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

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/2015/12/31/dawn-journal-december-31/

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 
surpassed.

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 
uses.

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 
plan.) 

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 
meters)
3	HAMO	Aug 17 - Oct 23	915 (1,470)	450 (140)	217	19 hours	14 inches (34 
cm)
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.