Planetary Exploration

Moons of Saturn May Be Younger Than the Dinosaurs

MOUNTAIN VIEW – New research suggests that some of Saturn’s icy moons, as well as its famous rings, might be modern adornments.  Their dramatic birth may have taken place a mere  hundred million years ago, more recent than the reign of many dinosaurs.

“Moons are always changing their orbits.  That’s inevitable,” says Matija Cuk, principal investigator at the SETI Institute.  “But that fact allows us to use computer simulations to tease out the history of Saturn’s inner moons.  Doing so, we find that they were most likely born during the most recent two percent of the planet’s history.”

While Saturn’s rings have been known since the 1600s, there’s still debate about their age.  The straightforward assumption is that they are primordial – as old as the planet itself, which is more than four billion years.  However, in 2012, French astronomers found that tidal effects – the gravitational interaction of the inner moons with fluids deep in Saturn’s interior – are causing them to spiral to larger orbital radii comparatively quickly.  The implication, given their present positions, is that these moons, and presumably the rings, are recent phenomena.

The new paper finds that Saturn's moon Rhea and all other moons and rings closer to Saturn may be only 100 million years old. Outer satellites (not pictured here), including Saturn's largest moon Titan, are probably as old as the planet itself. Download full size image credit:NASA/JPL


Cuk, together with Luke Dones and David Nesvorny of the Southwest Research Institute, used computer modeling to infer the past dynamic behavior of Saturn’s icy inner moons.  While our own moon has its orbit around Earth to itself, Saturn’s many satellites have to share space with each other.  All of their orbits slowly grow due to tidal effects, but at different rates.  This results in pairs of moons occasionally entering so-called orbital resonances.  These occur when one moon’s orbital period is a simple fraction (for example, one-half or two-thirds) of another moon’s period.  In these special configurations, even small moons with weak gravity can strongly affect each other’s orbits, making them more elongated and tilting them out of their original orbital plane.

By comparing present orbital tilts and those predicted by computer simulations, the researchers could learn how much the orbits of Saturn’s moons grew.  It turns out that for some of the most important satellites – Tethys, Dione and Rhea – the orbits are less dramatically altered than previously thought.  The relatively small orbital tilts indicate that they haven’t crossed many orbital resonances, meaning that they must have formed not far from where they are now.

Saturn's moon Tethys, with its giant canyon Ithaca Chasma. We propose that Ithaca Chasma was opened by strong tidal forces millions of years ago when Tethys was in an orbital resonance with the neighboring moon Dione. Download full size image credit:NASA


 

But how long ago was their birth?  Cuk and his team used results from NASA’s Cassini mission to help answer this question.  The Cassini spacecraft has observed ice geysers on Saturn’s moon Enceladus.  Assuming that the energy powering these geysers comes directly from tidal interactions, and that Enceladus’ level of geothermal activity is more or less constant, then the tides within Saturn are quite strong.  According to the team’s analysis, these would move the satellite by the small amount indicated by the simulations in only about 100 million years. This would date the formation of the major moons of Saturn, with the exception of more distant Titan and Iapetus, to the relatively recent Cretaceous Period, the era of the dinosaurs.

“So the question arises, what caused the recent birth of the inner moons?” asks Cuk.  “Our best guess is that Saturn had a similar collection of moons before, but their orbits were disturbed by a special kind of orbital resonance involving Saturn’s motion around the Sun.  Eventually, the orbits of neighboring moons crossed, and these objects collided.  From this rubble, the present set of moons and rings formed.”

If this result is correct, Saturn’s bright rings may be younger than the heyday of the dinosaurs, and we are fortunate to witness them today.

The research is being published in the Astrophysical Journal.

Read the preprint at http://arxiv.org/abs/1603.07071

 

Final print at http://iopscience.iop.org/article/10.3847/0004-637X/820/2/97 (requires subscription to view)

Pluto Reveals More Secrets

MOUNTAIN VIEW – Eight months after its encounter with the New Horizons spacecraft, Pluto continues to surprise the scientists who study this distant world.  

Pluto and its largest moon Charon comprise what is essentially a binary planet: two bodies of comparable size orbiting a common center of mass.  The four smaller moons – Styx, Nix, Kerberos and Hydra – are thought to have formed at the same time as Charon in the aftermath of a large impact. 

Most inner moons in the solar system keep one face pointed toward their central planet; this animation shows that certainly isn’t the case with the small moons of Pluto, which behave like spinning tops. Pluto is shown at center with, in order, from smaller to wider orbit: Charon, Styx, Nix, Kerberos, Hydra.


“Consequently, we expected the small moons to resemble Charon,” says Mark Showalter, Senior Scientist at the SETI Institute. “Instead, we find that their surfaces are much brighter and much older than that of their large sibling.” 

In addition, scientists are puzzled by the moons’ unusual rotation characteristics, as they orbit rapidly around axes that are tilted sideways.

“The gravitational tides from Pluto should have slowed down and re-oriented the spins of these moons by now, but they haven’t,” noted Showalter.  The New Horizons team speculates that collisions from outside objects might cause the strange rotation of these moons. 

New Horizons has revealed surprisingly complex geology on the surfaces of Pluto and Charon. Pluto sports a bright 1,000 km-wide plain, informally called Sputnik Planum, that is filled with volatile nitrogen and carbon monoxide ices.  This feature shows no craters, which attests to ongoing geological activity. Beyond the smooth plains, there are ancient cratered terrains that display impacts that Pluto has endured over millions and billions of years. The rims and ridges are frosted with ice.

Sputnik Planum, This high-resolution image captured by NASA's New Horizons spacecraft combines blue, red and infrared images taken by the Ralph/Multispectral Visual Imaging Camera (MVIC). The bright expanse is the western lobe of the "heart," informally called Sputnik Planum, which has been found to be rich in nitrogen, carbon monoxide and methane ices. Download full size image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute


“Among the more puzzling features are the two tall mounds with central depressions that look like volcanoes,” says Ross Beyer, Senior Scientist at the SETI Institute. “Wright Mons is about 2 miles tall and 90 miles wide, and Piccard Mons is even larger, about 3.5 miles tall and 140 miles across. They could be ice volcanoes, but this will take more analysis to establish.”

New Horizons observations have also revolutionized our understanding of Pluto’s atmosphere.

“We were very surprised to observe that Pluto’s atmosphere has a multi-layered bluish haze that extends from the surface to altitudes above 200 km. The upper atmosphere is much colder than expected, which greatly reduces the rate at which it escapes from Pluto. We’ve also measured the surface pressure for the first time – about 11 microbars – and we’ve discovered that the temperature structure of the lower atmosphere varies with location,” says David Hinson, Senior Scientist at the SETI Institute.

Pluto’s partner – Charon – offers its own surprises. Its water-rich icy surface is distinctly red around its northern region; a coloration due to tholin, a molecule produced by ultraviolet irradiation of simpler compounds like methane and ethane. There are patches of ammonia ice evident on Charon as well.

“Unlike Pluto, Charon’s surface is quite old and uniform. It’s covered with water ice with low amounts of ammonia mixed in, and is not as geologically active as Pluto,” says SETI Institute scientist Cristina Dalle Ore

charonCharon in Enhanced Color NASA's New Horizons captured this high-resolution enhanced color view of Charon just before closest approach on July 14, 2015. The image combines blue, red and infrared images taken by the spacecraft’s Ralph/Multispectral Visual Imaging Camera (MVIC); the colors are processed to best highlight the variation of surface properties across Charon. Charon’s color palette is not as diverse as Pluto’s; most striking is the reddish north (top) polar region, informally named Mordor Macula. Charon is 754 miles (1,214 kilometers) across; this image resolves details as small as 1.8 miles (2.9 kilometers). Dowload full size image Credits: NASA/JHUAPL/SwRI


A March 18 special issue of Science (http://science.sciencemag.org/content/351/6279?et_rid=49300793&et_cid=34...features articles on the continuing analysis of Pluto and its satellites.  With roughly half of the New Horizons data returned to Earth so far, scientists anticipate more discoveries and a deeper understanding of this dwarf planet system.

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