A (6)

http://www.skyandtelescope.com.au 31

averages 225 km across). Moreover, their spectra are
mostly featureless.
What we do know is that they’re very dark, with
average reflectivities of around 5% to 7%. Most Jupiter
Trojans can be classified as either ‘dark red’ (D-type)
or ‘less red’ (P-type) asteroids. A few have more neutral
colouring, akin to the carbon-rich C-type objects seen
in the outer asteroid belt. Intriguingly, a cluster of
Trojans in Jupiter’s L 4 cloud have very similar orbits,
and most have the same C-type surface colouration.
Dynamicists have proposed that this group, known as
the Eurybates family, came from a single broken-up
parent body.
The reddish D-types that dominate Jupiter’s Trojan
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 colour. 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
colour 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 of 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, a few Trojan
binaries have been identified. One of them, Patroclus,
has nearly equal-sized bodies. Franck Marchis (SETI
Institute) and colleagues have analysed the two
components’ orbits, and they find that the density of
the paired objects is only about 0.8 g/cm^3 — 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 may 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

movingaround.Sotheymightberelatedtoobjects

inthedistantKuiperBelt.Althoughthelatterare

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 colours.

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.

Their mysterious origins and our limited

compositional information about the Trojan asteroids

are why NASA has prioritised the Jupiter Trojans for

a spacecraft mission of the Discovery (small budget)

or New Frontiers (medium budget) class in the 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 LC 18

(the first known L 5 Neptune Trojan) and 2005 TN 53 (the first

high-inclination Neptune Trojan).

400 500 600

D type

“Classical”

KuiperBelt object

P type

C type

“Dark-red” group

“Less-red” group

700 800

0.8

0.9

1.0

1.1

1.2

1.3

Wavelength (nanometres)

Normalised reflectivity

RED & REDDER

Although

astronomers know

little about the

true composition

of Jupiter’s Trojan

asteroids, their

surfaces group

broadly into ‘dark

red’ and ‘less red’

populations that

share similarities

with the D-type

and P-type bodies

found in the main

asteroid belt.

S&T:

LEAH TISCIONE; SOURCE: J. EMERY ET AL. /

ASTRON. JOURNAL