Category Archives: Asteroids & Comets

Asteroids & Comets, Space Systems

Dawn: What happened to the large craters on Ceres?

The Dawn probe orbiting the dwarf planet Ceres in the Asteroid Belt has returned detailed imagery of the surface. So small features can now be studied but one mystery that has arisen is the absence of large craters. Somehow such craters have disappeared due to “Ceres’ peculiar composition and internal evolution”:

The Case of the Missing Ceres Craters

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Scientists with NASA’s Dawn mission were surprised to find that Ceres has no clear signs of truly giant impact basins. This image shows both visible (left) and topographic (right) mapping data from Dawn. Credit: NASA/JPL-Caltech/SwRI – Larger image.
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.

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. For a complete list of mission participants, visit: dawn.jpl.nasa.gov/mission

More information about Dawn is available at the following sites:

Asteroids & Comets, Events, Space films and videos

Videos: Asteroid Day is on line today – Watch film “51 Degrees North”

Happy Asteroid Day!

Asteroid Day is a global awareness campaign where people from around the world come together to learn about asteroids, the impact hazard they may pose, and what we can do to protect our planet, families, communities, and future generations from future asteroid impacts. Asteroid Day is held each year on the anniversary of the largest impact in recent history, the 1908 Tunguska event in Siberia.

Leaders in the field and several scientific organizations are participating in the event. For example, the European Space Agency says Every day is Asteroid Day at ESA.

In collaboration with Discovery Science, there is a live blog and on line discussions and presentations all  day. See the schedule of Live Streams.

Here is a seven part video series hosted by Neil deGrasse Tyson about asteroids and Asteroid Day:

More about the event:

In February 2014, Dr. Brian May, astrophysicist and famed guitarist for the rock band QUEEN, began working with Grigorij Richters, the director of a new film titled 51 Degrees North, a fictional story of an asteroid impact on London and the resulting human condition. May composed the music for the film and suggested that Richters preview it at Starmus, an event organized by Dr. Garik Israelian and attended by esteemed astrophysicists, scientists and artists, including Dr. Stephen Hawking, Richard Dawkins and Rick Wakeman. The result was the beginning of discussions that would lead to the launch of Asteroid Day in 2015.

Continue…

For 24 hours only, the film 51 Degrees North is available on line for free.

Asteroids & Comets, Space Science, Space Systems

Dawn: White spots on Ceres made of carbonates, possibly from hydrothermal activity

The latest from the Dawn spacecraft on the odd white patches on the dwarf planet Ceres in the asteroid belt:

Recent Hydrothermal Activity May Explain Ceres’ Brightest Area

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.

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The center of Ceres’ mysterious Occator Crater is the brightest area on the dwarf planet. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/ASI/INAF › Full image and caption

“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.

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The bright central spots near the center 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. Lower resolution color data have been overlaid onto a higher resolution view (see PIA20350) of the crater. The view was produced by combining the highest resolution images of Occator obtained in February 2016 (at image scales of 35 meters, or 115 feet, per pixel) with color images obtained in September 2015 (at image scales of 135 meters, or about 440 feet, per pixel). The three images used to produce the color were taken using spectral filters centered at 438, 550 and 965 nanometers (the latter being slightly beyond the range of human vision, in the near-infrared). The crater measures 57 miles (92 kilometers) across and 2.5 miles (4 kilometers) deep. 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.
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. For a complete list of mission participants, visit: http://dawn.jpl.nasa.gov/mission

More information about Dawn is available at the following sites:

Asteroids & Comets, Space Science, Space Systems

Dawn sends new hi-def images of Ceres

New images of the dwarf planet Ceres in the asteroid belt from the Dawn probe:

New Ceres Images Show Bright Craters

Craters with bright material on dwarf planet Ceres shine in new images from NASA’s Dawn mission.

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Ceres’ 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. This image was made using data from NASA’s Dawn spacecraft when it was in its high-altitude mapping orbit, at a distance of 915 miles (1,470 kilometers) from Ceres. This enhanced color view allows scientists to gain insight into materials and how they relate to surface morphology. Rays of bluish ejected material are prominent in this image. The color blue in such views has been associated with young features on Ceres. [Larger image]
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.

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This image is from a mosaic of views that NASA’s Dawn spacecraft took in its low-altitude mapping orbit, at a distance of 240 miles (385 kilometers) from the surface of Ceres. In the center is Haulani Crater, which has a diameter of 21 miles (34 kilometers).
“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.

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The 6-mile-wide (10-kilometer-wide) crater named Oxo Crater 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. NASA Dawn spacecraft took this image in its low-altitude mapping orbit, at a distance of 240 miles (385 kilometers) from the surface of Ceres. 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. The image has been rotated so that north on Ceres is up. [Larger image]
“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.

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. For a complete list of mission participants, visit: dawn.jpl.nasa.gov/mission.

Asteroids & Comets, Space Science, Space Systems

NASA’s NEOWISE spacecraft finding previously unknown near-earth objects

NASA’s NEOWISE space observatory has had a busy two years:

Asteroid-Hunting Spacecraft Delivers a Second Year of Data 

NASA’s Near-Earth Object Wide-field Survey Explorer (NEOWISE) mission has released its second year of survey data. The spacecraft has now characterized a total of 439 NEOs since the mission was re-started in December 2013. Of these, 72 were new discoveries.

Near-Earth Objects (NEOs) are comets and asteroids that have been nudged by the gravitational attraction of the giant planets in our solar system into orbits that allow them to enter Earth’s neighborhood. Eight of the objects discovered in the past year have been classified as potentially hazardous asteroids (PHAs), based on their size and how closely their orbits approach Earth.

With the release to the public of its second year of data, NASA’s NEOWISE spacecraft completed another milestone in its mission to discover, track and characterize the asteroids and comets that approach closest to Earth.

Since beginning its survey in December 2013, NEOWISE has measured more than 19,000 asteroids and comets at infrared wavelengths. More than 5.1 million infrared images of the sky were collected in the last year. A new movie, based on the data collected, depicts asteroids and comets observed so far by NEOWISE.

“By studying the distribution of lighter- and darker-colored material, NEOWISE data give us a better understanding of the origins of the NEOs, originating from either different parts of the main asteroid belt between Mars and Jupiter or the icier comet populations,” said James Bauer, the mission’s deputy principal investigator at NASA’s Jet Propulsion Laboratory in Pasadena, California.

Originally called the Wide-field Infrared Survey Explorer (WISE), the spacecraft was launched in December 2009. It was placed in hibernation in 2011 after its primary mission was completed. In September 2013, it was reactivated, renamed NEOWISE and assigned a new mission: to assist NASA’s efforts to identify the population of potentially hazardous near-Earth objects. NEOWISE also is characterizing previously known asteroids and comets to provide information about their sizes and compositions.

“NEOWISE discovers large, dark, near-Earth objects, complementing our network of ground-based telescopes operating at visible-light wavelengths. On average, these objects are many hundreds of meters across,” said Amy Mainzer of JPL, NEOWISE principal investigator. NEOWISE has discovered 250 new objects since its restart, including 72 near-Earth objects and four new comets.

NASA’s Jet Propulsion Laboratory in Pasadena, California, manages the NEOWISE mission for NASA’s Science Mission Directorate in Washington. The Space Dynamics Laboratory in Logan, Utah, built the science instrument. Ball Aerospace & Technologies Corp. of Boulder, Colorado, built the spacecraft. Science operations and data processing take place at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA.

For more information about NEOWISE, visit: www.nasa.gov/neowise

More information about asteroids and near-Earth objects is at: www.jpl.nasa.gov/asteroidwatch