A denizen of the Main Asteroid Belt, located in the warm, well-lit inner region of our Solar System, the dwarf planet Ceres circles our Star. A rocky, icy little sphere, Ceres basks in the bright, golden light of our Sun, that shines upon it with its magnificent stellar light. In October 2015, planetary scientists reported that a new collection of high-velocity impact experiments indicates that Ceres may function almost like a sheet of cosmic flypaper for impacting objects that crash down onto its surface–and then stick to it. Ceres is the largest object in the Main Asteroid Belt, that is located between the planets Mars and Jupiter, and it is the closest dwarf planet to Earth. It has long been known for its unremarkable, bland surface features, and the new research hints that the reason for its plain surface is that the impacting material that has pelted Ceres caused it to be very well-coated with literally billions of years worth of meteorite material.
Planetary scientists using the Vertical Gun Range at NASA’s Ames Research Center in California, conducted the new experiments suggesting that when asteroids and other impacting rocky chunks from space slam into Ceres, much of the impacting debris lingers on the surface instead of bouncing off into interplanetary space. The findings suggest the surface of Ceres could be composed primarily of a mingled mess of meteoritic material accumulated over billions of years of bombardment.
Until the recent arrival of the Dawn spacecraft, all that was known about this relatively nearby little world came from telescopic observations. The observations revealed that Ceres is a mysteriously low-density object, and this hints that it is composed either of extremely porous silicate material or, alternatively, it harbors a hidden thick, large layer of water ice. Observations of its bland surface were remarkably unremarkable.
The Dawn spacecraft is a space probe launched by NASA in 2007 to observe the duo of most-massive objects inhabiting the Main Asteroid Belt: Vesta and Ceres. Dawn entered orbit around Ceres on March 6, 2015, and obtained pictures with unprecedented resolution, during imaging sessions beginning in January 2015, as Dawn made its incredible journey towards its target–revealing that Ceres’s supposedly bland surface is actually pockmarked with craters.
“It’s really bland in the telescopic observations. It’s like someone took a single color of spray paint and sprayed the whole thing. When we think about what might have caused this homogeneous surface, our thoughts turn to impact processes,” explained Terik Daly in an October 14, 2015 Brown University Press Release. Mr. Daly is a doctoral student at Brown in Providence, Rhode Island, and the study’s lead author.
A Remarkably Unremarkable Dwarf Planet
Ceres was the first inhabitant of the Main Asteroid Belt to be discovered. On January 1, 1801, the Italian astronomer, mathematician, and Catholic priest, Giuseppe Piazzi, was the first to spot this seemingly bland-looking small world. At first, Ceres was classified as a major planet, but it was reclassified as an asteroid in the 1850s when a number of other kindred rocky objects in similar orbits were discovered.
Ceres sports a diameter of approximately 587 miles, making it the largest of the dwarf planets dwelling completely within the orbit of Neptune–the most distant major planet from our Sun. Indeed, Ceres is the thirty-third largest known object in our Sun’s family. Made up of a mixture of ice and rock, planetary scientists have estimated that Ceres contains one-third the mass of the entire Main Asteroid Belt. It is also the only body dwelling in the asteroid belt known to be massive enough to be rounded by its own relentless gravity. From our own planet, the apparent magnitude of Ceres ranges from 6.7 to 9.3. This means that even at its brightest, it is too faint to be seen with the unaided human eye–except under the darkest of skies.
Planetary scientists think that Ceres is differentiated into a rocky core and icy mantle, and may also contain relic, lingering liquid water beneath its thick layer of ice. The surface of Ceres may be composed of a mixture of various hydrated minerals, such as carbonates and clay, along with water ice. In January 2014, emissions of water vapor were spotted shooting out from several regions of Ceres. This was a surprise because large objects inhabiting the Main Asteroid Belt usually do not emit vapor–which is more characteristic of comets!
Imaging sessions beginning in January 2015, as Dawn approached Ceres, revealed its surface craters. Also, a duo of bright spots within a crater were seen in a February 19, 2015 image. This resulted in some scientific speculation about a possible icy volcanic (cryovolcanic) origin, or outgassing. On March 3, 2015, a spokesperson from NASA reported that this strange duo of mysterious bright spots are consistent with highly reflective materials that contain salts or ice, but that the presence of cryovolcanism is improbable. On May 11, 2015, NASA released a higher resolution picture revealing that, instead of merely two bizarre bright spots, there are actually several.
In 1772, the German astronomer Johann Elert Bode was the first to suggest that an undiscovered planet could exist between the orbits of Mars and Jupiter. Our Solar System’s eight major planets can be neatly divided into two distinct regions: the inner, rocky, and relatively small terrestrial planets consisting of Mercury, Venus, Earth, and Mars, and the outer giant gaseous planets consisting of Jupiter, Saturn, Uranus, and Neptune. The German mathematician and astronomer Johannes Kepler had already detected the mysteriously large gap between Mars and Jupiter in 1596. Bode based his theory on what is termed the Titius-Bode law–which is now a discredited theory first proposed by the German astronomer Johann Daniel Titius in 1766–observing that there was a regular pattern in the semi-major axes of the orbits of known planets, with the seemingly out-of-place large gap existing between Mars and Jupiter. The pattern predicted that the mysteriously missing hypothetical planet should possess an orbit with a semi-major axis near 2.8 astronomical units (AU). One AU is equivalent to the average distance between Earth and Sun, which is about 93,000,000 miles. In 1781, the German born British astronomer William Herchel’s discovery of the large ice-giant planet Uranus–the seventh major planet from our Star–near the predicted distance for the next body beyond Saturn, increased the scientific faith of that era in the validity of the Titius-Bode law. In 1800, a group headed by the Hungarian astronomer Franz Xavier von Zach, who was editor of the Monaliche Correspondenz, dispatched requests to twenty-four well-trained astronomers of that era–the so-called celestial police–asking them to combine their expertise and start a methodical hunt for the purported missing planet. Although the celestial police did not discover Ceres, they did manage to detect several large asteroids.
Giuseppi Piazzi of the Academy of Palermo in Sicily, was one of the astronomers chosen to be a member of the celestial police. Prior to receiving his invitation to become a member of this group, Piazzi had already discovered Ceres on January 1, 1801. Piazzi had been on the hunt for “the 87th [star] of the Catalogue of the Zodiacal stars of Mr la Calle”–but, in an interesting example of scientific serendipity, he found something else instead. Instead of a star, Piazzi had found a strange object that was a moving star-like object, which he at first incorrectly interpreted to be a comet. In fact, Piazzi had discovered Ceres a total of 24 times–the last time on February 11, 1801. He announced his discovery on January 24, 1801, reporting it as a comet but “since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet,” he noted in letters to two of his fellow astronomers. In April 1801, Piazzi sent his complete observations to other astronomers, and the information was published in the September 1801 issue of the Monatliche Correspondenz.
Originally Piazzi suggested that his discovery be named Cerere Ferdinandea, in honor of the goddess Ceres (Roman goddess of agriculture, Cerere in Italian, who was thought to have originated in Sicily and whose oldest temple was there) and King Ferdinand of Sicily. However, “Ferdinandea” was not acceptable to other nations and was dropped.
The categorization of Ceres has changed more than once and has been the subject of heated debate at times. Bode thought that Ceres was the “missing planet” he had proposed to exist between Mars and Jupiter, at a distance of 419 million kilometers from the Sun. Ceres received a planetary symbol, and remained listed as a planet in astronomy books and tables for fifty years. However, as other similar objects were discovered in the same general neighborhood of our Solar System as Ceres, it eventually became apparent that Ceres represented the first of a new class of objects.
Relic Rocky Planetesimals
Astronomers have known for years that Ceres is the largest asteroid inhabiting the Main Asteroid Belt. However, in 2006, the International Astronomical Union (IAU) reclassified Ceres as a dwarf planet–instead of an asteroid–because of its large size.
A dwarf planet is defined as a Solar System body that is smaller than the major planets, but larger than an asteroid. There are many relatively small bodies in orbit around our Star. The Main Asteroid Belt is the region where a host of small rocky and metallic objects dwell. These small bodies are leftovers from our Solar System’s formation, and they are what is left of a vast population of ancient objects that bumped into one another and then merged together–eventually forming the quartet of major planets that are situated in the inner Solar System. In addition, there are three regions hosting a multitude of icy, sparkling comet nuclei that are located much further away from our Star in our Solar System’s outer limits: the Kuiper Belt, Scattered disc, and Oort Cloud. The comets that inhabit these three regions, and sometimes streak into our inner Solar System, are really the relic population of what was once a very abundant population of frozen objects that merged together in our primordial Solar System. These ancient, icy objects eventually grew into the four gaseous, giant planets of the distant, frigid, twilight region of our Star’s family. Both asteroids and comets are really lingering planetesimals. The very abundant primordial population of planetesimals–of both the rocky and the icy kind–were the building-blocks of the eight major planets of our Solar System.
As planetary scientists obtain increasingly better views of the dwarf planet Ceres, thanks to the exploring Dawn spacecraft, they look forward to gaining a better and better understanding of its origin and evolution by observing its bewildering surface. The strange, intriguing bright spots and other fascinating features of this small world will come into sharper focus.
Whatever Lands On Ceres Sticks With It!
In order to understand impact processes, Terik Daly’s team of astronomers used NASA’s Vertical Gun Range, which is actually a cannon with a 14-foot barrel that can blast projectiles out at breathtaking speeds as high 16,000 miles per hour. For this study, Daly and his colleague, Dr. Peter Schulz, sought to simulate impacts into low-density surfaces that mimic the two suggested possible scenarios for the composition of Ceres’s bewildering surface: porous silicate or icy.
“The idea was to look at those two end-member cases, because we really don’t know yet exactly what Ceres is like,” Daly commented in the October 14, 2015 Brown University Press Release.
In order to simulate the porous silicate scenario, the astronomers shot impactors into powdered pumice. For the frozen, icy scenario, they made use of two targets: snow, and snow covered by a thin coat of fluffy silicate material, replicating the possibility that Ceres’s ice rests beneath a layer of soft silicate fluff. The scientists then blasted these targets with bits of aluminum and basalt of pebble-size, thus simulating both rocky and metallic impacting meteorites.
The simulations indicated that in all cases, large percentages of the impact material lingered in and around the impact crater–and this proved to be especially true in the icy impactor scenario, Daly continued to explain.
“We show that when you have a vertical impact into snow–an analog for the porous ice we think might be just beneath the surface of Ceres–you can have about 77 percent of the impactor’s mass stay in or near the crater,” Daly noted.
The astronomers were surprised at the results of their research, according to Dr. Schultz. Dr. Schultz has studied impact processes for years as a professor of Earth, Environmental, and Planetary sciences at Brown University.
“This is really contrary to previous estimates for small bodies. The thought was that you’d eject more material than you’d collect, but we show you can really deliver a ton of material,” Dr. Schultz continued to explain in the October 14, 2015 Brown University Press Release.
The impact speeds used in the experiments were comparable to speeds thought to be common in Main Asteroid Belt smash-ups. The findings indicate that most of the impacts on porous bodies like Ceres result in a build-up of impact material on the surface.
“People have thought that perhaps if an impact was unusually slow, that you could deliver this much material. But what we’re saying is that for a typical, average-speed impact in the asteroid belt, you’re delivering a ton of material,” Dr. Schultz added.
Billions and billions of years of such impacts may well have caused Ceres to accumulate a large quantity of non-native material. Daly and Schultz noted in the October 14, 2015 Brown University Press Release that this mixing together of impact material resulted in the relatively bland surface observed on Ceres from telescopes.The researchers are hopeful that as the Dawn spacecraft continues to study the surface at much higher resolution, it might be able to spot individual patches of this accumulated material. That would help confirm the relevance of these experiements to celestial bodies, according to the astronomers.
The findings have important implications for missions that are designed to return asteroid samples to Earth. Unless the landing sites are chosen carefully, the scientists say, those missions could wind up with samples that are not representative of the object’s original material. In order to get that, it might be necessary to hunt for an area that has suffered a relatively recent impact.
Dr. Schultz continued to comment that “You can’t do this like the old claw crane from the arcade. You can’t just reach down and grab whatever’s there. You may need to find a fresh impact where perhaps the native stuff has been churned up.”
This research is published in Geophysical Research Letters.