Download Whence Comets?

Stardust
Kuiper belt) complicate this picture dramatically. The scattered disk feeds comets into both
the Jupiter family and the Oort cloud [and thus
into the group of Halley-type comets (1, 2)].
So mixing of comet families complicates the
picture.
Michael F. A’Hearn
The nuclei of even the three Jupiter family
comets visited by spacecraft are surprisingly
Recent advances in cometary science have indicated the importance of mixing of materials in the diverse in their shapes and in their surface
disk where the planets of our solar system formed. Now, the results from the Stardust Discovery
topography (Fig. 2). Comet Borrelly has a bend,
Mission unambiguously show that even more extensive and earlier mixing of the material took
like a banana’s shape (3, 4), Wild 2 is more
place, raising new challenges for theories of the protoplanetary disk and the formation of comets. ellipsoidal (5), and Tempel 1 has surfaces that
are nearly planar (6). All three show signs of
wenty years ago, there was tremendous Neptune in the cold Kuiper belt. Over the 4.5- layering, although it is most obvious in the highinterest in the return of comet P/Halley. billion-year age of the solar system, the comets resolution images of Tempel 1 (6, 7), that may
Although that return was somewhat dis- gradually spread among the giant planets, with reflect the original formation scenario (8). The
appointing compared with its closer previous some of them ending up in orbits that are visible gas coming out of Tempel 1 (9) showed great
heterogeneity, consistent with the idea that the
return in 1910, it was scientifically a tremendous from Earth.
The long-period comets (and some short- nucleus is made of cometesimals (smaller
apparition. Primarily this was because the advance knowledge of its return allowed scientists period comets, including Halley), on the other chunks) of different composition. These results
to plan both Earth-based observing programs hand, formed from solids that condensed closer can be interpreted as suggesting extensive radial
mixing of cometesimals in the
and to design the first space missions
outer parts of the early solar
to a comet. The first spacecraft to fly
system, rather than formation of
past a comet was actually intended
Comparison of
cometary nuclei from cometesimals
to study the solar wind but was
comet orbits
condensed at a uniform distance
diverted to pass through the tail of
Halley
from the proto-Sun. This means
comet Grigg-Skjellerup in late 1985.
that it will be somewhat harder
For comet Halley, however, an ento pin down the locations about
tire armada of spacecraft—Suisei,
which comets can give us clues
Sakigake, Vega 1, Vega 2, and
to formation.
Giotto—flew past the comet in
Tempel 1
The Stardust results presented
March 1986. The dramatic breakin this issue provide dramatic
throughs in our understanding of
Wild 2
evidence for much more extensive
comets triggered great interest,
r
te
pi
radial mixing at an even earlier
largely because comets hold unique
Ju
rn
tu
stage in the formation. They show,
clues to the origin of our own
a
S
perhaps most importantly, that
planetary system. Nevertheless, it
us
nearly all of the crystalline silicate
took 15 years before the next
an
Ur
grains, which have been known
missions were under way.
e
un
for some time to exist in comets,
Deep Space 1 flew past comet
pt
e
N
o
must have formed in the solar
Borrelly in 2001, heralding a suite of
t
u
Pl
system very close to the protocomplementary missions to very
Hyakutake
Sun rather than being circumstellar
different comets: Stardust flew past
or other presolar grains that were
comet 81P/Wild 2 in January 2004,
Deep Impact excavated 10,000 tons Fig. 1. Schematic showing the orbits of comets (T. Farnham, University of transported from the interstellar
of material from comet Tempel 1 in Maryland). Wild 2 and Tempel 1 are Jupiter family comets (originally from medium and directly incorporated.
July 2005, and Stardust returned to the Kuiper belt in the classical picture); Halley is the prototype of the Only one circumstellar grain was
Earth the grains that it had collected Halley-class comets (originally from Oort cloud); Hyakutake is a comet found, showing that material is
from comet Wild 2 in January 2006. coming from the Oort cloud. The classical Kuiper belt would be circular preserved from the interstellar
These missions, coupled with recent orbits just outside the orbits of Neptune and Pluto. The scattered disk medium but that, at least among
dynamical studies, have caused a consists of elongated orbits with perihelia in as close as Uranus and the refractory grains, it is only a
small fraction of the material near
major rethinking of the origin of aphelia at 50 to 100 AU (up to about three times Neptune’s orbit).
the surface of a comet. Although
comets.
There is no doubt that comets did not form in to the Sun and at higher temperatures. Forming this mixing of near-Sun condensates with
orbits even remotely similar to the ones in which in among the giant planets, these comets were preserved circumstellar grains has been sugwe now observe them (Fig. 1). Our classical ejected out to the Oort cloud (extending halfway gested before as an important part of cometary
picture has been that the majority of short- to the Sun’s nearest neighbor, alpha Centauri) formation, we now have clear evidence that this
period comets (the Jupiter family, of which and then much later perturbed into orbits that are mixing must be taken into account in any theory
Borrelly, Wild 2, and Tempel 1 are members) also visible from Earth. Recently, dynamicists of our solar system.
The Stardust results also show significant
formed from solids that condensed beyond have shown that the trans-Neptunian objects in
what is now called the scattered disk (dis- differences when compared with the results
covered in the 1990s and containing objects in from Tempel 1. At a basic level, the size disDepartment of Astronomy, University of Maryland, College
eccentric orbits extending beyond the classical tribution of impacted Wild 2 grains is different
Park, MD 20742–2421, USA. E-mail: [email protected]
PERSPECTIVE
Whence Comets?
T
1708
15 DECEMBER 2006
VOL 314
SCIENCE
www.sciencemag.org
SPECIALSECTION
Fig. 2. Cometary nuclei visited in the last decade: (left) Tempel 1 (NASA,
University of Maryland, and Deep Impact Team), (middle) Borrelly [NASA,
Jet Propulsion Laboratory (JPL), and Deep Space 1 Team], and (right) Wild
2 (NASA, JPL, and Stardust Team). The longest dimensions are 8 km for
Borrelly, 6 km for Tempel 1, and 5.5 km for Wild 2. Note the differences in
from that inferred for the ejecta from the impact
onto Tempel 1 (10), different from that of the
particles seen at Halley, and even different from
that deduced from the Dust Flux Monitor
Instrument on Stardust itself as it flew through
the coma of Wild 2. Perhaps more significantly,
there are differences in the types of particles
inferred for Wild 2 and Tempel 1. What does all
this mean for the origin of comets? Are these
differences between two comets, both the compositional and grain-size differences reported
here and the large-scale morphological differences, related to their somewhat different orbital
histories—Wild 2 having only recently been
perturbed into the inner solar system, whereas
overall shape, even though all are comparable in size. The smooth areas on
Tempel 1 are low, whereas the only smooth area on Borrelly is
topographically high. On Wild 2, the smooth areas are at the bottoms of
circular depressions. The circular features on Wild 2 have a morphology
that is very different from those on Tempel 1.
Tempel 1 has been in the inner solar system far
longer? Or are these differences due to mixing of
comets from different reservoirs into the population of Jupiter family comets? Or do they
merely represent different mixing ratios for the
cometesimals that made up the cometary nuclei?
Stardust has certainly brought us plenty of
food for thought. Combining the Stardust results
with those from other recent comet missions will
keep the theoreticians working for some time, while
we hope for visits to other comets in the future.
References
1. M. Duncan, H. Levison, L. Dones, in Comets II,
M. C. Festou, H. U. Keller, H. A. Weaver, Eds. (Univ. of
Arizona Press, Tucson, AZ, 2004), pp. 193–204.
PERSPECTIVE
NASA Returns Rocks from a Comet
Don S. Burnett
Cometary particles returned by the Stardust Discovery Mission are primarily silicate materials of
solar system origin. Some of the grains were formed at high temperatures close to the Sun, but
then transported far out to the Kuiper belt region of the solar system before being incorporated in
the comet.
ntil now, extraterrestrial materials available for study have come from the inner
solar system, including meteorites that
have fallen from the sky and the returned lunar
samples. Meteorites include unequivocal samples of Mars and the Moon as well as an impressive variety of asteroidal materials.
U
Department of Geological and Planetary Sciences, California
Institute of Technology, Pasadena, CA 91125, USA. E-mail:
[email protected]
The Stardust Discovery Mission has, for the
first time, returned cometary materials for
analyses in terrestrial laboratories. Initial results
are reported in this issue of Science. Launched
in 1999, the Stardust spacecraft encountered
comet P81/Wild 2 in January 2004, passing
through the dust cloud surrounding the cometary nucleus and capturing an estimated 1000
particles in the size range of 5 to 300 micrometers (1). These were successfully returned to
Earth in January 2006.
www.sciencemag.org
SCIENCE
VOL 314
2. J. A. Fernández, T. Gallardo, A. Brunini, Icarus 172, 372
(2004).
3. R. L. Kirk, E. Howington-Kraus, L. A. Soderblom, B. Giese,
J. Oberst, Icarus 167, 54 (2004).
4. J. Oberst et al., Icarus 167, 70 (2004).
5. R. L. Kirk et al., Lunar Planet. Sci. XXXVI, 2244
(2005).
6. P. C. Thomas et al., in preparation.
7. M. F. A’Hearn et al., Science 310, 258 (2005); published
online 8 September 2005 (10.1126/science.1118923).
8. M. J. S. Belton et al., Icarus, in press; published online
7 November 2006 (10.1016/j.icarus.2006.09.05).
9. L. M. Feaga, M. F. A’Hearn, J. M. Sunshine, O. Groussin,
T. L. Farnham, in preparation.
10. C. M. Lisse et al., Science 313, 635 (2006); published
online 12 July 2006 (10.1126/science.1124694).
10.1126/science.1137083
Wild 2 is a Jupiter family comet; evidence
suggests that it formed in the Kuiper belt of objects beyond the orbit of Neptune and was then
diverted into the inner solar system by orbital
perturbations from Neptune and Jupiter. Thanks
to Stardust, we now have material to study from a
body that unequivocally originated in the outer
regions of the solar system.
In the inner solar system, volatile constituents, primarily H2O, sublimate from a comet
nucleus. Dust grains imbedded in the ices are
swept out with the outflowing gas, becoming
part of a large cloud of gas and dust, or coma.
The cometary dust is there for the taking—the
challenge is to capture the particles without
destroying both the dust and the spacecraft in
the process. Two clever technological achievements led to the success of Stardust: (i) design
of a spacecraft trajectory by Chen-Wan Yen (Jet
Propulsion Laboratory) that produced a relatively low encounter velocity of 6.1 km/s and
(ii) development of an aerogel capture medium
by Peter Tsou (Jet Propulsion Laboratory).
15 DECEMBER 2006
1709