Sky and Telescope - April 2015 - 24

Asteroid Adventure

Cere
s
in g

b it

er
fC

Mars
gravity assist
Feb. 2009
Vesta
arrival
July 2011

es
fV

it o

A Pioneer Expedition

it o

Orb

S&T: GREGG DINDERMAN

Launch
Sept. 27,
2007

Orb

th r
Or
b it o f E ar
a
M
of
O rbit

Sun

Vesta
departure
Sept. 2012

orbits Vesta

Path of Dawn

some longitudes, and not at every point in Ceres' orbit.
How ice seems to make a patchy, transient water-vapor
atmosphere is a new mystery for Dawn to try to solve.
And there's spectroscopic evidence for water's action
on Ceres' surface rocks, in the form of minerals never
seen on other asteroids: potentially brucite or another
hydroxide, and carbonates. The only other places in the
solar system where we have observed carbonates are Mars
and Earth.

Ceres arrival
March 2015

Da w

or
e rf

DAWN'S INTERPLANETARY VOYAGE Using ion-propulsion
thrusters and a gravity assist from Mars, Dawn traveled nearly four
years to reach Vesta. It's cruised another 2½ years to reach Ceres.

of rock in Ceres' crust would make it signiﬁcantly denser
than an icy mantle below it, which would be an unstable
arrangement. Most likely, a primitive crust would founder
and sink into the mantle, generating a fresh new surface
on the asteroid.
An internal ocean could also drive very active surface
geology. Ice is less dense than water, so as Ceres cooled
and its subsurface ocean froze, the internal pressure
would multiply as this material swelled and tried to take
up more space. That internal pressure would have to
be relieved somehow. The crust could have cracked in
order to allow the mantle to expand, forming planetary
stretch marks as a series of parallel fractures, like those
we see today on Saturn's moon Dione. If such fractures
propagated deep enough to open a conduit to the surface
for the pressurized ocean, we could see frozen ﬂows of
cryovolcanic material, made of the mineral-rich stuﬀ that
once circulated deep within the body. In fact, with its icy
composition and diﬀerentiated interior, Ceres will likely
look much more like the moons of Jupiter or Saturn than
it will look like Vesta.
Today, Ceres' surface is too hot for ice to be stable
anywhere except, possibly, at the poles. If there ever was
ice exposed at the surface, it has sublimed away. Any dust
or rocky material that was once buried in the ice would
remain on the surface, coating it in a darker, gunky lag
deposit made of rocky silicates and organic material. But
ice could be very close to the surface. Before it ran out of
cryogen in 2013, the European Space Agency's infrared
space telescope Herschel made the surprising discovery
of water vapor in the space around Ceres - but only at
24

April 2015 sky & telescope

Most of the questions driving Dawn's investigation of
Ceres are pretty basic, beﬁtting the ﬁrst reconnaissance of
a new world. What covers its surface? How is its interior
layered? What is its geologic story? Where and how did it
form? Could there ever have been life there?
To answer these questions, Dawn will perform a
survey of Ceres almost identical to the one it did at Vesta.
With its Framing Camera, it will map all of Ceres in full
color at medium resolution, and all of it in monochrome
at higher resolution. It will gather the data to create a
global infrared map with its Visible and Infrared Mapping Spectrometer at low resolution, and some locations
at higher resolution. It will also photograph the limb of
Ceres against the sky to develop detailed shape models.
In a three-month low orbit, Dawn will use its Gamma Ray
and Neutron Detector (GRAND) spectrometer to map the
distribution of diﬀerent elements, and radio tracking to
measure the gravity ﬁeld.
Dawn's observations of the shape and gravity of Ceres
will signiﬁcantly narrow the range of possible structures
for Ceres' interior, which will, in turn, tell us which of the
possible stories for Ceres' geologic history are more probable than others.
One of the most intriguing questions that Dawn could
answer is: where did Ceres form? Until recently, scientists
assumed that most of the objects in the solar system,
particularly the large ones, have been where they are now
since their formation. But we now know that giant planets
can migrate, and their migrations wreak havoc with the
motions of the solar system's smaller denizens.
Ceres has lots of water, so it can't have formed too
close to the Sun. Could it have formed farther away than
it is now? Its density overlaps with those of trans-Neptunian objects; researcher Bill McKinnon (Washington
University in St. Louis) has gone so far as to suggest that
Ceres formed in the Kuiper Belt and was transported to
the inner solar system by the same dynamical process
that populated the Trojan points of the giant planets'
orbits with icy bodies. Olivier Mousis (now at University
of Franche-Comté, France) and Yann Alibert (University
of Bern, Switzerland) proposed an intermediate history:
perhaps Ceres' rocky center formed in the asteroid belt,
but it accreted an icy envelope later as it caught up smaller
Kuiper Belt bodies that drifted into its path. The ques-

Sky and Telescope - April 2015

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