It might be oxymoronic to say that the more we find out about something, the more mysterious it becomes. But if that’s true of anything in our Solar System, it might be true about Ceres, the largest body in the main asteroid belt.Continue reading “A Hypervelocity Experiment Mimics the Surface Conditions of Ceres”
When Sicilian astronomer Giuseppe Piazzi spotted Ceres in 1801, he thought it was a planet. Astronomers didn’t know about asteroids at that time. Now we know there’s an enormous quantity of them, primarily residing in the main asteroid belt between Mars and Jupiter.
Ceres is about 1,000 km in diameter and accounts for a third of the mass in the main asteroid belt. It dwarfs most of the other bodies in the belt. Now we know that it’s a planet—albeit a dwarf one—even though its neighbours are mostly asteroids.
But what’s a dwarf planet doing in the asteroid belt?Continue reading “Ceres Probably Formed Farther out in the Solar System and Migrated Inward”
A new and thorough analysis of high-resolution images and data from NASA’s Dawn mission have now provided fresh insights into the dwarf planet Ceres, with intriguing evidence that Ceres has a global subsurface salty ocean, and has been geologically active in the recent past.Continue reading “It’s Starting to Look Like Ceres is an Ocean World, Too”
One of the most enduring questions about Earth regards the origins of its water. Where did it come from? One widely-held theory gives comets the honor of bringing water to Earth. Another one says that Earth’s water came when a protoplanet crashed into early Earth, not only delivering a vast quantity of water, but creating the Moon.
Now a new study shows that the minor planet Vesta got its water from space dust. Could that help explain the origin of Earth’s water?Continue reading “Space Dust Delivered Water to Vesta, Could it Have Done the Same for Earth?”
Ceres, at almost 1,000 km (620 miles) in diameter, is the largest body in the asteroid belt. Between 2015 and 2018, NASA’s ion-powered Dawn spacecraft visited the dwarf planet, looking for clues to help us understand how our Solar System formed. Ceres is the first dwarf planet ever visited by a spacecraft.
Now that scientists have worked with the data from Dawn, we’re starting to see just how unusual Ceres is. One of the most shocking of Dawn’s findings is the volcano Ahuna Mons, a feature that seems out of place on this tiny world. Now scientists from the German Aerospace Center (DLR) have figured out how this strange feature formed on this intriguing little planet.Continue reading “Ceres is a Strange Place, Including a Volcanic Peak 4,000 Meters High Made From Bubbling Salt Water, Mud and Rock”
In 2007, the Dawn mission launched from Earth and began making its way towards two historic rendezvous in the Main Asteroid Belt. The purpose of this mission was to learn more about the history of the early Solar System by studying the two largest protoplanets in the Main Belt – Ceres and Vesta – which have remained intact since their formation.
In 2015, the Dawn mission arrived in orbit around Ceres and began sending back data that has shed light on the protoplanet’s surface, composition and interior structure. Based on mission data, Pasquale Tricarico – the senior scientist at the Planetary Science Institute (PSI) – has also determined that the Ceres also experienced an indirect polar reorientation in the past, where its pole rolled approximately 36° off-axis.
In March of 2015, NASA’s Dawn mission became the first spacecraft to visit the protoplanet Ceres, the largest body in the Main Asteroid Belt. It was also the first spacecraft to visit a dwarf planet, having arrived a few months before the New Horizons mission made its historic flyby of Pluto. Since that time, Dawn has revealed much about Ceres, which in turn is helping scientists to understand the early history of the Solar System.
Last year, scientists with NASA’s Dawn mission made a startling discovery when they detected complex chains of carbon molecules – organic material essential for life – in patches on the surface of Ceres. And now, thanks to a new study conducted by a team of researchers from Brown University (with the support of NASA), it appears that these patches contain more organic material than previously thought.
The new findings were recently published in the scientific journal Geophysical Research Letters under the title “New Constraints on the Abundance and Composition of Organic Matter on Ceres“. The study was led by Hannah Kaplan, a postdoctoral researcher at Brown University, with the assistance of Ralph E. Milliken and Conel M. O’D. Alexander – an assistant professor at Brown University and a researcher from the Carnegie Institution of Washington, respectively.
The organic materials in question are known as “aliphatics”, a type of compound where carbon atoms form open chains that are commonly bound with oxygen, nitrogen, sulfur and chlorine. To be fair, the presence of organic material on Ceres does not mean that the body supports life since such molecules can arise from non-biological processes.
Aliphatics have also been detected on other planets in the form of methane (on Mars and especially on Saturn’s largest moon, Titan). Nevertheless, such molecules remains an essential building block for life and their presence at Ceres raises the question of how they got there. As such, scientists are interested in how it and other life-essential elements (like water) has been distributed throughout the Solar System.
Since Ceres is abundant in both organic molecules and water, it raises some intriguing possibilities about the protoplanet. The results of this study and the methods they used could also provide a template for interpreting data for future missions. As Dr. Kaplan – who led the research while completing her PhD at Brown – explained in a recent Brown University press release:
“What this paper shows is that you can get really different results depending upon the type of organic material you use to compare with and interpret the Ceres data. That’s important not only for Ceres, but also for missions that will soon explore asteroids that may also contain organic material.”
The original discovery of organics on Ceres took place in 2017 when an international team of scientists analyzed data from the Dawn mission’s Visible and Infrared Mapping Spectrometer (VIRMS). The data provided by this instrument indicated the presence of these hydrocarbons in a 1000 km² region around of the Ernutet crater, which is located in the northern hemisphere of Ceres and measures about 52 km (32 mi) in diameter.
To get an idea of how abundant the organic compounds were, the original research team compared the VIRMS data to spectra obtained in a laboratory from Earth rocks with traces of organic material. From this, they concluded that between 6 and 10% of the spectral signature detected on Ceres could be explained by organic matter.
They also hypothesized that the molecules were endogenous in origin, meaning that they originated from inside the protoplanet. This was consistent with previous surveys that showed signs of hydorthermal activity on Ceres, as well others that have detected ammonia-bearing hydrated minerals, water ice, carbonates, and salts – all of which suggested that Ceres had an interior environment that can support prebiotic chemistry.
But for the sake of their study, Kaplan and her colleagues re-examined the data using a different standard. Instead of relying on Earth rocks for comparison, they decided to examine an extraterrestrial source. In the past, some meteorites – such as carbonaceous chondrites – have been shown to contain organic material that is slightly different than what we are familiar with here on Earth.
After re-examining the spectral data using this standard, Kaplan and her team determined that the organics found on Ceres were distinct from their terrestrial counterparts. As Kaplan explained:
“What we find is that if we model the Ceres data using extraterrestrial organics, which may be a more appropriate analog than those found on Earth, then we need a lot more organic matter on Ceres to explain the strength of the spectral absorption that we see there. We estimate that as much as 40 to 50 percent of the spectral signal we see on Ceres is explained by organics. That’s a huge difference compared to the six to 10 percent previously reported based on terrestrial organic compounds.”
If the concentrations of organic material are indeed that high, then it raises new questions about where it came from. Whereas the original discovery team claimed it was endogenous in origin, this new study suggests that it was likely delivered by an organic-rich comet or asteroid. On the one hand, the high concentrations on the surface of Ceres are more consistent with a comet impact.
This is due to the fact that comets are known to have significantly higher internal abundances of organics compared with primitive asteroids, similar to the 40% to 50% figure this study suggests for these locations on Ceres. However, much of those organics would have been destroyed due to the heat of the impact, which leaves the question of how they got there something of a mystery.
If they did arise endogenously, then there is the question of how such high concentrations emerged in the northern hemisphere. As Ralph Milliken explained:
“If the organics are made on Ceres, then you likely still need a mechanism to concentrate it in these specific locations or at least to preserve it in these spots. It’s not clear what that mechanism might be. Ceres is clearly a fascinating object, and understanding the story and origin of organics in these spots and elsewhere on Ceres will likely require future missions that can analyze or return samples.”
Given that the Main Asteroid Belt is composed of material left over from the formation of the Solar System, determining where these organics came from is expected to shed light on how organic molecules were distributed throughout the Solar System early in its history. In the meantime, the researchers hope that this study will inform upcoming sample missions to near-Earth asteroids (NEAs), which are also thought to host water-bearing minerals and organic compounds.
These include the Japanese spacecraft Hayabusa2, which is expected to arrive at the asteroid Ryugu in several weeks’ time, and NASA’s OSIRIS-REx mission – which is due to reach the asteroid Bennu in August. Dr. Kaplan is currently a science team member with the OSIRIS-REx mission and hopes that the Dawn study she led will help the OSIRIS-REx‘s mission characterize Bennu’s environment.
“I think the work that went into this study, which included new laboratory measurements of important components of primitive meteorites, can provide a framework of how to better interpret data of asteroids and make links between spacecraft observations and samples in our meteorite collection,” she said. “As a new member to the OSIRIS-REx team, I’m particularly interested in how this might apply to our mission.”
The New Horizons mission is also expected to rendezvous with the Kuiper Belt Object (KBO) 2014 MU69 on January 1st, 2019. Between these and other studies of “ancient objects” in our Solar System – not to mention interstellar asteroids that are being detected for the first time – the history of the Solar System (and the emergence of life itself) is slowly becoming more clear.
In March of 2015, NASA’s Dawn mission arrived around Ceres, a protoplanet that is the largest object in the Asteroid Belt. Along with Vesta, the Dawn mission seeks to characterize the conditions and processes of the early Solar System by studying some of its oldest objects. One thing Dawn has determined since its arrival is that water-bearing minerals are widespread on Ceres, an indication that the protoplanet once had a global ocean.
Naturally, this has raised many questions, such as what happened to this ocean, and could Ceres still have water today? Towards this end, the Dawn mission team recently conducted two studies that shed some light on these questions. Whereas the former used gravity measurements to characterize the interior of the protoplanet, the latter sought to determine its interior structure by studying its topography.
The first study, titled “Constraints on Ceres’ internal structure and evolution from its shape and gravity measured by the Dawn spacecraft“, was recently published in the Journal of Geophysical Research. Led by Anton Ermakov, a postdoctoral researcher at JPL, the team also consisted of researchers from the NASA’s Goddard Space Flight Center, the German Aerospace Center, Columbia University, UCLA and MIT.
Together, the team relied on gravity measurements of the protoplanet, which the Dawn probe has been collecting since it established orbit around Ceres. Using the Deep Space Network to track small changes in the spacecraft’s orbit, Ermakov and his colleagues were able to conduct shape and gravity data measurements of Ceres to determine the internal structure and composition.
What they found was that Ceres shows signs of being geologically active; if not today, than certainly in the recent past. This is indicated by the presence of three craters – Occator, Kerwan and Yalode – and Ceres’ single tall mountain, Ahuna Mons. All of these are associated with “gravity anomalies”, which refers to discrepancies between the way scientists have modeled Ceres’ gravity and what Dawn observed in these four locations.
The team concluded that these four features and other outstanding geological formations, are therefore indications of cryovolcanism or subsurface structures. What’s more, they determined that the crust’s density was relatively low, being closer to that of ice than solid rock. This, however, was inconsistent with a previous study performed by Dawn guest investigator Michael Bland of the U.S. Geological Survey.
Bland’s study, which was published in Nature Geoscience back in 2016, indicated that ice is not likely to be the dominant component of Ceres strong crust, on a count of it being too soft. Naturally, this raises the question of how the crust could be light as ice in terms of density, but also much stronger. To answer this, the second team attempted to model how Ceres’ surface evolved over time.
Their study, titled “The Interior Structure of Ceres as Revealed by Surface Topography and Gravity“, was published in the journal Earth and Planetary Science Letters. Led by Roger Fu, an assistant professor with the Department of Earth, Atmospheric and Planetary Sciences at MIT, this team consisted of members from Virginia Tech, Caltech, the Southwest Research Institute (SwRI), the US Geological Survey, and the INAF.
Together, they investigated the strength and composition of Ceres’ crust and deeper interior by studying the dwarf planet’s topography. By modeling how the protoplanet’s crust flows, Fu and colleagues determined that it is likely a mixture of ice, salts, rock, and likely clathrate hydrate. This type of structure, which is composed of a gas molecule surrounded by water molecules, is 100 to 1,000 times stronger than water ice.
This high-strength crust, they theorize, could rest on a softer layer that contains some liquid. This would have allowed Ceres’ topography to deform over time, smoothing down features that were once more pronounced. It would also account for its possible ancient ocean, which would have frozen and become bound up with the crust. Nevertheless, some of its water would still exist in a liquid state underneath the surface.
This theory is consistent with several thermal evolution models which were published before the Dawn mission arrived at Ceres. These models contend that Ceres’ interior contains liquid water, similar to what has been found on Jupiter’s moon Europa and Saturn’s moon Enceladus. But in Ceres’ case, this liquid could be what is left over from its ancient ocean rather than the result of present-day geological activity in the interior.
Taken together, these studies indicate that Ceres has had a long and turbulent history. While the first study found that Ceres’ crust is a mixture of ice, salts and hydrated materials – which represents most of its ancient ocean – the second study suggests there is a softer layer beneath Ceres’ rigid surface crust, which could be the signature of residual liquid left over from the ocean.
As Julie Castillo-Rogez, the Dawn project scientist at JPL and a co-author on both studies, explained, “More and more, we are learning that Ceres is a complex, dynamic world that may have hosted a lot of liquid water in the past, and may still have some underground.”
On October 19, 2017, NASA announced that the Dawn mission would be extended until its fuel runs out, which is expected to happen in the latter half of 2018. This extension means that the Dawn probe will be in orbit around Ceres as it goes through perihelion in April 2018. At this time, surface ice will start to evaporate to form a transient atmosphere around the body.
During this period and long after, the spacecraft is likely to remain in a stable orbit around Ceres, where it will continue to send back information on this protoplanet/large asteroid. What it teaches us will also go a long way towards informing our understanding of the early Solar System and how it evolved over the past few billion years.
In the future, it is possible that a mission will be sent to Ceres that is capable of landing on its surface and exploring its topography directly. With any luck, future missions will also be able to explore the interior of Ceres, and other “ocean worlds” like Europa and Enceladus, and find out what lurks beneath their icy surfaces!
Further Reading: NASA
In 2011, NASA’s Dawn spacecraft established orbit around the large asteroid (aka. planetoid) known as Vesta. Over the course of the next 14 months, the probe conducted detailed studies of Vesta’s surface with its suite of scientific instruments. These findings revealed much about the planetoid’s history, its surface features, and its structure – which is believed to be differentiated, like the rocky planets.
In addition, the probe collected vital information on Vesta’s ice content. After spending the past three years sifting through the probe’s data, a team of scientists has produced a new study that indicates the possibility of subsurface ice. These findings could have implications when it comes to our understanding of how Solar bodies formed and how water was historically transported throughout the Solar System.
Their study, titled “Orbital Bistatic Radar Observations of Asteroid Vesta by the Dawn Mission“, was recently published in the scientific journal Nature Communications. Led by Elizabeth Palmer, a graduate student from Western Michigan University, the team relied on data obtained by the communications antenna aboard the Dawn spacecraft to conduct the first orbital bistatic radar (BSR) observation of Vesta.
This antenna – the High-Gain telecommunications Antenna (HGA) – transmitted X-band radio waves during its orbit of Vesta to the Deep Space Network (DSN) antenna on Earth. During the majority of the mission, Dawn’s orbit was designed to ensure that the HGA was in the line of sight with ground stations on Earth. However, during occultations – when the probe passed behind Vesta for 5 to 33 minutes at a time – the probe was out of this line of sight.
Nevertheless, the antenna was continuously transmitting telemetry data, which caused the HGA-transmitted radar waves to be reflected off of Vesta’s surface. This technique, known as bistatic radar (BSR) observations has been used in the past to study the surfaces of terrestrial bodies like Mercury, Venus, the Moon, Mars, Saturn’s moon Titan, and the comet 67P/CG.
But as Palmer explained, using this technique to study a body like Vesta was a first for astronomers:
“This is the first time that a bistatic radar experiment was conducted in orbit around a small body, so this brought several unique challenges compared to the same experiment being done at large bodies like the Moon or Mars. For example, because the gravity field around Vesta is much weaker than Mars, the Dawn spacecraft does not have to orbit at a very high speed to maintain its distance from the surface. The orbital speed of the spacecraft becomes important, though, because the faster the orbit, the more the frequency of the ‘surface echo’ gets changed (Doppler shifted) compared to the frequency of the ‘direct signal’ (which is the unimpeded radio signal that travels directly from Dawn’s HGA to Earth’s Deep Space Network antennas without grazing Vesta’s surface). Researchers can tell the difference between a ‘surface echo’ and the ‘direct signal’ by their difference in frequency—so with Dawn’s slower orbital speed around Vesta, this frequency difference was very small, and required more time for us to process the BSR data and isolate the ‘surface echoes’ to measure their strength.”
By studying the reflected BSR waves, Palmer and her team were able to gain valuable information from Vesta’s surface. From this, they observed significant differences in surface radar reflectivity. But unlike the Moon, these variations in surface roughness could not be explained by cratering alone and was likely due to the existence of ground-ice. As Palmer explained:
“We found that this was the result of differences in the roughness of the surface at the scale of a few inches. Stronger surface echoes indicate smoother surfaces, while weaker surface echoes have bounced off of rougher surfaces. When we compared our surface roughness map of Vesta with a map of subsurface hydrogen concentrations—which was measured by Dawn scientists using the Gamma Ray and Neutron Detector (GRaND) on the spacecraft—we found that extensive smoother areas overlapped areas that also had heightened hydrogen concentrations!”
In the end, Palmer and her colleagues concluded that the presence of buried ice (past and/or present) on Vesta was responsible for parts of the surface being smoother than others. Basically, whenever an impact happened on the surface, it transferred a great deal of energy to the subsurface. If buried ice was present there, it would be melted by the impact event, flow to the surface along impact-generated fractures, and then freeze in place.
Much in the same way that moon’s like Europa, Ganymede and Titania experience surface renewal because of the way cryovolcanism causes liquid water to reach the surface (where it refreezes), the presence of subsurface ice would cause parts of Vesta’ surface to be smoothed out over time. This would ultimately lead to the kinds of uneven terrain that Palmer and her colleagues witnessed.
This theory is supported by the large concentrations of hydrogen that were detected over smoother terrains that measure hundreds of square kilometers. It is also consistent with geomorphological evidence obtained from the Dawn Framing Camera images, which showed signs of of transient water flow over Vesta’s surface. This study also contradicted some previously-held assumptions about Vesta.
As Palmer noted, this could also have implications as far as our understanding of the history and evolution of the Solar System is concerned:
“Asteroid Vesta was expected to have depleted any water content long ago through global melting, differentiation, and extensive regolith gardening by impacts from smaller bodies. However, our findings support the idea that buried ice may have existed on Vesta, which is an exciting prospect since Vesta is a protoplanet that represents an early stage in the formation of a planet. The more we learn about where water-ice exists throughout the Solar System, the better we will understand how water was delivered to Earth, and how much was intrinsic to Earth’s interior during the early stages of its formation.”
This work was sponsored by NASA’s Planetary Geology and Geophysics program, a JPL-based effort that focuses on fostering the research of terrestrial-like planets and major satellites in the Solar System. The work was also conducted with the assistance of the USC’s Viterbi School of Engineering as part of an ongoing effort to improve radar and microwave imaging to locate subsurface sources of water on planets and other bodies.
Plasma propulsion is a subject of keen interest to astronomers and space agencies. As a highly-advanced technology that offers considerable fuel-efficiency over conventional chemical rockets, it is currently being used in everything from spacecraft and satellites to exploratory missions. And looking to the future, flowing plasma is also being investigated for more advanced propulsion concepts, as well as magnetic-confined fusion.
However, a common problem with plasma propulsion is the fact that it relies on what is known as a “neutralizer”. This instrument, which allows spacecraft to remain charge-neutral, is an additional drain on power. Luckily, a team of researchers from the University of York and École Polytechnique are investigating a plasma thruster design that would do away with a neutralizer altogether.
A study detailing their research findings – titled “Transient propagation dynamics of flowing plasmas accelerated by radio-frequency electric fields” – was released earlier this month in Physics of Plasmas – a journal published by the American Institute of Physics. Led by Dr. James Dendrick, a physicist from the York Plasma Institute at the University of York, they present a concept for a self-regulating plasma thruster.
Basically, plasma propulsion systems rely on electric power to ionize propellant gas and transform it into plasma (i.e. negatively charged electrons and positively-charged ions). These ions and electrons are then accelerated by engine nozzles to generate thrust and propel a spacecraft. Examples include the Gridded-ion and Hall-effect thruster, both of which are established propulsion technologies.
The Gridden-ion thruster was first tested in the 1960s and 70s as part of the Space Electric Rocket Test (SERT) program. Since then, it has been used by NASA’s Dawn mission, which is currently exploring Ceres in the Main Asteroid Belt. And in the future, the ESA and JAXA plan to use Gridded-iron thrusters to propel their BepiColombo mission to Mercury.
Similarly, Hall-effect thrusters have been investigated since the 1960s by both NASA and the Soviet space programs. They were first used as part of the ESA’s Small Missions for Advanced Research in Technology-1 (SMART-1) mission. This mission, which launched in 2003 and crashed into the lunar surface three years later, was the first ESA mission to go to the Moon.
As noted, spacecraft that use these thrusters all require a neutralizer to ensure that they remain “charge-neutral”. This is necessary since conventional plasma thrusters generate more positively-charged particles than they do negatively-charged ones. As such, neutralizers inject electrons (which carry a negative charge) in order to maintain the balance between positive and negative ions.
As you might suspect, these electrons are generated by the spacecraft’s electrical power systems, which means that the neutralizer is an additional drain on power. The addition of this component also means that the propulsion system itself will have to be larger and heavier. To address this, the York/École Polytechnique team proposed a design for a plasma thruster that can remain charge neutral on its own.
Known as the Neptune engine, this concept was first demonstrated in 2014 by Dmytro Rafalskyi and Ane Aanesland, two researchers from the École Polytechnique’s Laboratory of Plasma Physics (LPP) and co-authors on the recent paper. As they demonstrated, the concept builds upon the technology used to create gridded-ion thrusters, but manages to generate exhaust that contains comparable amounts of positively and negatively charged ions.
As they explain in the course of their study:
“Its design is based on the principle of plasma acceleration, whereby the coincident extraction of ions and electrons is achieved by applying an oscillating electrical field to the gridded acceleration optics. In traditional gridded-ion thrusters, ions are accelerated using a designated voltage source to apply a direct-current (dc) electric field between the extraction grids. In this work, a dc self-bias voltage is formed when radio-frequency (rf) power is coupled to the extraction grids due to the difference in the area of the powered and grounded surfaces in contact with the plasma.”
In short, the thruster creates exhaust that is effectively charge-neutral through the application of radio waves. This has the same effect of adding an electrical field to the thrust, and effectively removes the need for a neutralizer. As their study found, the Neptune thruster is also capable of generating thrust that is comparable to a conventional ion thruster.
To advance the technology even further, they teamed up with James Dedrick and Andrew Gibson from the York Plasma Institute to study how the thruster would work under different conditions. With Dedrick and Gibson on board, they began to study how the plasma beam might interact with space and whether this would affect its balanced charge.
What they found was that the engine’s exhaust beam played a large role in keeping the beam neutral, where the propagation of electrons after they are introduced at the extraction grids acts to compensate for space-charge in the plasma beam. As they state in their study:
“[P]hase-resolved optical emission spectroscopy has been applied in combination with electrical measurements (ion and electron energy distribution functions, ion and electron currents, and beam potential) to study the transient propagation of energetic electrons in a flowing plasma generated by an rf self-bias driven plasma thruster. The results suggest that the propagation of electrons during the interval of sheath collapse at the extraction grids acts to compensate space-charge in the plasma beam.”
Naturally, they also emphasize that further testing will be needed before a Neptune thruster can ever be used. But the results are encouraging, since they offer up the possibility of ion thrusters that are lighter and smaller, which would allow for spacecraft that are even more compact and energy-efficient. For space agencies looking to explore the Solar System (and beyond) on a budget, such technology is nothing if not desirable!