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Raumfahrt - NASA-Raumsonde Dawn im Orbit von Zwergplaneten Ceres - Update-11

6.04.2017

Ceres' Temporary Atmosphere Linked to Solar Activity

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NASA's Dawn spacecraft determined the hydrogen content of the upper yard, or meter, of Ceres' surface. Blue indicates where hydrogen content is higher, near the poles, while red indicates lower content at lower latitudes.
Credits: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Scientists have long thought that Ceres may have a very weak, transient atmosphere, but mysteries lingered about its origin and why it's not always present. Now, researchers suggest that this temporary atmosphere appears to be related to the behavior of the sun, rather than Ceres' proximity to the sun. The study was conducted by scientists from NASA's Dawn mission and others who previously identified water vapor at Ceres using other observatories.

 

"We think the occurrence of Ceres' transient atmosphere is the product of solar activity," said Michaela Villarreal, lead author of the new study in the Astrophysical Journal Letters and researcher at the University of California, Los Angeles.

 

Ceres is the largest object in the asteroid belt that lies between Mars and Jupiter. When energetic particles from the sun hit exposed ice and ice near the surface of the dwarf planet, it transfers energy to the water molecules as they collide. This frees the water molecules from the ground, allowing them to escape and create a tenuous atmosphere that may last for a week or so.

 

"Our results also have implications for other airless, water-rich bodies of the solar system, including the polar regions of the moon and some asteroids," said Chris Russell, principal investigator of the Dawn mission, also at UCLA. "Atmospheric releases might be expected from their surfaces, too, when solar activity erupts."

 

Before Dawn arrived in orbit at Ceres in 2015, evidence for an atmosphere had been detected by some observatories at certain times, but not others, suggesting that it is a transient phenomenon. In 1991, the International Ultraviolet Explorer satellite detected hydroxyl emission from Ceres, but not in 1990. Then, in 2007, the European Southern Observatory's Very Large Telescope searched for a hydroxide emission, but came up empty. The European Space Agency's Herschel Space Observatory detected water in the possible weak atmosphere, or "exosphere," of Ceres on three occasions, but did not on a fourth attempt.

 

As Dawn began its thorough study of Ceres in March 2015, scientists found ample evidence for water in the form of ice. The spacecraft’s gamma ray and neutron detector (GRaND) has found that the uppermost surface is rich in hydrogen, which is consistent with broad expanses of water ice. This ice is nearer to the surface at higher latitudes, where temperatures are lower, a 2016 study published in the journal Science found. Ice has been detected directly at the small bright crater called Oxo and in at least one of the craters that are persistently in shadow in the northern hemisphere. Other research has suggested that persistently shadowed craters are likely to harbor ice. Additionally, the shapes of craters and other features are consistent with significant water-ice content in the crust.  

 

Because of this evidence for abundant ice, many scientists think that Ceres' exosphere is created in a process similar to what occurs on comets, even though they are much smaller. In that model, the closer Ceres gets to the sun, the more water vapor is released because of ice sublimating near or at the surface.

 

But the new study suggests comet-like behavior may not explain the mix of detections and non-detections of a weak atmosphere.   

 

"Sublimation probably is present, but we don't think it's significant enough to produce the amount of exosphere that we're seeing," Villarreal said.

 

Villarreal and colleagues showed that past detections of the transient atmosphere coincided with higher concentrations of energetic protons from the sun. Non-detections coincided with lower concentrations of these particles. What's more, the best detections of Ceres' atmosphere did not occur at its closest approach to the sun. This suggests that solar activity, rather than Ceres' proximity to the sun, is a more important factor in generating an exosphere.

 

The research began with a 2016 Science study led by Chris Russell. The study, using GRaND data, suggested that, during a six-day period in 2015, Ceres had accelerated electrons from the solar wind to very high energies.

 

In its orbital path, Ceres is currently getting closer to the sun. But the sun is now in a particularly quiet period, expected to last for several more years. Since their results indicate Ceres' exosphere is related to solar activity, study authors are predicting that the dwarf planet will have little to no atmosphere for some time. However, they recommend that other observatories monitor Ceres for future emissions.

 

Dawn is now in its extended mission and studying Ceres in a highly elliptical orbit. Engineers are maneuvering the spacecraft to a different orbital plane so that Ceres can be viewed in a new geometry. The primary science objective is to measure cosmic rays to help determine which chemical elements lie near the surface of Ceres. As a bonus, in late April, the sun will be directly behind Dawn, when the spacecraft is at an altitude of about 12,300 miles (20,000 kilometers). Ceres will appear brighter than before in that configuration, and perhaps reveal more secrets about its composition and history.

The Dawn 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.

Quelle: NASA

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Update: 20.04.2017

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Landslides on Ceres Reflect Ice Content

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NASA's Dawn spacecraft has revealed many landslides on Ceres, which researchers interpret to have been shaped by a significant amount of water ice. Shown are examples of Type I (left), Type II (middle) and Type III (right).
Credits: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
 

As NASA's Dawn spacecraft continues exploring Ceres, evidence mounts that the enigmatic dwarf planet retains a significant amount of water ice. A new study in the journal Nature Geoscience adds to this picture, showing how ice may have shaped the variety of landslides seen on Ceres today.

 

"Images from Dawn show that landslides, many of which are similar to those seen on Earth, are very common on Ceres, and further the case that Ceres has a lot of water ice involved in its structure," said Britney Schmidt, who led the study. She is an associate of the Dawn science team and assistant professor at Georgia Institute of Technology in Atlanta.  

 

Types of Landslides

 

Schmidt and colleagues identified three types of landslides. Type I, which are relatively round and large, have thick "toes" at their ends. They look similar to rock glaciers and icy landslides on Earth. Type I landslides are mostly found at high latitudes on Ceres, which is also where the most ice is thought to reside just beneath the surface, suggesting they involve the most ice of any of the flow features. Three small Type 1 flows are found in Oxo Crater, a tiny bright crater in the northern hemisphere that hosts an ice deposit at the surface.  

 

Type II features are often thinner and longer than Type I, and are the most common type of landslide on Ceres. The landslide deposits appear similar to those left behind by avalanches seen on Earth.

 

Ceres' Type III features may involve a brief melting of some of the ice within the soil-like regolith, causing the material to flow like mud before refreezing. These landslides are always associated with large impact craters, and may have formed when an impact event melts subsurface ice on Ceres. These features have similar appearances to ejected material from craters in the icy regions of Mars and on Jupiter's moon Ganymede.

 

"The locations of these different types of features reinforces the idea that the shallow subsurface of Ceres is a mixture of ice and rock, and that ice is most plentiful near the surface at the poles," Schmidt said.

 

Scientists were also surprised at just how many landslides have occurred on Ceres in general. About 20 to 30 percent of craters greater than 6 miles (10 kilometers) wide have some type of landslide associated with them. Such widespread "ground ice" features, which formed from of a mixture of rock and ice, had only been observed before on Earth and Mars.

 

Implications and Future Observations

 

Based on the shape and distribution of landslides on Ceres, study authors estimate that the ice in the upper few tens of meters of Ceres may range from 10 percent to 50 percent by volume.

 

"These kinds of flows are not seen on bodies such as Vesta, which Dawn studied from 2011 to 2012, because the regolith is devoid of water," said Carol Raymond, deputy principal investigator for the Dawn mission, based at NASA's Jet Propulsion Laboratory, Pasadena, California.

Now in its extended mission phase, Dawn is using its ion engine to swivel the plane of its orbit around Ceres to prepare for observations from a new orbit and orientation. At the end of April, the spacecraft will be directly between the sun and the mysterious Occator Crater. In this geometry, Dawn may deliver new insights about the reflective material of Ceres' most famous "bright spot," the highly reflective center of Occator that has been named Cerealia Facula.

The Dawn 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.

Quelle: NASA

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Update: 27.04.2017

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Artist's rendering shows NASA's Dawn spacecraft
This artist's rendering shows NASA's Dawn spacecraft maneuvering above Ceres with its ion propulsion system.

Mission Status Report

NASA's Dawn spacecraft is preparing to observe Ceres on April 29 from an "opposition" position, directly between the dwarf planet's mysterious Occator Crater and the sun. This unique geometry may yield new insights about the bright material in the center of the crater. 

While preparing for this observation, one of Dawn's two remaining reaction wheels stopped functioning on April 23. By electrically changing the speed at which these gyroscope-like devices spin, Dawn controls its orientation in the zero-gravity, frictionless conditions of space. 

The team discovered the situation during a scheduled communications session on April 24, diagnosed the problem, and returned the spacecraft to its standard flight configuration, still with hydrazine control, on April 25. The failure occurred after Dawn completed its five-hour segment of ion thrusting on April 22 to adjust its orbit, but before the shorter maneuver scheduled for April 23-24. The orbit will still allow Dawn to perform its opposition measurements. The reaction wheel's malfunctioning will not significantly impact the rest of the extended mission at Ceres.

Dawn completed its prime mission in June 2016, and is now in an extended mission. It has been studying Ceres for more than two years, and before that, the spacecraft orbited giant asteroid Vesta, sending back valuable data and images. Dawn launched in 2007.

The Dawn operations team has been well prepared to deal with the loss of the reaction wheel. The spacecraft is outfitted with four reaction wheels. It experienced failures of one of the wheels in 2010, a year before it entered orbit around Vesta, and another in 2012, as it was completing its exploration of that fascinating world. (See issues with these devices). When a third reaction wheel stopped working this week, the spacecraft correctly responded by entering one of its safe modes and assigning control of its orientation to its hydrazine thrusters.

 

Today, Dawn's elliptical orbit will bring it from an altitude of 17,300 miles (27,900 kilometers) to 15,800 miles (25,400 kilometers) above Ceres.

The Dawn mission is managed by NASA's Jet Propulsion Laboratory in 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. For a complete list of mission participants, visit: https://dawn.jpl.nasa.gov/mission

Quelle: NASA

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Update: 19.06.2017

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Dawn mission managers await NASA decision on spacecraft’s future

Artist’s concept of the Dawn spacecraft with one of its ion engines firing. Credit: NASA/JPL-Caltech

The future of NASA’s Dawn spacecraft, running low on hydrazine fuel and now flying around the dwarf planet Ceres without the help of internal pointing wheels, will be decided in the coming weeks by top space agency managers.

Scientists have not ruled out sending Dawn on a journey across the solar system to another destination, a voyage that counterintuitively might burn less of the craft’s remaining hydrazine propellant than if Dawn stayed in orbit around Ceres, where it has resided since March 2015.

Dawn’s primary mission ended in June 2016, and NASA officials approved a one-year extension that expires June 30. The fate of Dawn after June 30 remains uncertain, but senior managers at NASA Headquarters are expected to soon decide whether the spacecraft should be turned off, continue exploring Ceres, or depart the dwarf planet and perhaps fly by an asteroid.

Officials are expected to consider the financial cost of Dawn’s operations and the scientific payoff of continuing the mission, either at Ceres or another destination.

“We are in the process of assessing with NASA options for a second extended mission,” said Carol Raymond, Dawn’s deputy principal investigator at NASA’s Jet Propulsion Laboratory.

Raymond said Tuesday at a meeting of NASA’s Small Bodies Assessment Group, a community of asteroid and comet scientists, that one option for Dawn’s future could be to send the probe away from Ceres to encounter an asteroid.

Otherwise, Dawn could remain at Ceres for further exploration of the previously-unvisited world, a dwarf planet with a diameter matching Texas’s, and the largest object in the asteroid belt.

The gray landscape of Ceres is scattered with impact craters, some of which contain salt deposits in the form of bright spots that greeted scientists with mystery as Dawn arrived in early 2015.

Dawn also found evidence of an ice-rich later in Ceres’s crust just below the world’s charcoal-colored surface, and scientists believe Ceres harbored an underground ocean in the past.

The mission also discovered a tenuous, temporary atmosphere containing water vapor around Ceres, and scientists have linked its fluctuations to the intensity of the solar wind.

Ceres was Dawn’s second destination after the craft orbited the giant asteroid Vesta in 2011 and 2012.

Occator Crater, measuring 57 miles (92 kilometers) across and 2.5 miles (4 kilometers) deep, contains the brightest area on Ceres. This region has been the subject of intense interest since Dawn’s approach to the dwarf planet in early 2015. Dawn’s close-up view reveals 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. This image shows the bright spots in a mosaic of two Dawn images taken using a shorter exposure time. The shorter exposure reveals details within the bright features that are overexposed, or nearly so, in the full mosaic. The images used to make these mosaics were taken from Dawn’s low-altitude mapping orbit (LAMO), 240 miles (385 kilometers) above Ceres. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

The solar-powered spacecraft, fitted with solar array wings spanning 65 feet (19.7 meters) tip-to-tip, was built by Orbital ATK and launched from Cape Canaveral aboard a United Launch Alliance Delta 2 rocket in September 2007.

The mission has exceeded all of its scientific objectives, and the last year of bonus operations at Ceres included extra imaging of the dwarf planet, and a unique “opposition” observation in late April that positioned the Dawn spacecraft directly between the sun and Occator Crater.

Scientists hoped the favorable sun angle would yield new insights about the bright salt material inside Occator.

Dawn lost the third of its four reaction wheels — spinning devices similar to gyroscopes which use momentum to control the craft’s pointing — April 23, less than a week before the opposition observation opportunity.

The science campaign went ahead as planned after ground controllers restored Dawn to its regular flight mode, but using hydrazine-fueled rocket thrusters instead of reaction wheels.

Dawn’s first reaction wheel failed in 2010, before it reached Vesta. A second wheel stopped working in 2012 as the craft’s ion propulsion system drove Dawn away from Vesta for the trip to Ceres.

Engineers designed Dawn to control its attitude, or orientation, in space with three reaction wheels, one for each pointing axis. A spare fourth reaction wheel was added for redundancy.

Experts from JPL and Orbital ATK devised a hybrid method of controlling Dawn’s attitude with the two remaining reaction wheels and hydrazine thrusters, the spacecraft now must fully rely on its rocket jets, wrote Marc Rayman, Dawn’s chief engineer at JPL, in a mission update posted on a NASA website.

“With the third wheel failure, we can be grateful that each wheel provided as much benefit as it did,” Rayman wrote. “The wheels allowed Dawn to conduct extremely valuable work while using the hydrazine very sparingly.”

Raymond said Tuesday that the third reaction wheel failure was “certainly not a mission-ending event, but it does reduce our lifetime because we have to use the hydrazine at a faster rate.”

NASA’s Dawn spacecraft successfully observed Ceres at opposition on April 29, 2017, taking images from a position exactly between the sun and Ceres’ surface. This movie shows these opposition images, with contrast enhanced to highlight brightness differences. The bright spots of Occator stand out particularly well on an otherwise relatively bland surface. Dawn took these images from an altitude of about 12,000 miles (20,000 kilometers). Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn will use more hydrazine to maintain its attitude when it is closer to Ceres, but spiraling the probe away from the dwarf planet with its three xenon-fueled ion engines would require even less of the hydrazine maneuvering propellant.

“The amount of hydrazine Dawn uses depends on its activities,” Rayman wrote last month. “Whenever it fires an ion engine, the engine controls two of the three axes, significantly reducing the consumption of hydrazine.

“In orbit around Vesta and Ceres, the probe often trains its sensors on the alien landscapes beneath it. The lower the orbital altitude, the faster the orbital velocity, so Dawn needs to turn faster to keep the ground in its sights,” Rayman wrote. “Also, the gravitational attraction of these massive worlds tends to tug on the unusually large solar arrays in a way that would turn the ship in an unwanted direction. That force is stronger at lower altitude, so Dawn needs to work harder to counter it.

“The consequence is that Dawn uses more hydrazine in orbit around Vesta and Ceres than when it is journeying between worlds, orbiting the sun and maneuvering with its ion engine. And it uses more hydrazine in lower orbits than in higher ones,” Rayman wrote.

There is plenty of xenon gas left aboard Dawn, officials said.

Raymond said Dawn is currently in an egg-shaped orbit around Ceres that ranges in distance between 12,000 miles (20,000 kilometers) and 30,000 miles (50,000 kilometers). The probe traveled as close as 240 miles (385 kilometers) to Ceres last year.

“We have enough resources, hydrazine and xenon, to support operations through at least the end of 2018,” Raymond said. “That will be depending the decision at (NASA) Headquarters what we will do with those resources.”

That lifetime prediction depends on Dawn remaining far away from Ceres.

“One day at our low-altitude mapping orbit, which was at 385 kilometers, would be equivalent to about 18 days (of hydrazine fuel) at higher altitude, which is what we’re in now,” Raymond said.

“Lifetime at a lower altitude would likely be limited to weeks at this point,” she said.

A decision on Dawn’s future in the coming weeks — whether it will stay at Ceres or head elsewhere — echoes a similar stay or go choice that faced NASA managers last June.

Dawn’s science team last year proposed dispatching Dawn toward asteroid Adeona, a primitive, carbon-rich remnant from a collision that destroyed a much larger body, for a relatively slow-speed flyby in May 2019, but NASA officials decided keeping the probe in orbit around Ceres would yield a greater scientific return.

One possible fate has been ruled out.

Scientists don’t want Dawn to collide with Ceres and potentially spoil future exploration of the airless world.

“When our planetary protection requirements were negotiated, (scientists) already made the prediction that Ceres was an ocean world in the past, and could possibly be an ocean world today,” Raymond said. “We’ve been vindicated, so our planetary protection requirement was don’t land, don’t crash.”

Before shutting off Dawn for good, navigators will ensure the spacecraft is on a “quarantine” trajectory that avoids impacting Ceres, she said.

Quelle: SN

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Update: 9.07.2017

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Pitted Materials in Craters Could Indicate Buried Ice on Asteroids 

 
July 7, 2017

Tucson, Ariz. -- Pitted terrains inside fresh complex craters on Ceres are similar to terrains seen Mars and Vesta, and are likely formed through the rapid evaporation of subsurface H2O, a new paper by Planetary Science Institute Research Scientist Hanna G. Sizemore says. 

“Pitted terrains may be common morphological markers of volatile-rich near-surface material in the asteroid belt,” Sizemore said.

Sizemore is lead author of “Pitted Terrains on (1) Ceres and Implications for Shallow Subsurface Volatile Distribution” that is published in Geophysical Research Letters. PSI scientists Norbert Schorghofer, Thomas H. Prettyman, David A. Crown, Scott C. Mest and R. Aileen Yingst are among the co-authors on the paper. 

“Wherever we send a spacecraft into the Solar System, we’re always asking, ‘Is there water? Is there ice?’  Both questions are important because we’re interested in habitability, and because water is a resource that will be needed for manned exploration,” Sizemore said. 

When pitted crater materials were first described on Mars, they were cited as evidence that even the “dry” low latitude regions of Mars are somewhat ice-rich. When pitted materials were discovered on Vesta, there was a lively debate about whether the water that formed the pits was sourced from Vesta, or whether the impactors that hit Vesta brought it in. 

“Now, we’ve found this same type of morphological feature on Ceres, and the evidence suggests that ice in the Cerean subsurface dominated the formation of pits there,” Sizemore said. “Finding this type of feature on three different bodies suggests that similar pits might be found on other asteroids we will explore in the future, and that pitted materials may mark the best places to look for ice on those asteroids. 

“We used numerical models to investigate the formation of pitted materials on Ceres, and investigated the relative importance of water ice and other volatiles in pit development there,” Sizemore said. “We concluded that water ice likely plays a key role in pit development on Ceres. Similar pitted terrains will be of interest to future asteroid missions motivated by both astrobiology and in situ resource utilization.” 

Funding for the research was provided by NASA’s Dawn at Ceres Guest Investigator Program and through a contract to PSI from the Jet Propulsion Laboratory, California Institute of Technology. 

Quelle: Planetary Science Institute

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Update: 3.10.2017

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The mysterious bright spots on Ceres may have a common origin

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The bright spots of Ceres, a dwarf planet in the main asteroid belt, have provoked curiosity and speculation ever since NASA’s Dawn spacecraft spotted them in 2015. Now it seems they might all have formed the same way, even though they are made of different materials.

Ceres is speckled with hundreds of bright splotches. An international team led by Ernesto Palomba at the National Institute for Astrophysics in Rome is analysing the light reflected by them – as observed by Dawn, presently in orbit around Ceres – to identify any differences between them.

“The bright spots are only bright relative to Ceres’ already-dark surface,” says Nathanial Stein, a collaborator at the California Institute of Technology. “If you saw those spots on Earth or even on [the asteroid] Vesta you would consider them to be dark spots.”

 

 

 

While the biggest and brightest spots are in Occator crater, more exist elsewhere on the dwarf planet. “Almost all of them are associated with impact craters,” says Stein. The team found 90 per cent of the bright spots are in craters or are debris ejected from a crater.

Researchers theorise that the spots are the result of the heat of an impact melting subsurface materials, which then well up to the surface to create the bright spots.

“As of 20 years ago we would have said that Ceres was just a big, round rock that was the same the whole way through,” explains Andy Rivkin, a planetary astronomer at the Johns Hopkins University Applied Physics Laboratory in Maryland. The jagged mountains and craters led researchers to theorise that Ceres might have an icy core with a rock-ice mantle.

 
 

But these spots are telling a story of a younger, more geologically active Ceres than researchers expected. That’s because we would expect material ejected by impacts to mix eventually and create a uniform surface. “Mixing hasn’t had time to occur yet, which means these spots must be young.”

Changing its spots

Over time, the bright spots could be darkening as exposure to the harsh conditions of space scours their surface, or because darker material is being tossed around by subsequent impacts.

Most of the spots are made from the same basic material as the rest of Ceres’s surface: calcium or magnesium carbonates mixed with ammonia-rich clays. But a handful of the spots in the youngest craters – including the exceptionally bright spots in Occator crater – are made of sodium carbonates without nearly as much ammonium clay.

These are small variations, but they are enough to point to new avenues of enquiry.

“Does this mean that there are different formation mechanisms that account for the different compositions?” asks Stein, “or is it just that they’re end-members of the same process?” The compositions of the bright spots seem similar enough that they could be explained by the impact-melt and upwelling theory, just modified by local conditions on the surface.

“Carbonates are interesting because they’re made via reactions between water and minerals with carbon in them,” says Rivkin. While researchers don’t expect to find life on Ceres, understanding the processes that created carbonates in the bright spots will help them think about the types of reactions necessary for life to occur in such a hostile environment.

The next step is to build a computer model of Ceres based on everything scientists have learned about its composition, then start pelting it with simulated rocks to try to match the craters and bright spots we see there today. “We have all the information we need,” says Stein. Now researchers just need to figure out how to piece it together.

And after that? “These bright spots are almost certainly where you would want to send the next mission,” says Rivkin, dreaming of future exploration with robotic landers or rovers on Ceres.

Quelle: NewScientist

 

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