[meteorite-list] Dawn Journal - December 31, 2015

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 8 Jan 2016 14:41:41 -0800 (PST)
Message-ID: <201601082241.u08MffxY019335_at_zagami.jpl.nasa.gov>


Dawn Journal
by Dr. Marc Rayman
December 31, 2015

Dear Transcendawnts,

Dawn is now performing the final act of its remarkable celestial choreography,
held close in Ceres' firm gravitational embrace. The distant explorer
is developing humankind's most intimate portrait ever of a dwarf planet,
and it likely will be a long, long time before the level of detail is

The spacecraft is concluding an outstandingly successful year 1,500 times
nearer to Ceres than it began. More important, it is more than 1.4 million
times closer to Ceres than Earth is today. From its uniquely favorable
vantage point, Dawn can relay to us spectacular views that would otherwise
be unattainable. At an average altitude of only 240 miles (385 kilometers),
the spacecraft is closer to Ceres than the International Space Station
is to Earth. From that tight orbit, the dwarf planet looks the same size
as a soccer ball seen from only 3.5 inches (9.0 centimeters) away. This
is in-your-face exploration.

The spacecraft has returned more than 16,000 pictures of Ceres this year
(including more than 2,000 since descending to its low orbit this month).
One of your correspondent's favorites (below) was taken on Dec. 10 when
Dawn was verifying the condition of its backup camera. Not only did the
camera pass its tests, but it yielded a wonderful, dramatic view not far
from the south pole. It is southern hemisphere winter on Ceres now, with
the sun north of the equator. From the perspective of the photographed
location, the sun is near the horizon, creating the long shadows that
add depth and character to the scene. And usually in close-in orbits,
we look nearly straight down. Unlike such overhead pictures typical of
planetary spacecraft (including Dawn), this view is mostly forward and
shows a richly detailed landscape ahead, one you can imagine being in
- a real place, albeit an exotic one. This may be like the breathtaking
panorama you could enjoy with your face pressed to the porthole of your
spaceship as you are approaching your landing sight. You are right there.
It looks - it feels! - so real and physical. You might actually plan a
hike across some of the terrain. And it may be that a visiting explorer
or even a colonist someday will have this same view before setting off
on a trek through the Cerean countryside.

Of course, Dawn's objectives include much more than taking incredibly
neat pictures, a task at which it excels. It is designed to collect scientifically
meaningful photos and other valuable measurements. We'll see more below
about what some of the images and spectra from higher altitudes have revealed
about Ceres, but first let's take a look at the three highest priority
investigations Dawn is conducting now in its final orbit, sometimes known
as the low altitude mapping orbit (LAMO). While the camera, visible mapping
spectrometer and infrared mapping spectrometer show the surface, these
other measurements probe beneath.

With the spacecraft this close to the ground, it can measure two kinds
of nuclear radiation that come from as much as a yard (meter) deep. The
radiation carries the signatures of the atoms there, allowing scientists
to inventory some of the key chemical elements of geological interest.
One component of this radiation is gamma ray photons, a high energy form
of electromagnetic radiation with a frequency beyond visible light, beyond
ultraviolet, even beyond X-rays. Neutrons in the radiation are entirely
different from gamma rays. They are particles usually found in the nuclei
of atoms (for those of you who happen to look there). Indeed, outweighing
protons, and outnumbering them in most kinds of atoms, they constitute
most of the mass of atoms other than hydrogen in Ceres (and everywhere
else in the universe, including in your correspondent).

To tell us what members of the periodic table of the elements are present,
Dawn's gamma ray and neutron detector (GRaND) does more than detect those
two kinds of radiation. Despite its name, GRaND is not at all pretentious,
but its capabilities are quite impressive. Consisting of 21 sensors, the
device measures the energy of each gamma ray photon and of each neutron.
(That doesn't lend itself to as engaging an acronym.) It is these gamma
ray spectra and neutron spectra that reveal the identities of the atomic
species in the ground.

Some of the gamma rays are produced by radioactive elements, but most
of them and the neutrons are generated as byproducts of cosmic rays impinging
on Ceres. Space is pervaded by cosmic radiation, composed of a variety
of subatomic particles that originate outside our solar system. Earth's
atmosphere and magnetic field protect the surface (and those who dwell
there) from cosmic rays, but Ceres lacks such defenses. The cosmic rays
interact with nuclei of atoms, and some of the gamma rays and neutrons
that are released escape back into space where they are intercepted by
GRaND on the orbiting Dawn.

Unlike the relatively bright light reflected from Ceres's surface that
the camera, infrared spectrometer and visible spectrometer record, the
radiation GRaND measures is very faint. Just as a picture of a dim object
requires a longer exposure than for a bright subject, GRaND's "pictures"
of Ceres require very long exposures, lasting weeks, but mission planners
have provided Dawn with the necessary time. Because the equivalent of
the illumination for the gamma ray and neutron pictures is cosmic rays,
not sunlight, regions in darkness are no fainter than those illuminated
by the sun. GRaND works on both the day side and the night side of Ceres.

In addition to the gamma ray spectra and neutron spectra, Dawn's other
top priority now is measuring Ceres' gravity field. The results will help
scientists infer the interior structure of the dwarf planet. The measurements
made in the higher altitude orbits turned out to be even more accurate
than the team had expected, but now that the probe is as close to Ceres
as it will ever go, and so the gravitational pull is the strongest, they
can obtain still better measurements.

Gravity is one of four fundamental forces in nature, and its extreme weakness
is one of the fascinating mysteries of how the universe works. It feels
strong to us (well, most of us) because we don't so easily sense the two
kinds of nuclear forces, both of which extend only over extremely short
distances, and we generally don't recognize the electromagnetic force.
With both positive and negative electrical charges, attractive and repulsive
electromagnetic forces often cancel. Not so with gravity. All matter exerts
attractive gravity, and it can all add up. The reason gravity -- by far
the weakest of the four forces -- is so salient for those of you on or
near Earth is that there is such a vast amount of matter in the planet
and it all pulls together to hold you down. Dawn overcame that pull with
its powerful Delta rocket. Now the principal gravitational force acting
on it is the cumulative effect of all the matter in Ceres, and that is
what determines its orbital motion.

The spacecraft experiences a changing force both as the inhomogeneous
dwarf planet beneath it rotates on its axis and as the craft circles that
massive orb. When Dawn is closer to locations within Ceres with greater
density (i.e., more matter), the ship feels a stronger tug, and when it
is near regions with lower density, and hence less powerful gravity, the
attraction is weaker. The spacecraft accelerates and decelerates very
slightly as its orbit carries it closer to and farther from the volumes
of different density. By carefully and systematically plotting the exquisitely
small variations in the probe's motion, navigators can calculate how the
mass is distributed inside Ceres, essentially creating an interior map.
This technique allowed scientists to establish that Vesta, the protoplanet
Dawn explored in 2011-2012, has a dense core (composed principally of
iron and nickel) surrounded by a less dense mantle and crust. (That is
one of the reasons scientists now consider Vesta to be more closely related
to Earth and the other terrestrial planets than to typical asteroids.)

Mapping the orbit requires systems both on Dawn and on Earth. Using the
large and exquisitely sensitive antennas of NASA's Deep Space Network
(DSN), navigators measure tiny changes in the frequency, or pitch, of
the spacecraft's radio signal, and that reveals changes in the craft's
velocity. This technique relies on the Doppler effect, which is familiar
to most terrestrial readers as they hear the pitch of a siren rise as
it approaches and fall as it recedes. Other readers who more commonly
travel at speeds closer to that of light recognize that the well-known
blueshift and redshift are manifestations of the same principle, applied
to light waves rather than sound waves. Even as Dawn orbits Ceres at 610
mph (980 kilometers per hour), engineers can detect changes in its speed
of only one foot (0.3 meters) per hour, or one five-thousandth of a mph
(one three-thousandth of a kilometer per hour). Another way to track the
spacecraft is to measure the distance very accurately as it revolves around
Ceres. The DSN times a radio signal that goes from Earth to Dawn and back.
As you are reminded at the end of every Dawn Journal, those signals travel
at the universal limit of the speed of light, which is known with exceptional
accuracy. Combining the speed of light with the time allows the distance
to be pinpointed. These measurements with Dawn's radio, along with other
data, enable scientists to peer deep into the dwarf planet

Although it is not among the highest scientific priorities, the flight
team is every bit as interested in the photography as you are. We are
visual creatures, so photographs have a special appeal. They transport
us to mysterious, faraway worlds more effectively than any propulsion
system. Even as Dawn is bringing the alien surface into sharper focus
now, the pictures taken in higher orbits have allowed scientists to gain
new insights into this ancient world. Geologists have located more than
130 bright regions, none being more striking than the mesmerizing luster
in Occator crater. The pictures taken in visible and infrared wavelengths
have helped them determine that the highly reflective material is a type
of salt.

It is very difficult to pin down the specific composition with the measurements
that have been analyzed so far. Scientists compare how reflective the
scene is at different wavelengths with the reflective properties of likely
candidate materials studied in laboratories. So far, magnesium sulfate
yields the best match (although it is not definitive). That isn't the
kind of salt you normally put on your food (or if it is, I'll be wary
about accepting the kind invitation to dine in your home), but it is very
similar (albeit not identical) to Epsom salts, which have many other familiar

Scientists' best explanation now for the deposits of salt is that when
asteroids crash into Ceres, they excavate underground briny water-ice.
Once on the surface and exposed to the vacuum of space, even in the freezing
cold so far from the sun, the ice sublimes, the water molecules going
directly from the solid ice to gas without an intermediate liquid stage.
Left behind are the materials that had been dissolved in the water. The
size and brightness of the different regions depend in part on how long
ago the impact occurred. A very preliminary estimate is that Occator was
formed by a powerful collision around 80 million years ago, which is relatively
recent in geological times. (We will see in a future Dawn Journal how
scientists estimate the age and why the pictures in this low altitude
mapping orbit will help refine the value.)

As soon as Dawn's pictures of Ceres arrived early this year, many people
referred to the bright regions as "white spots," although as we opined
then, such a description was premature. The black and white pictures revealed
nothing about the color, only the brightness. Now we know that most have
a very slight blue tint. For reasons not yet clear, the central bright
area of Occator is tinged with more red. Nevertheless, the coloration
is subtle, and our eyes would register white.

Measurements with both finer wavelength discrimination and broader wavelength
coverage in the infrared have revealed still more about the nature of
Ceres. Scientists using data from one of the two spectrometers in the
visible and infrared mapping spectrometer instrument (VIR) have found
that a class of minerals known as phyllosilicates is common on Ceres.
As with the magnesium sulfate, the identification is made by comparing
Dawn's detailed spectral measurements with laboratory spectra of a great
many different kinds of minerals. This technique is a mainstay of astronomy
(with both spacecraft and telescopic observations) and has a solid foundation
of research that dates to the nineteenth century, but given the tremendous
variety of minerals that occur in nature, the results generally are neither
absolutely conclusive nor extremely specific.

There are dozens of phyllosilicates on Earth (one well known group is
mica). Ceres too likely contains a mixture of at least several. Other
compounds are evident as well, but what is most striking is the signature
of ammonia in the minerals. This chemical is manufactured extensively
on Earth, but few industries have invested in production plants so far
from their home offices. (Any corporations considering establishing Cerean
chemical plants are invited to contact the Dawn project. Perhaps, however,
mining would be a more appropriate first step in a long-term business

Ammonia's presence on Ceres is important. This simple molecule would have
been common in the material swirling around the young sun almost 4.6 billion
years ago when planets were forming. (Last year we discussed this period
at the dawn of the solar system.) But at Ceres' present distance from
the sun, it would have been too warm for ammonia to be caught up in the
planet-forming process, just as it was even closer to the sun where Earth
resides. There are at least two possible explanations for how Ceres acquired
its large inventory of ammonia. One is that it formed much farther from
the sun, perhaps even beyond Neptune, where conditions were cool enough
for ammonia to condense. In that case, it could easily have incorporated
ammonia. Subsequent gravitational jostling among the new residents of
the solar system could have propelled Ceres into its present orbit between
Mars and Jupiter. Another possibility is that Ceres formed closer to where
it is now but that debris containing ammonia from the outer solar system
drifted inward and some of it ultimately fell onto the dwarf planet. If
enough made its way to Ceres, the ground would be covered with the chemical,
just as VIR observed.

Scientists continue to analyze the thousands of photos and millions of
infrared and visible spectra even as Dawn is now collecting more precious
data. Next month, we will summarize the intricate plan that apportions
time among pointing the spacecraft's sensors at Ceres to perform measurements,
its main antenna at Earth to transmit its findings and receive new instructions
and its ion engine in the direction needed to adjust its orbit.

The plans described last month for getting started in this fourth and
final mapping orbit worked out extremely well. You can follow Dawn's activities
with the status reports posted at least twice a week here. And you can
see new pictures regularly in the Ceres image gallery.

We will be treated to many more marvelous sights on Ceres now that Dawn's
pictures will display four times the detail of the views from its third
mapping orbit. The mapping orbits are summarized in the following table,
updated from what we have presented before. (This fourth orbit is listed
here as beginning on Dec. 16. In fact, the highest priority work, which
is obtaining the gamma ray spectra, neutron spectra and gravity measurements,
began on Dec. 7, as explained last month. But Dec. 16 is when the spacecraft
started its bonus campaign of measuring infrared spectra and taking pictures.
Recognizing that what most readers care about is the photography, regardless
of the scientific priorities, that is the date we use here.

Mapping orbit Dawn code name Dates Altitude in miles (kilometers) Resolution
in feet (meters) per pixel Resolution compared to Hubble Orbit period Equivalent
distance of a soccer ball
1 RC3 April 23 - May 9 8,400 (13,600) 4,200 (1,300) 24 15 days 10 feet
(3.2 meters)
2 Survey June 6-30 2,700 (4,400) 1,400 (410) 73 3.1 days 3.4 feet (1.0
3 HAMO Aug 17 - Oct 23 915 (1,470) 450 (140) 217 19 hours 14 inches (34
4 LAMO Dec 16 - end of mission 240 (385) 120 (35) 830 5.4 hours 3.5 inches
(9.0 cm)

Dawn is now well-positioned to make many more discoveries on the first
dwarf planet discovered. Jan. 1 will be the 215th anniversary of Giuseppe
Piazzi's first glimpse of that dot of light from his observatory in Sicily.
Even to that experienced astronomer, Ceres looked like nothing other than
a star, except that it moved a little bit from night to night like a planet,
whereas the stars were stationary. (For more than a generation after,
it was called a planet.) He could not imagine that more than two centuries
later, humankind would dispatch a machine on a cosmic journey of more
than seven years and three billion miles (five billion kilometers) to
reach the distant, uncharted world he descried. Dawn can resolve details
more than 60 thousand times finer than Piazzi's telescope would allow.
Our knowledge, our capabilities, our reach and even our ambition all are
far beyond what he could have conceived, and yet we can apply them to
his discovery to learn more, not only about Ceres itself, but also about
the dawn of the solar system.

On a personal note, I first saw Ceres through a telescope even smaller
than Piazzi's when I was 12 years old. As a much less experienced observer
of the stars than he was, and with the benefit of nearly two centuries
of astronomical studies between us, I was thrilled! I knew that what I
was seeing was the behemoth of the main asteroid belt. But it never occurred
to me when I was only a starry-eyed youth that I would be lucky enough
to follow up on Piazzi's discovery as a starry-eyed adult, responsible
for humankind's first visitor to that fascinating alien world, answering
a celestial invitation that was more than 200 years old.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.66 AU (340
million miles, or 547 million kilometers) from Earth, or 1,360 times as
far as the moon and 3.72 times as far as the sun today. Radio signals,
traveling at the universal limit of the speed of light, take one hour
and one minute to make the round trip.
Received on Fri 08 Jan 2016 05:41:41 PM PST

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