Sonntag, 4. September 2016 - 08:45 Uhr

Raumfahrt - NASA-Raumsonde Dawn im Orbit von Zwergplaneten Ceres - Update-9



Bright Spots and Color Differences Revealed on Ceres

Scientists from NASA's Dawn mission unveiled new images from the spacecraft's lowest orbit at Ceres, including highly-anticipated views of Occator Crater, at the 47th annual Lunar and Planetary Science Conference in The Woodlands, Texas, on Tuesday.


Occator Crater, measuring 57 miles (92 kilometers) across and 2.5 miles (4 kilometers) deep, contains the brightest area on Ceres.


Occator Crater, measuring 57 miles (92 kilometers) across and 2.5 miles (4 kilometers) deep, contains the brightest area on Ceres, the dwarf planet that Dawn has explored since early 2015. The latest images, taken from 240 miles (385 kilometers) above the surface of Ceres, reveal a dome in a smooth-walled pit in the bright center of the crater. Numerous linear features and fractures crisscross the top and flanks of this dome.  Prominent fractures also surround the dome and run through smaller, bright regions found within the crater.
"Before Dawn began its intensive observations of Ceres last year, Occator Crater looked to be one large bright area. Now, with the latest close views, we can see complex features that provide new mysteries to investigate," said Ralf Jaumann, planetary scientist and Dawn co-investigator at the German Aerospace Center (DLR) in Berlin. "The intricate geometry of the crater interior suggests geologic activity in the recent past, but we will need to complete detailed geologic mapping of the crater in order to test hypotheses for its formation."
Ceres' Haulani Crater (21 miles, 34 kilometers wide) is shown in these views from the visible and infrared mapping spectrometer (VIR) aboard NASA's Dawn spacecraft.
Color Differences
The team also released an enhanced color map of the surface of Ceres, highlighting the diversity of surface materials and their relationships to surface morphology. Scientists have been studying the shapes of craters and their distribution with great interest. Ceres does not have as many large impact basins as scientists expected, but the number of smaller craters generally matches their predictions. The blue material highlighted in the color map is related to flows, smooth plains and mountains, which appear to be very young surface features.
"Although impact processes dominate the surface geology on Ceres, we have identified specific color variations on the surface indicating material alterations that are due to a complex interaction of the impact process and the subsurface composition," Jaumann said. "Additionally, this gives evidence for a subsurface layer enriched in ice and volatiles."
This map shows a portion of the northern hemisphere of Ceres with neutron counting data acquired by the gamma ray and neutron detector (GRaND) instrument aboard NASA's Dawn spacecraft.
Data relevant to the possibility of subsurface ice is also emerging from Dawn's Gamma Ray and Neutron Detector (GRaND), which began acquiring its primary data set in December. Neutrons and gamma rays produced by cosmic ray interactions with surface materials provide a fingerprint of Ceres’ chemical makeup.  The measurements are sensitive to elemental composition of the topmost yard (meter) of the regolith.
In Dawn's lowest-altitude orbit, the instrument has detected fewer neutrons near the poles of Ceres than at the equator, which indicates increased hydrogen concentration at high latitudes. As hydrogen is a principal constituent of water, water ice could be present close to the surface in polar regions.
"Our analyses will test a longstanding prediction that water ice can survive just beneath Ceres' cold, high-latitude surface for billions of years," said Tom Prettyman, the lead for GRaND and Dawn co-investigator at the Planetary Science Institute, Tucson, Arizona.
This colorized global map of Ceres was created from a clear-filter mosaic.
The Mystery of Haulani Crater
But the subsurface does not have the same composition all over Ceres, according to data from the visible and infrared mapping spectrometer (VIR), a device that looks at how various wavelengths of sunlight are reflected by the surface, allowing scientists to identify minerals.
Haulani Crater in particular is an intriguing example of how diverse Ceres is in terms of its surface material composition. This irregularly-shaped crater, with its striking bright streaks of material, shows a different proportion of surface materials than its surroundings when viewed with the VIR instrument. While the surface of Ceres is mostly made of a mixture of materials containing carbonates and phyllosilicates, their relative proportion varies across the surface. 
"False-color images of Haulani show that material excavated by an impact is different than the general surface composition of Ceres. The diversity of materials implies either that there is a mixed layer underneath, or that the impact itself changed the properties of the materials," said Maria Cristina de Sanctis, the VIR instrument lead scientist, based at the National Institute of Astrophysics, Rome.
The Big Picture
Dawn made history last year as the first mission to reach a dwarf planet, and the first to orbit two distinct extraterrestrial targets -- both of them in the main asteroid belt between Mars and Jupiter. The mission conducted extensive observations of Vesta during its 14-month orbit there in 2011-2012.
"We're excited to unveil these beautiful new images, especially Occator, which illustrate the complexity of the processes shaping Ceres' surface. Now that we can see Ceres’ enigmatic bright spots, surface minerals and morphology in high resolution, we're busy working to figure out what processes shaped this unique dwarf planet. By comparing Ceres with Vesta, we'll glean new insights about the early solar system," said Carol Raymond, deputy principal investigator for the Dawn mission, based at NASA's Jet Propulsion Laboratory, Pasadena, California.Dawn's mission is managed by JPL for NASA's Science Mission Directorate in Washington. Dawn is a project of the directorate's Discovery Program, managed by NASA's Marshall Space Flight Center in Huntsville, Alabama. UCLA is responsible for overall Dawn mission science. Orbital ATK Inc., in Dulles, Virginia, designed and built the spacecraft. The German Aerospace Center, Max Planck Institute for Solar System Research, Italian Space Agency and Italian National Astrophysical Institute are international partners on the mission team.
The bright spots of Occator Crater are shown in enhanced color in this view from NASA's Dawn spacecraft. Such views can be used to highlight subtle color differences on Ceres' surface.
Quelle: NASA
Update: 24.03.2016
MYSTERIOUS BRIGHT SPOTS ON CERES: Last year, NASA's Dawn spacecraft went into orbit around Ceres, a dwarf planet in the asteroid belt. Immediately, researchers were mesmerized by a number of bright spots peppering the planet's charcoal-gray surface. What were they? No one knew, but they hoped that close-up images taken during a low orbit in Feb. 2016 might crack the mystery. On Tuesday, those images were released, and here is an example:
This is the largest bright spot on Ceres, located at the center of Occator Crater. Dawn took the picture at point-blank range, just 240 miles (385 kilometers) above the crater's floor. By far the best photo ever taken of a bright spot on Ceres, it reveals a dome criss-crossed by numerous linear features and fractures.
It does not, however, solve the mystery. "The intricate geometry of the crater interior suggests geologic activity in the recent past, but we will need to complete detailed geologic mapping of the crater in order to test multiple hypotheses for its formation," says Ralf Jaumann of the German Aerospace Center. He is a member of the Dawn science team that released the new images at the 47th annual Lunar and Planetary Science Conference in The Woodlands, Texas, on March 22nd.
One idea gaining favor is that the bright area might be a crust of salt marking the spot where a briny underground ocean briefly broke through to the surface. If so, Ceres might not be as dry as it looks. Indeed, the density of Ceres suggests that 1/3rd of its mass is H2O, and some researchers think Ceres contains more freshwater than Earth itself. Sounds like a mystery worth solving
Quelle: Spaceweather
Update: 19.04.2016
New Ceres Images Show Bright Craters
Ceres' Haulani Crater, with a diameter of 21 miles (34 kilometers), shows evidence of landslides from its crater rim.
Craters with bright material on dwarf planet Ceres shine in new images from NASA's Dawn mission.
In its lowest-altitude mapping orbit, at a distance of 240 miles (385 kilometers) from Ceres, Dawn has provided scientists with spectacular views of the dwarf planet.
Haulani Crater, with a diameter of 21 miles (34 kilometers), shows evidence of landslides from its crater rim. Smooth material and a central ridge stand out on its floor. An enhanced false-color view allows scientists to gain insight into materials and how they relate to surface morphology. This image shows rays of bluish ejected material. The color blue in such views has been associated with young features on Ceres.
"Haulani perfectly displays the properties we would expect from a fresh impact into the surface of Ceres. The crater floor is largely free of impacts, and it contrasts sharply in color from older parts of the surface," said Martin Hoffmann, co-investigator on the Dawn framing camera team, based at the Max Planck Institute for Solar System Research, Göttingen, Germany.
The crater's polygonal nature (meaning it resembles a shape made of straight lines) is noteworthy because most craters seen on other planetary bodies, including Earth, are nearly circular. The straight edges of some Cerean craters, including Haulani, result from pre-existing stress patterns and faults beneath the surface.  
A hidden treasure on Ceres is the 6-mile-wide (10-kilometer-wide) Oxo Crater, which is the second-brightest feature on Ceres (only Occator's central area is brighter). Oxo lies near the 0 degree meridian that defines the edge of many Ceres maps, making this small feature easy to overlook. Oxo is also unique because of the relatively large "slump" in its crater rim, where a mass of material has dropped below the surface. Dawn science team members are also examining the signatures of minerals on the crater floor, which appear different than elsewhere on Ceres.
"Little Oxo may be poised to make a big contribution to understanding the upper crust of Ceres," said Chris Russell, principal investigator of the mission, based at the University of California, Los Angeles.
NASA's Dawn spacecraft took images of Haulani Crater at a distance of 240 miles (385 kilometers) from the surface of Ceres.
Oxo Crater is unique because of the relatively large "slump" in its crater rim.
Quelle: NASA
Update: 29.06.2016
Recent Hydrothermal Activity May Explain Ceres' Brightest Area
The center of Ceres' mysterious Occator Crater is the brightest area on the dwarf planet. The inset perspective view shows new data on this feature: Red signifies a high abundance of carbonates, while gray indicates a low carbonate abundance.
The brightest area on Ceres, located in the mysterious Occator Crater, has the highest concentration of carbonate minerals ever seen outside Earth, according to a new study from scientists on NASA's Dawn mission. The study, published online in the journal Nature, is one of two new papers about the makeup of Ceres.
"This is the first time we see this kind of material elsewhere in the solar system in such a large amount," said Maria Cristina De Sanctis, lead author and principal investigator of Dawn's visible and infrared mapping spectrometer. De Sanctis is based at the National Institute of Astrophysics, Rome.
At about 80 million years old, Occator is considered a young crater. It is 57 miles (92 kilometers) wide, with a central pit about 6 miles (10 kilometers) wide. A dome structure at the center, covered in highly reflective material, has radial and concentric fractures on and around it.
De Sanctis' study finds that the dominant mineral of this bright area is sodium carbonate, a kind of salt found on Earth in hydrothermal environments. This material appears to have come from inside Ceres, because an impacting asteroid could not have delivered it. The upwelling of this material suggests that temperatures inside Ceres are warmer than previously believed. Impact of an asteroid on Ceres may have helped bring this material up from below, but researchers think an internal process played a role as well.
More intriguingly, the results suggest that liquid water may have existed beneath the surface of Ceres in recent geological time. The salts could be remnants of an ocean, or localized bodies of water, that reached the surface and then froze millions of years ago.
"The minerals we have found at the Occator central bright area require alteration by water," De Sanctis said. "Carbonates support the idea that Ceres had interior hydrothermal activity, which pushed these materials to the surface within Occator."
The spacecraft's visible and infrared mapping spectrometer examines how various wavelengths of sunlight are reflected by the surface of Ceres. This allows scientists to identify minerals that are likely producing those signals. The new results come from the infrared mapping component, which examines Ceres in wavelengths of light too long for the eye to see.
Last year, in a Nature study, De Sanctis' team reported that the surface of Ceres contains ammoniated phyllosilicates, or clays containing ammonia. Because ammonia is abundant in the outer solar system, this finding introduced the idea that Ceres may have formed near the orbit of Neptune and migrated inward. Alternatively, Ceres may have formed closer to its current position between Mars and Jupiter, but with material accumulated from the outer solar system.
The new results also find ammonia-bearing salts -- ammonium chloride and/or ammonium bicarbonate -- in Occator Crater. The carbonate finding further reinforces Ceres' connection with icy worlds in the outer solar system. Ammonia, in addition to sodium carbonate and sodium bicarbonate found at Occator, has been detected in the plumes of Enceladus, an icy moon of Saturn known for its geysers erupting from fissures in its surface. Such materials make Ceres interesting for the study of astrobiology.
"We will need to research whether Ceres' many other bright areas also contain these carbonates," De Sanctis said.
A separate Nature study in 2015 by scientists with the Dawn framing camera team hypothesized that the bright areas contain a different kind of salt: magnesium sulfate. But the new findings suggest sodium carbonate is the more likely constituent.
"It’s amazing how much we have been able to learn about Ceres' interior from Dawn's observations of chemical and geophysical properties. We expect more such discoveries as we mine this treasure trove of data," said Carol Raymond, deputy principal investigator for the Dawn mission, based at NASA's Jet Propulsion Laboratory, Pasadena, California.
Dawn science team members have also published a new study about the makeup of the outer layer of Ceres in Nature Geoscience, based on images from Dawn's framing camera. This study, led by Michael Bland of the U.S. Geological Survey, Flagstaff, Arizona, finds that most of Ceres' largest craters are more than 1 mile (2 kilometers) deep relative to surrounding terrain, meaning they have not deformed much over billions of years. These significant depths suggest that Ceres' subsurface is no more than 40 percent ice by volume, and the rest may be a mixture of rock and low-density materials such as salts or chemical compounds called clathrates. The appearance of a few shallow craters suggests that there could be variations in ice and rock content in the subsurface.
Dawn’s mission is managed by JPL for NASA’s Science Mission Directorate in Washington. Dawn is a project of the directorate’s Discovery Program, managed by NASA’s Marshall Space Flight Center in Huntsville, Alabama. UCLA is responsible for overall Dawn mission science. Orbital ATK Inc., in Dulles, Virginia, designed and built the spacecraft. The German Aerospace Center, Max Planck Institute for Solar System Research, Italian Space Agency and Italian National Astrophysical Institute are international partners on the mission team.
Occator Crater is 57 miles (92 kilometers) wide, with a central pit around 6 miles (10 kilometers) wide. This enhanced-color view highlights subtle color differences on Ceres'
Quelle: NASA
Update: 30.06.2016

Dawn Completes Primary Mission

NASA's Dawn mission exceeded all expectations during its primary mission to Vesta and Ceres.
Credits: NASA/JPL-Caltech
On June 30, just in time for the global celebration known as Asteroid Day, NASA's Dawn spacecraft completes its primary mission. The mission exceeded all expectations originally set for its exploration of protoplanet Vesta and dwarf planet Ceres.
The historic mission is the first to orbit two extraterrestrial solar system targets, and the first to orbit any object in the main asteroid belt, between Mars and Jupiter. On March 6, 2015, Dawn also became the first spacecraft to enter orbit around a dwarf planet.
An infographic highlights some of the accomplishments of Dawn's journey since launching in September 2007. Dawn has traveled 3.5 billion miles (5.6 billion kilometers) since launch, and has made 2,450 orbits around Vesta and Ceres. The spacecraft has returned about 69,000 images, combined, of both bodies.
Dawn's advanced ion propulsion system made it possible for the spacecraft to orbit two targets in the main asteroid belt. The spacecraft has logged about 48,000 hours of ion engine thrusting.
Scientists have learned a great deal about these unique, massive residents of the asteroid belt through data from the mission. Dawn has revealed that while Vesta is a dry body, Ceres could be as much as 25 percent water ice by mass. Dawn also discovered many intriguing features at both bodies -- Vesta is home to a mountain whose height is more than twice that of Mount Everest, and Ceres has a crater called Occator with mysterious bright features that continue to spark scientific investigation. 
Dawn's mission is managed by NASA's Jet Propulsion Laboratory, Pasadena, California, for NASA's Science Mission Directorate in Washington. Dawn is a project of the directorate's Discovery Program, managed by NASA's Marshall Space Flight Center in Huntsville, Alabama. UCLA is responsible for overall Dawn mission science. Orbital ATK Inc., in Dulles, Virginia, designed and built the spacecraft. The German Aerospace Center, Max Planck Institute for Solar System Research, Italian Space Agency and Italian National Astrophysical Institute are international partners on the mission team.
Quelle: NASA
Update: 8.07.2016

Dawn Maps Ceres Craters Where Ice Can Accumulate

Scientists with NASA's Dawn mission have identified permanently shadowed regions on the dwarf planet Ceres. Most of these areas likely have been cold enough to trap water ice for a billion years, suggesting that ice deposits could exist there now.
"The conditions on Ceres are right for accumulating deposits of water ice," said Norbert Schorghofer, a Dawn guest investigator at the University of Hawaii at Manoa. "Ceres has just enough mass to hold on to water molecules, and the permanently shadowed regions we identified are extremely cold -- colder than most that exist on the moon or Mercury."
Permanently shadowed regions capable of accumulating surface ice were identified in the northern hemisphere of Ceres using images taken by NASA’s Dawn mission combined with sophisticated computer modeling of illumination.
Credits: NASA/JPL-Caltech
Permanently shadowed regions do not receive direct sunlight. They are typically located on the crater floor or along a section of the crater wall facing toward the pole. The regions still receive indirect sunlight, but if the temperature stays below about minus 240 degrees Fahrenheit (minus 151 degrees Celsius), the permanently shadowed area is a cold trap -- a good place for water ice to accumulate and remain stable. Cold traps were predicted for Ceres but had not been identified until now.
In this study, Schorghofer and colleagues studied Ceres' northern hemisphere, which was better illuminated than the south. Images from Dawn's cameras were combined to yield the dwarf planet's shape, showing craters, plains and other features in three dimensions. Using this input, a sophisticated computer model developed at NASA's Goddard Space Flight Center, Greenbelt, Maryland, was used to determine which areas receive direct sunlight, how much solar radiation reaches the surface, and how the conditions change over the course of a year on Ceres.
The researchers found dozens of sizeable permanently shadowed regions across the northern hemisphere. The largest one is inside a 10-mile-wide (16-kilometer) crater located less than 40 miles (65 kilometers) from the north pole.
Taken together, Ceres' permanently shadowed regions occupy about 695 square miles (1,800 square kilometers). This is a small fraction of the landscape -- much less than 1 percent of the surface area of the northern hemisphere.
The team expects the permanently shadowed regions on Ceres to be colder than those on Mercury or the moon. That's because Ceres is quite far from the sun, and the shadowed parts of its craters receive little indirect radiation.
"On Ceres, these regions act as cold traps down to relatively low latitudes," said Erwan Mazarico, a Dawn guest investigator at Goddard. "On the moon and Mercury, only the permanently shadowed regions very close to the poles get cold enough for ice to be stable on the surface."
The situation on Ceres is more similar to that on Mercury than the moon. On Mercury, permanently shadowed regions account for roughly the same fraction of the northern hemisphere. The trapping efficiency -- the ability to accumulate water ice -- is also comparable.
By the team's calculations, about 1 out of every 1,000 water molecules generated on the surface of Ceres will end up in a cold trap during a year on Ceres (1,682 days). That's enough to build up thin but detectable ice deposits over 100,000 years or so.
"While cold traps may provide surface deposits of water ice as have been seen at the moon and Mercury, Ceres may have been formed with a relatively greater reservoir of water," said Chris Russell, principal investigator of the Dawn mission, based at the University of California, Los Angeles. "Some observations indicate Ceres may be a volatile-rich world that is not dependent on current-day external sources."
Quelle: NASA
Update: 26.07.2016

The Case of the Missing Ceres Craters

Ceres is covered in countless small, young craters, but none are larger than 175 miles (280 kilometers) in diameter. To scientists, this is a huge mystery, given that the dwarf planet must have been hit by numerous large asteroids during its 4.5 billion-year lifetime. Where did all the large craters go?
A new study in the journal Nature Communications explores this puzzle of Ceres' missing large craters, using data from NASA's Dawn spacecraft, which has been orbiting Ceres since March 2015. 
"We concluded that a significant population of large craters on Ceres has been obliterated beyond recognition over geological time scales, which is likely the result of Ceres' peculiar composition and internal evolution," said lead investigator Simone Marchi, a senior research scientist at the Southwest Research Institute in Boulder, Colorado. 
Marchi and colleagues modeled collisions of other bodies with Ceres since the dwarf planet formed, and predicted the number of large craters that should have been present on its surface. These models predicted Ceres should have up to 10 to 15 craters larger than 250 miles (400 kilometers) in diameter, and at least 40 craters larger than 60 miles (100 kilometers) wide. However, Dawn has shown that Ceres has only 16 craters larger than 60 miles, and none larger than 175 miles (280 kilometers) across.
One idea about Ceres' origins holds that it formed farther out in the solar system, perhaps in the vicinity of Neptune, but migrated in to its present location. However, scientists determined that even if Ceres migrated into the main asteroid belt relatively late in solar system history, it should still have a significant number of large craters.
"Whatever the process or processes were, this obliteration of large craters must have occurred over several hundred millions of years," Marchi said.
Dawn's images of Ceres reveal that the dwarf planet has at least three large-scale depressions called "planitiae" that are up to 500 miles (800 kilometers) wide. These planitiae have craters in them that formed in more recent times, but the larger depressions could be left over from bigger impacts. One of them, called Vendimia Planitia, is a sprawling area just north of Kerwan crater, Ceres' largest well-defined impact basin. Vendimia Planitia must have formed much earlier than Kerwan.
One reason for the lack of large craters could be related the interior structure of Ceres. There is evidence from Dawn that the upper layers of Ceres contain ice. Because ice is less dense than rock, the topography could "relax," or smooth out, more quickly if ice or another lower-density material, such as salt, dominates the subsurface composition. Recent analysis of the center of Ceres' Occator Crater suggests that the salts found there could be remnants of a frozen ocean under the surface, and that liquid water could have been present in Ceres' interior.
Past hydrothermal activity, which may have influenced the salts rising to the surface at Occator, could also have something to do with the erasure of craters. If Ceres had widespread cryovolcanic activity in the past -- the eruption of volatiles such as water -- these cryogenic materials also could have flowed across the surface, possibly burying pre-existing large craters. Smaller impacts would have then created new craters on the resurfaced area.
"Somehow Ceres has healed its largest impact scars and renewed old, cratered surfaces," Marchi said. 
Ceres differs from Dawn's previous destination, protoplanet Vesta, in terms of cratering. Although Vesta is only half the size of Ceres, it has a well-preserved 300-mile- (500-kilometer) -wide crater called Rheasilvia, where an impacting asteroid knocked out a huge chunk of the body. This and other large craters suggest that Vesta has not had processes at work to smooth its surface, perhaps because it is thought to have much less ice. Dawn visited Vesta for 14 months from 2011 to 2012.
"The ability to compare these two very different worlds in the asteroid belt -- Vesta and Ceres -- is one of the great strengths of the Dawn mission," Marchi said. 
Quelle: NASA
Update: 4.08.2016

What’s Inside Ceres? New Findings from Gravity Data

This artist's concept shows a diagram of how the inside of Ceres could be structured.
This artist's concept shows a diagram of how the inside of Ceres could be structured, based on data about the dwarf planet's gravity field from NASA's Dawn mission.

In the tens of thousands of photos returned by NASA’s Dawn spacecraft, the interior of Ceres isn’t visible. But scientists have powerful data to study Ceres’ inner structure: Dawn’s own motion.


Since gravity dominates Dawn's orbit at Ceres, scientists can measure variations in Ceres’ gravity by tracking subtle changes in the motion of the spacecraft. Using data from Dawn, scientists have mapped the variations in Ceres' gravity for the first time in a new study in the journal Nature, which provides clues to the dwarf planet's internal structure.


"The new data suggest that Ceres has a weak interior, and that water and other light materials partially separated from rock during a heating phase early in its history," said Ryan Park, the study’s lead author and the supervisor of the solar system dynamics group at NASA’s Jet Propulsion Laboratory, Pasadena, California.


Ceres' gravity field is measured by monitoring radio signals sent to Dawn, and then received back on Earth, by NASA’s Deep Space Network. This network is a collection of large antennas at three locations around the globe that communicate with interplanetary spacecraft. Using these signals, scientists can measure the spacecraft's speed to a precision of 0.004 inches (0.1 millimeters) per second, and then calculate the details of the gravity field.


Ceres has a special property called "hydrostatic equilibrium," which was confirmed in this study. This means that Ceres' interior is weak enough that its shape is governed by how it rotates. Scientists reached this conclusion by comparing Ceres' gravity field to its shape. Ceres' hydrostatic equilibrium is one reason why astronomers classified the body as a dwarf planet in 2006.


The data indicate that Ceres is “differentiated,” which means that it has compositionally distinct layers at different depths, with the densest layer at the core. Scientists also have found that, as they suspected, Ceres is much less dense than Earth, the moon, giant asteroid Vesta (Dawn’s previous target) and other rocky bodies in our solar system. Additionally, Ceres has long been suspected to contain low-density materials such as water ice, which the study shows separated from the rocky material and rose to the outer layer along with other light materials.


"We have found that the divisions between different layers are less pronounced inside Ceres than the moon and other planets in our solar system," Park said. “Earth, with its metallic core, semi-fluid mantle and outer crust, has a more clearly defined structure than Ceres," Park said.


Scientists also found that high-elevation areas on Ceres displace mass in the interior. This is analogous to how a boat floats on water: the amount of displaced water depends on the mass of the boat. Similarly, scientists conclude that Ceres’ weak mantle can be pushed aside by the mass of mountains and other high topography in the outermost layer as though the high-elevation areas "float" on the material below. This phenomenon has been observed on other planets, including Earth, but this study is the first to confirm it at Ceres.


The internal density structure, based on the new gravity data, teaches scientists about what internal processes could have occurred during the early history of Ceres. By combining this new information with previous data from Dawn about Ceres' surface composition, they can reconstruct that history: Water must have been mobile in the ancient subsurface, but the interior did not heat up to the temperatures at which silicates melt and a metallic core forms.


"We know from previous Dawn studies that there must have been interactions between water and rock inside Ceres," said Carol Raymond, a co-author and Dawn’s deputy principal investigator based at JPL. "That, combined with the new density structure, tells us that Ceres experienced a complex thermal history."

Quelle: NASA


Update: 2.09.2016


Dawn probe paints picture of icy, rocky Ceres – with an ice volcano

NASA scientists take stock of the treasure trove of data on the largest object in the asteroid belt. Angus Bezzina reports.

An artist's impression of Dawn firing above dwarf planet Ceres.

NASA’s Dawn spacecraft has beamed back an abundance of information about dwarf planet Ceres, painting a new picture of the gigantic, pockmarked asteroid made of ice and rock – and sporting an ice volcano. 

The papers were published in Science today.

Discovered in 1801, Ceres is the largest object in the asteroid belt that whirls around the sun between Mars and Jupiter. It is thought to be a protoplanet – a mass that was on its way to becoming a planet before the gravitational pull of Jupiter stopped it from accumulating enough mass – and one of the oldest pieces of the solar system, forming within the celestial array’s first five to 15 million years.

Planetary scientists think Ceres contains a layer of water-ice below its dusty outer crust – similar to Jupiter’s moon Europa and Saturn’s moon Enceladus, which are both considered to be potential sources for harbouring life. 

Information about the dwarf planet has come from a combination of sources including microwave studies, and analyses of meteorites thought to have originated from another, similar dwarf planet, Vesta. 

But the lion’s share comes from the Dawn spacecraft, launched by NASA in 2007 to get a better understanding of Ceres, Vesta and the origins of the solar system.

Now, a deluge of recent Dawn data has been presented in six papers.

Surface watermarks

While we’ve known for a while that the dwarf planet contains water – we’ve seen water vapour emissions from the surface – exactly what the nature of the water was has been vague.

Theories have long suggested Ceres has a mantle rich in ice – and they’re right.

Jean-Philippe Combe from the Bear Fight Institute in the US and his colleagues describe ice on the surface of the dwarf planet. 

The Dawn spacecraft found the mass of ice as it scanned the 10-kilometre-wide, geologically fresh Oxo crater with is visible light and infrared mapping spectrometer.

More water-based revelations on Ceres from the spectrometer were provided by Eleonora Ammannito from the National Institute of Astrophysics in Italy and her colleagues. But rather than water ice, they uncovered a number of phyllosilicate (hydrated, rock-forming) minerals in Ceres’ clay-like crust.

They found magnesium and ammonium bearing minerals all over Ceres’ surface but their abundance varied considerably from place to place.

A view of the Ahuna Mons region, derived from a digital terrain model and images from the framing camera on board the Dawn spacecraft. 

Ice volcanoes

Ceres’ distinctive outer shell is the product of a combination of ice and rock and shaped by external impacts, cryomagmatism and cryovolcanism. And it looks as though it has the solar system’s first confirmed ice volcano.

Ahuna Mons is largest mountain on Ceres – but Ottaviano Ruesch from NASA’s Goddard Space Flight Centre in the US and his colleagues believe it is an ice volcano (also called a cryovolcano).

Perspective view of Ahuna Mons in false colour derived from the Dawn framing camera data. The bluish hue of the mountain's flanks probably reflects a compositional change. 

Their claim is based on extensive modelling of the formation’s topographic profile. 

If correct, it would mark the first evidence of a cryovolcano, although scientists have predicted their existence before as they observed molten ices.

Ruesch and his colleagues claim that features such as Ahuna Mons’ elliptical base and concave top are especially indicative of cryovolcanism because it consists of a brittle outer shell with pressurised, fluid-like material inside.

They also determined that Ahuna Mons is younger than surrounding craters, suggesting that the dome-shaped mountain is a relatively recent formation, extruding from below as chlorine salts mixed with water and pushed the ice up.
NASA's Dawn spacecraft spotted this tall, conical mountain on Ceres from a distance of 1,470 kilometres. The mountain, located in the southern hemisphere, stands 6 kilometres high. Its perimeter is sharply defined, with almost no accumulated debris at the base of the brightly streaked slope.

Meanwhile, Debra Buczkowski from Johns Hopkins University in the US and her colleagues conducted a broader analysis of Ceres’ topography and geomorphology. Their report was based on a geomorphic analysis and physical modelling of the images collected by Dawn’s framing camera.

Buczkowski and her team discovered that while Ceres is covered with a large number of craters, features such as grooves, pit crater chains and troughs are also scattered around.

They argue that many of these features indicate near-surface cryovolcanism.

They also claim that although only one instance of surface ice on Ceres has been detected so far, the presence of these other potentially ice-related features suggests that there is ice in other parts of Ceres’ crust. 

Harald Hiesinger from the University of Münster in Germany and his team found something similar when they looked at more images taken by the same camera on the Dawn spacecraft.

According to them, the thermochemical models that had been developed by scientists in the past predicted Ceres should have an icy crust with few or no impact craters.

NASA's Dawn Spacecraft took this image of Gaue crater, the large crater on the bottom, on Ceres. Gaue is a Germanic goddess to whom offerings are made in harvesting rye.

And although Hiesinger and his colleagues’ observations revealed no large craters, the pictures contradicted predictions, as they showed a varied and heavily cratered surface.

Some impact craters seemed “relaxed” in shape – evidence the ground there was icy – while others were not – indicating something harder such as rock. 

They suggest that this, along with the varied features of the dwarf planet’s surface such as polygon-shaped craters, smooth deposits and bright spots suggest that Ceres’ surface is a combination of ice and rock, not one or the other. 

NASA's Dawn spacecraft took this image that shows a mountain ridge, near lower left, that lies in the center of Urvara crater on Ceres.

Up in the sky

Christopher Russell from the University of California, Los Angeles in the US and his colleagues looked off the little world to its atmosphere. 

They peered through the eyes of Dawn’s gamma-ray and neuron detector instrument and mapped charged particles from the sun curving around the dwarf planet. How did this happen?

The most likely explanation was that Ceres’ weak atmosphere was ionised – that is, molecules had electrons stripped away – by particles streaming from the sun. This solar wind was deflected around the dwarf planet. 

Their other explanation was that the salty interior generated an electric current, which created a magnetic field which deflected solar particles – in the same way Earth’s magnetic field does. 

And this isn’t the last we’ll hear of Dawn. The mission is still active – just today it was moved into a slightly higher orbit to see things from a slightly different angle – and is expected to send back information about the mysterious dwarf planet until 2017 at least.

Quelle: COSMOS


Update: 4.09.2016


Ice Not a Major Factor of Dwarf Planet Ceres’ Surface Features 

Images of Ceres' Occator crater
Although there is significant evidence of ice on the surface of the dwarf planet Ceres, an analysis of the surface geology indicates that ice is not a major factor in forming surface features. Ceres’ Occator crater: (A) High-altitude mapping orbit (HAMO) mosaic of Occator crater (140 m/pixel). North is up. Inset shows location of (C). Red arrows point to 90° bends in the crater rim. (B) HAMO topography of Occator overlain on the HAMO mosaic. (C) Low-altitude mapping orbit (LAMO) resolution (35 m/pixel) mosaic of the lobate flows in Occator. Red arrows point to flow margins. White arrows point to fractures on the crater floor. (D) Map of the fractures on the floor of Occator. The crater rim is outlined. Credit: NASA

Although there is significant evidence of ice on the surface of the dwarf planet Ceres, the largest object in orbit between Mars and Jupiter, an analysis of the surface geology indicates that ice is not a major factor in forming surface features, according to a paper in the September issue of Science magazine.

The paper is based on images obtained by NASA’s Dawn spacecraft, which entered orbit around Ceres in March 2015 after completing its mission on the asteroid Vesta. Since that time, Dawn’s instruments have created digital terrain models, three-dimensional visualizations of the surface of Ceres.

The resulting mapping allowed for a more detailed observation of Ceres’ topography, which includes impact craters, linear structures, domical features and lobate flows. Craters appear to dominate Ceres, many of which have sharp walls and deep floors, although one topographically high region — a plateau named Hanami Planum — stood out to researchers. Researchers also spotted multiple mysterious bright spots in Ceres’ craters.

Angular polygonal craters on the surface suggest that Ceres’ crust is fractured, furthering the conclusion that the near-surface crust “must be both brittle enough to fracture and strong enough to retain fractures for long periods of time.”

“Based on our analysis, the crust of Ceres is too strong to be dominated by ice,” said Debra Buczkowski of the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, the study’s lead author. “While surface features such as the lobate flows show that water ice is present in the dwarf planet’s upper crust and on the surface in some locations, it appears not to be a major factor in creating surface features.”

In addition to studying the surface, researchers drew conclusions about the dwarf planet’s interior makeup. Beneath a strong crust composed of rock, ice and salt hydrates lays a water-rich mantle and a silicate core. Evidence of cryomagmatism is found in the floor-fractured craters, while Ahuna Mons and other domical features have been shown to be cryovolcanic in nature. These surface features suggest that Ceres has been geologically active at some point in its past, perhaps even its recent past.

This in-depth study was the first of its kind for Ceres. The primary purpose of the study was to answer questions about the dwarf planet; although some were answered, many more still remain.

Buczkowski is also a coauthor of three other papers about the dwarf planet appearing in this month’s issue of Science magazine.

Quelle: The Johns Hopkins University Applied Physics Laboratory LLC



Sulfur, Sulfur Dioxide and Graphitized Carbon Observed on Asteroid For First Time 

Tucson, Ariz. -- Hubble Space Telescope observations of the dwarf planet Ceres have discovered the first evidence of sulfur, sulfur dioxide and graphitized carbon found on an asteroid. The sulfur species are likely associated with regions of recent activity, reports Planetary Science Institute Senior Scientist Amanda Hendrix.
The discoveries were made by comparing Ceres’ ultraviolet-visible spectra to laboratory measurements and are presented in the paper “Ceres: Sulfur Deposits and Graphitized Carbon” that appears in the journal Geophysical Research Letters.
Planetary Science Institute Senior Scientists Faith Vilas and Jian-Yang Li are co-authors. 
The new HST observations are complementary to observations being made by instrument on the Dawn spacecraft in orbit at Ceres, covering additional wavelengths.
The presence of graphitized carbon is consistent with weathering of carbonaceous material on the asteroid’s surface, caused by processes such as charged particle bombardment.
“For the first time, a carbon-rich asteroid has been observed in the spectral region where graphitized carbons show unique spectral features,” said Hendrix. “Other dark asteroids probably have graphitized carbon on their surfaces as well.” 
“This is a window to evidence of the effects caused by direct exposure to space for a primitive asteroid surface,” said Vilas. 
“Both sulfur and SO2 are volatile species at typical Ceres temperatures – they aren’t likely to stick around for long before they sublimate and are lost to space. These species could also migrate to cold regions on Ceres, such as some shadowed craters, where they are stable,” said Hendrix. “The presence of these volatile species on the surface suggests that they have recently been emplaced, perhaps by some sort of geothermal activity. Both Dawn observations and Herschel Space Telescope observations have suggested recent activity at Ceres, so it may be that sulfurous materials are involved in the activity.“
“It is remarkable that Ceres has this graphitized carbon covering much of its surface – which tells us that it’s been exposed to weathering processes for eons – and yet Ceres also shows evidence of relatively young, fresh materials as well,” said Hendrix.
“With two space probes planning to rendezvous with dark, carbon-rich asteroids in the next few years, these Ceres observations are helping us to build a good foundation for our understanding of these type of bodies,” Vilas said.
Ceres is the largest object in the main asteroid belt, and, along with Pluto, is classified as a dwarf planet.
Quelle: The Planetary Science Institute

Tags: Raumfahrt