New findings are running rings around planetary theories of old age, particularly in the Saturn system.
The planet journal Icarus had a special section on Saturn’s rings recently. About the same time, Cassini sent new images from Titan, Saturn’s largest satellite, leading observers to inquire, “Watch a dune?”
Titan’s dunes and other features emerge in new images (PhysOrg). Sand dunes were so unexpected to exist on Titan, Cassini scientists wondered what those “cat scratches” on the surface could be when the first images emerged through the haze in 2004. Radar mapping has since revealed quite a bit about the massive dunes that encircle the equatorial regions (e.g., 4/09/11). The “beautiful, undulating patterns” betray dynamic, transitory features driven by prevailing winds. The dunes wrap around mountains like the Xanada region, believed to be one of the oldest terrains on Titan. Recent radar swaths show a “Xanadu annex” adjacent to it. “Xanadu—and now its annex—remains something of a mystery,” the article says. “Elsewhere on Titan, mountainous terrain appears in small, isolated patches, but Xanadu covers a large area, and scientists have proposed a variety of theories about its formation.” Visit the PhysOrg article to watch a flyover animation of the area. There will only be four more radar mapping encounters of Titan before Cassini’s end of mission next year.
Flooded canyons (Astrobiology Magazine): In August, Cassini radar revealed the presence of steep-sided canyons alongside Titan’s polar lakes that appear to be flooded with liquid hydrocarbons. Scientists compared these with features on the Earth, such as at Lake Powell. Although the liquids, solids and temperatures are different, similar processes appear to be at work. How old they are, though, is a matter of interpretation:
The presence of such deep cuts in the landscape indicates that whatever process created them was active for a long time or eroded down much faster than other areas on Titan’s surface. The researchers’ proposed scenarios include uplift of the terrain and changes in sea level, or perhaps both.
“It’s likely that a combination of these forces contributed to the formation of the deep canyons, but at present it’s not clear to what degree each was involved. What is clear is that any description of Titan’s geological evolution needs to be able to explain how the canyons got there,” said Valerio Poggiali of the University of Rome, a Cassini radar team associate and lead author of the study.
Watch a stunning animation of the historic January 14, 2005 descent of the Huygens Probe to Titan posted by NASA/JPL on YouTube. The narrated video, based on actual photos, takes you a billion miles closer from the first image all the way to the surface in just 3.5 minutes.
A-Ring news (Icarus): The A-ring, outermost of the main rings, has very sharp edges and some distinct gaps. The ring specialists believe the smallest particles are 1 mm in diameter, even though particles grind each other down through collisions: “interparticle collisions caused by satellite perturbations in the region result in more shedding of regolith or fragmentation of particles in the outermost parts of the A ring.”
B-Ring news (Icarus): Thickest of the main rings, the B-ring was thought to have enough material to be old – maybe a billion years old, which would still fall short of the assumed age of Saturn (see 2/04/16). This open-access paper by Hedman and Nicholson now suggests that there’s less material than meets the eye. The authors know this is important; “The large uncertainties in the B-ring’s mass and its typical surface mass density not only hamper efforts to understand the structure and dynamics of this ring, but also complicate efforts to ascertain the age and history of Saturn’s ring system,” they say by way of introduction. Their analysis of spiral density waves constrains the mass of the ring, they argue, to less than what optical depth would indicate. “This suggests that the total mass of the B ring is most likely between one-third and two-thirds the mass of Saturn’s moon Mimas,” the first main moon outside the rings – a small icy body barely big enough to be spherical. Since a high mass estimate for the B ring based on optical depth was the main justification for assuming the rings were old, a quote from the paper merits attention:
Indeed, as shown in Fig. 21, it appears that rings with a given mass density can have optical depths that vary by almost an order of magnitude. Optical depth therefore cannot be regarded as a reliable proxy for the ring’s mass density in any part of Saturn’s rings.
[italics in original.I
These findings also have implications for the total mass of Saturn’s rings. Recent work has considered the possibility that the B-ring’s high opacity might require extremely high surface mass densities (Charnoz, Crida, Castillo-Rogez, et al., 2011, Hedman, Nicholson, Cuzzi, et al., 2013 and Robbins, Stewart, Lewis, et al., 2010), but these new measurements suggest that this may not be justified. Instead, the total mass of the B ring could be quite low, which may be consistent with estimates based on the charged-particle populations near the rings (Cooper et al., 1985) and the high porosity of the ring particles inferred from thermal infrared data (Reffet, et al., 2015).
Lower mass implies younger age. Why? The rings are brighter than expected. Previous estimates of higher mass allowed scientists to “hide” some of the ring darkening from meteoritic contamination in fluffy material that allowed turnover of bright material to the outside of particles. Lowering the mass significantly reduces the ability of ring particles to hide contaminants for long. The authors admit they could not analyze the whole B-ring with all its complexities, but “there is not much space to hide a large amount of mass.” Next year (2017), it may be possible to get better estimates on the B-ring’s total mass when Cassini gets close-in views of the rings from vertical orbits intersecting the D-ring before the spacecraft plunges into Saturn.
Update 9/16/16: New Scientist admits the origin of the rings “is elusive” but to “put a spin on it,” offers a suggestion that a passing rock broke up to become the rings. Existing theories fail to explain why Saturn’s rings are icy, while other planets’ rings are rocky. A SETI Institute guy thinks the idea makes some progress, but says “a question of timing remains.”
But the clean water ice of Saturn’s ring system suggests that it may be much younger, since interplanetary dust should pollute it over time.
“Even if you can get it in the first place, how does it survive for 4 billion years and still look pristine?” [Matthew] Tiscareno asks.
Update 9/16/16: Some minor planets have rings, too. Science Daily offers the suggestion that dust from the surface of Centaurs (a class of minor planets between Jupiter and Neptune) gets levitated off of them by gravitational pulls from gas giants. This sounds like a delicate situation; could it last for billions of years?
Dance of Janus and Epimetheus (Icarus): Every four years, the two co-orbital moons Janus and Epimetheus, which orbit just outside Saturn’s rings, swap orbits in a unique gravitational pas de deux. This paper describes how the transfer sends a detectable wave into the A-ring and B-ring. The solitary wave only occurs when Janus moves inwards. The distinctive signature of this wave allows scientists to make estimates of ring density.
Dusty rings (Icarus): Two outer rings of Saturn, the G-ring and the E-ring (formed by Enceladus), consist of much finer particles than the main rings. Scientists analyzing data from the Radio and Plasma Wave Spectrometer (RPWS) confirmed measurements of particle diameters made with other instruments, the Cosmic Dust Analyzer (CDA) and Imaging Subsystem (ISS). “Based on RPWS observation,” they say, “the region around the G ring is a very thin layer of dust particles with no observable vertical offset of the peak density from the ring plane.” Finer particles, it should be noted, are more subject to rapid erosion and disappearance unless there is frequent replenishment.
New constraints on Enceladus tides (Icarus): To keep the geysers going on tiny Enceladus, scientists have looked to gravitational tides to provide heat and pumping action. Tides can provide a lot of energy; Space.com says that alignments with the moon and sun can trigger large earthquakes on our home planet. New estimates by Mikael Beuthe further limit the motion of the Enceladus crust, however. His model “strongly reduces oceanic dissipation in Enceladus.” His abstract explains the bad news:
Could tidal dissipation within Enceladus’ subsurface ocean account for the observed heat flow? Earthlike models of dynamical tides give no definitive answer because they neglect the influence of the crust. I propose here the first model of dissipative tides in a subsurface ocean, by combining the Laplace Tidal Equations with the membrane approach. For the first time, it is possible to compute tidal dissipation rates within the crust, ocean, and mantle in one go. I show that oceanic dissipation is strongly reduced by the crustal constraint, and thus contributes little to Enceladus’ present heat budget. Tidal resonances could have played a role in a forming or freezing ocean less than 100 m deep.
As if adding insult to injury, he adds, “The model is general: it applies to all icy satellites with a thin crust and a shallow ocean.”
It’s hard to decide which is more fun: the thrill of discovering new things about planets, or the humor of watching secular materialists groaning in their struggle to keep their billions of years.