TOEFL Reading練習 #036 — An Old Planet's Suspiciously Young Rings(日本語解説)

An Old Planet's Suspiciously Young Rings

¶1 Saturn's rings look like a relic, a wide disk of ice that should have circled the planet since the solar system came together roughly four and a half billion years ago. A single measurement taken during a NASA spacecraft's final months in 2017 challenged that picture. It suggested the rings could be as young as ten to a hundred million years old, possibly younger than the asteroid impact that ended the age of the dinosaurs sixty-six million years ago. Other scientists pushed back almost immediately: the same evidence used to call the rings young, they argued, could just as easily describe rings that are ancient but quietly disguising their age. Neither side has yet produced a measurement that ends the disagreement.

¶2 Weighing a ring from hundreds of millions of kilometers away is harder than it sounds. Telescopes can record how much sunlight Saturn's rings reflect, but brightness depends on what the ring is made of as much as on how much material it holds, so it cannot fix a mass on its own. A more direct method uses a spacecraft's own radio signal. When a spacecraft passes close enough for the rings' gravity to tug on it, that tiny pull shifts the signal's frequency by a measurable amount, a technique called gravity science. During the final months of its mission in 2017, the Cassini spacecraft dived repeatedly through the narrow gap between Saturn's cloud tops and its innermost ring. On several of these passes, a radio link with Earth captured that gravitational tug precisely. A team led by Luciano Iess used the data to calculate a total ring mass equal to only about forty percent of the mass of Mimas, one of Saturn's smaller icy moons.

¶3 Two different histories could produce a ring this light and this bright. One holds that Saturn's rings are simply young, formed within roughly the last hundred million years, perhaps out of the shattered remains of an icy moon or a captured comet. With so little time behind them, the rings would not yet have gathered much mass or picked up much of the dark, non-icy material that micrometeoroids, dust-sized particles drifting through the solar system, slowly mix into ring ice. A second history holds that the rings are primordial, as old as Saturn itself, roughly four and a half billion years, but that some ongoing process keeps stripping away the polluted material almost as fast as micrometeoroids deposit it, leaving an ancient ring looking freshly made. The 2017 mass measurement, on its own, cannot tell the two apart: a light, bright ring is exactly what either history predicts.

¶4 Two further findings have shaped the debate without resolving it. [A] Researchers led by James O'Donoghue showed that Saturn's magnetic field channels charged ice particles from the rings into the upper atmosphere, a process called ring rain, at roughly four hundred to twenty-eight hundred kilograms per second. [B] At that pace, the rings would drain away within about three hundred million years, a finding that fit neatly with the idea of a ring system unlikely to last especially long. [C] But the astronomer Aurélien Crida and his colleagues modeled self-cleaning and found that a steady loss of dark, silicate-rich grains, unlike water ice, could keep a primordial ring looking as light and bright as the one Cassini measured. [D] Their model did not prove the rings are ancient, but it showed that the 2017 measurement no longer points clearly toward either age.

¶5 The disagreement reaches beyond one planet's decorations. Astronomers use ring systems as natural laboratories for the physics that shapes moons and planets more generally. An assumed age changes almost every conclusion drawn from a ring, including how quickly icy particles clump together and how such systems eventually break apart. If Saturn's rings, the best studied in the solar system, resist a confident age, the fainter rings recently detected around smaller worlds will be harder still to date. No mission can drill into a ring and count layers the way geologists read rock. Decades after scientists first tried to estimate the rings' age from their brightness alone, astronomers still cannot say with confidence which story is right. Saturn's rings may be relics of the early solar system, a recent arrival, or something that has aged in a way neither label quite captures.