Small Planetary Bodies Unexpectedly Active
You would think things would have cooled down after 4.5 billion years. That’s not what planetary scientists are observing.
Ceres Cold Volcanoes
Several news outlets reported that the dwarf planet Ceres, largest body in the asteroid belt, has ice volcanoes. Phys.org says that the famous Ahuna Mons cryovolcano, announced in 2015 from Dawn Spacecraft observations, is probably not alone. If there’s one, there’s probably been more.
Adding to the puzzle are the steep sides and well-defined features of Ahuna Mons – usually signs of geologic youth, [Michael] Sori [U of Arizona] said. That leads to two possibilities: Ahuna Mons is just as it appears, inexplicably alone after forming relatively recently on an otherwise inactive world. Or, the cryovolcano is not alone or unusual, and there is some process on Ceres that has destroyed its predecessors and left the young Ahuna Mons as the solitary cryovolcano on the dwarf planet, according to Sori.
Earlier volcanoes may have flattened out by viscous relaxation over millions of years, the article claims. But then why would there be any activity at all today? “Ahuna Mons is at most 200 million years old, “Sori claims. “It just hasn’t had time to deform.” He guesses that age by assuming models of water-ice content and other model parameters, and fitting its lifetime into what he assumes for the age of the solar system (A.S.S.) in Darwin Years. It should be noted, though, that 200 million years represents only 1/23rd of the A.S.S. Was activity occurring steadily for the other 22/23rds of the dwarf planet’s existence?
Adding to the puzzle is the brightness of those intriguing white spots in Occator Crator. Another news item on Phys.org says they are only 4 million Darwin Years old, 30 million Darwin Years younger than the crater itself. What happened? The article presents a story, but it’s difficult to explain a one-off type of feature. See Astrobiology Magazine for pretty pictures and more storytelling.
Space. com puts a happy face on the quandary facing old-age planetary scientists:
“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,” Singer said in the same statement. “Because all of the putative cryovolcanic features on other worlds are different, I think this helps to expand our inventory of what is possible.“
Yes; explaining features that your philosophy did not predict can be loads of fun.
The Way the Comet Crumbles
Is Comet 67P breaking up? That’s what scientists are guessing after the ESA crashed their Rosetta orbiter into the comet. It’s not surprising, of course, that comets crumble and dissipate as they round the sun. Several comets have been seen to break up (most famously Comet Shoemaker-Levy 9 whose fragments crashed into Jupiter in 1994). Rosetta scientists are watching cliffs collapse, bright spots appear and jets erupt from beneath the surface of Comet 67P, as Monica Grady describes with pictures on The Conversation.
Another comet broke in half last month. Space.com tells about how Comet 73P/Schwassmann-Wachmann was observed to split apart as astronomers watched from a telescope in Chile. “”It certainly feels like it’s only a matter of time before comet 73P is destroyed, disintegrating into a trail of cosmic dust,” said one astronomer, aware that the solar wind and solar radiation spell doom for all comets eventually. The question is why there are any comets left after 4.6 billion years.
A Black Eye for Enceladus?
Astronomers are calling on their favorite theory rescue device again – asteroid impacts – to try to explain one of the most amazing discoveries by the Cassini spacecraft: the geysers of Saturn’s little moon Enceladus. The impact theory has arisen of necessity, Leah Crane reports on New Scientist, because of the puzzling features of this moon. Why is all the activity at the south pole? The astronomers’ previous explanation, tidal flexing, would tend to make the activity spread all over the body. So sound the bugles: in comes an impactor!
Enceladus’ south pole is wounded, bleeding heat and water. Its injury may have come from a huge rock smashing into this frigid moon of Saturn less than 100 million years ago, leaving the area riddled with leaky cracks.
The region near Enceladus’ south pole marks one of the solar system’s most intriguing mysteries. It spews plumes of liquid from an interior ocean, plus an enormous amount of heat. The south pole’s heat emission is about 10 gigawatts higher than expected – equivalent to the power of 4000 wind turbines running at full capacity. The rest of the moon, though, is cold and relatively homogeneous.
“We don’t have a really good explanation for why all this activity is so concentrated,” says John Spencer at the Southwest Research Institute in Colorado.
Every solution breeds new problems, though. Where did this impactor come from? And why did it hit 100 million Darwin Years ago, long after the solar system had theoretically quieted down? 100 million years is just 1/45th of the A.S.S.
10 gigawatts is enough to make Doc cry, “Great Scott!” That’s a lot of heat for a moon the diameter of Arizona. In Icarus, five planetary scientists try to model “heat transport in the high-pressure ice mantle of large icy moons,” particular bigger ones like Ganymede and Titan. They assume that melt extraction is instantaneous, but end up with long ages: “Overall, we predict long periods of time during these moons’ history where water generated in contact with the rock core is transported to the above ocean,” they say, because they have to. Models must fit the A.S.S.
Speaking of Titan, Astrobiology Magazine says that dissolved nitrogen may make the giant moon’s methane lakes fizzy. Up comes the bubbling crude to the delight of the Titan hillbillies, which certain astrobiologists expect might exist (see Space.com).
Note: The Cassini mission ends on September 15. For a timetable of Grand Finale events, including daring dives between the rings, see the JPL Cassini page.
The Pluto Papers
We won’t dwell on Pluto’s possible comeback as a planet, since that’s simply a philosophical question of taxonomy. Of interest in this article is, why is Pluto still active? The planetary journal Icarus featured a series of papers about Pluto last month. For those wishing to do further research on signs of youth vs age, here’s a list of the papers, starting with one led by the project scientist of the New Horizons mission:
- MacKinnon et al., “Origin of the Pluto–Charon system: Constraints from the New Horizons flyby,” Icarus. Guess the latest origin theory. Impact!
- Howard et al., “Present and Past Glaciation on Pluto,” Icarus. First highlight: “Nitrogen glaciers are presently flowing on Pluto.” Question: why, after 4.5 billion years?
- Umurhan et al., “Modeling Glacial Flow on and onto Pluto’s Sputnik Planitia,” Icarus. Undoubtedly they do their best to keep flows going for billions of Darwin Years.
- Schmitt et al., “Physical state and distribution of materials at the surface of Pluto,” Icarus. As they watch material sublimate away, they’re seeking “to better understand the geophysical processes in action at the surface of this astonishingly active frozen world.“
- Moore et al., “Sublimation as a landform-shaping process on Pluto,” Icarus. One way out of the time problem: ” In our models, the temporal evolution of pitted surfaces is such that initially lots of time passes with little happening, then eventually, very rapid development of relief and rapid sublimation.”
- Robbins et al., “Craters of the Pluto-Charon System,” Icarus. This team conducts initial mapping of craters before interpreting them. The lack of craters was a big puzzle for the scientists, indicating that much of Pluto’s surface, especially Sputnik Planitia, must be very young.
- Protopapa et al., “Pluto’s global surface composition through pixel-by-pixel Hapke modeling of New Horizons Ralph/LEISA data,” Icarus. They consider Sputnik Planitia as a possible cold trap of volatiles.
- Desch and Neveu, “Differentiation and cryovolcanism on Charon: A view before and after New Horizons,” Icarus. This overconfident duo claims, “In this article we compare the predictions against the new observations, and find that they largely support the expected history of the Pluto system and the evolution of Charon.” Why, then, the surprised looks on the team’s faces? Their ‘predictions’ rely on radiogenic heat, not observable, only accessible to models.
- Beyer et al., “Charon Tectonics,” Icarus (open access). “New Horizons images of Pluto’s companion Charon show a variety of terrains that display extensional tectonic features, with relief surprising for this relatively small world.” With some strain, they find a way to fit the observations to the A.S.S.
Commenting on Beyer’s paper, Jesse Emspak at Space.com says that “Pluto’s moon Charon had its own icy plate tectonics” – a surprising claim for such a small body. As you ponder the heat sources, try saying ‘proto-Pluto’ five times real fast.
Beyer noted that current models depict Charon forming as a result of a proto-Pluto colliding with something big and effectively splitting into two unequal pieces, one of which would become Charon. The initial collision would have generated some heat, and radioactive elements in Charon’s core would have generated some more, he said. Initially the surface, made mostly of water with some ammonia, would freeze, but the layer between the rocky core and crust of ice would still be liquid, he added.
Doctors will do whatever it takes to keep their patient’s A.S.S. intact.
Planetary scientists could have such an easier time by relaxing their anal-retentive A.S.S. and enjoying youth.