What Saturn’s moons reveal about the creation of the Solar System – 07/07/2023 – Science

Ever since mankind began looking up at the sky, our Moon has stared at us from its orbit at a relatively short distance from our planet. It is the most visible of our Solar System’s natural satellites, but it is not the only one.

Calculating how many there are, however, is a constant challenge.

In May of this year, astronomers announced that they had discovered 62 new moons orbiting Saturn, one of the gas giants in the Solar System.

With that, the number of confirmed moons orbiting this distant planet — which is about 1.3 billion kilometers from the Sun — has increased to 145.

The discoveries also consecrated Saturn as the planet with the largest number of moons in its orbit, displacing its giant neighbor Jupiter, in what was called by some the “lunar race”.

And the number of Saturn’s moons continued to grow with another discovery added to the list by the same team just a few weeks later.

The new moons were located by a team led by Edward Ashton, a postdoctoral researcher at the Institute of Astronomy and Astrophysics at Academia Sinica in Taiwan. The discovery, made through a telescope on top of Mauna Kea, a volcano in Hawaii, took more than two years.

But astronomers have been watching Saturn and its satellites for more than three and a half centuries. Humanity has even sent four spacecraft to Saturn, and yet these moons have gone unnoticed.

How do so many of Saturn’s moons remain hidden? What makes distant moons so hard to find? And how many more moons might be out there in the blackness of space?

hard to detect

At last count, there were no fewer than 290 “traditional” moons in our Solar System. However, simply observing a moon does not mean it is officially a moon.

Some of the new satellites have been observed before, but there is a long process before the International Astronomical Union officially calls one of them a moon. There are several years of constant observations.

During centuries of human civilization, many of our celestial neighbors were too far away for us to discern.

In 1655, Dutch astronomer Christiaan Huygens discovered the largest of Saturn’s moons, Titan, larger than the planet Mercury. It wasn’t until 16 years later that Jean-Dominique Cassini found Iapetus and then Rhea, Dione and finally Tethys in 1684. And it wasn’t until 1789 that the German astronomer William Herschel identified Mimas and the icy moon Enceladus.

Other moons of Saturn escaped human observation much longer. Hyperion, which is potato-shaped, was not discovered until 1848, followed some 50 years later by Phoebe, which orbits Saturn in the opposite direction to most other moons.

But it wasn’t until the arrival of the space age and modern telescopes that Saturn’s list of moons began to grow considerably. Spacecraft like the Voyager and Cassini probes have been able to add more discoveries by looking closely at Saturn’s complex system.

Still, the vast majority of Saturn’s moons were discovered relatively recently, starting in the year 2000.

One reason for this is that satellites discovered in the early days of astronomy followed certain patterns: They were relatively large and traveled in predictable orbits, which astronomers call regular orbits.

“If you look at Jupiter’s four bright moons, the Galilean satellites, they’re called regular moons, and all the giant planets have regular moons,” says Brett Gladman, a Canadian astronomer at the University of British Columbia and one of Ashton’s colleagues. who participated in the recent discoveries around Saturn.

“And their moons are orbiting in the planet’s equatorial plane, as are the rings around the planet. They are believed to have formed in orbit, a flat disk of gas and dust that formed around the giant planets… the same way our planets formed in orbit around the Sun.”

The common sense, according to Gladman, was that if moons formed from nearby planets, they would be very close together and orbit around their equatorial planes — much like planetary rings do.

But it turns out that some moons do not follow these rules. Planets also have irregular satellites whose orbits do not follow a predictable trajectory around the equatorial plane of the host planet.

Their orbits are more elliptical and inclined, moving further away from the planet and often in a different direction than the planet’s course around the Sun. Many are also much smaller in size.

For decades, moon sleuths have had to use photographic plates from here on Earth to try to find evidence of the Solar System’s moons. The smaller they were and the more irregular their orbit, the harder they were to observe.

But in the 1990s and 2000s, digital photography suddenly changed the way scientists like Gladman could locate them.

Digital camera CCD sensors were much more sensitive to light than photographic plates or emulsions, meaning that much fainter objects could be detected.

But a new problem arose. CCD sensors were small, so the field of view they could capture was very limited.

“Giant planets are very big, the region around them where you could orbit the planets — as opposed to orbiting the Sun — is very large,” says Gladman.

“In 1997, I discovered the first two of these moons near Uranus using a camera. It was difficult, but it worked.”

He adds that the planet’s relative distance from Earth meant a relatively limited search field of view.

Then came another breakthrough: mosaic CCD cameras, which group several CCD sensors into a network.

“It gives you a much wider field of view,” explains Gladman.

“When that happened, there was an explosion [de descobertas] in the late 1990s, early 2000s.”

In 2000, Gladman himself proved the effectiveness of the new technique.

“There were 12 that I discovered in 2000 with a pair of telescopes,” he says.

“Large-format multi-CCD mosaic cameras became available in large-aperture telescopes. And that way, you could capture enough sky that finding them wasn’t like fishing in the dark.”

Detecting moons is painstaking work.

“In the past, we’d capture one image, and maybe an hour later another image, and an hour later another image of the same displacement,” says Gladman.

These three images would help show whether an object – perhaps a moon – was traveling in a defined direction.

“In the past, when CCD cameras weren’t very big, I did everything by eye. But now the databases are huge, this is not feasible to do. We have computer software that now takes the framing, finds all the objects, eliminate everything that doesn’t move and then look for what moves.”

The remaining moons are small and reflect only a small amount of light, forcing scientists to use new approaches.

The discovery in May, for example, involved a technique called shift stackwhich is similar to the multiple exposure mode on a camera.

And astronomers believe that the search for moons is a field worth pursuing. The recent discoveries — of tenuous chunks of rock that barely reflect light — offer some enticing clues about the solar system’s past.

Mike Alexandersen, a Minor Planet Center (MPC) postdoctoral researcher who was also involved in the discovery of Saturn’s last moons, says the discoveries will guide our understanding of what formed these moons in the first place.

“It is believed that the reason they are clustered together and have similar orbits is that there used to be an object that had a collision. And then, over billions of years, the fragments continued to collide.”

Gladman calls this a “collisional cascade”: a series of collisions that lead to smaller and smaller moons.

He and his colleagues recently suggested that a relatively recent collision event, within the last few hundred million years, may have created some of Saturn’s smallest irregular moons.

Alexandersen has conducted extensive research on the Kuiper Belt: a vast collection of icy debris 20 times larger than our Solar System’s asteroid belt.

He says mapping some 4,000 objects in the Kuiper Belt has offered some theories about how planets form — and why so many small moons are scattered across the Solar System.

An ancient cataclysm may have sent these tiny satellites spinning in the dark, to a point where the gravitational pull of the gas giants (Jupiter and Saturn) was greater than that of the now distant Sun—although Alexandersen notes that the Sun continues to exert influence even on these. great distances.

The moons these astronomical sleuths are looking for are at the edge of what current technology can capture — satellites measuring at least a kilometer in diameter.

Artificial intelligence can offer an additional leap forward.

“We can use techniques of machine learning of artificial intelligence to provide the databases to a computer and tell it to find the moons”, says Gladman.

“We’re still working on it…it’s a challenging thing to do. But in the last few years, people are starting to make real progress.”

In any case, the discoveries show no signs of stopping. A few weeks after the announcement of the 62 new discoveries, scientists got another surprise: there was one more moon to add to the list.

“Another moon has been announced that was not included in the press release because we were unable to adjust the orbit correctly,” Alexandersen tells the BBC.

“But we worked it out. So it’s not 62, it’s 63.”

This brings Saturn’s total number of moons to 146.

Read the full story (in English) on the BBC Future website.

This article was originally published here