In science, one discovery often leads to more questions and mysteries. That’s certainly true of the ice volcanoes on the dwarf planet Ceres. When the Dawn spacecraft discovered the massive cryovolcano called Ahuna Mons on the surface of Ceres, it led to more questions: How cryovolcanically active is Ceres? And, why do we only see one?
Fraser Cain (universetoday.com / @fcain)
Dr. Paul M. Sutter (pmsutter.com / @PaulMattSutter)
Dr. Kimberly Cartier ( KimberlyCartier.org / @AstroKimCartier )
Dr. Morgan Rehnberg (MorganRehnberg.com / @MorganRehnberg ChartYourWorld.org)
This week’s guest is Dr. Claudia Lagos (@CDPLagos).
Claudia is the Research Assistant at the International Centre for Radio Astronomy Research, in the University of Western Australia. Dr. Lagos is one of the core researchers for the Cosmic Dawn Centre (DAWN). Her expertise is in modelling of physical processes in galaxies, such as gas accretion onto galaxies, star formation, stellar feedback, gas accretion onto black holes, among other similar mechanisms.
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Sometimes they see it, sometimes they don’t. That’s why scientists have never been completely sure if Ceres has an atmosphere or not. But now data from the Dawn spacecraft — in orbit of Ceres — confirms the dwarf planet really does have a very weak atmosphere, but it comes and goes.
The on-again-off-again nature of Ceres’ atmosphere appears to be linked to solar activity. When energetic particles from the Sun hit exposed ice within the craters on Ceres, the ice can sublimate and create an “exosphere” that lasts for a week or so.
Michaela Villarreal from UCLA, lead author of the new study, and her team wrote in their paper that the “atmosphere appeared shortly after the passage of a large enhancement in the local flux of high-energy solar protons,” and explained that 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.
A process like this could also be taking place on the Moon, and is likely similar to the process similar to what takes place on comets.
“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.”
There have been hints of an atmosphere at Ceres since the early 1990’s. 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 vapor as a possible weak atmosphere, on three occasions, but did not on a fourth attempt.
The Dawn spacecraft itself saw evidence of a transient atmosphere when it arrived at Ceres in March 2015, with data from its Gamma Ray and Neutron Detector instrument. It also has found ample evidence for water in the form of ice, found just underground at higher latitudes, where temperatures are lower. 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.
The team’s research shows the atmosphere doesn’t necessarily show up when Ceres is close to the Sun or when sunlight hits the ice directly, but from energetic particles released by the Sun when its activity level is high. For example, the best detections of Ceres’ atmosphere did not occur at its closest approach to the Sun.
Also, the times where no atmosphere was detected coincided with lower solar activity, so the researchers say this suggests that solar activity, rather than Ceres’ proximity to the Sun, is a more important factor in generating an exosphere.
Ceres actually is now getting closer to the Sun. However, since the Sun appears to be in a very quiet period, Villarreal, Russell and team predict an atmosphere won’t show up, that the dwarf planet will have little to no atmosphere for some time. However, they said both Dawn and other observatories should keep an eye on what’s happening at Ceres.
The bright regions on the dwarf planet Ceres have been some of the most talked about features in planetary science in recent years. While data from the Dawn spacecraft has shown these bright areas are salt deposits (alas, not lights of an alien city), the question remained of how these salts reached the surface.
Researchers with the Dawn mission say they have now thoroughly investigated the complex geological structures in Occator crater, the region with the brightest regions on Ceres. The scientists conclude that a bright dome-like feature called Cerealia Facula is the remnant of a cryovolcano — an ice volcano — that repeatedly and relatively recently spewed salty ice from within Ceres up to the surface.
“The age and appearance of the material surrounding the bright dome indicate that Cerealia Facula was formed by a recurring, eruptive process, which also hurled material into more outward regions of the central pit,” said Andreas Nathues, a Dawn scientist from the Max Planck Institute for Solar System Research. “A single eruptive event is rather unlikely.”
Occator crater located in the northern hemisphere of Ceres measures 92 kilometers (57 miles) in diameter. In its center is a pit with a diameter of about 11 kilometers (7 miles). On some parts of its edges, jagged mountains and steep slopes rise up to 750 meters (820 yards) high. Within the pit a bright dome formed. It has a diameter of 3 km (1.8 miles), is 400 meters (437 yards) high, with prominent fractures.
In analyzing images from Dawn’s Framing Camera, Nathues and his team deduced that the central pit is a remnant of a former central mountain, formed from the impact that created Occator Crater about 34 million years ago. But with a method for estimating the age of a planet’s surface – called crater counting — the science team could determine the dome of bright material is only about four million years old.
This suggests, the team said, that Occator crater has been the scene of eruptive outbursts of subsurface brine over a long period and until almost recently.
Jupiter’s moons Callisto and Ganymede show similar types of domes, and researchers interpret them as signs of cryovolcanism. While Ceres is too far from the Sun to be warm enough for regular volcanic activity, it very likely has harbored cryovolcanic activity, and it may even be active today.
Images from the Hubble Space Telescope taken more than a decade ago hinted at the bright spots in Occator Crater, but as the Dawn spacecraft approached Ceres in 2015, new images showed the bright areas almost shining like “cosmic beacons, like interplanetary lighthouses drawing us forth,” as described by Marc Rayman, the chief engineer and mission director for Dawn, in an interview with me last year.
Dawn scientist had previously determined the bright areas were salts left over from subsurface briny water that had made its way to the surface, and in the vacuum of space, the water sublimated away, leaving behind the dissolved salts. These salts were determined to be sodium carbonate and ammonium chloride.
But don’t call these bright areas “spots,” said Rayman. “Some of these bright areas are miles across,” he said, “and just as if you were standing on salt flats on Earth that were several thousand acres, you wouldn’t say, ‘I’m standing on a spot.’ You are standing on a big area. But just to see the distribution of this material in the Dawn images shows there is something complex going on there.”
It is currently unknown if the region in Occator Crater is active, but there are hints it is, at least at a low level.
In 2014 the Herschel spacecraft detected water vapor above Occator, and images from Dawn’s cameras of the crater show a ‘haze’ when imaged at certain angles, and this has been explained as the sublimation of water.
Dawn scientists are also studying the large volcanic feature on Ceres, Ahuna Mons, to determine if it could be a cryovolcano, and will continue to study other bright areas on Ceres, as well.
Further Reading: Max Planck Institute for Solar System Research
There’s one thing that could mean the end of the Dawn mission: if the hydrazine fuel for its maneuvering thruster system runs out. Now, engineers for the Dawn mission have figured out a way to save on this fuel while still sending Dawn to a new science orbit around the dwarf planet Ceres. They are effectively extending the mission while expanding on the science Dawn can do.
And in the meantime, Dawn’s cameras can take stunning new images, like the one above of Occator Crater on Ceres and its intriguing, mysterious bright regions.
“This image captures the wonder of soaring above this fascinating, unique world that Dawn is the first to explore,” said Marc Rayman, Dawn’s chief engineer and mission director at the Jet Propulsion Laboratory.
Dawn started making its way to a sixth science orbit earlier this month, raising its orbital height to over 4,500 miles (7,200 kilometers) from Ceres. For previous changes in its orbit, Dawn needed to make several changes in direction while it spiraled either higher or lower. But Dawn’s ever-ingenious engineers have figured out a way for the spacecraft to arrive at this next orbit while the ion engine thrusts in the same direction that Dawn is already going. This uses less hydrazine and xenon fuel than Dawn’s normal spiral maneuvers.
Previously, Dawn’s engineers have done things nothing short of miraculous, such as figuring out how to operate the spacecraft with only two reaction wheels (when at least three are needed, normally), they have developed new, emergency flight paths on short notice, and they keep figuring out ways to conserve the hydrazine. Earlier in the mission, they analyzed more than 50 different options to figure out how to reduce their fuel usage by a whopping 65 percent.
Occator Crater, with its central bright region and other reflective areas, provides evidence of recent geologic activity. The latest research suggests that the bright material in this crater is comprised of salts left behind after a briny liquid emerged from below, froze and then sublimated, meaning it turned from ice into vapor.
The impact that formed the crater millions of years ago unearthed material that blanketed the area outside the crater, and may have triggered the upwelling of salty liquid.
Another new image from Dawn scientists at the German Aerospace Center in Berlin shows how the dwarf planet’s colors would appear to the human eye. The color was calculated based on the way Ceres reflects different wavelengths of light.
Dawn scientists say that one goal of Dawn’s sixth science orbit is to refine previously collected measurements. The spacecraft’s gamma ray and neutron spectrometer, which has been investigating the composition of Ceres’ surface, will characterize the radiation from cosmic rays unrelated to Ceres. This will allow scientists to subtract “noise” from measurements of Ceres, making the information more precise.
The spacecraft has gathered tens of thousands of images and other information from Ceres since arriving in orbit on March 6, 2015. After spending more than eight months studying Ceres at an altitude of about 240 miles (385 kilometers), closer than the International Space Station is to Earth, Dawn headed for a higher vantage point in August. Then, in October, Dawn raised its orbit to about 920-mile (1,480 km) altitude, returning more images and other valuable data about Ceres.
Thanks to the ingenuity of Dawn’s engineers, we’ll have more time to study Ceres.
Scientists have found a bit of a mystery at the dwarf planet Ceres. Yes, there are those intriguing bright spots inside numerous craters, which is a mystery that has mostly been solved (the bright areas likely made of bright salts leftover from the sublimation of a briny solution of sodium carbonate and ammonium chloride; read more details in this NASA article.)
But a new puzzle involves the craters themselves. In the rough and tumble environment of the asteroid belt, ancient Ceres was certainly pummeled by numerous large asteroids during its 4.5 billion-year lifetime. But yet, there are just a few large craters on Ceres.
How could that be?
“It is as though Ceres cures its own large impact scars and regenerates new surfaces, over and over,” said Dr. Simone Marchi, a senior research scientist at the Southwest Research Institute.
Ceres has lots of little craters, but the Dawn spacecraft, orbiting Ceres since early 2015, has found only 16 craters larger than 100 km, and none larger than 280 km (175 miles) across. Scientists who model asteroid collisions in our Solar System predicted Ceres should have amassed up to 10 to 15 craters larger than 400 kilometers (250 miles) wide, and at least 40 craters larger than 100 km (62 miles) wide.
By comparison, Dawn’s other target of study, the smaller asteroid Vesta, has several large craters, including one 500 kilometers (300 miles) in diameter, covering almost the entire south pole region.
While they aren’t visible now, the scientists say there are clues that large impact basins may be hidden beneath Ceres’ surface.
“We concluded that a significant population of large craters on Ceres has been obliterated beyond recognition over geological time scales, likely the result of Ceres’ peculiar composition and internal evolution,” Marchi said.
There are hints of about three shallow depressions around 800 km (500 miles) wide, and Marchi said they could be what are called or planitiae, or ancient impact basins, left over from large collisions that took place early in Ceres’ history.
There are a few possible reasons why the big craters have been erased, and the scientists now have to figure out which reason or combination of reasons best explain their findings. One reason could be because of large amounts of water or ice in Ceres’ interior, which has long been suspected. Does the absence of large craters lend any insight into Ceres’ water content?
“It might,” Marchi said via email. “There is evidence for ice locally at the surface but it is not clear how much water ice there is in the subsurface.”
Marchi said the craters allow scientists to “probe” down to different depths, depending on their sizes, and that the missing large craters (greater than 100 km in diameter) can provide information on the properties on just the upper 100-200 km or so of Ceres’s outer shell.
Because ice is less dense than rock, the topography could “relax” over time — like what happens if you push on your skin, then take the pressure off, and it relaxes back to its original shape, although this happened extremely more slowly on Ceres. The scientists said that over geological timescales of several million years the water or ice would slowly flow and the craters would smooth out.
Additionally, recent analysis of the center of Ceres’ Occator Crater — where the largest bright areas are located — 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.
A recent paper constrains the amount of subsurface ice to be no more than 30-40%.
“However, the lack of large craters cannot be solely explained by the presence of 30-40% of water,” Marchi told Universe Today.
Another reason for the lack of large craters could be hydrothermal activity, such as geysers or cryovolcanoes, which could have flowed across the surface, possibly burying pre-existing large craters. Smaller impacts would have then created new craters on the resurfaced area. Hydrothermal activity has been linked to bright areas on Ceres, as well.
A close look at some of the craters on Ceres show cracked-like surfaces and other areas that looks like there was flow of the surface that “softened” some of the features. Marchi said the team is still working to clarify the Ceres’ peculiar composition and how cryolava or “low viscous material” could have caused the crater rims and bowls to “relax.”
“This is still work in progress,” he told Universe Today. “Ceres is far more richer than Vesta in terms of smooth, flow features. Given that they are in the same environment (e.g. similar impact speed with asteroids), one would think that the production of impact melts would be the same. Thus the fact that we see more flow features on Ceres is a confirmation of its peculiar composition. This may facilitate production of impact “melt” (or ‘mud’ if there is enough water and clays).”
Another reason for the lack of large craters is that smaller, later impacts could have erased the bigger older impact basins. But if that were the case, the older basins would seemingly be more visible than they are now.
But the answer to this puzzle might all come back to the intriguing bright areas on Ceres.
“The presence of ammoniated phyllosilicates, carbonates and salts is truly amazing,” Marchi said. “I think this peculiar composition and Ceres’ internal structure are responsible for the lack of large craters, although we do not know precisely what the obliteration mechanism is.”
Marchi said the large crater obliteration was active well after the late heavy bombardment era, or about 4 billion year ago, so the resurfacing is inextricably linked to Ceres itself and its internal evolution, not impact events.
“All this shows over and over how peculiar Ceres is,” Marchi said. “Beside being a transition object (at inner/outer solar system boundary), it is peculiar in composition, and now also in cratering record.”
Finding out more about Ceres’ interior is one of the more intriguing aspects of Dawn’s continued mission there.
Marchi is lead author of the paper, “The Missing Large Impact Craters on Ceres,” published in the July 26, 2016, issue of Nature Communications.
The Dawn spacecraft, NASA’s asteroid hopping probe, may not be going gently into that good night as planned. Dawn has visited Vesta and Ceres, and for now remains in orbit around Ceres. The Dawn mission was supposed to end after its rendezvous with Ceres, but now, reports say that the Dawn team has asked NASA to extend the mission to visit a third asteroid.
Dawn was launched in 2007, and in 2011 and 2012 spent 14 months at Vesta. After Vesta, it reached Ceres in March 2015, and is still in orbit there. The mission was supposed to end, but according to a report at New Scientist, the team would like to extend that mission.
Dawn is still is fully operational, and still has some xenon propellant remaining for its ion drive, so why not see what else can be achieved? There’s only a small amount of propellant left, so there’s only a limited selection of possible destinations for Dawn at this point. A journey to a far-flung destination is out of the question.
Chris Russell, of the University of California, Los Angeles, is the principal investigator for the Dawn mission. He told New Scientist, “As long as the mission extension has not been approved by NASA, I’m not going to tell you which asteroid we plan to visit,” he says. “I hope a decision won’t take months.”
If the Dawn mission is not extended, then its end won’t be very fitting for a mission that has accomplished so much. It will share the fate of some other spacecraft at the end of their lives; forever parked in a harmless orbit in an out of the way place, forgotten and left to its fate. The only other option is to crash it into a planet or other body to destroy it, like the Messenger spacecraft was crashed into Mercury at the end of its mission.
The crash and burn option isn’t available to Dawn though. The spacecraft hasn’t been sterilized. If it hasn’t been sterilized of all possible Earthly microbial life, then it is strictly forbidden to crash it into Ceres, or another body like it. Planetary protection rules are in place to avoid the possible contamination of other worlds with Earthly microbial life. It’s not likely that any microbes that may have hitched a ride aboard Dawn would have survived Dawn’s journey so far, nor is it likely that they would survive on the surface of Ceres, but rules are rules.
The secret of Dawn’s long-life and success is not only due to the excellent work by the teams responsible for the mission, it’s also due to Dawn’s ion-drive propulsion system. Ion drives, long dreamed of in science and science fiction, are making longer voyages into deep space possible.
Ion drives start very slow, but gain speed incrementally, continuing to generate thrust over long distances and long periods of time. They do all this with minimal propellant, and are ideal for long space voyages like Dawn’s.
The success of the Dawn mission is key to NASA’s plans for further deep space exploration. NASA continues to work on improving ion drives, and their latest project is the Advanced Electric Propulsion System (AEPS.) This project is meant to further develop the Hall Thruster, a type of ion-drive that NASA hopes will extend spacecraft mission capabilities, allow longer and deeper space exploration, and benefit commercial space activities as well.
The AEPS has the potential to double the thrust of current ion-drives like the one on Dawn. It’s a key component of NASA’s Journey to Mars. NASA also has plans for a robotic asteroid capture mission called Asteroid Redirect Mission, which will use the AEPS. That mission will visit an asteroid, retrieve a boulder- sized asteroid from the surface, and place it in orbit around the Moon. Eventually, astronauts will visit it and return samples to Earth for study. Very ambitious.
As far as the Dawn mission goes, it’s unclear what its next destination might be. Vesta and Ceres were chosen because they are thought be surviving protoplanets, formed at the same time as the other planets. But they stopped growing, and they remain largely undisturbed, so in that sense they are kind of locked in time, and are intriguing objects of study. There are other objects in the vicinity, but it would be pure guesswork to name any.
We are prone to looking at the past nostalgically, and thinking of prior decades as the golden age of space exploration. But as Dawn, and dozens of other current missions and scientific endeavours in space show us, we may well be in a golden age right now.
Ah, dome sweet dome. 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. The new images, taken from Dawn’s low-altitude mapping orbit (LAMO) of 240 miles (385 kilometers) above Ceres, reveal a dome in a smooth-walled pit in the bright center of the crater. Linear fractures crisscross the top and flanks of the dome with still more fractures slicing across the nearby plains.
“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.”
Like me, you’ve probably been anticipating LAMO for months, when we’d finally get our clearest view of the famous “bright spots”. Spectral observations have shown that the patches are consistent with a magnesium sulfate called hexahydrite that resembles the more familiar Epsom salts here on Earth. Scientists think these salt-rich areas were residue left behind when water-ice sublimated in the past. Impacts from asteroids could have broken into Ceres’ crust and possibly unearthed salt-rich ices. Exposed to the vacuum of space, the ice would have sublimated (vaporized), leaving the salt behind.
The team also released an enhanced color map of the surface of Ceres that reveals a diversity of surface materials and how they relate to Ceres’ landforms. The dwarf planet doesn’t 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.”
We’re learning more about that subsurface ice thanks to Dawn’s Gamma Ray and Neutron Detector (GRaND). Neutrons and gamma rays produced by cosmic rays interacting with the topmost yard (meter) of the loose rock and dust called regolith provide a fingerprint of Ceres’ chemical makeup. Lower counts indicate the presence of hydrogen, and since water’s rich in hydrogen (H2o), the results from GRanD suggest concentrations of water ice in the near-surface at high latitudes.
“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.
Dawn scientists also reported that the Visual and Infrared Mapping Spectrometer (VIR) has detected water at Oxo Crater, a young, 6-mile-wide (9-kilometer-wide) feature in Ceres’ northern hemisphere. This water could either be bound up in minerals or exist as ice and may have been exposed during a landslide or impact or a combination of the two events. Oxo is the only place on Ceres where water has been detected at the surface so far.
Not only have scientists found evidence of possible extensive subsurface ice, but the composition of the surface is variable. Using VIR, which measures mineral composition by how those minerals reflect sunlight, they found that Haulani Crater shows a different proportion of surface materials than its surroundings. While the surface of Ceres is mostly made of a mixture of materials containing carbonates and phyllosilicates (clays), 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.
All these cool stuff we’re finding out about this small body makes it nearly as exciting as Pluto. Taking a closer look is the best form of education.
All right, maybe not blinking like a flashlight (or a beacon on the tippity-top of a communication tower—don’t even start that speculation up) but the now-famous “bright spots” on the dwarf planet Ceres have been observed to detectably increase and decrease in brightness, if ever-so-slightly.
And what’s particularly interesting is that these observations were made not by NASA’s Dawn spacecraft, currently in orbit around Ceres, but from a telescope right here on Earth.
Researchers using the High Accuracy Radial velocity Planet Searcher (HARPS) instrument on ESO’s 3.6-meter telescope at La Silla detected “unexpected” changes in the brightness of Ceres during observations in July and August of 2015. Variations in line with Ceres’ 9-hour rotational period—specifically a Doppler effect in spectral wavelength created by the motion of the bright spots toward or away from Earth—were expected, but other fluctuations in brightness were also detected.
“The result was a surprise,” said Antonino Lanza from the INAF–Catania Astrophysical Observatory, co-author of the study. “We did find the expected changes to the spectrum from the rotation of Ceres, but with considerable other variations from night to night.”
Watch a video below illustrating the rotation of Ceres and how reflected light from the bright spots within Occator crater are alternately blue- and red-shifted according to the motion relative to Earth.
First observed with Hubble in December 2003, Ceres’ curious bright spots were resolved by Dawn’s cameras to be a cluster of separate regions clustered inside the 60-mile (90-km) -wide Occator crater. Based on Dawn data they are composed of some type of highly-reflective materials like salt and ice, although the exact composition or method of formation isn’t yet known.
Since they are made of such volatile materials though, interaction with solar radiation is likely the cause of the observed daily brightening. As the deposits heat up during the course of the 4.5-hour Ceres daytime they may create hazes and plumes of reflective particles.
“It has been noted that the spots appear bright at dawn on Ceres while they seem to fade by dusk,” noted study lead author Paolo Molaro in the team’s paper. “That could mean that sunlight plays an important role, for instance by heating up ice just beneath the surface and causing it to blast off some kind of plume or other feature.”
Once day turns to night these hazes will re-freeze, depositing the particles back down to the surface—although never in exactly the same way. These slight differences in evaporation and condensation could explain the random variation in daily brightening observed with HARPS.
These findings have been published the journal Monthly Notices of the Royal Astronomical Society (full text on arXiv here.)