Geoscience Reference
In-Depth Information
Time variation of relative magnitude of
V
-
I
in Fig. 3(e) does not show
any definite anomaly, unlike what was seen in Fig. 3(b) in September 2003.
The wavelength dependence of the relative reflectance of this asteroid in
Fig. 3(f) is more like that of phase
=0
.
2 in Fig. 3(b) in September 2003
than that of phase
0.2. In a word, (832) Karin did not show a mature
(red) surface in September 2004, exhibiting only a fresh surface with low
relative reflectance at longer wavelengths.
∼
4. Discussion
4.1.
Interpretation of the observing results
So far we do not have a very good explanation for the unexpected color
mismatch between the 2003 and 2004 multi-color observations. The key to
solving this problem might lie in the difference in the amplitude of two
lightcurves in Figs. 3(a) and (d): The lightcurve of September 2003 has a
larger amplitude than that of September 2004. In general, when we look
at an asteroid from its pole direction, especially at around opposition, the
brightness of the asteroid can be nearly constant. Considering the relative
orbital configuration between (832) Karin and the Earth, we have drawn
a rough and possible schematic figure for deducing why we did not see
a red surface on this asteroid in our 2004 observation (Fig. 4). Following
Sasaki
et al.
16
considerations, (832) Karin might be a cone-shaped asteroid
fragment with a small portion of mature surface that used to be part of
the parent body's surface. If the rotation axis of this fragment is highly
inclined, nearly parallel to its orbital plane as in Fig. 4, it might account for
the fact that we see its red surface occasionally as it rotates at the position
of September 2003. If the orbital configuration, the spin axis orientation,
and the location of the red surface are as in Fig. 4, it might also be that
we could not see any red surface on this asteroid in September 2004 when
we were supposed to look at this asteroid from nearly the pole direction.
This geometric configuration could explain why the lightcurve amplitude
is smaller in our 2004 observation than in the 2003 observation, depending
on the shape of this asteroid.
The surface color variation of (832) Karin suggests that this asteroid
possesses an inhomogeneous surface. Judging from the recent breakup his-
tory of the Karin family, a part of it could be fresh and newly exposed by
the family-forming disruption. Meanwhile there could be a mature surface,
once the parent body surface, and had been exposed to space radiation or
particle bombardment over a long time.
