Humanity exists at a very special time in our solar system's history — the era of Saturn's rings.
In the next 100 million years, Saturn's rings will completely disappear, and, as revealed by a new Science study, planetary scientists have realized that it acquired its rings only very recently.
During the Cassini mission's final months at Saturn, the NASA spacecraft carried out a series of daring orbits through the space between the planet's cloud tops and innermost edge of its rings. The so-called "Grand Finale" signaled that the end was nigh for the probe and, on Sept. 15, 2017, it burned up in the gas giant's atmosphere, bringing a spectacular 13 years of science in Saturn's orbit to a poignant close.
The spacecraft was low on fuel and, to avoid an accidental crash into one of Saturn's potentially habitable moons, such as Enceladus or Titan, NASA had long ago decided that the best way to dispose of the mission was to burn it up in Saturn's upper atmosphere. The agency wanted to avoid Earthly contamination on these pristine alien environments.
Before its fiery death, however, Cassini took unprecedented measurements of the mysterious ring gap region to reveal some surprising ring dynamics. Though mission scientists expected to detect some wispy elemental gases in this "empty" region, Cassini's particle instrumentation found a smorgasbord of elements and molecules "raining" from the rings down to the planet's atmosphere. They estimate that around 10 tons (9,072 kilograms) of material is falling onto Saturn from the rings per second, as Meghan Bartels for Space.com writes.
That means that Saturn's rings will eventually disappear, as a study from late 2018 concluded.
"[T]he rings have less than 100 million years to live. This is relatively short, compared to Saturn's age of over 4 billion years," James O'Donoghue, of NASA's Goddard Space Flight Center, said in a December statement.
Now, in the Science study published today (Jan. 17), researchers have used Cassini's ring dives to estimate when Saturn acquired its famous rings.
When Cassini zipped through Saturn's ring plane, mission managers allowed the planet, its rings and moons to gravitationally tug at the speeding spacecraft. The extremely slight tugs resulted in tiny changes in the probe's trajectory, which could be precisely measured. Those changes allowed scientists to, for the first time, make a very good measurement of how much mass is holed up in Saturn's rings.
After analyzing the final set of orbits in September 2017, however, the extent to which Cassini's trajectory was deflected initially didn't make sense. It didn't match the predictions by theoretical models. It turned out that Cassini's motion was being additionally altered by massive flows of material inside Saturn's thick atmosphere at the equator about 6,000 miles (9,656 kilometers) deep. These massive flows are moving about 4 percent slower than the upper atmospheric clouds that are visible, causing a gravitational anomaly that wasn't predicted.
"The discovery of deeply rotating layers is a surprising revelation about the internal structure of the planet," said Cassini project scientist Linda Spilker of NASA's Jet Propulsion Laboratory in a statement. "The question is what causes the more rapidly rotating part of the atmosphere to go so deep and what does that tell us about Saturn's interior."
With this puzzle solved, scientists were free to measure the gravitational influence of Saturn's rings. The researchers estimate that the total mass of Saturn's rings is approximately 40 percent that of Saturn's moon Mimas. Considering Mimas is 2,000 times smaller than Earth's moon, there certainly isn't a lot of material in Saturn's rings.
Scientists had previously relied on density waves, or ripples, through the rings caused by the motion of the 62 moons in Saturn orbit to estimate ring mass. Although these estimates were also low, astronomers have always assumed that there was some hidden mass in large blocks of material that went unseen. "We always suspected there was some hidden mass that we could not see in the waves," added planetary scientist Burkhard Militzer, of the University of California, Berkeley, in a statement.
Now, with the precision measurements made by Cassini's final orbits, we know that there is no hidden mass. The lower the mass, the younger the rings are, and because they are predominantly made of ice, if they were older, the ring material would have become contaminated by interplanetary debris, dulling them. Saturn's rings, as we're acutely aware, are beautifully bright. The new age estimate is within the range of the earlier density wave estimates, allowing the researchers to make a more accurate stab at their true age.
Previous estimates of ring age have been far ranging, from 4.5 billion years (the leftovers of when Saturn itself was forming) to a few tens of millions of years. But with this new finding in hand, it looks like the rings are very young — they formed less than 100 million years ago, perhaps as recently as 10 million years ago.
Where did they come from? Well, it's possible that an icy object from the Kuiper Belt, or an errant comet, became entwined in Saturn's gravitational field and succumbed to the planet's powerful tides, ripping them apart, eventually grinding down their material to create the banded rings we know and love today.