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http://dawnblog.jpl.nasa.gov/2016/03/31/dawn-journal-march-31-3/

Dawn Journal
by Dr. Marc Rayman
March 31, 2016

Dear Resplendawnt Readers,

One year after taking up its new residence in the solar system, Dawn is continuing to witness extraordinary sights on dwarf planet Ceres. The indefatigable explorer is carrying out its intensive campaign of exploration from a tight orbit, circling its gravitational master at an altitude of
only 240 miles (385 kilometers).

Even as we marvel at intriguing pictures and other discoveries, scientists
are still in the early stages of putting together the pieces of the big
puzzle of how (and where) Ceres formed, what its subsequent history has
been, what geological processes are still occurring on this alien world
and what all that reveals about the solar system.

For many readers who have not visited Ceres on their own, Occator Crater
is the most mysterious and captivating feature. (To resolve the mystery
of how to pronounce it, listen to the animation below.) As Dawn peered
ahead at its destination in the beginning of 2015, the interplanetary
traveler observed what appeared to be a bright spot, a shining beacon
guiding the way for a ship sailing on the celestial seas. With its mesmerizing glow, the uncharted world beckoned, and Dawn answered the cosmic invitation
by venturing in for a closer look, entering into Ceres' gravitational
embrace. The latest pictures are one thousand times sharper than those
early views. What was not so long ago a single bright spot has now come
into focus as a complex distribution of reflective material in a 57-mile (92-kilometer) crater.

[Image]
Occator Crater is shown in this mosaic of photos Dawn took at its lowest altitude of 240 miles (385 kilometers). Go to the full image to see exquisite details of the crater walls and the many fractures in the floor. The exposure for these pictures was set for typical Ceres lighting to show the structure
of the crater itself and the surrounding area. The pictures below use
a shorter exposure to bring out more detail in the famously bright area.
Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

[Image]
Occator Crater and Ceres' Brightest Spots: Figure 1

Dawn took these pictures of Occator Crater on March 16. This is the most reflective area on Ceres. The exposure was optimized for the brightest
part of the scene, revealing details that were indiscernible in longer exposures and in photos from higher altitudes. Full image and caption.
Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Scientists are still working on refining their understanding of this striking region. As we described in December, it seems that following the powerful impact that excavated Occator Crater, underground briny water reached
the surface. The detailed photographs show many fractures cutting across
the bright areas, and perhaps they provided a conduit. Water, whether
as liquid or ice, would not last long there in the cold vacuum, eventually subliming. When the water molecules disperse, either escaping from Ceres
into space or falling back to settle elsewhere, the dissolved salts are
left behind. This reflective residue covers the ground, making the spellbinding

and beautiful display Dawn now reveals.

While the crater is estimated to be a geological youngster at 80 million
years old, that is an extremely long time for the material to remain so reflective. Exposed for so long to cosmic radiation and pelting from the
rain of debris from space, it should have darkened. Scientists don't know (yet) what physical process are responsible, but perhaps it was replenished long after the crater itself formed, with more water, carrying dissolved salts, finding its way to the surface. As their analyses of the photos
and spectra continue, scientists will gain a clearer picture and be able
to answer this and other questions.

[Image]
Center of Occator Crater (Enhanced Color)

The high resolution photo of the central feature of Occator Crater is
combined here with color data from the third mapping orbit. With enhanced color to highlight subtle variations, this illustrates the red tinge that
we described in December. (The scene would not look this colorful to your
eye, even if you and your eye were fortunate enough to be in a position
to see it.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI/LPI

These latest Occator pictures did not come easily. Orbiting so close to
Ceres, the adventurer's camera captures only a small scene at a time,
and it is challenging to cover the entirety of the expansive terrain.
(Perhaps it comes as a surprise to those who have not read at least a
few of the 123 Dawn Journals that precede this one that operating a spacecraft closer to a faraway dwarf planet than the International Space Station
is to Earth is not as easy as, say, thinking about it.) But the patience
and persistence in photographing the exotic landscapes have paid off handsomely.

We now have high resolution pictures of essentially all of Ceres save
the small area around the south pole cloaked in the deep dark of a long
winter night. Seasons last longer on Ceres than on Earth, and Dawn may
not operate there long enough for the sun to rise at the south pole. By
the beginning of southern hemisphere spring in November 2016, Dawn's mission to explore the first dwarf planet discovered may have come to its end.

[Animation]

This animation from NASA's Dawn mission shows the spacecraft's imaging coverage of dwarf planet Ceres during its low-altitude mapping orbit,
240 miles (385 kilometers) above the surface. The movie shows that the brightest area on Ceres, located in Occator Crater, was one of the last features to be imaged as Dawn progressively built its map.

This is an accelerated excerpt from this complete animation showing Dawn's accumulated photographic coverage of Ceres during the lowest altitude
mapping campaign from December 16 to March 11. To ensure that it can see
all latitudes, Dawn travels in a polar orbit, flying from the north pole
to the south pole over the illuminated hemisphere and back to the north
over the nighttime hemisphere. Each orbital revolution takes 5.4 hours. Meanwhile, Ceres rotates from east to west, completing one Cerean day
in just over nine hours. The combined motion causes the spacecraft's path
over the landscape to follow these graceful curves. Consecutive orbits
pass over widely separated regions because Ceres continues to rotate beneath Dawn while the spaceship glides over the hidden terrain of the night side.
The swaths that don't fit the typical pattern are the extra pictures Dawn
took as it turned away from the scenery below it, as described in January.
The spacecraft does not take pictures on every orbit, because sometimes
it performs other functions (such as pointing its main antenna to Earth),
so that causes gaps that are filled in later. Note that the center of
the popular Occator Crater (slightly above and to the right of center),
just happened to be one of the last places to be imaged as Dawn progressively built its high-resolution map. Animation credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In addition to photographing Ceres, Dawn conducts many other scientific observations, as we described in December and January. Among the probe's objectives at Ceres is to provide information for scientists to understand
how much water is there, where it is, what form it is in and what role
it plays in the geology.

We saw that extensive measurements of the faint nuclear radiation can
help identify the atomic constituents. While the analysis of the data
is complicated, and much more needs to be done, a picture is beginning
to emerge from Dawn's neutron spectrometer (part of the gamma ray and
neutron detector, GRaND). These subatomic particles are emitted from the nuclei of atoms buried within about a yard (meter) of the surface. Some
manage to penetrate the material above them and fly into space, and the helpful ones then meet their fate upon hitting GRaND in orbit above. (Most others, however, will continue to fly through interplanetary space, decaying into a trio of other subatomic particles in less than an hour.) Before
it escapes from the ground, a neutron's energy (and, equivalently, its
speed) is strongly affected by any encounters with the nuclei of hydrogen atoms (although other atomic interactions can change the energy too). Therefore, the neutron energies can indicate to scientists the abundance
of hydrogen. Among the most common forms in which hydrogen is found is
water (composed of two hydrogen atoms and one oxygen atom), which can
occur as ice or tied up in hydrated minerals.

GRaND shows Ceres is rich in hydrogen. Moreover, it detects more neutrons
in an important energy range near the equator than near the poles, likely indicating there is more hydrogen, and hence more (frozen) water, in the ground at the high latitudes. Although Ceres is farther from the sun than Earth, and you would not consider it balmy there, it still receives some warmth. Just as at Earth, the sun's heating is less effective closer to
the poles than at low latitudes, so this distribution of ice in the ground
may reflect the temperature differences. Where it is warmer, ice close
to the surface would have sublimed more quickly, thus depleting the inventory compared to the cooler ground far to the north or south.
Ceres Neutron Counts Reflect Hydrogen Abundance

This map, centered over the northern hemisphere, uses color to depict
the rate at which GRaND detected neutrons of a particular energy from
an altitude of 240 miles (385 kilometers). (The underlying image of Ceres
is based on pictures Dawn took with its camera at a higher altitude.)
Red indicates more neutrons than blue. The relative deficiency of neutrons near the north pole (and near the south pole, although not shown here)
is because hydrogen is more abundant there. The hydrogen atoms rob the neutrons of energy, so GRaND does not find as many at the special energy
used for this study. (It does find them at other energies.) Full image
and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Dawn spends most of its time measuring neutrons (and gamma rays), so it
is providing a great deal of new data. And as scientists conduct additional analyses, they will learn more about the ice and other materials beneath
the surface.

Another spectrometer is providing more tantalizing clues about the composition of Ceres, which is seen to vary widely. As the dwarf planet is not simply
a huge rock but is a geologically active world, it is no surprise that
it is not homogenous. We discussed in December than the infrared mapping spectrometer had shown that minerals known as phyllosilicates are common
on Ceres. Further studies of the data show evidence for the presence of
two types: ammoniated phyllosilicates (described in December) and magnesium phyllosilicates. Scientists also find evidence of compounds known as carbonates,
minerals that contain carbon and oxygen. There is also a dark substance
in the mix that has not been identified yet.

And in one place (so far) on Ceres, this spectrometer has directly observed water, not below the surface but on the ground. The infrared signature
shows up in a small crater named Oxo. (For the pronunciation, listen to
the animation below.) As with the neutron spectra, it is too soon to know whether the water is in the form of ice or is chemically bound up in minerals.

At six miles (10 kilometers) in diameter, Oxo is small in comparison to
the largest craters on Ceres, which are more than 25 times wider. (While geologists consider it a small crater, you might not agree if it formed
in your backyard. Also note that when we showed Oxo Crater before, the diameter was slightly different. The crater's size has not changed since
then, but as we receive sharper pictures, our measurements of feature
sizes do change.) Dawn's first orbital destination, the fascinating protoplanet

Vesta, is smaller than Ceres and yet has two craters far broader than
the largest on Ceres. Based on studies of craters observed throughout
the solar system, scientists have established methods of calculating the number and sizes of craters that could be formed on planetary surfaces.
Those techniques show that Ceres is deficient in large craters. That is,
more should have formed than appear in Dawn's pictures. Many other bodies (including Vesta and the moon) seem to preserve their craters for much
longer, so this may be a clue about internal geological processes on Ceres that gradually erase the large craters.

Scientists are still in the initial stages of digesting and absorbing
the tremendous wealth of data Dawn has been sending to Earth. The benefit
of lingering in orbit (enabled by the remarkable ion propulsion system), rather than being limited to a brief glimpse during a fast flyby, is that
the explorer can undertake much more thorough studies, and Dawn is continuing to make new measurements.

As recently as one year ago, controllers (and this writer) had great concern about the spacecraft's longevity given the loss of two reaction wheels,
which are used for controlling the ship's orientation. And in 2014, when
the flight team worked out the intricate instructions Dawn would follow
in this fourth and final mapping orbit, they planned for three months
of operation. That was deemed to be more than enough, because Dawn only
needed half that time to accomplish the necessary measurements. Experienced spacecraft controllers recognize that there are myriad ways beautiful
plans could go awry, so they planned for more time in order to ensure
that the objectives would be met even if anomalies occurred. They also
were keenly aware that the mission could very well conclude after three
months of low altitude operations, with Dawn using up the last of its hydrazine. But their efforts since then to conserve hydrazine proved very effective. In addition, the two remaining wheels have been operating well since they were powered on in December, further reducing the consumption
of the precious propellant.

As it turned out, operations have been virtually flawless in this orbit,
and the first three months yielded a tremendous bounty, even including
some new measurements that had not been part of the original plans. And because the entire mission at Ceres has gone so well, Dawn has not expended
as much hydrazine as anticipated.

[Image]
This is Ceres, the dwarf planet that Dawn's been orbiting for more than
a year now, providing us with fascinating views of an alien world. During
its exploration, Dawn has moved closer and closer, allowing us to get
a broad overview and then see exquisite detail.

This is an excerpt from an animation showing some of the highlights of
Dawn's exploration of Ceres so far, including Occator and Oxo craters,
both of which are discussed above. You can also hear your correspondent's pronunciation of the names of those and other features on Ceres. Full animation and transcript. Animation credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn is now performing measurements that were not envisioned long in advance but rather developed only in the past two months, when it was apparent
that the expedition could continue. And since March 19, Dawn has been following a new strategy to use even less hydrazine. Instead of pointing
its sensors straight down at the scenery passing beneath it as the spacecraft orbits and Ceres rotates, the probe looks a little to the left. The angle
is only five degrees (equal to the angle the minute hand of a clock moves
in only 50 seconds, or less than the interval between adjacent minute
tick marks), but that is enough to decrease the use of hydrazine and thus extend the spacecraft's lifetime. (We won't delve into the reason here.
But for fellow nerds, it has to do with the alignment of the axes of the operable reaction wheels with the plane in which Dawn rotates to keep
its instruments pointed at Ceres and its solar arrays pointed at the sun.
The hydrazine saving depends on the wheels' ability to store angular momentum and applies only in hybrid control, not in pure hydrazine control. Have
fun figuring out the details. We did!)

The angle is small enough now that the pictures will not look substantially different, but they will provide data that will help determine the topography. (Measurements of gravity and the neutron, gamma ray and infrared spectra
are insensitive to this angle.) Dawn took pictures at a variety of angles during the third mapping orbit at Ceres (and in two of the mapping orbits
at Vesta, HAMO1 and HAMO2) in order to get stereo views for topography.
That worked exceedingly well, and photos from this lower altitude will
allow an even finer determination of the three dimensional character of
the landscape in selected regions. Beginning on April 11, Dawn will look
at a new angle to gain still another perspective. That will actually increase the rate of hydrazine expenditure, but the savings now help make that
more affordable. Besides, this is a mission of exploration and discovery,
not a mission of hydrazine conservation. We save hydrazine when we can
in order to spend it when we need it. Dawn's charge is to use the hydrazine
to accomplish important scientific objectives and to pursue bold, exciting goals that lift our spirits and fuel our passion for knowledge and adventure. And that is exactly what it is has done and what it will continue to do.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.90 AU (362
million miles, or 583 million kilometers) from Earth, or 1,505 times as
far as the moon and 3.90 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour
and five minutes to make the round trip.

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