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Marc Rayman

Dawn Journal: In the Details

Posted by Marc Rayman

08-06-2016 18:33 CDT

Topics: mission status, asteroids, Dawn, asteroid 1 Ceres

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.

Canyons in Ezinu Crater

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Canyons in Ezinu Crater
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.

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. 

High-resolution mosaic of Occator crater, Ceres

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

High-resolution mosaic of Occator crater, Ceres

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.

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.

Craters on Ceres

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Craters on Ceres
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.

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.

Crater on the western rim of Azacca Crater

NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Crater on the western rim of Azacca Crater
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.

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

 
See other posts from June 2016

 

Or read more blog entries about: mission status, asteroids, Dawn, asteroid 1 Ceres

Comments:

ScienceNotFiction: 06/16/2016 04:18 CDT

A Possible Evidence of MgO Plasma Core inside Ceres Scientists have used the ESO 3.6-metre HARPS telescope at La Silla in Chile to observe Ceres' Occator light spot for a little over two nights in July and August 2015 when the light holes were still unfilled then. They have detected unexpected random variations in its spectral patterns on both short and longer timescales. Assume the internal mantle salt liquid (such as calcium chloride) can conduct light generated by the MgO core to a certain extend. We would expect constant spectral variation due to the plasma core reaction as well as the circulation pattern among various liquid layers within the mantle.Simply says, the observed spectral variance goes far deeper than the surface plume evaporation phenomenon. Since we can no longer observe these open light holes at Occator, perhaps the HARPS team can focus on the light coming from OXO. There, the detected H2O should be coming from the evaporation of hydrates, and mostly from vapor rising from the slumped opening. Because OXO's opening is not a vertical exposure like the large Occator light spot, we will never be able to get a precise measurement. However, any observable variance will help us to confirm the existence of spectral variance inside Ceres, and will have a good indication that the core is very active.

GaryChurch: 06/16/2016 08:32 CDT

Is there an ocean? How about sending astronauts there? It cannot be that much farther than Mars.

ScienceNotFiction: 06/17/2016 04:08 CDT

Unmanned Surface Exploration of Ceres DAWN has given us great hope that there is an alternate course of planetary evolution. Size and chemical composition played a pivotal role in the survival of the dwarf planets. We knew water is more abundant on this tiny planet. The preliminary chemical analysis have confirmed a rich deposit of clay and salt hydrates on its surface. The next logical step is to send a landing probe to Ceres for collecting actual samples. A low budget mission may do the job by relying a team of AI controlled flying drones with duo robotic limbs for sample collection. The landing pod should carry a self-launching payload vessel in which the drones can place sample containers in it. Once the vessel is filled, the payload vessel can be launched into space and use its small ion engine to fly directly to Earth's HAMO for subsequent collection. Robotic flying drones are the better choice for such mission. It can fly toward the opening of OXO and narrow canyons of Occator. We can even relay all transmission signals in-between drones to expand the exploration parameter. The biggest problem for the drones is the availability of enough atmosphere on the surface. With low gravity, a solar-powered drone may be able to hover on the surface. We must make sure the drone will get charged during daylight hours on Ceres.

ScienceNotFiction: 06/17/2016 04:31 CDT

Inherent Flaws with Statistical Counting of Small Craters on Ceres 1. Ceres orbits along with huge amount of asteroids and debris in the Asteroid Belt. The amount of tiny and small debris impacts on Ceres will be much higher than any other planets in our solar system. Ceres micro-gravity will also pull in more asteroid debris due to close proximity.. 2. The re-surfacing ability of Ceres' craters may affect the age calculation by a large factor. It seems only the crater rims can give us a better estimation on the age of each formation. 3. Some of the super tiny craters were possibly created by electrical discharges.

ScienceNotFiction: 06/29/2016 07:28 CDT

The Primary Chemical Reaction of Ceres' Liquid Mantle may be Analogous to the Solvay Process At the bottom near the MgO plasma heating zone, calcium carbonate (limestone, >2.71g/cm3 density) begins to release carbon dioxide. With sodium chloride and ammonia rich solution sitting above, the carbon dioxide bubbled up through it and sodium bicarbonate begins to precipitate. Meanwhile the ammonia is regenerated from the ammonium chloride byproduct by treating it with the calcium hydroxide (lime) left from carbon dioxide generation. Since the Solvay process recycles its ammonia, it consumes only brine and limestone, thus creates the byproduct - calcium chloride (2.15-1.7g/cm3) which floats upward. Eventually, the 1.51 g/cm3 heptahydrate of sodium carbonate floats to the surface of Ceres' liquid mantle. This will build up the surface with rich sodium carbonate hydrates. When a near-mantle-surface puncture like the Occator spot occurs, the internal tidal force will eventually fill up the holes by pushing up the viscous liquid. Before the hole is completely filled by the spill over sodium carbonate solution, stalactite-like bridges connecting the linear fractures around the Occator spot will occur rapidly due to low temperature crystalization on the surface of Ceres. These bridges and overpasses are formed similar to slow separation of two sticky cliffs (which forms the fracture). It is like breaking up a flat surface spilled with thick glue (sticky lines formed when the two halves are slowly separated). Based on the closeups of Occator spot, we can see these small stalactite bridges and overpasses occurred near the top of most fractures/canyons.

ScienceNotFiction: 07/05/2016 04:59 CDT

The out-flowing of Sodium Carbonate solution from Occator Spots gives us a solid evidence of an existing active liquid mantle on Ceres. As I have pointed out last year that Calcium Chloride can lower the freezing point of water to -50C, the additional anti-freeze effect from ammonia (down to -80C) may push the freezing point of the mantle ocean even lower. Coupled with the heat insulation property of several solutions in the mantle, the mantle can maintain its liquid state indefinitely. At this point, we will be wrong to use the term "water-ice" to describe the mantle of Ceres. We should consider other processes that can generate hydrogen, from acid-base reactions of Na2CO3 with strong acids and metal solutions. The poles may contain high concentration of H ions because of the thicker crust. Like CaCl2, Na2CO3 solution is also an electrolyte. This explain why Ceres can have electrical discharges occurring both on its surface and the interior cavity. We have seen evidence of tiny crater-chains formed on the crust. Based on the orbit of Ceres, its closest encounter with Mars and the Sun will be sometime during Feb-Mar 2017. The tidal activity of the mantle will increase during that period. Since the Occator spots were all sealed, there may not be any observable changes on the surface. However, based on my "self-fallen stalactite" theory (multiple holes to the right of Occator), it is possible that the tidal surges can knock down a few large stalactite columns under the influence of Mars-Sun combined gravitational pull, revealing more light spots in the lower basin areas.

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