Article Review: “Cryovolcanism on Ceres” by Ruesch et al. (2016)
Paul Christian Yang-ed
It was 9 years ago in September 27 when NASA’s Dawn spaceprobe set forth to Vesta and to Ceres, two of the largest asteroids in the asteroid belt lying between Mars and Jupiter. Since Dawn started its close-up observations of Ceres, bright spots and a lonely elevated mound were among the features that captivated the science community. Remote spectroscopy subsequently revealed that the bright spots are made up of salt sulfates and carbonates that meteorite impacts exposed from beneath Ceres’ surface. But the lonely mound along the Southern hemisphere of Ceres (called Ahuna Mons, Fig. 1) remained baffling. There was no evidence of plate tectonics nor was the mound a hill at a crater’s center that formed from a meterorite’s impact on a surface. Ahuna Mons was one of its kind in Ceres. What could this pimple-like feature on the crater pockmarked face of Ceres be?
As part of its 9th launch anniversary, Science magazine released six recent articles detailing the findings of the Dawn team on Ceres. One of these six articles whose first author is Ottaviano Ruesch suggested that lonely Ahuna Mons is a “dome” due to cryovolcanism — albeit inactive or extinct.
Unlike typical volcanoes on Earth and Jupiter’s moon Io which spew hot molten silica, cryovolcanoes spew out ice, water vapor, and liquid water that is relatively warmer than the ice surrounding the volcanoes’ vents. A paper released in Nature January this year has earlier revealed that Ceres has at least a differentiated interior (based on Dawn’s measurements of the asteroid’s gravity field and shape) with a “shell” made up of ice and salt volatiles. Its soil was determined to be made up of a mix of carbonates and a group of water and ammonia-bearing clay minerals called “ammoniated phyllosilicates”. Even before Dawn was sent to the asteroid belt, there were already theoretical models which suggest that rock-and-ice bodies with a temperature regime like Ceres might still host volcanic activity involving salt solutions (brines) as the melt medium compared to silicate lavas on Earth. Compared to pure water, brines have lower freezing temperature and therefore can modify the surface of Ceres even when under below-zero degrees celsius temperatures on Ceres.
Ruesch and co-authors explored the various ways the mound of Ahuna Mons formed. Based on the the way how the various soil and rock units they identified surrounding and on Ahuna Mons itself are stacked on top of each other, they concluded that Ahuna Mons formed locally on site instead of being delivered by comets or other asteroids as a pile (Fig. 2), since no layer or unit repeated stacking over other units. The layering was relatively organized instead of the young and old units being mixed. They also discounted Ahuna Mons as a compressive tectonic feature (which usually forms large regional mountain belts instead of isolated mounds like Ahuna Mons). Nor were any gullies found around the mound that would indicate that Ahuna Mons was an erosional feature carved by water. Based on their mathematical models, the authors claim that Ceres’ crust is too thick for a blob or diapir to intrude the subsurface and warp the surface above, hence Ahuna Mons could not have been a deformation feature made by some molten blob of ice and brine from below. There is also no nearby planet that can flex and provide internal heat to Ceres which can produce the melt that formed Ahuna Mons, and Ceres is too small to generate an internal thermal gradient needed to heat itself from within, just like larger bodies such as Earth.
Instead, they suggest that Ahuna Mons formed like how viscous lava domes form on Earth during volcanic eruptions. During the eruption, some of the very viscous salt and ice melts choke the cracks where the melts pass through, and then they solidify. The next eruption of salt melts form over the solidified older melts. The process repeats itself over and over until the solidified melts become taller than when the cryovolcano used to be. As the layers of the dome solidified, it cooled, expanded, and broke up into pieces, forming jagged rocks of ice and salt which slid unto the sides of the dome to form the shiny salty lineations on the slopes of Ahuna Mons. Brightness (“albedo”) data already indicate that the slopes of Ahuna Mons (Fig. 3) are made up of of materials that reflect light nicely, and salt minerals, which have already been identified and exposed from below the surface by impact cratering elsewhere in Ceres, are good light reflectors. Unlike silicate melts, chloride salt melts in Ceres can exist and flow at relatively low temperatures on Earth but which is already considered “warm” enough in Ceres’ subsurface. Nevertheless, the authors note that microscopic data of the samples from Ahuna Mons would be still needed to characterize the behavior of the materials that form the mound of Ahuna. There is also a lack of data regarding how chloride salt melts flow and respond to deformation which makes it difficult to definitely characterize the possible parent melt that formed the dome. A detailed study to know the exact nature of Ceres’ interior and internal heat mechanisms would be also needed and very helpful.
To support their claim further, they calculated that the proportion of the mound’s height to the width of its base, called the “aspect ratio”, and compared the same to the aspect ratio of a known volcanic dome on the moon and a volcanic dome formed by viscous dacitic lava at the Chaiten volcano in Chile. The aspect ratio of the latter two features was 0.2 whereas the aspect ratio of Ahuna Mons was calculated to be 0.24, which is quite close to the value. Overall, even when the materials involved in the formation of domes differ between the moon, Earth, and Ahuna Mons in Ceres, as long as the key physical properties of these materials are similar, such as viscosity, these materials would tend to form comparably similar geomorphologic features, despite being separated distantly from each other in space.
By counting the craters on each soil unit around and on Ahuna Mons and observing what layers were cut by craters, the authors were able to date the relatively oldest geologic unit to the youngest to create a stratigraphic column of layers (Fig. 2), The number of craters also constrain Ahuna Mons’ time of formation, assuming that Ceres and the Moon were both already present at the same time during the period of heavy bombardment billions of years ago. From their count, the dome is relatively young, at least 210 Million years old, which means the dome formed around when the earliest dinosaurs here on Earth still walked around during the mid-Triassic period.
Ruesch et al., 2016)
The study successfully demonstrates how “different chemistry but same physics” worked to create Ahuna Mons as a comparatively similar feature to volcanic domes on Earth and the moon. Organic dunes and lakes of liquid methane exist in Titan and are analogous with sand dunes and water lakes on Earth. Nitrogen glaciers and ice mountains exist in Pluto, analogous with water ice glaciers and silicate mountains on Earth. With this study, another geomorphologic analogy has been added to our list: a salt-ice dome on Ceres analogous to a volcanic dome on Earth.
(Courtesy of Ruesch et al., 2016)
