Dawn Journal: In the Details
Posted by Marc Rayman
08-06-2016 18:33 CDT
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.
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.
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 gases 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.
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.
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.
Dr. Marc D. Rayman
3:30 p.m. PDT May 31, 2016