°E. The remainder is new coverage obtained by SN. (B) Gravitational topography of the nucleus from Thomas et al.
(2013). Topography in some of the blank areas in the mosaic was constrained by limb profiles. The maximum topographic variation is about 830 m. (C) Sketch map of
prominent surface features. Four areas of smooth terrain (S1–S4) are indicated. Pits marked by circles of scaled diameters. Dark dots show mesa-like forms.
anything seen on the surfaces of Comet Halley (Keller et al., 1988),
Borrelly (Soderblom et al., 2004) or Hartley 2 (A’Hearn et al., 2011).
Early discussions as to whether these pits are entirely endogenic
(representing areas in which volatiles are sublimating) or whether
at least some of them are impact scars modified by later sublimational erosion had proved inconclusive (Thomas et al., 2007). These
depressions vary widely in size, morphological sharpness, and circularity suggesting a broad range of development and age. In terms
of sharpness, the depressions range from almost circular pits to
depressions that have irregularly scalloped walls. In many cases,
on both Wild 2 and Tempel 1, such depressions have flat floors,
suggestive of possible layering of the surface or ongoing floor
deposition. In no cases are raised rims detectable. A typical contact
relationship between the higher, rougher pitted terrain and the
lower, smoother flow-like deposits is shown in Fig. 4.
Relative to nucleus size, the depressions on Wild 2 are substantially more prominent than those on Tempel 1. The largest well-defined depressions on either comet are about 800 m in diameter.
(The mean radii of Wild 2 and Tempel 1 are 2.1 and 2.8 km, respectively.) Between diameters of 800 m and 100 m the size distribution of depressions is almost the same on the two comets
following a 2 slope on a cumulative log–log plot (Thomas et al.,
2013). In this size range there are approximately 10 times more
depressions per unit area on the surface of Wild 2 than on Tempel
1, evidently the result of the absence of extensive smooth regions
on Wild 2. Wild 2 appears to have many more flat-floored pits than
does Tempel 1. It is unclear whether this reflects the influence of
layering on Wild 2 or more loose debris on the floors of pits on that
comet.
The origin of the pitted terrain is discussed at length by Belton
et al. (2013). Based on the work of Duncan et al. (2004), Belton
et al. argue that given the calculated impact rates on a typical Jupiter Family comet during its lifetime there are far too many pits on
Tempel 1 to be accounted for by impacts. They suggest that most
pits originate as sources of material ejected into the coma during
‘‘mini-outbursts.’’ They estimate that a mini-outburst of 1 mag involves the removal of enough surface material to produce a pit
30 m across. Such pits could be enlarged by repeated outbursts
from the same location, or possibly by sublimation of volatiles
from the pit walls and floor.
Belton et al. (2013) use the improved shape model and spin axis
orientation derived by Thomas et al. (2013) to update estimated
source locations of repetitive mini-outbursts observed by the DI
spacecraft (Belton et al., 2008; A’Hearn et al., 2005) and confirm
their earlier finding that these locations are restricted to the
comet’s pitted terrain. It is noteworthy that no significant albedo
J. Veverka et al. / Icarus 222 (2013) 424–435
429
deposition and erosion. Part of its margin is a topographic terrace
about 50 m high showing up to five benches, possibly layers, each
of which is about 10 m thick. Mesa-like forms suggest removal of
materials over much of this area.
To the west is a distinct smooth area (S4 in Fig. 3) resembling a
plateau with well-defined digitate margins. It displays the smoothest surface on this face of the comet and it appears to be the youngest flow in this area. Unlike the flows on the DI side, the interior of
this flow is pitted. Based on shadow measurements within the pits,
this flow is about 4–16 m thick. This thickness is similar to that of
flow S2, which is about 10–15 m thick at its distal margin.
A detailed discussion of the characteristics of the four areas of
smooth terrain on Tempel 1 is provided by Thomas et al. (2013).
In summary, these areas occupy some 30% of the nucleus surface
restricted to gravitational lows and to varying degrees display
evidence of flow-like morphology. At least one area shows strong
evidence of multiple flow events, and all display evidence of active
erosion. While it is impossible to date these features, they must be
relatively recent. An interesting speculation is that some or all may
be evidence of increased activity following the reduction of
Tempel’s perihelion distance from 3.5 to the current 1.5 AU which
occurred in 1609 according to Yeomans et al. (2005).
6.3. Layering
Fig. 4. (Top): SN picture n30038 showing the contact between pitted terrain (top
right) and smooth areas on the nucleus (bottom left). (Bottom): Close-up stereo pair
of contact area. The terraced scarp is approximately 50 m high. Isolated mesas
within the smooth region are approximately 20 m high.
variations occur in the pitted terrain (Li et al., in review) and that
no localized albedo markings around the edges or within the interiors of pits can be identified at the resolutions available (generally
about 20 m/pixel).
Layers of varying thickness are conspicuous in both the DI and
Stardust-NExT images.
The layers may be of two distinct types: thinner layers 10–15 m
thick associated with the smooth flows that cover the low lying
portions of the surface and the thicker layers (50–100 m thick) that
are being exposed in places by erosion.
The thicker layers on the DI side of the comet were mapped and
described by Thomas et al. (2007). One published interpretation of
these thicker layers is that they date to the time of accretion of the
nucleus: the TALPS model of Belton et al. (2007). Recent numerical
simulations of this process are discussed by Thomas et al. (2013).
The elevated area on the equator centered near 45E (Fig. 6, left)
may consist of a stack of layers. If so, even for layers as thick as
50–100 m, a considerable number of individual layers would be
needed to make up the 830 m of relief of this elevated area
(Fig. 6, right).
Scarp morphology at boundaries varies considerably across the
surface (Fig. 7). Some scarp edges are sharp, some concave, some
terraced, and others scalloped indicating different styles of erosion
and implying differences in composition and/or texture.
6.2. Smooth terrain
7. Impact site
The combined DI and SN coverage shows that smooth terrains
cover about 30% of the surface and that they are located in the
three prominent gravitational lows on the nucleus (Thomas et al.,
2013). Such locations are consistent with the interpretation that
these are deposits of material erupted from the subsurface and
accumulated in the lowest areas. A specific model based on the
release of highly volatile materials such as CO2 and CO in the interior followed by the fluidization of overlying material has been
developed by Belton and Melosh (2009).
Two of the smooth regions were identified in the Deep Impact
images (S1 and S2 in Fig. 3). The better imaged of the smooth areas
(S2) showed flow-like characteristics suggesting that it resulted
from the eruption of mobile materials onto the surface. This flow,
S2, the northern edge of which occurs in the area of the DI impact
site, was also imaged by Stardust-NExT (Fig. 5).
A third, more extensive and more complex area of smooth terrain (S3 in Fig. 3) is found in the expanded Stardust-NExT coverage
in Fig. 5. This region shows evidence of several generations of
Three contributions to this special issue deal with the DI impact
site. Wellnitz et al. update information on the location of the impact site, while Richardson and Melosh and Schultz et al. offer
complementary interpretations of SN images of the impact area.
Previous analyses of the DI impact event were published by
A’Hearn et al. (2005) and by Richardson et al. (2007), and Schultz
et al. (2007), among others. A major result based on observations
of the ejecta plume was the estimate of surface gravity and hence
of the mean density (400 kg/m3) of the nucleus made by Richardson et al. As is well known, the DI spacecraft did not succeed in
imaging the crater made by the impactor (see for example, A’Hearn
et al., 2005).
Wellnitz et al. (in review) re-analyze DI data related to the location of the impact site, particularly the nested sequence of images
from the impactor on its way to its collision with the surface. These
results, consistent with previous analyses, locate the impact site to
better than 100 m on the surface (Fig. 8), and probably to about
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Fig. 5. (Top left): SN image n30034 of smooth flow S2 which was imaged by both SN and DI. (Top right): Smooth flows S3 and S4 imaged only by SN, n30040. (Bottom panels):
Correlation between topographic lows and location of smooth regions. On Tempel 1 smooth terrains are associated with low regions on the nucleus, consistent with the
interpretation that these terrains originate as flows that are erupted onto the surface. Corresponding to each image in the top row is a topographic map (bottom row) with
boundaries of smooth regions outlined in RED. Elevations color-coded as in Fig. 3. See also Thomas et al. (2013).
Fig. 6. (Left): Stardust-NExT image n30036 showing lineations suggestive of large scale layering in the left hand side of the image. (Right): Shape model showing elevation
differences in this area (red = high and blue = low as in Fig. 3) and the trend of the layers (dashed lines). The maximum elevation difference is 830 m.
25 m (see also Richardson and Melosh, in review; Schultz et al., in
review).
Richardson and Melosh (2013) identify a shallow depression
within tens of meters of the calculated impact site as the DI crater.
The feature is 4 ± 1 pixels across corresponding to a diameter of
50 ± 12 m. On the basis of this identification they place constraints
on the material properties of the surface at the impact site. They
find that the Earth-analog material with most similarity to Tempel
1 material in terms of density, strength, and scaling parameters is a
lightly packed, mountain snow.
The observed crater size and the scaling parameters can also be
applied to the excavation model developed in Richardson et al.
(2007). This model leads to a DI impact total ejected mass of
1.2 106 kg (5.4 105–2.6 106 kg), with 60 ± 20% of this mass
ejected at greater than the comet’s escape velocity of 1.44 m/s.
An ejecta blanket thickness can be estimated, using the technique
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Fig. 7. Variety of scarp morphology observed on Tempel 1 including a terraced scarp, a scalloped depression wall, a sharply bounded flow edge and a concave eroding scarp.
These differences in erosion style suggest differences in composition and/or texture.
Fig. 8. DI impact site. Pre-impact (left) and post-impact (center and right) images of the DI impact area. The SN image is the highest resolution (scale about 12 m/pixel) postimpact view of the impact area. The arrows at bottom indicate the directions of incident sunlight in the DI and SN views. The cluster of arrows in the rightmost view point to
the impact site.
described in Richardson (2009), which gives a relatively thin ejecta
blanket thickness at the final crater rim of 2.7 mm (0.5–13.3 mm).
At the time of excavation, a significant fraction (30–60%) of this
ejecta blanket would have been water ice (Lisse et al., 2006), which
would have sublimated over a period of hours.
A complementary analysis of the impact site is provided by
Schultz et al. (in review). Schultz et al. also identify a 50 m ‘‘dimple’’ at the impact site, but in addition point out a possible partial
outer rim some 150 m wide that appears to overlap an area in
which a tiny mound is seen in the DI images, but not in the SN coverage. Schultz et al. consider the possibility that the DI crater may
have originally been larger but was significantly modified by postimpact collapse or by erosion and mass wasting over the 5.5 years
separating the DI and SN encounters.
8. Surface changes
A major objective of the Stardust-NExT imaging experiment
was to document surface changes that were expected to occur between the 2005 and 2011 perihelion passages. Lisse et al. (2005,
2006) determined that the comet loses some 109 kg of material
per perihelion passage due to sublimation, corresponding to about
1 m of surface material lost from active areas if it is assumed that
about 10% of the surface is active.
The Stardust-NExT images cover about 20 km2 of territory previously imaged by Deep Impact. The overlap region extends from
the area of the impact site and includes the northern extent of
the southern-most smooth flow discovered by Deep Impact. Allowing for the somewhat different viewing geometry (about 50° emission angle for DI, compared to about 30° for Stardust-NExT) the
overlap area looks remarkably unchanged (Fig. 9). There are no
obvious changes in albedo on scales of a hundred meters or more.
The larger dark circular makings near the impact site appear unchanged. Several small albedo spots in the region have disappeared, and others have appeared. A few may have changed in
contrast and extent. Given the small linear scale of these features
and the limited resolution of the images (the best have a pixel scale
of 11 m/pixel), it is impossible to determine whether we are seeing
differences due to modifications of surface texture, surface slope,
or amount of exposed water ice rather than the effects of slight differences in illumination and viewing conditions between the two
image sets.
The only significant changes in morphology occur along the
bounding scarp of smooth flow S2, which at least in two places
has receded by up to 50 m between 2005 and 2011. Thomas
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J. Veverka et al. / Icarus 222 (2013) 424–435
Fig. 9. Changes in the margins of flow S2 between 2005 and 2011. Left is Stardust-NExT image n30035, right from Deep Impact ITS image 173727747. Images have been map
projected and further warped to reliable common points. Some bright albedo spots have both been added and disappeared, and changed in contrast and extent. The scarp at
the bottom has lost two roughly triangular segments (arrows) of maximum horizontal extent 50 m. Other areas may have changed as well, but limited resolution and
differences in lighting and viewing geometry restrict the confidence of such conclusions.
et al. (2013) estimate that this flow is some 8–15 m thick and that
the total loss of material may amount to some 3 105 m3 or about
2 108 kg.
Thomas et al. provide a revised estimate of the mass loss per orbit of Tempel 1. Their value of 7–14 109 kg is about a factor of 10
higher than previous estimates. Thus the observed material loss
from the boundary of S2 accounts for about 2% of the total mass
loss suggesting that mass loss is highly concentrated to specific
areas on this comet: the 2% coming from about 0.01% of the surface
area. More detailed discussions of mass loss from Tempel 1 are
provided by Thomas et al. (2013) and by Farnham et al. (in review).
In this context it is interesting to note that the sources of most jets
observed during the SN encounter can be traced to sources along
an apparently eroding terraced scarp (see below and Farnham, in
review). This terraced scarp occurs on the face of the comet imaged
only by SN, making it impossible to document any possible
changes between 2005 and 2011.
9. Coma and jet activity
Coma and jet activity during the SN encounter is described by
Farnham et al. (in review). The activity level of Comet Tempel 1
was lower during Stardust-NExT than it was during Deep Impact
because the encounter occurred 34 days after perihelion (compared to a day before perihelion for DI), and possibly because of
a steady decline in water production reported in recent apparitions
(Schleicher, 2007). Strong seasonal and diurnal variations suggest
that the gaseous activity of Tempel 1 is confined to a limited number of active areas.
The Stardust-NExT images show fewer coma features than were
observed by Deep Impact. A subset of the encounter images
(n30025–n30036) reveal a number of small, well-defined jets,
highlighted against the dark background at the edge of the nucleus
as they move over the horizon (Fig. 10). These jets appear to originate in the vicinity of +30° latitude and 30–90° longitude. Most of
the jet sources appear to originate at or close to the prominent terraced scarp (Fig. 4) and indicate that this scarp is actively eroding.
The spacecraft passed over this area, but preliminary analyses of
DFMI data indicate that it did not fly through any individual jets.
Active areas have been identified previously at similar, low northern latitudes (Farnham, 2009; Schleicher, 2007; Vasundhara, 2009;
Belton, 2010; Vincent et al., 2010). Farnham et al. estimate the total
dust production to be 130 kg/s at the time of the SN flyby.
Fig. 10. Jets on the sunward side of the nucleus. The coma structure has been
enhanced by dividing out a 5° rotational average to improve the contrast of the jets.
The nucleus is inset using a separate stretch. The shadow of the nucleus against the
background coma can be seen on the right of the image. The Sun is toward the left,
circular structures in the coma are artifacts created by pixelization at low signal
levels.
10. Dust environment: DFMI measurements
Results from the Dust Flux Monitor Instrument (DFMI) are summarized by Economou et al. (in review). The instrument measures
particle impacts using two kinds of sensors—one based on polyvinylidene fluoride (PVDF) thin films, the other on acoustic detectors
(Tuzzolino et al., 2003). The PVDF sensors comprise two circular
films of 20 cm2 and 200 cm2, with four different mass thresholds
each. The two acoustic sensors, with two mass thresholds each,
are mounted on the front and second protective shields of the
spacecraft (with sensitive area 0.3 m2 and 0.7 m2 respectively).
Particles reaching the second shield have to penetrate the front
shield. At the higher encounter speed of 10.9 km/s compared to
the 6.1 km/s at Wild 2, the mass sensitivity of DFMI sensors
J. Veverka et al. / Icarus 222 (2013) 424–435
increased by approximately a factor of between 2 to 12 (depending
on the sensor and mass channel) providing sensitivity to dust particles in the mass range from 3 10 15 to >10 6 kg. The PVDF
sensors accumulated impact counts in 0.1-s time bins. The derivation of impact counts from the acoustic sensor signals is more complex, with time bins between 0.1 and 1 s (Tuzzolino et al., 2003;
Green et al., 2004).
DFMI was powered on 22 min (14,000 km) before closest approach. The first particle detection occurred at 4300 km; 90% of
the particle events were observed within 300 km of the nucleus.
The DFMI data (Fig. 11, top) show clearly that dust is not emitted
uniformly into the coma but occurs in clusters. This observation
is consistent with the conclusions from Wild 2 (Tuzzolino et al.,
2004) that larger aggregates of material are emitted sporadically
from the cometary nucleus and undergo significant fragmentation
into smaller particles in the coma (Clark et al., 2004). Bursts of up
to 1000 impacts of particles a few microns in diameter in 0.1 s
were detected at Wild 2 as the spacecraft flew through expanding
433
clouds of fragments (Green et al., 2004). As was the case for Wild 2,
it appears that at Tempel 1 the steady emission of small (micronsized) particles from active areas is of secondary importance compared to the emission of aggregates. Additional evidence for emission of aggregates has recently been obtained from images of
Hartley 2 from the EPOXI mission (A’Hearn et al., 2011). Possible
evidence of aggregate fragmentation within the coma of Comet
Halley is discussed by Simpson et al. (1987).
The mass distribution of particles observed at Tempel 1 is
shown in Fig. 11, bottom. As for Comet Wild 2 and Comet Halley,
the total dust mass is dominated by the larger particles. As expected for a dust coma dominated by fragmenting dust aggregates,
the mass distribution is highly variable along the trajectory.
11. Dust composition: CIDA measurements
The Cometary and Interstellar Dust Analyzer is a time-of-flight
mass spectrometer designed to analyze the chemical composition
Fig. 11. Dust measurements at Tempel 1. Top: Counting rates for all DFMI counters as a function of time from the closest approach. The flyby speed was 10.9 km/s. m1–m3
refer to mass thresholds of the 20-cm2 PVDF sensor. m1–m2 are thresholds for the 200-cm2 PVDF sensor. The other thresholds shown (AC1–AC4) are those of the acoustic
sensors. Bottom: Cumulative mass distribution of dust particles registered by DFMI in the inner coma during the encounter with Comet Tempel 1. The best fit overall mass
distribution index of a = 0.65, where the number of particles of mass greater than m, N(>m) = km a, is somewhat lower than that found for Wild 2 by the same detectors.
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J. Veverka et al. / Icarus 222 (2013) 424–435
of dust grains impacting the instrument’s silver target (Kissel
et al., 2003). Typical encounter speeds are sufficient to ionize a
fraction of each impacting dust grain. This instrument was used
at the flyby of Wild 2 in 2004 (Kissel et al., 2004) and is similar
to those used during encounters with Comet Halley in 1986
(Kissel et al., 1986a, 1986b). Previous measurements were carried
out using positive ions, but later studies showed that important
information on the organic component of dust grains can be
obtained from negative ions (Krueger et al., 2004). Accordingly,
during the encounter with Tempel 1, CIDA was operated in the
negative ion mode on the inbound leg and switched to its positive ion mode at closest approach. The instrument operated from
E 2 h to E + 2 h (±78,000 km of the nucleus). CIDA recorded a
total of 80 spectra, 46 in the negative ion mode and 34 in the positive mode. Prominent peaks due to H (mass number 1) and
CN (mass number 26) are evident in the data. Many spectra
show long tails at high mass numbers indicative of the presence
of complex molecules.
12. Conclusion
What have we learned about comet nuclei from the flybys of
Borrelly, Wild 2, Tempel 1, and Hartley 2?
Possibly the most important fact is these nuclei show significant
morphological diversity—we are far from knowing what a ‘‘typical’’
Jupiter family comet looks like. While significant portions of the
surfaces of Wild 2 and Tempel 1 are pitted, this does not appear
to be the case for Borrelly and Hartley 2. Furthermore, the pits
on Wild 2 and Tempel 1, while they display a similar size—frequency distribution, are morphologically distinct: it is very easy
to distinguish images of pitted terrain on Wild 2 from those of
Tempel 1.
The smooth flows that cover some thirty percent of the surface
of Tempel 1 seem to be totally absent on Wild 2. Suggestions of a
possibly similar feature occur in images of Borrelly (Soderblom
et al., 2004) but due to the limited resolution of the Borrelly coverage, a definitive identification cannot be made. It is true that a
smooth region is observed on Hartley 2, but the context in which
it occurs makes it unlikely that it is the result of an eruptive event
such as those implicated in the formation of smooth flows in Tempel 1.
While some evidence of layering has been noted for Borrelly
(Britt et al., 2004) as well as for Wild 2 and Hartley 2 (Thomas
et al., 2013), only in the case of Tempel 1 is evidence of layering
dramatically evident on global scales. The common occurrence of
scarps on all four nuclei is consistent with some degree of layering
of the surfaces and suggests that scarp erosion, most likely driven
by volatile sublimation, is a common erosion process on these
objects.
It is interesting that in spite of marked morphological difference, the surfaces of these comets are uniformly black with very
low albedos and generally very restricted exposures of water ice.
The five comet nuclei explored by spacecraft to date (Halley,
Borrelly, Wild 2, Hartley 2, and Tempel 1) show widely different
surface characteristics indicating that processes by which comet
nuclei evolve are varied and complex. Preserved on the surface of
Tempel 1 is evidence of an astonishingly wide range of geological
processes making Tempel 1 arguably the most geologically interesting and puzzling among the so-called ‘‘primitive bodies’’ (comets and asteroids) explored by spacecraft so far. The geologic
complexity and the knowledge about the surface properties
(occurrence of smooth, easily sampled areas) provided by Deep Impact and Stardust-NExT make Tempel 1 an ideal target for future
sample return missions.
13. Post script
Having successfully completed all aspects of the original Stardust and of the Stardust-NExT missions, the spacecraft was shut
down on March 24, 2011. The spacecraft is in a 1.5-year solar orbit
and will not come closer to Earth than 1.7 million km for at least
100 years.
Acknowledgments
We thank our two reviewers for very helpful comments and
suggestions.
Stardust-NExT was supported by NASA through its Discovery
Program. The Science Team expresses its thanks and acknowledges
its debt to the Project Management and Navigation Teams at the Jet
Propulsion Laboratory, to the Deep Space Network (DSN), and to
the Spacecraft Team at Lockheed Martin Aerospace (LMA) in Denver. We record our special thanks to the world-wide network of
observers for providing crucial observations of Tempel 1 to support
the determination of the appropriate time-of-arrival at the comet.
Part of the research described was carried out at JPL under contract
with NASA. O. Groussin’s participation in the project was supported by the Centre Nationale d’Etudes Spatiales (CNES).
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