Sky and Telescope - June 2016 - 21

1.3

"Classical"
Kuiper Belt object

"Dark-red" group

Normalized reﬂectivity

"Less-red" group
1.2

D type

1.1

P type

1.0

C type

0.9

0.8

RED & REDDER
Although astronomers know little
about the true composition of Jupiter's
Trojan asteroids,
their surfaces group
broadly in "dark-red"
and "less-red" populations that share
similarities with the
D-type and P-type
objects found in the
main asteroid belt.
S&T: LEAH TISCIONE; SOURCE: J.
EMERY ET AL. / ASTRON. JOURNAL

400

500

600

700

800

Wavelength (nanometers)

populations are relatively rare in the main asteroid belt.
But it turns out that only the largest of Jupiter's objects
tend to be this color. Recently, Ian Wong (Caltech)
and colleagues showed that Jupiter's less-red (P-type)
Trojans actually appear to dominate the population at
sizes smaller than about 5 km. Perhaps Jupiter's Trojans
derived from two groups of objects that formed in diﬀerent locations in the solar nebula prior to their capture.
Or maybe the dependence of color on size is telling us
that the less-red objects are collisional fragments of the
larger, redder objects. If the latter idea is correct, then
the smaller Trojans would have "fresher" surfaces that
have been altered less by exposure to the Sun's ultraviolet light, its solar wind, and cosmic-ray bombardment.
Geochemists usually interpret reddish surface material to be rich in carbon and complex organic molecules.
But mid-infrared spectra recently obtained by Joshua
Emery (University of Tennessee, Knoxville) and two
colleagues suggest the surfaces of Trojans are covered
in silicate dust that could be similar in composition to
that in comets. If Trojans formed with lots of ices, then
their interiors could still contain this initial volatile-rich
material underneath outer layers that have been highly
altered by "space weathering."
Determining the range of possible compositions
among Jupiter's Trojans would be a lot easier if we knew
their masses and thus could estimate their bulk densities. To that end, however, few Trojan binaries have been
identiﬁed. One of them, Patroclus, has nearly equalsized bodies. Franck Marchis (SETI Institute) and colleagues have analyzed the two components' orbits, and
they ﬁnd that the density of the paired objects is only
about 0.8 g/cm3 - less than that of water. This in turn
suggests that Trojans are mostly made of porous ice, a

composition more typical of a comet's nucleus than of
a rocky asteroid. Since Trojans are slowly leaking out of
their Lagrangian conﬁnement, conceivably some display
cometary activity if and when they approach the Sun.

Wanted: More Data
All the dynamical and compositional evidence in hand
suggests that Jupiter's Trojans came from the outer solar
system after the giant planets ﬁnished moving around.
So they might be related to objects in the distant Kuiper
Belt. Although the latter are generally redder, this diﬀerence might be a superﬁcial consequence of the Trojans'
closer proximity to the Sun's intense radiation.
One challenge to this idea is that Neptune's Trojans,
which likely never approached the Sun all that closely,
look quite similar to Jupiter's and likewise show a fairly
uniform distribution of surface colors. Moreover, they
are not like the more varied and redder Kuiper Belt
objects, even though both populations have remained in
"cold storage" since the solar system's formation.
The mysterious origins and our limited compositional
information about the Trojan asteroids are why NASA has
prioritized the Jupiter Trojans for a spacecraft mission of
the Discovery (small budget) or New Frontiers (medium
budget) class in the very near future. And while awaiting
those results, ground-based observers will continue to
probe these orbital curiosities for more clues to their origins and to the evolutions of their giant-planet partners. ✦
A frequent user of some of the world's largest telescopes, Scott
Sheppard is an astronomer at the Carnegie Institution for
Science in Washington, D.C. He codiscovered 2008 LC18
(the ﬁrst known L5 Neptune Trojan) and 2005 TN53 (the
ﬁrst high-inclination Neptune Trojan).
Sk yandTelescope.com June 2016

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Sky and Telescope - June 2016

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