Advances in Space Research 38 (2006) 1995–1999
www.elsevier.com/locate/asr

A newborn asteroid 832 Karin with old and new
surfaces – SUBARU spectroscopy
Sho Sasaki a,b,*, Takanori Sasaki b, Jun-ichi Watanabe a, Fumi Yoshida a,
Hideyo Kawakita c, Naruhisa Takato d, Budi Dermawan e, Tetsuharu Fuse d,
Takashi Ito a, Tomohiko Sekiguchi aaNational Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japanc
Department of Physics, Faculty of Science, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japand
Subaru Telescope, National Astronomical Observatory of Japan, 650 North Aohoku Place, Hilo HI96720, USAe
Department of Astronomy, Bandung Institute of Technology, Bandung 40132, IndonesiabReceived 1 November 2004; received in revised form 14 April 2006; accepted 19 April 2006

Abstract
The mismatch between reﬂectance spectra of most common asteroids (S-type asteroids) and most common meteorites (ordinarychondrites) is thought to be caused by space weathering. Recent study of celestial mechanics has led to the discovery of a young groupof S-type asteroids, ‘‘Karin cluster group’’, which is thought to be remnants of a collisional breakup only 5.8 million years ago. We performed near-infrared spectroscopy of the brightest asteroid 832 Karin among this cluster group. For diﬀerent rotational phases of Karin,
we derived diﬀerent spectra such as reddened spectrum like that of S-type asteroid and unreddened spectrum like that of ordinarychondrites. These ﬁndings indicate that a part of Karin may retain spectrally mature surface of its parent body. Although Karin might
have formed from gravitational accumulation from catastrophic collision of its parent body, there would be some color heterogeneity on
its surface if Karin collected materials which derived the weathered surface of the parent body.
Ó 2006 Published by Elsevier Ltd on behalf of COSPAR.
Keywords: S-type asteroid; Space weathering: Karin cluster group; Impact disruption; Surface heterogeneity; Spectral mismatch

1. Introduction
Spectral studies have been clariﬁed ‘‘the connection’’between asteroid-types and meteorite-classes. However,there is well-known spectral mismatch between S-type
asteroids and ordinary chondrites. Although S-type asteroids comprise the most prevalent type of asteroids in the
inner part of the main asteroid belt, their reddened visible/NIR reﬂectance spectra and derived mineralogies are
diﬀerent from those of ordinary chondrites, which are*
Corresponding author. Present address: Mizusawa Observatory,
National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Oshu,
Iwate 023-0861, Japan. Tel.: +81 197 22 7139; fax: +81 197 22 7120.
E-mail address: sho@miz.nao.ac.jp (S. Sasaki).

0273-1177/$30 Ó 2006 Published by Elsevier Ltd on behalf of COSPAR.
doi:10.1016/j.asr.2006.04.014

the most common meteorites. Space weathering isthought to be able to explain this spectral mismatch(Chapman, 1996).
In fact, a large number of small near-Earth asteroids
show intermediate reﬂectance spectra between S-type asteroids and ordinary chondrites (Binzel et al., 1996; Burbine
and Binzel, 2002). Recently, Galileo and NEAR spacecraft
observations have conﬁrmed that interior of S-type bodies
Ida and Eros have ordinary chondrite-like composition,
despite their surface reddened spectra (Chapman, 1996;
McFadden et al., 2001; Bell et al., 2002). On the other
hand, the detailed mechanisms involved in space weathering have been clariﬁed recently. Hapke’s hypothesis, which
the formation of iron nanoparticles may be responsible forthe space weathering (Hapke et al., 1975), was conﬁrmed

S. Sasaki et al. / Advances in Space Research 38 (2006) 1995–1999

by pulse laser irradiation experiments simulating the heating by high velocity dust impacts. Reﬂection spectra oflaser-irradiated silicate minerals match the typical spectraof S-type asteroids (Yamada et al., 1999; Hiroi and Sasaki,2001). Additional evidence includes the ﬁnding of ironnanoparticles in the laser-irradiated sample (Sasaki et al.,2001, 2002, 2003). As in lunar soils (Keller and McKay,1993), iron nanoparticles were found in some meteorite
recently (Noble et al., 2004).
The degree of space weathering has been used as a tool
to estimate the ages of asteroid surfaces (Sasaki et al.,2001), and the relationship between the color and age of
S-type asteroid groups has been suggested through the
SDSS project (Jedicke et al., 2004). And now, we have a
newborn group of asteroids named Karin cluster group,
which is thought to have been derived from a breakup that
occurred approximately 5.8 million years ago (as indicatedby numerical integration of asteroids’ orbits (Nesvorn’yet al., 2002) and by numerical simulations of catastrophic
disruption which succeeded in reproducing the formationof this cluster (Michel et al., 2003). In this work, we propose that the brightest asteroids 832 Karin in this groupis one of excellent targets to examine the eﬀect of space
weathering.
2. Observations
Using 8 m Subaru telescope on Mauna Kea, we
obtained near-infrared spectra of 832 Karin on September 14th, 2003 (UT). To perform wide-range near-infrared observation, we used CISCO (Cooled Infrared
Spectrograph and Camera for OHS) with grisms zJ(0.88–1.40 lm), JH (1.06–1.82), and wK (1.85–2.51).
Spectral region between 1.06 and 1.40 lm was observed
both by zJ and JH bands. As for Karin observation,
integration time is 800 s for each band, i.e., 2400 s foreach set of observation. Reference stars (A0 star
SAO165395 and A2 star SAO165274) were observed during the Karin observation and a reference star (G2V star
HIP3990) was observed after the Karin observation.
Detailed observation method is described in Sasaki
et al. (2004). During the same night after the Karinobservation, we observed comet C/2002 T7 (Kawakita
et al., 2004).
We obtained three sets of reﬂectance spectra of Karin:
7:57–8:40UT, 8:46–9:29UT and 10:45–11:50UT. Fromlightcurve observations, the rotational period of Karin was
estimated to be 18.348 h (Yoshida et al., 2004). According
to the lightcurve, rotational phases of our observation
sequences are 0.30–0.34 for the ﬁrst observation set, 0.35–
0.38 for the second set and 0.45–0.50 for the last set (Fig. 1).
3. Results
Fig. 2 shows the relative reﬂectance spectra of Karin forthese three observation sets. The three spectra in Fig. 2 arethat relative to the ﬁrst, the second, and the last observa-

Fig. 1. Lightcurve of 832 Karin (Yoshida et al., 2004). Observations weredone by 1.8 m VATT at Vatican observatory at Mt. Graham, Arizona, by
1 m Schmidt telescope at Kiso observatory, Japan, and by 40 cm telescopeat Fukuoka University of Education, Japan. Lightcurve observations weredone through 9 nights from August to September 2003. Rotational periodof Karin is 18.348 h. Rotational phase is zero at 2004 July 31, 0:00 UT.
Amplitude of the lightcurve is 0.7 mag at zero solar-phase-angle.
Assuming that Karin is an elongated ellipsoid with poles being on the
short axis and that observed direction is vertical to the pole direction, theaxis ratio of Karin ellipsoid (long to short) is about 2. Three sets of ourobservations correspond to the rotational phases denoted by gray zones.

1.6
Relative reflectance

1996

1.4
1.2
1
0.8
0.6
0.9

1.2

1.8
1.5
Wavelength (micron)

2.1

2.4

Fig. 2. Relative spectra of 832 Karin. Bottom one is the spectrum relative
to the ﬁrst set, middle one is that relative to the second set, and top one isthat relative to the last set. Spectra are smoothed by running average of 5data points. The top and the bottom spectra are vertically shifted by +0.2
and by 0.2, respectively.

tional sets. There is an obvious diﬀerence between the top
two and bottom spectra, which we interpret as indicativeof Karin’s surface being heterogeneous in each rotational
phase. We conﬁrmed that spectra of the standard star
SAO165395 were not changed before the ﬁrst set and after
the second set of Karin observations (Fig. 3). The observed
spectral change of Karin (Fig. 2) was not caused by an
instrumental, atmospheric, or hour angle eﬀects.

S. Sasaki et al. / Advances in Space Research 38 (2006) 1995–1999

1997

Normalized reflectance

1.6
1.4
1.2
1
0.8
0.9

1.2

1.8
1.5
Wavelength (micron)

2.1

2.4

Fig. 4. Reﬂectance spectra of Karin (the ﬁrst set by crosses and the last set
by circles) along with the previous observations of S(IV)-type asteroid 584
Semiramis (denoted by squares) and L6 ordinary chondrite Paranaiba(denoted by diamonds) normalized to the unity at 1.0 lm. The meanvalues for every 10 data points are plotted with their error bars of standard
deviation. From Fig. 2, we removed data in the wavelength range where
the telluric absorptions were strong and the error bars are large. Note thatthe range between 1.1 and 1.4 lm were observed both by zJ and JH bandand their data were overlapped. The change of redness in this region wasobserved both by zJ and JH bands.

normalized reﬂectance spectra of Karin (ﬁrst set and lastset) along with the previous observations of S(IV) asteroid
584 Semiramis and L6 ordinary chondrite Paranaiba.
Whereas, the ﬁrst set’s spectrum of Karin matches that of
the S(IV) asteroid, the last set’s spectrum of Karin matchesa typical normalized spectrum of L6 ordinary chondritewith relatively gentle slope at the wavelength shorter than
1.6 lm. Laboratory simulations of space weathering predict that relatively intense reddening should occur at wavelength shorter than 1.6 lm (Yamada et al., 1999; Sasaki
et al., 2001), which suggests that the ﬁrst set could be theredden spectrum of the last set by space weathering.
4. Discussions

Fig. 3. Relative spectra of SAO165395 (zJ) that were observed before the
1st set (above) and after the 2nd set (middle) of Karin observation, and theratio of these two spectra (bottom). The diﬀerence of absolute value
between these spectra reﬂects the diﬀerence of airmass and the diﬀerence ofintegration time.

The shape of the ﬁrst set’s spectrum in 0.8–2.5 lm is
consistent with that of an S-type object. The spectrum iscompared with S-subclass classiﬁcation where S-type asteroids are divided into 7 subclasses on the basis of the area
ratio between the 2 lm/1 lm band, the central position ofthese two bands, the depth and slope of the 1 lm absorption (Cloutis et al., 1986; Gaﬀey et al., 1993). The ﬁrst set’s
spectrum of Karin belongs to S(IV) class. Fig. 4 shows the

We observed a rapid change of the asteroid NIR spectrum around the rotational phase 0.35, i.e., between theﬁrst and the second sets of our observation. From the lightcurve as shown in Fig. 1, this change from the (mature)
reddened spectrum to the (fresh) reddened spectrum corresponds to the stage during which observing cross section of
Karin increased. During a diﬀerent night of the same season, Yoshida et al. (2004) performed multi-color visible
observation using 1.8 m VATT at the Vatican observatory.
They found that V–I magnitude is high (V being dark) at
rotational phase 0.2 (just earlier than our ﬁrst set), which
we propose can may also be the result of space weathering.
Although we cannot rule out the possibility that Karin wasa fragment of compositionally diﬀerentiated parent body,the above-mentioned spectral similarity between the ﬁrst
set of our observation and S type asteroid suggests that

1998

S. Sasaki et al. / Advances in Space Research 38 (2006) 1995–1999

Karin is more likely to be an impact fragment which keeps
the weathered surface. Probably, more reddened reﬂectance slope should be observed at a phase earlier than0.3, out ﬁrst set’s spectrum would be some average of more
reddened spectrum in earlier phase and unreddened fresh
spectrum in later phase.
Let us discuss our results with impact fragmentation
processes that formed asteroid groups. The spectral change
according to the rotational phase could be explained if
Karin is a cone-shaped fragment from a low-velocity
impact forming Karin group. Experimental studies ofimpact disruption suggest that an impact target is disrupted
into cone-shaped fragments under a low-velocity (<1 km/s)
impact (Fig. 5) (Fujiwara et al., 1989). Cone base is composed of original weathered surface. If the cone base is
not replenished by impact ejecta, the cone should have heterogeneous surface, i.e., old cone base and fresh cone sides.
In the case of Karin, the cone base (with relatively smaller
cross section) should be observed at a rotational phase of
0.2. This would also explain that Karin is a little darker
at 0.2 than at 0.7 in the lightcurve (Fig. 1). However,according to the cone fragments model, other bodies similar to Karin’s size should have been produced. Currently,
Karin is the only large fragment in the Karin group, which
is not consistent with the cone fragment model.

The size of Karin (20 km) is much larger than thetransition size (a few 100 m) from ‘‘strength-regime’’ to‘‘gravity-regime’’ for catastrophic disruption (Asphaug
et al., 2002). The interior strength of Karin against disruption is basically controlled by gravitational attraction
rather than material strength. The disruption of a parent
body (which should be as large as or larger than Karin)
should be controlled by gravity rather than strength.
Probably the simple cone-shaped disruption model
requiring the ‘‘strength-regime’’ would only be applicableto sub-km bodies.
Another possibility is that ‘‘Karin’’ is a heterogeneous
rubble pile, i.e., a gravitationally accumulated aggregate(Fig. 5). In this case, impact disruption of the parent
body of the Karin group would be core fragmentation
or more intensive fragmentation according to high velocity collision. Numerical simulation of asteroid collision
simulating asteroid groups suggested fragmentations
would be prevalent throughout a parent body. Afterthe fragmentation, some of bodies should have formedby gravitational accumulation (Michel et al., 2003).
Thus, if one body collected materials which derived fromthe surface region of the weathered parent body, there
would be some color heterogeneity on its surface. Gravitational accumulation forming a rubble pile would have

Fig. 5. Impact fragmentation of a parent body of asteroids and two models of Karin with heterogeneous surface. (Left-above) Cone fragmentation under
low-impact velocity (<1 km/s). (Left-below) Core fragmentation under high impact velocity. (Right-above) Cone fragment model of Karin. (Right-below)
Rubble pile model of Karin. Left-hand side ﬁgures were modiﬁed from Fujiwara et al. (1989).

S. Sasaki et al. / Advances in Space Research 38 (2006) 1995–1999

also collected ﬁner regolith-forming materials, whichmight cover and replenish the rubble pile surface. But
if those ﬁne materials have some ejecta velocity, theyshould have escaped before the gravitational aggregate
forming Karin was produced.
The other idea is based on non-catastrophic formationof Karin cluster group. Assuming that a body much smaller than Karin should have impacted on proto-Karin withweathered surface at relatively low velocity. Although Karin was not destroyed, impact cratering should have exposed
fresher surface. An alternative is that impact-induced shaking would have mixed the surface, which would have
replenished the very surface regolith of Karin. Nevertheless, because the Karin group would be comprised of morethan 20 bodies, the impact event forming the group shouldhave been catastrophic, even if the impact speed would be
low (<1 km/s).
Acknowledgements
We are grateful P. Michel and an anonymous referee for
their thoughtful reviews. We thank J. A. P. Rodriguez forthe checking of the manuscript.
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