Sky and Telescope - December 2016 - 20

Solar System Expedition

Clay-rich outer crust

Water-ice layer

NASA / JPL-CALTECH

Rocky inner
core

WATER WORLD? Planetary scientists think that Ceres has
a large reservoir of water ice beneath its crust.

tive material on Ceres, making it the brightest and most
conspicuous place on the dwarf planet, but scientists
have catalogued more than 130 other locations that also
are very reﬂective. Nearly all of them are bluish.
Although 80 million years is geologically brief, it
seems much too long for the material in Occator to
remain so reﬂective. Scientists expect that the constant bombardment of small interplanetary debris and
cosmic radiation would darken it relatively quickly, so
persistent brightness is very diﬃcult to explain. This
suggests the intriguing possibility that some geologic
process replenished it not long ago. It could be that current or recent activity causes more of that underground
salt water to reach the surface and then vaporize, providing fresh salt deposits. This and other explanations
are being investigated now.

How Dawn Measures Ceres

Water World

Dawn's camera is equipped with eight ﬁlters mounted
in a wheel in front of its CCD. One admits light across
the visible and near-infrared spectrum, from 450 to 920
nanometers. Each of the other seven allows a narrow
range of wavelengths from deep blue at 430 nm to nearinfrared at 980 nm.
To reveal Ceres' topography, Dawn mapped the
dwarf planet from six angles during its third mapping
orbit. One map was made by looking straight down at
the scenery beneath it. Each of the other ﬁve maps was
developed by holding the camera ﬁxed at a diﬀerent
angle, looking ahead or behind and to the right or left.
Taken together, these create stereo views, making the
landscape pop into 3D.
Dawn has four spectrometers packaged in two instruments. Dawn's visible spectrometer, operating from 0.4
to 1 micron (in the near-infrared) would have a view very
much like the rainbow created by a prism. The infrared
spectrometer works from 1 to 5 microns. A spectrometer does more than simply disperse the light into its
components, however. It measures the intensity of that
light at the diﬀerent wavelengths. The materials on the
surface leave their signature in the sunlight they reﬂect,
making some wavelengths relatively brighter and some
dimmer. This characteristic pattern is called a spectrum.
By comparing these spectra with spectra measured in
laboratories, scientists can infer the nature of the minerals on the ground. The infrared measurements also yield
the surface temperature.
Dawn's Gamma Ray and Neutron Detector (GRAND)
consists of two spectrometers that work on the same
principle, except with gamma rays and neutrons kicked
up from Ceres by cosmic rays. But because the glow of
Ceres' nuclear radiation is so faint, GRAND needs to collect data for a very long time, in the same way that photographing a dim object requires a long exposure.

The inference of water, most likely frozen, beneath the
surface is based on more than the bright areas in Occator.
The overall density of Ceres is 2.2 grams per cubic centimeter (g/cm3). This is surprisingly low, well less than
half Earth's density, strongly suggesting that water is a
major constituent. (The average dry rock density is 3.4 g/
cm3.) Water may account for about a quarter of the dwarf
planet's mass. Ceres then would be the ﬁrst rocky body
with such a large inventory of water orbited by spacecraft,
providing opportunities to learn about how such a world
works. Other large water-rich places, including Europa
and Titan, have not been studied at the level of detail
Dawn has achieved from its orbital vantage point.
Dawn also has found signs of water thanks to the
compound's interaction with cosmic rays, energetic
particles raining down from space. Cosmic rays impinge
on Ceres, slam into atomic nuclei in the crust, and
produce gamma rays or neutrons that escape back into
space. The spacecraft's Gamma Ray and Neutron Detector (GRAND) consists of two spectrometers that record
the number and energy of these photons and particles.
Measuring them tells us which atoms the cosmic rays
hit, down to a depth of about a meter.
While still preliminary, the measurements of
neutrons emanating from Ceres indicate that it is rich
in hydrogen. The same measurements at Vesta found
much less of that element. Hydrogen is a good indicator
of the presence of water, which comprises two hydrogen
atoms bound to one oxygen atom and is the most common hydrogen-bearing molecule that occurs on solid
surfaces. Although the evidence suggests hydrogen is
everywhere on Ceres, it is even more plentiful near the
poles than near the equator. Researchers are investigating reasons the neutron count might vary with latitude.
Their analysis of these complex data may allow them to
test the hypothesis that GRAND is sensing ice extremely
close to the surface. At higher latitudes, where the

December 2016 sky & telescope

Sky and Telescope - December 2016

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