Saturn's chaotic moon may be a lot younger than we thought

Saturn is a busy planet. The gas giant is known for its mysterious rings made up of comets, asteroids and broken rocky objects, and has the largest number of moons of any other planet in the Solar System, beating Jupiter by three more moons.

Saturn's many moons are believed to have formed around the same time as the Solar System, at least 4 billion years ago. However, a new model suggests that the orbiting moons may be a lot younger than previously thought, with some forming only a couple of million years ago.

A study, published Wednesday in the Journal of Geophysical Research Planets, has major implications for our understanding of Saturn's moon Titan and its chaotic, liquid-filled terrain.

Samuel Bell, a research scientist at the Planetary Science Institute, and lead author of the new study, looked at the rate of impact on the different moons that orbit Saturn in order to develop a new chronology for the natural satellites.

"One of the things that’s really exciting about this kind of science is that there’s a lot we don’t know — a lot of unanswered questions," Bell tells Inverse.

How old are moons? One of the main ways that scientists find out the age of an object, like an orbiting moon, is by counting the number of craters on its surface. Earth's Moon, for example, has a number of impact craters from asteroids, meteorites and other bodies that crashed into the Moon's surface over billions of years.

The more impact craters a moon's surface has, the older the moon.

However, it's not always that simple.

When looking at the impact craters on Saturn's moons, previous models had assumed that the impacts were a result of comets orbiting around the Sun. But there is an increased amount of evidence that the craters were formed from objects orbiting Saturn itself, smaller moonlets that are too small to detect with current telescopes.

The recent study looked at the relative crater density of the oldest terrains on each moon, and compared them to each other, based on how craters could form.

"The relative pattern on the moon really didn’t make that much sense..."

"In doing that, I discovered that the relative pattern on the moon really didn’t make that much sense if the impact was coming from comets orbiting the Sun," Bell says.

If it were true that Sun-orbiting comets caused the impact craters on Saturn's moon, then there would've been more craters on the moons that are closer to Saturn since the comets would've been drawn to the planet's gravitational pull. Instead, there aren't that many impact craters on Mimas, the closest moon to Saturn, while there are many more craters on Iapetus, the furthest moon away from Saturn.

A younger Titan — The new chronology for Saturn's moons suggests that Titan is a much younger moon than previously believed, shaving off a few million years of its life.

"The surface of Titan is younger than its neighboring moons, it could in fact be way younger," Bell says. "It may be around 15 million years, which is a very young, active, kind of environment."

Titan is the largest moon orbiting Saturn, is the only other place besides Earth known to have rivers, lakes, and seas — although they are made of flammable greenhouse gases. The moon's crust is made of water ice, and it has liquid oceans of hydrocarbon.

It is also the only moon known to have a dense atmosphere, slightly thicker than Earth's atmosphere.

The assumption that the impact craters formed on Saturn's moons came from objects orbiting the Sun would suggest that Titan is over 4 billion years old, formed around the same time as the Solar System, and affected by the early chaos that surrounds a young star. However, Titan shows signs of being active.

"There’s a lot of ongoing weather," Bell says. "One of the big ways to test it is getting a big picture view of how old the surface of Titan is."

Titan has been of special interest to astronomers, who are drawn to its unusual characteristics. NASA's Cassini spacecraft, which launched in 1997, got a closer look at Titan as it flew past the large moon toward the end of its 20-year mission.

And yet, scientists have not been able to figure out how Titan has managed to remain active until today.

"The overall date of Titan's surface really matters when answering big questions," Bell says. "Having a young date, Titan could be really active and really robust."

Abstract: The chronology of the moons of Saturn, especially Titan, has been limited by a lack of strong constraints on the cratering rate, low number statistics for small‐N counts of large‐diameter craters, and uncertainty about whether impactors are mostly heliocentric impactors orbiting the Sun or planetocentric impactors orbiting Saturn itself. Here, I propose to address these three problems. Instead of looking for an absolute cratering rate, I focus on the relative crater densities, calculating scaling relationships between the moons. I update crater analysis methodology by numerically modeling probability density functions of the uncertainty of crater density, enabling me to accurately assess the error of even single‐crater observations. Using these updated statistics, I show how the heliocentric cratering model leads to a dramatic increase in relative crater density for Mimas, Tethys, Dione, Rhea, and Iapetus with distance from Saturn. Under this model, the surface age of Titan is probably older than the cratered plains of Mimas—implying a very low erosion rate and minimal endogenic resurfacing on Titan. I explore possible explanations, concluding that the likeliest explanation is planetocentric cratering, although saturation effects cannot be ruled out. Under the planetocentric model, the relative crater densities of the cratered plains of Mimas, Tethys, Dione, Rhea, and Iapetus are all very close, with the relative crater density at Titan between 1 and 2 orders of magnitude lower. Under planetocentric cratering, the cratering rate on Titan allows for vigorous erosion and endogenic resurfacing.