Ceres: A Small World With An Icy Secret

In addition to the eight major planets inhabiting our Solar System, a multitude of small objects are also in orbit around our Sun. The myriad small rocky and metallic worldlets, that travel around our Sun within the Main Asteroid Belt between Mars and Jupiter, are really the lingering relics of the primeval planet-birthing era when our entire Solar System was first forming approximately 4.56 billion years ago. Astronomers have known for years that Ceres is the largest “asteroid” dwelling within the Main Asteroid Belt, but it was not until 2006 that the International Astronomical Union (IAU) finally reclassified this object as a dwarf planet because of its large size–that had rendered it massive enough for its gravity to have pulled it into the shape of a ball. Ceres has kept its many secrets well, but it is now revealing itself to the watchful eyes of astronomers. In September 2018, a team of planetary scientists reported that they have solved another of Ceres’ intriguing mysteries. The astronomers announced their new findings indicating that icy volcanoes (cryovolanoes) have erupted throughout the entire history of Ceres’ existence. Even though this continuous icy volcanic activity has not had the same extensive influence on the dwarf planet’s surface as the fiery lava eruptions of volcanoes have had on Earth, they still played a starring role in the ancient drama of Ceres’ primordial formation.

A new paper, titled Cryovolcanic rates on Ceres revealed by topography, is published in the journal Nature Astronomy. Unlike Earth’s familiar fiery volcanoes that erupt molten red-hot flowing lava from beneath its surface, cryovolcanoes erupt liquid or gaseous volatiles such as water, ammonia or methane. Salty water is thought to be the main ingredient hurled out by erupting cryovolcanoes on Ceres.

Dr. Michael M. Sori of the Lunar and Planetary Laboratory (University of Arizona) is lead author of the paper, and Planetary Science Institute (PSI) Senior Scientist Dr. Hanna G. Sizemore is second author. Both are in Tucson, Arizona.

The robotic Dawn spacecraft is still in orbit around Ceres, and it provides planetary scientists with their best chance of determining the significance of icy volcanism on distant bodies existing in the alien families of stars beyond our own Sun.

Launched back in September 2007, NASA’s Dawn Space Probe’s mission is to observe a duo of the trio of largest inhabitants of the Main Asteroid Belt–Ceres and its smaller sibling Vesta. After finishing a long and treacherous journey through the space between planets, Dawn finally reached Ceres in March 2015. On July 16, 2015, Dawn went into orbit around Ceres, and it is predicted to remain in orbit long after its mission has ended.

Our Solar System’s Main Asteroid Belt is the home of a countless number of mostly rocky chunks that are similar to those that went into the construction of the quartet of inner solid planets: Mercury, Venus, Earth and Mars. Likewise, further from our Star, there is a distant belt composed of a vast number of icy comet nuclei called the Kuiper Belt. These frozen denizens of our Solar System’s twilight zone are akin to the icy planetesimals that merged together long ago to form the quartet of gigantic, gaseous planets: Jupiter, Saturn, Uranus and Neptune.

Both asteroids and comets are lingering planetesimals–the primeval building blocks of our Solar System’s planets. Very long ago, when our Sun and its family of objects were still evolving, planetesimals of both the rocky and icy kind bumped into one another, and frequently merged together–although they also shattered one another during more violent collisions.

Ceres possesses about one third of the total mass of the Main Asteroid Belt, and its spectral signature shows that it is made up of the same material as water-rich carbonaceous chondrites. Carbonaceous chondrites are meteorites that include some of the most primitive known.

In contrast, Vesta–the smaller member of the rocky duo visited by Dawn–bears a resemblance to achondritic asteroids. Vesta contains about one tenth of the mass of the entire Main Asteroid Belt. It has also experienced a great deal of both heating and differentiation. Many planetary scientists propose that little Vesta also harbors a core composed of metal, and displays basaltic flows similar to those seen on Earth’s Moon. Vesta also possesses a density similar to that of Mars.

Both Ceres and Vesta were likely born early in the history of our Solar System. For this reason, the pair retain a precious record of what happened long ago–going all the way back to the ancient birth of the inner solid planets.

The Only Dwarf Planet In The Inner Solar System

Ceres is slightly closer to the orbit of Mars than to the orbit of Jupiter, and it is about 587 miles in diameter. This makes Ceres the largest of the minor planets located inside the orbit of the outermost major planet Neptune. It is the 33rd largest known object in our Sun’s family, and it is made up of a combination of rock and ice. Ceres is also the only known object inhabiting the Main Asteroid Belt whose own gravitational pull is powerful enough to have made it spherical in shape. Even at its brightest, Ceres is too faint to be observed with the unaided human eye–except when the night sky above Earth is extremely dark and clear.

The Italian astronomer Giuseppi Piazzi (1746-1826) discovered Ceres on January 1, 1801. It was the first asteroid to be discovered at the time Piazzi detected it while using the Palermo Astronomical Observatory. At first considered to be a major planet, Ceres was eventually reclassified as an asteroid in the 1950s after many other kindred objects in similar orbits were discovered.

Ceres is differentiated into a rocky core encased within an icy mantle. It may also have a lingering internal ocean of liquid water sloshing around beneath its layer of blanketing ice. The surface of Ceres is a combination of water ice mixed well with sundry minerals such as clay and carbonates. In January 2014, emissions of water vapor were observed shooting skyward from several regions on Ceres. This was surprising because large bodies inhabiting the Main Asteroid Belt normally do not emit water vapor–a defining characteristic of icy comets.

On September 2, 2016, NASA planetary scientists published a paper in the journal Science, proposing that a massive ice volcano named Ahuna Mons provides the most credible evidence so far of the existence of such alien icy formations.

As of this writing, Dawn is about to wrap up its 11 year mission. However, the plucky spacecraft will remain in orbit around Ceres for at least 20 years–and possibly for decades longer–before it finally meets its doom by crashing down to the surface of the dwarf planet it had watched for many years.

Icy Mysteries

“There was a great deal of interest in searching for cryovolcanoes on Ceres as soon as Dawn arrived there, because thermal models had predicted they might exist. Ahuna Mons was a great candidate right away. I carried out a global search that identified 31 other large domes, based on analysis of Dawn’s Framing Camera images and topography data. Making the case that they were volcanic was difficult because they were more ancient than Ahuna and the surfaces were heavily cratered. In the study, we were able to compare the shapes of the mountains to numerical models of how they should relax over time if they were made of icy lava. That strengthened the case that they were volcanic features, and let us make comparisons to volcanism on other planets,” Dr. Sizemore explained in a September 17, 2018 PSI Press Release.

Finite Element Method models were used by the planetary scientists to analyze images obtained from Dawn in order to show that Ceres has undergone periods of icy volcanism throughout its geologic history with an average surface extrusion rate of approximately 10,000 cubic meters each year. This is orders of magnitude lower that that of basaltic volcanism on the quartet of solid inner terrestrial planets.

“We measured the height and diameter of 22 domes, and from this calculated the impact ratio and volume of each. We assumed that they all started out sharply peaked like the youngest mountain, Ahuna Mons. We then calculated the time it would take them to flatten to their current shape, using a numerical model of viscous relaxation. This allowed us to assign approximate ages to the majority of domes formed over the past 1 billion years. Having the volumes of the domes and the rate of formation on Ceres, we could then make direct comparisons to other worlds,” Dr. Sizeman continued to explain in the September 17, 2018 PSI Press Release.

Dr. Sizeman added: “Given how small Ceres is, and how quickly it cooled off after its formation, it would be exciting to identify only one or two possible cryovolcanoes on the surface. To identify a large population of features that may be cryovolcanoes would suggest a long history of volcanism extending up to nearly the present day, which is tremendously exciting. Ceres is a little world that ought to be ‘dead’, but these new results suggest it might not be. Seeing so much potential evidence for cryovolcanism on Ceres also lends more weight to discussions of cryovolcanic processes on larger icy moons in the outer Solar System, where it’s likely to be more rigorous.”