There She Blows... Ice--and how!
Ahuna Mons rises 13,000 feet, and is 11 miles wide at its base. This certainly would be an impressively large volcano on our own planet, but Ceres is less than 600 miles wide. 'Ahuna is one true 'mountain' on Ceres. After studying it closely, we interpret it as a dome raised by cryovolcanism," explains David A. Williams, associate research professor in Arizona State University's School of Earth and Space Exploration.
Cryovolcanism is a form of low temperature volcanic activity, where molten water ice--usually combined with ammonia or salts--plays the same role as molten silicate rock erupted from Earth's familiar volcanoes. The giant ice-mountain Ahuna Mons is a volcanic dome constructed by repeated eruptions of freezing salty water.
Williams is a member of a team of planetary scientists working with NASA's Dawn mission. His specialty is volcanism. "Ahuna is truly unique, being the only mountain of its kind on Ceres. It shows nothing to indicate a tectonic formation, so that led us to consider cryovolcanism as a method of its origin."
The Dawn spacecraft is a probe launched in 2007. Its mission is to study the two most-massive inhabitants of the Main Asteroid Belt--the dwarf planet Ceres, and the somewhat smaller asteroid, Vesta. A dwarf planet like Ceres is defined as a solar system denizen that is smaller than a major planet, but larger than an asteroid--and astronomers have understood for decades that Ceres is the largest inhabitant of the Main Asteroid Belt.
In 2006, the International Astronomical Union classified Ceres as a dwarf planet, rather than an asteroid, because of its size. Even though Vesta is the second-largest body in the Main Asteroid Belt, it still is small enough not to have been molded into a sphere by the pull of its own gravity, and it has retained its status as an asteroid.
NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the Dawn mission with significant contributions made by European colleagues from the Netherlands, Italy, and Germany. It also is the first NASA exploratory space probe to make use of ion propulsion in order to enter into orbit. Previous multitarget missions used conventional drivers that limited them to fly-bys.
Dawn has been taking high-resolution extended images of Ceres since Dec. 1, 2014, and it also was the first spacecraft to visit Vesta. As planetary scientists receive increasingly improved views of Ceres, they look forward to obtaining a greater understanding of this small world's origin as well as its evolution.
Discovered on New Year's Day in 1801 by astronomer Guiseppi Piazzi in Palermo, Italy, Ceres initially was classified as a major planet. Piazzi suggested that Ceres should be named Cerere Ferdinandea, in honor of the ancient Roman goddess of agriculture, and King Ferdinand of Sicily. However, the name Ferdinandea ultimately was rejected because other nations found it unacceptable.
Ceres is a differentiated small world. Many planetary scientists think that it contains a rocky core and mantle of ice. It also may harbor an internal ocean of liquid water hidden under its intriguing surface, which is thought to be a combination of various hydrated minerals, such as carbonates and clay--well mixed with water ice.
In January 2014, plumes of water vapor were observed flying out from several areas on Ceres' surface. This was a surprise because large objects inhabiting the Main Asteroid Belt normally do not shoot out water vapor, which is more characteristic of icy, dirty comets that are small denizens of our solar system's dark and frigid outer limits.
This particular finding strengthens the theory that icy bodies, such as migrating comets, may have traveled into the Main Asteroid Belt from their cold and distant twilight home--wandering far from their frozen place of origin into the golden light and melting heat of the sun. This invasion of icy comets occurred when our 4,560,000,000-year-old solar system still was in the process of forming.
There is a significant population of relatively tiny bodies inhabiting the solar system.
The Main Asteroid Belt is the home of a marvelous multitude of small rocky and metallic bodies that really are the lingering relics of a vast population of primordial objects that eventually had collided and then merged together to give rise to the four major terrestrial planets of the sun's inner kingdom: Mercury, Venus, Earth, and Mars.
In addition, there are three regions containing a multitude of icy comet nuclei that are located much farther from the sun: Kuiper Belt, Scattered Disk, and Oort Cloud. Comets are the frozen leftovers of an ancient, abundant population of frozen bodies that ultimately merged together to evolve into the quartet of giant, gaseous planets inhabiting the outer regions of the solar system: Jupiter, Saturn, Uranus, and Neptune.
Both the asteroids and comets are relic planetesimals. The vast primordial population of ancient planetesimals served as the building blocks of the eight major planets of the sun's family.
As for Ahuna Mons, Dawn scientist Ottaviano Ruesch--of NASA's Goddard Space Flight Center, Greenbelt, Md., and the lead author of a paper about Ceres' volcanism published in the journal Science--explains: "This is the only known example of a cryovolcano that potentially formed from a salty mud mix and which formed in the geologically recent past.
"Ahunahas only has a few craters on its surface, which points to an age of just a couple hundred million years at most." Heavily cratered surfaces usually indicate a very old surface, while surfaces with few craters are thought to be comparatively young.
According to the Dawn mission team, the implication that Ahuna Mons is volcanic in origin is of tremendous importance because it confirms that, even though Ceres' surface temperature averages almost -40[degrees]F, its interior has been kept warm enough for the liquid water or brines to linger for quite some time. This made it possible for volcanic activity on the surface of Ceres to occur in recent geological time.
Ahuna Mons is not the only place on Ceres where cryovolcanoes erupt. The instruments aboard Dawn have detected features that suggest cryovolcanism is responsible for resurfacing areas, rather than building tall mountains of ice. Many craters that pockmark the surface of Ceres show floors that appear to be flatter than those that are caused by impacting meteorites. This suggests that these crater floors may have been flooded from below. Furthermore, such flat-floored craters frequently display cracks indicating that icy "magma" has pushed them upward, and then subsided.
A few regions on Ceres' surface display a "geo-museum" of features. "Occator Crater has several bright spots on its floor. The central spot contains what looks like a cryovolcanic dome, rich in sodium carbonates," notes Williams. Additional bright spots on this small world, he adds, occur over fractures that indicate the venting of water vapor mixed with bright salts. "As the vapor has boiled away, it leaves the bright salts and carbonate minerals behind."
Even though volcanic-related features show themselves all over the entire surface of Ceres, perhaps the most-intriguing aspect is what these features are saying about the hidden interior of this small world. Dawn observations hint that Ceres has an outer shell that is not purely composed of ice or rock, but instead is a combination of both.
Williams recently participated in the research project responsible for discovering that large impact craters are missing from Ceres' surface--presumably eradicated by internal heat. However, smaller craters are preserved. "This shows that Ceres' crust has a variable composition--it is weak at large scales but strong at smaller scales. It has also evolved geologically. Ceres appears to be differentiated internally, with a core and a complex crust made of 30 to 40% water ice mixed with silicate rock and salts."
He further suggests that perhaps there are small pockets of brine that still linger in its interior. "We need to continue studying the data to better understand the interior structure of Ceres."
Ceres was the second stop for the Dawn mission, which paid a visit to Vesta from 2011-12. The spacecraft carries a suite of spectrometers, cameras, and neutron and gamma-ray detectors. These were created to map, measure, and image the shape and surface materials of Ceres, and they gather important information to enable planetary scientists to understand the history of these small worlds--and what they can reveal about our solar system's long-ago birth.
BY JUDJTH BHAFFMAN-MILLER
Judith Braffman-MUler is a freelance journalist.
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|Title Annotation:||SCIENCE & TECHNOLOGY; cryovolcanism on dwarf planet Ceres|
|Publication:||USA Today (Magazine)|
|Date:||Jul 1, 2018|
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