Author: 一毫秒的永恒

An image taken by NASA's Cassini spacecraft shows Saturn's northern hemisphere approaching the summer solstice in 2016. A year on Saturn lasts 29 Earth years; according to the latest analysis of Cassini data, a day lasts only 10:33:38.

  Using new data from NASA’s Cassini spacecraft, researchers believe they have solved a long-standing mystery in Solar System science: the length of a day on Saturn. The answer is 10 hours, 33 minutes, and 38 seconds. Planetary scientists spent decades trying to obtain that figure because the rotating gas giant has no fixed surface features to track, while its unusual magnetic field conceals its rotation rate.

  The answer, it turns out, was hidden in Saturn’s rings.

  While Cassini orbited Saturn, its instruments examined the icy, rocky rings in unprecedented detail. Christopher Mankovich, then a graduate student in astronomy and astrophysics at the University of California, Santa Cruz, used these data to study patterns of waves within the rings. His work established that the rings respond to Saturn’s own oscillations, much as a seismometer measures the strength of vibrations during an earthquake. Oscillations inside Saturn produce extremely small, periodic changes in the distribution of its mass, which in turn make the surrounding gravitational field fluctuate. The rings also respond to Saturn’s magnetic field. Mankovich explained that particles throughout the rings feel these oscillations in the gravitational field. At particular locations in the rings, the oscillations affect the particles at just the right time, gradually adding energy to their orbits. That energy is then carried away as observable waves.

  Mankovich’s study, published in The Astrophysical Journal on January 17, 2019, describes how he developed models of Saturn’s internal structure that matched the waves in the rings. This allowed him to trace motion inside the planet and ultimately determine its rotation period. The analysis produced a rotation period of 10 hours, 33 minutes, and 38 seconds, several minutes shorter than the estimate obtained from radio signals by the Voyager spacecraft in 1981.

  Voyager’s estimate was 10 hours, 39 minutes, and 23 seconds, a value based on magnetic-field data. Cassini also used magnetic data, but early estimates ranged from 10 hours and 36 minutes to 10 hours and 48 minutes.

  Scientists usually rely on a planet’s magnetic field to measure its rotation rate. Like Earth’s magnetic axis, Jupiter’s is not aligned with its rotation axis. As the planet spins, the magnetic-field axis therefore wobbles, allowing scientists to measure a periodic radio signal and derive the rotation rate. Saturn is different. Its unusual magnetic-field axis is almost perfectly aligned with its rotation axis, so that method does not work. Researchers therefore turned to the waves in Saturn’s rings to probe its interior and reveal this long-sought fundamental property. Scientists who study Saturn were delighted to obtain their best answer yet to this basic question about the planet. The idea of using Saturn’s rings to study the planet’s seismology had first been proposed in 1982, well before sufficiently precise observations became possible.

  Co-author Mark Marley, now at NASA’s Ames Research Center, developed the idea in detail in his 1990 doctoral dissertation. Alongside the calculations, he predicted where the relevant features would appear in Saturn’s rings. Marley also noted that the planned Cassini mission could make the observations needed to test his idea. Twenty years later, during Cassini’s final years, scientists analyzed mission data and found ring features exactly where Marley had predicted them.

  Cassini’s mission ended in September 2017. With the spacecraft running low on fuel, the mission team deliberately sent it into Saturn’s atmosphere to prevent a collision with one of the planet’s moons. The mission was a collaboration among NASA, the European Space Agency (ESA), and the Italian Space Agency. NASA’s Jet Propulsion Laboratory (JPL) managed it for the agency’s Science Mission Directorate in Washington. The Cassini orbiter was designed, developed, and assembled by JPL, while its radar instrument was developed jointly by JPL, the Italian Space Agency, and team members from the United States and several European countries.