[meteorite-list] Dawn Journal - October 30, 2015

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/category/dawn-journal/

Dawn Journal 
by Dr. Marc Rayman
October 30, 2015

Dear Exuldawnt Readers,

Dawn has completed another outstandingly successful campaign to acquire 
a wealth of pictures and other data in its exploration of dwarf planet 
Ceres. Exultant residents of distant Earth now have the clearest and most 
complete view ever of this former planet.

The stalwart probe spent more than two months orbiting 915 miles (1,470 
kilometers) above the alien world. We described the plans for this third 
major phase of Dawn's investigation (also known as the high altitude mapping 
orbit, or HAMO) in August and provided a brief progress report in September. 
Now we can look back on its extremely productive work.
Ceres wuth planetary names

[Image]
This map of Ceres shows the feature names approved by the International 
Astronomical Union. We described the naming convention in December, and 
the most up-to-date list of names is here. The small crater Kait (named 
for the ancient Hattic grain goddess) is used to define the location of 
the prime meridian. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Each revolution, flying over the north pole to the south pole and back 
to the north, took Dawn 19 hours. Mission planners carefully chose the 
orbital parameters to coordinate the spacecraft's travels with the nine-hour 
rotation period of Ceres (one Cerean day) and with the field of view of 
the camera so that in 12 orbits over the lit hemisphere (one mapping "cycle"), 
Dawn could photograph all of the terrain.

In each of six mapping cycles, the robot held its camera and its infrared 
and visible mapping spectrometers at a different angle. For the first 
cycle (Aug. 17-26), Dawn looked straight down. For the second, it looked 
a little bit behind and to the left as it completed another dozen orbits. 
For the third map, it pointed the sensors a little behind and to the right. 
In its fourth cycle, it aimed ahead and to the left. When it made its 
fifth map, it peered immediately ahead, and for the sixth and final cycle 
(Oct. 12-21) it viewed terrain farther back than in the third cycle but 
not as far to the right.

The result of this extensive mapping is a very rich collection of photos 
of the fascinating scenery on a distant world. Think for a moment of the 
pictures not so much from the standpoint of the spacecraft but rather 
from a location on the ground. With the different perspectives in each 
mapping cycle, that location has been photographed from several different 
angles, providing stereo views. Scientists will use these pictures to 
make the landscape pop into its full three dimensionality.

Dawn's reward for these two months of hard work is much more than revealing 
Ceres' detailed topography, valuable though that is. During the first 
and fifth mapping cycles, it used the seven color filters in the camera, 
providing extensive coverage in visible and infrared wavelengths.
Hints at Ceres' Composition from Color

[Image]
This false-color map of Ceres was constructed using images taken in the 
first mapping cycle at an altitude of 915 miles (1,470 kilometers). It 
combines pictures taken in filters that admit light in what the human 
eye perceives as violet (440 nanometers), near the limit of visible red 
(750 nanometers), and invisible infrared (920 nanometers). Because humans 
are so good at processing visual information, depictions such as this 
are a helpful way to highlight and illustrate variations in the composition 
or other properties of the material on Ceres' surface. Full image and 
caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In addition to taking more than 6,700 pictures, the spacecraft operated 
its visible and infrared mapping spectrometers to acquire in excess of 
12.5 million spectra. Each spectrum contains much finer measurements of 
the colors and a wider range of wavelengths than the camera. In exchange, 
the camera has sharper vision and so can discern smaller geological features. 
As the nerdier among us would say, the spectrometers achieve better spectral 
resolution and the camera achieves better spatial resolution. Fortunately, 
it is not a competition, because Dawn has both, and the instruments yield 
complementary measurements.

Even as scientists are methodically analyzing the vast trove of data, 
turning it into knowledge, you can go to the Ceres image gallery to see 
some of Dawn's pictures, exhibiting a great variety of terrain, smooth 
or rugged, strangely bright or dark, unique in the solar system or reminiscent 
of elsewhere spacecraft have traveled, and always intriguing.
Occator Mosaic

[Image]
Ten photos from Dawn's first mapping cycle were combined to make this 
view centered on Occator crater. Because of the range of brightness, pictures 
with two different exposures were required to record the details of the 
bright regions and the rest of the crater itself, as explained last month. 
Eight additional pictures show the area around the crater. Occator is 
almost 60 miles (more than 90 kilometers) in diameter. Full image and 
caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Among the questions scientists are grappling with is what the nature of 
the bright regions is. There are many places on Ceres that display strikingly 
reflective material but nowhere as prominently as in Occator crater. Even 
as Dawn approached Ceres, the mysterious reflections shone out far into 
space, mesmerizing and irresistible, as if to guide or even seduce a passing 
ship into going closer. Our intrepid interplanetary adventurer, compelled 
not by this cosmic invitation but rather by humankind's still more powerful 
yearning for new knowledge and new insights, did indeed venture in. Now 
it has acquired excellent pictures and beautiful spectra that will help 
determine the composition and perhaps even how the bright areas came to 
be. Thanks to the extraordinary power of the scientific method, we can 
look forward to explanations. (And while you wait, you can register your 
vote here for what the answer will be.)

Scientists also puzzle over the number and distribution of craters. We 
mentioned in December the possibility that ice being mixed in as a major 
component on or near the surface would cause the material to flow, albeit 
very slowly on the scale of a human lifetime. But over longer times, the 
glacially slow movement might prove significant. Most of Ceres' craters 
are excavated by impacts from some of the many bodies that roam that part 
of the solar system. Ceres lives in a rough neighborhood, and being the 
most massive body between Mars and Jupiter does not give it immunity to 
assaults. Indeed, its gravity makes it even more susceptible, attracting 
passersby. But once a crater is formed, the scar might be expected to 
heal as the misshapen ground gradually recovers. In some ways this is 
similar to when you remove pressure from your skin. What may be a deep 
impression relaxes, and after a while, the original mark (or, one may 
hope, Marc) is gone. But Ceres has more craters than some scientists had 
anticipated, especially at low latitudes where sunlight provides a faint 
warming. Apparently the expectation of the gradual disappearance of craters 
was not quite right. Is there less evidence of flowing ground material 
because the temperature is lower than predicted (causing the flow to be 
even slower), because the composition is not quite what was assumed, or 
because of other reasons? Moreover, craters are not distributed as would 
be expected for random pummeling; some regions display significantly more 
craters than others. Investigating this heterogeneity may give further 
insight into the geological processes that have taken place and are occurring 
now on this dwarf planet.
Occator Topography

[Image]
This color-coded topographic map of Occator crater is based on Dawn's 
observations in its second mapping orbit at an altitude of 2,700 miles 
(4,400 kilometers). Of course there is no sea level on Ceres, but the 
deep blue here is 5,150 feet (1,570 meters) below a reference level, and 
brown is 14,025 feet (4,275 meters) above it. (Brown is used in place 
of white for the elevation, so white can show the bright regions.) Imagine 
the exotic scenery here, with strangely bright areas and towering crater 
walls. The stereo views acquired in the third mapping orbit will reveal 
finer detail in the topography. Full image and caption. 
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn's bounty from this third major science campaign includes even more 
than stereo and color pictures plus visible and infrared spectra. Precise 
tracking of the spacecraft as it moves in response to Ceres' gravitational 
pull allows scientists to calculate the arrangement of mass in the behemoth. 
Performing such measurements will be among the top three priorities for 
the lowest altitude orbit, when Dawn experiences the strongest buffeting 
from the gravitational currents, but already the structure of the gravitational 
field is starting to be evident. We will see next month how this led to 
a small change in the choice of the altitude for this next orbit, which 
will be less than 235 miles (380 kilometers).

The other top two priorities for the final mission phase are the measurement 
of neutron spectra and the measurement of gamma ray spectra, both of which 
will help in establishing what species of atoms are present on and near 
the surface. The weak radiation from Ceres is difficult to measure from 
the altitudes at which Dawn has been operating so far. The gamma ray and 
neutron detector (GRaND) has been in use since March 12 (shortly after 
Dawn arrived in orbit), but that has been to prepare for the low orbit. 
Nevertheless, the sophisticated instrument did detect the dwarf planet's 
faint nuclear emissions even in this third orbital phase. The signal was 
not strong enough to allow any conclusions about the elemental composition, 
but it is interesting to begin seeing the radiation which will help uncover 
more of Ceres' secrets when Dawn is closer.

To scientists' great delight, one of GRaND's sensors even found an entirely 
unexpected signature of Ceres in Dawn's second mapping orbit, where the 
spacecraft revolved every 3.1 days at an altitude of 2,700 miles (4,400 
kilometers). In a nice example of scientific serendipity, it detected 
high energy electrons in the same region of space above Ceres on three 
consecutive orbits. Electrons and other subatomic particles stream outward 
from the sun in what is called the solar wind, and researchers understand 
how planets with magnetic fields can accelerate them to higher energy. 
Earth is an example of a planet with a magnetic field, but Ceres is thought 
not to be. So scientists now have the unanticipated joy not only of establishing 
the physical mechanism responsible for this discovery but also determining 
what it reveals about this dwarf planet.
Dawn HAMO Image 29

[Image]
Dawn had this view near 0 degrees longitude in the northern hemisphere 
on Sept. 9 in its third mapping cycle at an altitude of 915 miles (1,470 
kilometers). Oxo crater on the right, which shows bright material inside 
and out as well as a peculiar shape, is slightly over five miles (nearly 
nine kilometers) in diameter. The crater is named for the god of agriculture 
for the Yoruba people of Brazil. Full image and caption. 
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Several times during each of the six mapping cycles, Dawn expended a few 
grams of its precious hydrazine propellant to rotate so it could aim its 
main antenna at Earth. While the craft soared high above ground cloaked 
in the deep black of night, it transmitted some of its findings to NASA's 
Deep Space Network. But Dawn conducted so many observations that during 
half an orbit, or about 9.5 hours, it could not radio enough data to empty 
its memory. By the end of each mapping cycle, the probe had accumulated 
so much data that it fixed its antenna on Earth for about two days, or 
2.5 revolutions, to send its detailed reports on Ceres to eager Earthlings.

Following the conclusion of the final mapping cycle, after transmitting 
the last of the information it had stored in its computer, the robotic 
explorer did not waste any time gloating over its accomplishments. There 
was still a great deal more work to do. On Oct. 23 at 3:30 p.m., it fired 
up ion engine #2 (the same one it used to descend from the second mapping 
orbit to the third) to begin more than seven weeks of spiraling down to 
its fourth orbit. (You can follow its progress here and on Twitter @NASA_Dawn.) 
Dawn has accomplished more than 5.4 years of ion thrusting since it left 
Earth, and the complex descent to less than 235 miles (380 kilometers) 
is the final thrusting campaign of the entire extraterrestrial expedition. 
(The ion propulsion system will be used occasionally to make small adjustments 
to the final orbit.)

The blue lights in Dawn mission control that indicate the spacecraft is 
thrusting had been off since Aug. 13. Now they are on again, serving as 
a constant (and cool) reminder that the ambitious mission is continuing 
to power its way to new (and cool) destinations.

Dawn is 740 miles (1,190 kilometers) from Ceres. It is also 2.91 AU (271 
million miles, or 436 million kilometers) from Earth, or 1,165 times as 
far as the moon and 2.93 times as far as the sun today. Radio signals, 
traveling at the universal limit of the speed of light, take 48 minutes 
to make the round trip.

P.S. While the spacecraft is hard at work continuing its descent tomorrow, 
your correspondent will be hard at work dispensing treats to budding (but 
cute) extortionists at his front door. But zany and playful as ever, he 
will expand his delightful costume from last year by adding eight parts 
dark energy. Trick or treat!