The Mystery Of The Vanishing Ice Volcanoes
By Judith E Braffman-Miller
Strange things happen in our Solar System–things that do not occur on Earth, and are decidedly alien and unfamiliar to us. Take the dwarf planet Ceres, for example. This small, spherical denizen of the Main Asteroid Belt, located between Mars and Jupiter, apparently has had, in the past, odd volcanoes that do not hurl fiery lava onto its surface, like our own Earthly volcanoes, but instead erupt molten ice. Cryovolcanism–colloquially called icy volcanism–is a type of low temperature volcanic activity, where melted water ice, mixed with ammonia or salts, essentially performs the same role as molten silicate rock that erupts from our own planet’s more familiar fiery and fierce volcanoes. In February 2017, a team of planetary scientists announced that a recently discovered lone ice volcano on Ceres, dubbed Ahuna Mons, may have had some long-gone, older frozen relatives that have gone missing, mysteriously vanishing over the passage of millions of years.
NASA’s Dawn spacecraft discovered the volcanic ice-mountain Ahuna Mons back in 2015. Dawn is a space probe that was launched by NASA back in 2007, whose ongoing mission is to observe the duo of most massive denizens of the Main Asteroid Belt–the dwarf planet Ceres, and the smaller asteroid, Vesta.
Standing about 2.5 miles high, Ahuna Mons is conspicuously alone on Ceres. In addition, the dwarf planet also resides closer to the melting heat of our searing-hot, fiery Sun than any of the other denizens of our Solar System where ice volcanoes have been discovered. Other icy worlds in our Sun’s family that host cryovolcanoes are the distant dwarf planet Pluto; the cracked-icy moon of Jupiter, Europa; Triton, an icy moon of the ice-giant Neptune; Charon, a large moon of Pluto; and Titan, a large and fascinating smoggy, hydrocarbon-slashed, moon-world of the gas-giant planet Saturn.
However, in February 2017, planetary scientists were able to demonstrate that there may have been ice volcanoes other than Ahuna Mons on Ceres millions–or even billions–of years ago. However, Ceres’ ancient cryovolcanoes may have flattened out as time passed–thus becoming indistinguishable from the dwarf planet’s surface.
“We think we have a very good case that there have been lots of cryovolcanoes on Ceres but they have deformed,” Dr. Michael Sori commented in a February 2, 2017 American Geophysical Union (AGU) Press Release. Dr. Sori is of the Lunar and Planetary Laboratory at the University of Arizona in Tucson, and lead author of the new paper.
Ceres was discovered on January 1, 1801 by Guiseppi Piazzi in Palermo, Italy, and was originally designated a major planet. Now considered to be a dwarf planet, Ceres received its most recent classification in 2006, courtesy of the International Astronomical Union (IAU), because of its unusually large size. Dwarf planets are worlds that are smaller than a major planet, but larger than an asteroid. Even though Vesta is the second-largest denizen of the Main Asteroid Belt, it is still small enough not to have been shaped by its own gravity into a sphere, and has kept its designation as an asteroid.
However, the larger Ceres has been pulled by its own gravity into a ball. Ceres is a differentiated little world, and is thought to harbor both a core of rock and a mantle of ice. It has even been proposed that Ceres may sport a subsurface ocean of liquid water hidden beneath its surface. The surface of Ceres is thought to be composed of a combination of various hydrated minerals, such as clay and carbonates, that are mixed with water ice. In January 2014, plumes of water vapor were seen shooting out from a number of spots on Ceres’ surface. This surprised many planetary scientists because objects dwelling within the Main Asteroid Belt usually do not emit water vapor. The emission of water vapor is normally a characteristic of icy, dusty comets, which are small inhabitants of our Solar System’s frigid outer region, far from our Sun, that is swathed in perpetual twilight. The discovery of water vapor on Ceres strengthens a theory that icy little worldlets, such as wandering comets, may have traveled into the Main Asteroid Belt from their remote, frozen domain in the outer limits of our Solar System. This migration of frozen comets into the warmer and well-lit Main Asteroid Belt would have occurred when our 4.56 year old Solar System was still in the process of forming.
The Dawn spacecraft has obtained increasingly high-resolution images of Ceres ever since its arrival there on December 1, 2014. It was also the first spacecraft to visit Vesta. As astronomers receive better and better views of little Ceres, courtesy of Dawn, they anticipate gaining an improved understanding of this little world’s origin and evolution.
Instead of molten rock, ice volcanoes erupt a substance termed cryomagma–or ice-volcanic melt. These substances are usually liquids that can create plumes, but they can also be in a vapor form. Following eruption, it is thought that cryomagma condenses into a solid form when exposed to frigid ambient temperatures. Cryovolcanoes can potentially exist on icy moons, and other bodies, that possess a large amount of water and dwell in our Solar System’s outer regions, far from our Sun, where temperatures are frigid. A number of possible ice volcanoes have been detected on Pluto and Titan. Furthermore, even though they are not known to create volcanoes, ice geysers have been detected on Enceladus, a moon of the planet Saturn, and on Triton of Neptune.
Tidal friction is a possible energy source for some Solar System bodies. Tidal friction can both melt ices and produce cryovolcanoes. It has also been proposed by some planetary scientists that translucent deposits of frozen materials could cause a subsurface greenhouse effect that could potentially accumulate the necessary amount of heat.
Signs of ancient warming of the Kuiper Belt Object (KBO) Quaoar have also sung a sirens’ song to scientists, luring them to speculate that it exhibited cryvolcanism in the past. Radioactive decay could provide the necessary energy for such activity. This is because cryovolcanoes can erupt a mixture of water and ammonia, which would melt at the frigid temperature of -95 degrees Celsius. This melting could result in a cold liquid that would gush out of the volcano.
On November 27, 2005 NASA’s Cassini spacecraft photographed geysers on the south pole of Enceladus. Furthermore, there are also indirect signs of cryovolcanic activity on a handful of other icy moons dwelling in our Solar System, including Europa, Titan, Ganymede (Jupiter), and Miranda (Uranus). Cassini, during its long and successful visit to the Saturn-system has also spotted several features that could possibly be ice volcanoes on Titan, especially Doom Mons with adjacent Sotra Patera–a feature that is often considered to be the best evidence observed so far for volcanic topography anywhere on an icy moon of our Solar System. Cryovolcanism is one of the possible sources for the large amount of methane found in Titan’s smoggy, orange, hydrocarbon-laden atmosphere.
Observations conducted in 2007 at the Gemini Observatory, unveiling areas of ammonia hydrates and water crystals on Pluto’s large moon Charon, also hinted at the presence of active ice volcanoes or cryogeysers. Later observations by NASA’s New Horizons spacecraft–on its historic visit to the Pluto-system and the remote Kuiper Belt–discovered that Charon had a young surface. This observation strengthed the idea that Pluto itself has a duo of features that have been identified as potential cryovolcanoes, because they are mountains displaying indented peaks. In 2015, two distinct bright patches observed within a crater on the surface of Ceres were imaged by the Dawn spacecraft. This observation led scientists to speculate about a possible ice volcano origin.
It was in September 2016, that scientists at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, and NASA Goddard in Greenbelt, Maryland, released their findings of the large Ahuna Dome on Ceres. It was noted that Ahuna Dome is a “volcanic dome unlike any seen elsewhere in the Solar System. [The large] mountain is likely volcanic in nature. Specifically, it would be a cryovolcano–a volcano that erupts a liquid made of volatiles such as water, instead of silicates,… the only known example of a cryovolcano that potentially formed from a salty mud mix, and that formed in the geologically recent past.”
Ahuna Mons is a prominent feature on Ceres. It rises to about half the height of Mount Everest on Earth. Ahuna Mons also shows few craters, indicating that it is a youthful structure–a mere two hundred million years of age, at most–a blink of the eye on geologic time scales. Heavily cratered surfaces indicate an old surface, while very few craters indicate youth.
However, the lonely, solitary existence of this volcanic ice mountain has puzzled planetary scientists ever since they first discovered it.
Ice Volcanoes Gone Missing
“Imagine if there was just one volcano on all of Earth. That would be puzzling,” Dr. Sori commented in the February 2, 2017 AGU Press Release.
Adding to the mystery of this lonely volcanic ice mountain are its well-defined features and steep size, which are normally signs of youth on geologic time scales, Dr. Sori continued to explain. This observation suggests two distinct possibilities: Ahuna Mons is precisely what it appears to be, a mysteriously solitary surface feature that formed relatively recently on an otherwise inactive small world. Or, alternatively, this strange cryovolcano is neither as lonely or unusual as it appears to be at first glance, and there is really some process on Ceres that has destroyed its ancestors–leaving the young Ahuna Mons as the solitary ice volcano left to tell the story of the history of its kind on the dwarf planet.
Because Ceres has no atmosphere, processes that wear down volcanoes on Earth–such as rain, wind, and ice–cannot occur on this little world. Dr. Sori and his team propose that a different process, termed viscous relaxation, is the true culprit behind the tragically solitary existence of the lonely ice mountain, Ahuna Mons.
Vicous relaxation is a term used to describe the idea that almost any solid will flow, provided it is given sufficient time. This has been likened to the way that a cold block of honey appears to be solid, but if it is given a long enough period of time, the block will ultimately flatten out until there is no sign left of the now-vanished block structure.
On our own planet, viscous relaxation is the process that makes the glaciers flow, Dr. Sori explained in the February 2, 2017 AGU Press Release. However, this process does not influence volcanoes on our own planet because they are constructed of rock. However, Ceres’ volcanoes contain ice, and this ice is what makes viscous relaxation possible. On Ceres, viscous relaxation could cause older ice volcanoes to flatten out–just like the block of cold, solid honey. As millions of years pass, this process would make the former ice mountains difficult to discern. In addition, because Ceres resides in the inner Solar System, relatively close to our Sun’s fires, the process itself could become more pronounced.
In order to test the idea that viscous relaxation was the culprit behind the missing ice volcanoes, causing them to flatten out on Ceres, Dr. Sori and his team created a model using the actual dimensions of Ahuna Mons to predict how fast the ice mountain might be in the process of flowing. The scientists ran the model assuming varying water contents of the material that composes the mountain–ranging from 100% water ice to 40% water ice.
Dr. Sori and his colleagues found that Ahuna Mons would have to be composed of more than 40% water ice to be influenced by the process of viscous relaxation. At this particular composition, Dr. Sori estimates that Ahuna Mons should be flattening out at a rate of 30 to 160 feet per million years. That is enough time to cause the missing cryovolcanoes to vanish in hundreds of millions to billions of years. This indicates that there could have been other cryovolcanoes on Ceres, according to this study.
“Ahuna Mons is at most 200 million years old. It just hasn’t had time to deform,” Dr. Sori commented in the February 2, 2017 AGU Press Release.
In the future, Dr. Sori and his colleagues plan on trying to identify the flattened relics that may still linger of older cryovolcanoes on the surface of Ceres. The findings could help planetary scientists gain a better understanding of the still-elusive history of how the dwarf planet was born.
This new study helps planetary scientists increase their knowledge of what might occur on planetary denizens of our own Solar System, noted Dr. Kelsi Singer in the February 2, 2017 AGU Press Release. Dr. Singer is a postdoctoral researcher who studies icy worlds at the Southwest Reseearch Institute in Boulder, Colorado, and was not involved in this study.
Dr. Singer continued to comment that “It would be fun to check some of the other features that are potentially older domes on Ceres to see if they fit in with the theory of how the shapes should viscously evolve over time. Because all of the putative cryovolcanic features on other worlds are different, I think this helps to expand our inventory of what is possible.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.