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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 2 Nov 2015 16:22:53 -0800 (PST)
Message-ID: <201511030022.tA30MrB2001429_at_zagami.jpl.nasa.gov>

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 _at_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!
Received on Mon 02 Nov 2015 07:22:53 PM PST