Reflectance Spectra of Members of Very Young Asteroid Families

1
Reflectance Spectra of Members of Very Young
Asteroid Families
40th Lunar and Planetary Science Conference, The
Woodlands TX, USA. Poster 2258. Poster
Session I, Tues., 24 March 2009.
C.R. Chapman, B. Enke, W.J. Merline, D. Nesvorný,
P.M. Tamblyn, E.F. Young (SwRI, Boulder CO)
Introduction Several asteroid families or
clusters have very young dynamical ages 1. The
Veritas family of C-type asteroids, Karin cluster
within the S-type Koronis family, and the Iannini
cluster (S-type) formed about 8.3, 5.8, and lt5 My
ago, respectively. A family associated with
Datura is extremely young, 0.45 My 2. If space
weathering modifies asteroid spectra on
timescales comparable to or longer than several
My, then we may expect different spectra for
members of these recently formed families than
for older families. We use the SpeX instrument
on the IRTF (in low-resolution prism mode) to
study infrared reflectance spectra of members of
these young families. We observe selected young
asteroids, controls (e.g. members of the Themis
family and non-Karin members of the Koronis
family), and G-type comparison stars. Observation
s Spectra were obtained on 11 nights between
June 2005 and July 2006. Although the sky
quality was generally good, we had some
difficulty characterizing extinction resulting in
slight uncertainties in the overall slopes of
spectra. Also there is jitter for faint
asteroids beyond 2µm and near the 1.4 and 1.9µm
water bands. An average spectrum is calculated
for each asteroid each night. Nightly averages
are then averaged to yield spectra shown in the
bottom half of this poster for asteroids observed
on multiple nights. Averages for each asteroid
family are shown in white-on-black immediately
below.
PROJECT GOALS The goals of our project are to
study, with various telescopic techniques, the
physical properties of members of very young
dynamical families. These families, identified
by D. Nesvorny and his colleagues, were formed at
discrete and well-determined times less than 10
million years ago. Most asteroid families are
believed to be hundreds of millions or even
billions of years old. Our expectation is that
young families, as compared with typical
families, may provide clues to processes that
presumably take place on relatively short
timescales. Processes in which we are interested
include space weathering, temporary preservation
of near-surface volatiles, satellite formation
and evolution, the Yarkovsky Effect (which we can
help to calibrate), and understanding the
initial/early configurations of asteroid families
as a check on hydrocode simulations and other
theoretical insights concerning the formation of
families. PROJECT APPROACH Our approach is to
observe known members of the Karin cluster
(a2.87 AU, e0.044, i2.1º), the Veritas family
(a3.17 AU, e0.065, i9.3º), the Iannini cluster
(a2.64 AU, e0.267, i12.2º), and younger
families like Datura, plus controls (e.g.
non-Karin members of the Koronis family, also
small members of the C-type Themis family), using
a variety of telescopes, instruments, and
techniques. Our approach is to make coordinated
space-based and ground-based observations of
relevant family members, obtaining low- and
medium-resolution spectrophotometry, radiometry,
adaptive optics imaging, and lightcurve
photometry. In this poster we present our final
reductions for IR reflectance spectra obtained
with SpeX on the IRTF during June 2005 through
July 2006.
Discussion The timescale for space weathering
processes that modify reflectance spectra of
S-type asteroids is an important issue 3. It
can be seen that spectra for the young Iannini
and Karin family asteroids are slightly less red
(and/or have shallower bands) than typical
Koronis members, which may be a subtle reflection
of their comparative youth. However, if space
weathering modifies ordinary chondrite-like
spectra to mature S-type spectra (like the
Koronis family), then the Iannini and Karin
family asteroids are near the end of their
spectral evolution after only a few million
years. The very young asteroid Datura is a
Q-type with a deep 1µm band and has only begun to
evolve toward S-type maturity. Assuming
that the Veritas and Themis families are of the
same inherent composition, then the different
average spectra for these families may reflect
space weathering trends. Whereas Themis spectra
show a U-like shape in this wavelength range, the
young Veritas members tend to show a slightly
bluish slope with a hint of an inverted U-like
shape, as has been previously noted 4,5. It is
possible that the effects of space weathering are
more prominent at these longer wavelengths for
C-types than for S-types. References
1 Nesvorny D. et al. (2003) Ap.J. 591, 486-497.
2 Nesvorny D. et al. (2006) Science 312, 1490.
3 Chapman C.R. (2004) Ann. Rev. EPS 32,
539-567. 4 Chapman C.R. (2008) ACM 8391. 5
Ziffer J. (2008) AAS/DPS40, 60.06. This work
is supported primarily by the NASA Planetary
Astronomy Program.
Comparison of S-like families Koronis exhibits
trends for slightly more weathering than Iannini
(which has a slightly deeper 1µm band, measured
between 0.9µm and 1.4µm) or than Karin (less
reddened overall). But the differences from
Koronis are subtle and not the same for Karin and
Iannini. Datura has a deep 1µm band and
resembles a mostly unweathered, olivine-rich
ordinary chondrite.
Some members of our team, observing with the IRTF
SpeX remotely from the Boulder, Colorado, offices
of SwRI, from back-left to front-right Bill
Merline, Clark Chapman, Eliot Young, and Peter
Tamblyn. Photo by Y Chapman.
Karin
Iannini
Koronis
Themis
Veritas
Average spectra for asteroids, grouped by family.
All spectra are from 0.8 to 2.45µm, normalized
to 1.0 reflectance at 1.5µm (but vertically
displaced by 0.2). Within each part of the
figure, asteroid numbers from top to bottom are
Datura 1270. Iannini 1547, 151032, 4652,
81550, 87239. Karin 10783, 832. Koronis 1289,
1423, 167, 208, 2574, 3032, 311, 4863, 5338, 534.
Veritas (Part 1) 10793, 1086, 15066, 19845,
2147, 28022, 28546, 29891. Veritas (Part 2)
31743, 490, 49622, 5592, 5594, 6343, 7612.
Themis 1027, 2114, 2297, 3591, 4470, 468, 492,
656, 8591, 90.
Concluding Thoughts The extremely young Datura
family is represented by a fresh, olivine-rich
spectrum with a deep 1 micron band, suggesting
little space weathering. The small, dynamically
young S-type asteroid families (Karin and
Iannini) have evolved rather far toward mature
S-type reflectance characteristics in the 0.8
2.5 micron region in just a few million years.
There may be less rapid spectral evolution
exhibited by Veritas family members, provided
that their inherent carbonaceous mineralogy is
similar to that of Themis family members. For
these C-type families, the differences in
spectral slope are opposite in our wavelength
range from the differences observed in the
visible.