Active Moons Challenge Old-Age Beliefs
Beyond the asteroid belt, where sunlight is weak, small bodies that should be old and cold seem young and active. Here are some recent papers and reports about some of the moons around the gas giants showing more signs of vitality than expected for bodies assumed to be billions of years old.
Jupiter’s Io
Astronomers at the Keck Observatory in Hawaii detected a big outburst of the Tvashtar volcano on Io. Reported in Icarus,1a, the eruption lasted over a month from April to June 2006. The temperature extended over 60 square kilometers and emitted 7.7 trillion watts. The authors noted the activity at Tvashtar seen by the New Horizons spacecraft in February this year (10/15/2007), but could not say if it was a continuation of the eruption they observed.
Another paper in Icarus found plumes near Pele.1b Using the Hubble Space Telescope, a team detected sulfur and sulfur dioxide plumes in February 2003, March 2003 and January 2004. These plumes showed dramatic variability over a matter of days and weeks. Additional plumes were detected at other locations.
Another paper in Icarus recalculated the heat output of Io’s volcanoes downward.2 This somewhat mitigates the problem of explaining the anomalously high temperatures measured, but still shows that, within error, Io’s heat output is right at the edge of what can be explained by theory – 1340K measured, 1300K theoretical. Even so, getting the hot lava out in a 20-30% liquid crust while keeping the mountains up presents other theoretical problems. The authors said that it is still possible the lavas are ultramafic (see “Io, Io, It’s Off to Work I Go,” 05/04/2004). Whether or not the heat output can fit tidal flexing models, “Io has a staggering rate of volcanism,” they said. “…if the observed heat loss were all provided by mafic lavas, ~500 km3 would have to be erupted each year (Blaney et al., 1995) compared to ~4 km3 per year for the Earth.”
Note this admission: “if there were no efficient means of recycling the crust into the mantle, we would actually see only relatively low temperature silicate volcanism on Io.” Models of Io’s interior, therefore, include the recycling assumption: “The fact that eruption temperatures in excess of 1100 °C are observed requires that there be an efficient recycling mechanism and that there is limited differentiation between the crust and mantle.” This requirement comes from the assumption that Io is 4.5 billion years old.
Jupiter’s Europa
A paper in Icarus about Europa this month,3 though focused primarily on its magnetic interactions with Jupiter, repeated the well-known evidence of the moon’s youthfulness:
Galileo measurements of Europa’s gravitational field and modeling show Europa to be a differentiated satellite consisting of a metallic core, a silicate mantle and a water ice-liquid outer shell. The minimum water ice-liquid outer shell thickness is about 80 km for plausible mantle densities (Anderson et al., 1998). High resolution data obtained with the Solid State Imaging (SSI) system show evidence of a young and thin, cracked and ruptured ice shell (e.g., [Belton et al., 1996] and [Carr et al., 1998]). The geological observations imply that warm, convecting material existed at shallow depths within the subsurface at the time of its recent geological deformation.
A dazzling montage of Europa, reprocessed from Galileo images, was posted on Astronomy Picture of the Day for December 2.
Saturn’s Titan
A range of mountains on Saturn’s largest moon Titan was discovered by radar imaging in 2005. This month in Icarus,4 Cassini scientists analyzed this feature which stands out from the rest of the moon’s mostly gentle terrain. As with the Appalachians on earth, they said “erosion seems to act fast enough (or mountain-building slow enough) to preclude the formation of abundant features taller than a few hundred meters.” They estimated the age of the range at 20 to 100 million years old maximum – which would be less than 1/40 the assumed age of the moon. “This is short for planetary geological timescales, and further corroborates the conclusion from the dearth of impact craters [refs] that the surface overall is less than a billion years old.” These figures should be understood as upper limits.
Saturn’s Enceladus
The geyser moon of Saturn got another write-up in Icarus this month.5 This time, however, the focus was not on the plumes at the south pole, but on the wrinkles and ridges near the equator. The ridges appear to be up to 400m high and 3-4 km apart. This area may lack the tiger stripes and eruptions of the south polar terrain, but has been active recently as well: there are “extensive sets of parallel, north-south trending ridges and troughs occur within the planitiae themselves, and evidence for resurfacing and extensional tectonics abounds.”
Models of fracturing caused by unstable extension (stretching) of the crust are 2 to 3 times too low to account for the observed ridges. Perhaps some other process amplified the stretching. Because Jupiter’s moon Ganymede is more massive, the grooves on that moon are even harder to explain, they said.
The tremendous range of ages of surface features on Enceladus led the scientists to begin their paper with an obvious but astonishing statement: “Saturn’s moon Enceladus is one of the great enigmas of the outer Solar System.”
Neptune’s Triton
One would think the farther out you go from the sun, the colder it gets, and, therefore, the more quiescent. That is surely not the case at Neptune, which has some of the strongest winds, and for its moon Triton, another aging body sporting a young physique. “Triton’s geological complexity ranks with Europa and Titan,” said another paper in Icarus this month that claimed the moon Triton has a “negligible surface age.”6
All the impact craters on Triton cluster on the leading hemisphere, suggesting a pummeling from objects in orbit around Neptune. None of the craters appear to have come from outside the system. The authors put an upper limit of 50 million years on the heavily cratered leading hemisphere, and 6 million years on the Neptune-facing “cantaloupe terrain” observed by Voyager 2 in 1989. This would be negligible indeed – about 1/10 of 1% of the assumed age of the solar system.
The authors considered whether the young-looking terrain was resurfaced because of the cratering or other factors in the past, and ruled them out: “no matter what, Triton has to have actively resurfaced on a time scale short compared to its age,” they said. Obviously, “From the perspective of Triton’s thermal history, there is little distinction between a world that remains active for 4.0 billion years and a world that remains active for 4.5 billion years.” The resurfacing rate on Triton has probably been high for a long time, they judged. In fact, “Our results push Triton’s surface to even younger, perhaps negligible, ages. If we accept the hypothesis that most of Triton’s large craters are not heliocentric [i.e., not caused by interlopers from all angles], then the surface may be significantly younger than Europa’s. The consensus is that Triton’s surface is very young, and therefore is probably geologically active today.”
Summary
Sources of heating that might keep small bodies active include (1) radioactive heating in the core, (2) tidal flexing, (3) impacts. It seems unlikely any of these could work non-stop for 4.5 billion years on moons as small as Enceladus; indeed, a paper in the September issue of Icarus said that resonance and tidal heating is insufficient to account for the observed flux; “Therefore, the source of endogenic activity of Enceladus remains unexplained.”7 And if the activity began just recently, why at a time when humans are present to observe it?
1a. Laver, de Pater and Marchis, “Tvashtar awakening detected in April 2006 with OSIRIS at the W.M. Keck Observatory,” Icarus, Volume 191, Issue 2, 15 November 2007, Pages 749-754.
1b. Jessup, Spencer and Yelle, “Sulfur volcanism on Io,” Icarus, Volume 192, Issue 1, 1 December 2007, Pages 24-40.
2. Keszthelyi et al, “New estimates for Io eruption temperatures: Implications for the interior,” Icarus, Volume 192, Issue 2, 15 December 2007, Pages 491-502.
3. Schilling, Neubauer, Saur, “Time-varying interaction of Europa with the jovian magnetosphere: Constraints on the conductivity of Europa’s subsurface ocean,” Icarus, Volume 192, Issue 1, 1 December 2007, Pages 41-55.
4. Radebaugh, Lorenz, Kirk, Lunine, Stofan, Lopes, Wall and the Cassini Radar team, “Mountains on Titan observed by Cassini Radar,” Icarus, Volume 192, Issue 1, 1 December 2007, Pages 77-91.
5. Bland, Beyer and Showman, “Unstable Extension of Enceladus’ Lithosphere,” Icarus, Volume 192, Issue 1, 1 December 2007, Pages 92-105.
6. Schenk and Zahnle, “On the negligible surface age of Triton,” Icarus, Volume 192, Issue 1, 1 December 2007, Pages 135-149.
7. Rappaport, Iess, Tortora, Anabtawi, Asmar, Somenzi and Zingoni, “Mass and interior of Enceladus from Cassini data analysis,” Icarus, Volume 190, Issue 1, September 2007, Pages 175-178.
As if this weren’t enough, there is recent evidence that Pluto, Charon and some of the Kuiper Belt objects and minor planets at the farthest reaches of the solar system also show activity on their surfaces.
CEH is one of very few websites reporting the latest papers from leading scientific journals with a critical analysis of their evolutionary implications. TV documentaries and textbooks rarely mention these very serious problems with standard theories of the solar system. You have just seen that the best planetary scientists in the world, constrained within their chosen billions-of-years mindset, have many questions and few answers. We hope you find this liberating.