Sky and Telescope - December 2016 - 21

Cosmic
ray

NASA / JPL-CALTECH / UCLA / MPS / DLR / IDA

Neutron
capture
Inelastic
collision
1m
Collisions

Natural
radioactivity

Fast
neutrons
Collisions

S&T: L. TISCIONE, SOURCE: T. H. PRETTYMAN ET AL. / JOURNAL OF GEOPHYSICAL RESEARCH 2006

Neutrons

Gamma rays
(major and
radioactive elements)

BLUISH CRATER Left: Shown here in enhanced color, the crater Haulani has landslides along its rim, as well as a central ridge.
Scientist think the bluish material here (in outlying ejecta) and elsewhere on Ceres is younger than its surroundings. Right: When a
high-energy cosmic ray slams into a nucleus in Ceres' surface, the target nucleus "explodes," producing a spray of secondary particles, including neutrons. These particles can collide with other atoms in the regolith, creating gamma rays that escape with other
neutrons from the dwarf planet and reach Dawn in orbit.

distant Sun provides less heat, ice could have persisted
throughout Ceres' lifetime, whereas at the warmer low
latitudes, it would have retreated to greater depths.
In one location, however, Dawn has already directly
detected water. The infrared mapping spectrometer
picked up its clear signature in a crater named Oxo,
42°° north of the equator. Oxo is relatively small, only
10 km in diameter, but it's the second brightest region
on Ceres. (Only Occator is brighter.) We don't yet know
whether the water Dawn found there is ice or is bound
up in hydrated minerals. However, because exposed ice,
if that's what it is, would be ephemeral, it must have
arrived on the surface recently. Scientists are still scouring Dawn's infrared spectra for signs of water elsewhere.
Those spectra have already revealed much more
about the composition of Ceres. Most of the surface is
a currently unidentified mixture of dark materials, but
we now know that clay-like minerals known as phyllosilicates are ubiquitous on that faraway world. Many
phyllosilicates are familiar on Earth, including mica.
What makes their presence on Ceres particularly important is that they contain distinctive evidence of ammonia. The ammonia is bound up in minerals that form
when water interacts with rock. This simple molecule,
consisting of nitrogen and hydrogen, should have been
common in the nebula of dust and gas from which the
planets formed. But swirling around the young Sun, the
material at Ceres' present location should have been too
warm for ammonia to have condensed and been trapped
in nascent planetesimals.
How then did it become so common on Ceres? It may
be that Ceres formed much farther from the Sun, even

beyond Saturn, where it was cool enough to incorporate
ammonia, and the subsequent gravitational jostling of
the planets pushed it to its current orbit. Another possibility is that Ceres formed close to where it is now but
accreted material that originated farther away and moved
in closer. That then raises the question of why so few
other bodies in the vicinity display signs of ammonia. It
also may be that our picture of the physical and chemical
nature of the early solar system is wrong.
Carbonates, another group of minerals indicative of
chemical reactions that take place in water, are common on Ceres as well. Indeed, the bright salt in Occator
contains the highest concentration of carbonates known
anywhere in the solar system except on Earth. And some
of the minerals, such as serpentine and sodium carbonate, form only under pressure, as would occur beneath
several kilometers of water.
Suppose the minerals were produced far underground
and later forced to the surface by some geological process.
Any mechanism for transporting the minerals from deep
within Ceres should depend on the surface temperature
and so should be different near the equator than near
the poles, which on average diverge by more than 50K.
(The surface temperature matters for what occurs inside
because it affects the rate at which heat leaks out.) In that
case, the distribution of minerals we see today should
vary widely over the surface. But that is not what we see.
Instead, the minerals show up nearly everywhere we look.
An alternative is that we may be looking at the floor
of an ancient ocean! Heated by the radioactive elements
incorporated when Ceres formed, the water would
have been liquid, creating an ocean on the surface. But
Sk yandTelescope.com December 2016

21


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Sky and Telescope - December 2016

Table of Contents for the Digital Edition of Sky and Telescope - December 2016

Contents
Sky and Telescope - December 2016 - Cover1
Sky and Telescope - December 2016 - Cover2
Sky and Telescope - December 2016 - 1
Sky and Telescope - December 2016 - Contents
Sky and Telescope - December 2016 - 3
Sky and Telescope - December 2016 - 4
Sky and Telescope - December 2016 - 5
Sky and Telescope - December 2016 - 6
Sky and Telescope - December 2016 - 7
Sky and Telescope - December 2016 - 8
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