Australasian Science - May 2016

New research suggests that the birth of Saturn’s icy moons, as well as its
famous rings, may have taken place a mere 100 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. We find that they were most likely born during the most
recent 2% 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 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 a recent phenomena.
Cuk’s team used computer modelling to infer the past dynamic
behaviour 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 close “orbital resonance”  means that 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 determine by how much the orbits of

Saturn’s moons grew.

The relatively small orbital tilts found indicate that they haven’t

crossed many orbital resonances, so they must have formed not far from

where they are now. But how long ago was their birth?

NASA’s Cassini mission helped to answer this question when it saw ice

geysers on Saturn’s moon Enceladus. Assuming that the energy powering

these geysers comes directly from tidal interactions, then the tides

within Saturn are quite strong. Analysis suggests 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 neighbouring 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.

Recently released images from the Atacama Large Millimetre/

submillimetre Array (ALMA) have revealed never-before-seen

details of what astronomers believe may be a “baby Earth” or an

even more massive “super-Earth” forming.

The images show a planet-forming disc around TWHydrae,

aSun-like star close toEarth. The planet is about 10 million years

old, making it a veritable newborn, loating in a ring of planet-

forming material.Combinedwith its close proximity to Earth,

TWHydrae has become a popular study subject for researchers.

The developing planet is about 175 light years away and has a

face-on orientation that provides a rare, undistorted view of the

complete disc. Mostproiles give a more oblique and indistinct

outline.

“Previous studies with optical and radio telescopes conirm

that this star hosts a prominent disc with features that strongly

suggest planets are beginning tocoalesce,” said leadresearcherSean

Andrews.“The new ALMA images show the disc in unprece-

dented detail, revealing a series of concentric dusty bright rings

and dark gaps, intriguing features that suggest a planet with an

Earth-like orbit is formingthere.”

Pronounced gaps near the star are 3 billion and 6 billion km from

the central stars, which are similar distances from theSun to

Uranus and Pluto.These are also likely the result of particles that

came together and formed planets before sweeping their orbits

clear of dust and gas before corralling the remaining material into

bands.

Scientists believethat studying young star systems like TW

Hydrae can help them learn about our own solar system’s past. It

provides a snapshot of when things got started around here about

5 billion years ago. “TW Hydrae is quite special. It is the nearest

known proto-planetary disc to Earth and it may closely resemble

the solar system when it was only 10 million years old,” co-author

David Wilner said.

By studying TW Hydrae, astronomers hope to get a better

understanding of Earth’s evolution and prospects for similar

systems throughout the galaxy. Astronomers are now trying to

ind out how common these features are in discs around other

young stars and how they may change in the future.

MAY 2016|| 45

David Reneke is an astronomy lecturer and teacher, a featurewriter for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio.

Subscribe toDavid’s freeAstro-Spacenewsletter at http://www.davidreneke.com

OUT OF THIS WORLD David Reneke

A “Baby Earth” in the Making

Saturn’s Moons May Be Younger than the Dinosaurs

This ALMA image shows the planet-forming disc TW Hydrae. The

image inset zooms in on the gap to the nearest to the star, which

is the same distance as the Earth is from the Sun.

Credit: S. Andrews (Harvard-Smithsonian CfA)