[meteorite-list] Dawn Journal - May 31, 2016

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_05_31_16.html

Dawn journal
Dr. Marc Rayman
May 31, 2016

Dear Phodawngraphers,

Dawn is continuing to record the extraordinary sights on dwarf planet 
Ceres. The experienced explorer is closer to the alien world than the 
International Space Station is to Earth.

Dawn has completed more than 1,000 orbital revolutions since entering 
into Ceres' gentle but firm gravitational grip in March 2015. The probe 
is healthy and performing its ambitious assignments impeccably. In the 
last few months, we have described how Dawn has greatly exceeded all of 
its original objectives at Ceres and the excellent progress it has been 
making in collecting bonus data. On schedule on May 25, the spacecraft 
completed the mapping campaign it began on April 11, in which it took 
photographs with the camera pointed to the left and forward as it circled 
Ceres. Now it is looking to the right and forward to get another stereo 
view.

In January we mentioned that, having already acquired far more measurements 
with the visible and infrared mapping spectrometer than anticipated, scientists 
were devoting further observations to infrared rather than visible. Now 
Dawn is operating both spectrometers again. Having seen much more of Ceres 
in the infrared from this low altitude than planned, mission controllers 
now can afford to allocate some of the spacecraft's data storage and interplanetary 
radio transmissions to visible spectra in exchange for limiting the infrared 
to a few select targets. In addition, a device in the infrared spectrometer 
that lowers the sensor's temperature to -307 degrees Fahrenheit (-188 
degrees Celsius) is showing signs of age. (We saw here that the sensor 
can detect heat. So to avoid interference from its own heat, it needs 
to be cooled.) Its symptoms are not a surprise, given that the instrument 
has acquired far, far more data at Vesta and Ceres than it was designed 
for. It is continuing to function quite productively, but now its use 
is being curtailed.
Dawn LAMO Image 95

[Image]
Dawn took this picture of canyons in Ezinu Crater from an altitude of 
240 miles (385 kilometers) on April 17. (Ezinu was a Sumerian goddess 
of grain.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

One of the mission's objectives was to photograph 80 percent of Ceres’ 
vast landscape with a resolution of 660 feet (200 meters) per pixel. Dawn 
has now photographed nearly the entirety (99.9 percent) with a resolution 
of 120 feet (35 meters) per pixel. The adventurer has shown us 25 percent 
more terrain than planned with 5.7 times the clarity. We can see detail 
830 times sharper than the Hubble Space Telescope revealed.

What is the value of that much detail? The more detailed the portrait, 
the better understanding geologists can obtain. Imagine the difference 
(not only visually but also emotionally and socially) between seeing a 
person at the opposite end of a soccer field and seeing them from five 
inches (12 centimeters) away.

The pictures speak quite eloquently (and succinctly) for themselves, but 
let's take a look at one of the many uses of these sharp photographs: 
determining the age of geological features.

In December, we gave an approximate age of 80 million years for Occator 
Crater, site of the famous "bright spots" (or famously bright spots). 
It takes more than an experienced geological eye to estimate such an age.

[Image]
Occator Crater is shown in this mosaic of photos Dawn took at its lowest 
altitude of 240 miles (385 kilometers). The crater is 57 miles (92 kilometers) 
in diameter. Go to the full image to see exquisite details of the bright 
areas as well as fractures in the crater floor and other intriguing features. 
Note how few craters are within Occator or the area around it. Scientists 
can translate the number and size of craters into an age. From pictures 
taken at higher altitudes, they estimate Occator is 80 million years old, 
as explained below. That age will be refined with these sharper pictures, 
which reveal smaller craters. Full image and caption. Image credit: 
NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Now don't forget that we are trying to ascertain the age, but we are going 
to get there on a long and winding path, mostly because it's an opportunity 
to touch on some fun and interesting topics.

To begin, we go back in time, not quite 80 million years, to the Apollo 
program. Astronauts returned from the moon with many treasures, including 
842 pounds (382 kilograms) of lunar material collected on six missions. 
In addition, three Soviet robotic Luna spacecraft came back with a total 
of 11 ounces (0.3 kilograms).

Earth's total inventory of lunar samples is larger. By comparing the chemical 
composition of that material with a great many meteorites, scientists 
have identified nearly 120 pounds (54 kilograms) of meteorites that were 
blasted from the moon by asteroid impacts and then landed on our planet.

Other meteorites are known to have originated on Mars. The principal method 
by which that connection was made was comparison of gasses trapped in 
the meteorites with the known constituents of the Martian atmosphere as 
measured by the two Viking spacecraft that landed there 40 years ago. 
Scientists thus have 276 pounds (125 kilograms) of Martian material.

Of course, unlike the Apollo and Luna samples, the lunar and Martian meteorites 
were selected for us by nature's randomness from arbitrary locations that 
are not easy to determine.

The moon and Mars are two of only three (extant) extraterrestrial bodies 
that are clearly established as the source of specific meteorites. The 
third is Vesta, the fascinating protoplanet Dawn explored in 2011-2012. 
That world is farther away even than Mars, and yet we have 3,090 pounds 
(1,402 kilograms) from Vesta, or more than 11 times as much as from the 
red planet and more than three times as much as from the moon. We reflected 
on these meteorites during our travel from Vesta to Ceres.

It is thanks to Dawn's detailed measurements of the composition of Vesta 
that scientists were able to clinch the connection with the meteorites 
that were under study in terrestrial laboratories. The impact of an asteroid 
perhaps 20 to 30 miles (30 to 50 kilometers) in diameter more than one 
billion years ago excavated Vesta's Rheasilvia Crater. It left behind 
a yawning basin more than 300 miles (500 kilometers) across, a mountain 
more than twice the height of Mt. Everest, and a network of about 90 canyons 
with dimensions rivaling those of the Grand Canyon. And it launched a 
tremendous amount of material into space. Some of it settled back onto 
Vesta, resurfacing much of the southern hemisphere, but some of it departed 
with so much energy that it escaped Vesta's gravitational hold. Some of 
the biggest pieces liberated by that tremendous impact are now visible 
as small asteroids known as vestoids. And some of the small pieces eventually 
made their way to the part of the solar system where many of our readers 
(perhaps including you) reside. After Earth's gravity took hold of any 
of those wandering interplanetary rocks and pulled them in, they became 
meteors upon entering the atmosphere, meteorites upon hitting the ground, 
and keys to studying the second largest object in the main asteroid belt 
upon entering laboratories. One esteemed scientist on the Dawn team opined 
that with Dawn's detailed data and our Vestan samples, Vesta joined the 
ranks of the moon and Mars as the only extraterrestrial bodies that have 
been geologically explored in a rigorous way.

With so many meteorites from Vesta, why have we not linked any to Ceres? 
Is it because the rocks didn't get blasted away in the first place, or 
they didn't make it to the vicinity of Earth or to the ground, or we have 
not recognized that they are in our collections? While there are some 
ideas, the answer is not clear. For that matter, although Vesta and Ceres 
are the two largest residents of the main asteroid belt, why have we not 
tied meteorites to any of the smaller but still sizable bodies there? 
We will return to this question in a future Dawn Journal, but for now, 
let's get back to the question of how Dawn's pictures help with measuring 
the ages of features on Ceres.

[Image]
Dawn took this picture on March 22 from an altitude of 240 miles (385 
kilometers). The impact that formed the crater in the upper left deposited 
material outside the crater, partially covering the smaller craters that 
were already there. The area on the lower right of the picture, including 
the other large crater in this scene, has many more small craters and 
so must be older. Sunlight in this photograph is coming from the right, 
so all the craters are dark on the right side where their walls descend 
into shadow. The crater walls on the left face the sun and so are illuminated. 
Look closely around the young crater and on its floor to see many very 
small features with the opposite lighting: they are bright on the right 
and dark on the left. Unlike all the craters, they are not depressions 
but rather are very large boulders, catching sunlight on the right side. 
(Each pixel in this picture is 120 feet, or 35 meters.) The tremendous 
punch that excavated the young crater must have produced these boulders. 
The Dawn project does not recommend doing the same thing at home. Full 
image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Scientists have measured the relative abundance of different atomic species 
in the Apollo and Luna samples from different locations. Elements with 
known radioactive decay rates serve as clocks, providing a record of how 
old a sample is. This process enabled scientists to pin down the ages 
of many craters on the moon, and from that, they developed a history of 
the rate at which craters of different sizes formed.

During some periods in the moon's history, it was pelted with more interplanetary 
debris, forming more craters, than at other times. This uneven history 
is a reflection of solar-system-wide events. For example, it seems that 
the giant planets of the outer solar system jockeyed for their orbital 
positions around the sun about four billion years ago. Their gravitational 
jostling over the course of about 300 million years may have sent a flurry 
of material into the inner solar system, where the moon recorded the bombardments.

The moon lives at one astronomical unit (1 AU, which is 93 million miles 
or 150 million kilometers) from the sun (because that's where Earth is). 
Scientists can extrapolate the cratering history the moon experienced 
to other locations in the solar system, so they can calculate what other 
bodies should have been subjected to. Ceres lives between 2.6 and 3.0 
AU from the sun.

[Image]
Dawn observed this scene on March 28 from an altitude of 240 miles (385 
kilometers). The prominent crater on the left lies on the western rim 
of Azacca Crater, which goes vertically through the center of the picture. 
(Azacca is a Haitian god of agriculture.) With a diameter of 31 miles 
(50 kilometers) Azacca, is too large to fit in a single picture from this 
low altitude. Note the many deposits of bright material, which is likely 
some kind of salt. Full image and caption. Image credit: 
NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Scientists count the number and size of craters in an area of interest, 
like inside Occator Crater and on the blanket of ejected material surrounding 
it. (See the picture above.) With their mathematical description of how 
many impacts should have occurred over time, they can estimate how long 
the surface has been exposed and accumulating craters. Although the ages 
have not been computed yet, compare the third and fourth pictures presented 
in April for a clear illustration of areas that are of very different 
ages.

The method of determining the age involves many subtleties we did not 
touch on here, and there are many complicating factors that limit the 
accuracy. But the dating results are improved substantially by including 
smaller craters in the count.

It is readily apparent in pictures of Ceres, Vesta, the moon, and elsewhere 
that small craters are more prevalent than large ones. There has simply 
been more small stuff than large stuff flying around in the solar system 
and crashing into surfaces to make craters. There are more bits like sand 
grains than pebbles, more pebbles than boulders, more small boulders than 
big boulders, etc.

Extending Dawn's photographic documentation of the Cerean landscapes to 
finer resolution provides the means to develop a better census of the 
population of craters, yielding a better measure of the age.

Dawn's bonus observations thus give us not only a sharper view of the 
dwarf planet beneath it today but also a more accurate view of the mysterious 
world's past. As this extraordinary journey through space and time continues, 
next month, we will look to the future.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.42 AU (318 
million miles, or 512 million kilometers) from Earth, or 1,400 times as 
far as the moon and 3.38 times as far as the sun today. Radio signals, 
traveling at the universal limit of the speed of light, take 57 minutes 
to make the round trip.