The Phoebe ring, Saturn's largest ring
The Phoebe ring is an enormous torus of dark dust circling Saturn between about 6 and 16 million km (4 and 10 million miles) from the planet, far beyond the visible rings. Discovered in 2009 with the Spitzer Space Telescope, it is the largest known ring in the solar system. Its dust, knocked off the moon Phoebe, orbits backward and eventually slams into Iapetus, darkening one whole face of that moon.
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A discovery in infrared
Saturn's main rings shine because they reflect sunlight. The Phoebe ring is made of dark dust spread so thin that it is invisible in visible light: even Cassini, orbiting Saturn, had not spotted it. It does give off a faint thermal glow, because its grains sit at about -190 °C, slightly warmer than the background sky.
That glow is what the MIPS instrument on the Spitzer Space Telescope detected at 24 micrometers. Anne Verbiscer (University of Virginia), Michael Skrutskie and Douglas Hamilton (University of Maryland) announced the discovery in Nature in October 2009, calling it Saturn's largest ring. According to NASA, it would take about a billion Earths to fill its volume.
A ring like no other
| Property | Main rings | Phoebe ring |
|---|---|---|
| Distance from Saturn | about 67,000 to 140,000 km | about 6 to 16 million km |
| Thickness | about 10 m to 1 km | millions of km |
| Plane | Saturn's equator | close to Saturn's orbital plane, tilted about 27° |
| Material | bright water ice | dark dust |
| Direction of orbit | prograde | retrograde, like Phoebe |
The Phoebe ring is not a thin disk but a vast, thick doughnut. Spitzer's first measurements placed it between about 128 and 207 Saturn radii, or 7.7 to 12.5 million km. In 2015 a study based on data from the WISE telescope, published in Nature by Douglas Hamilton and colleagues, showed it actually extends from about 100 to 270 Saturn radii, or 6 to 16 million km, and is dominated by very small particles.
Its tilt follows from its distance. Millions of kilometers out, Saturn's equatorial bulge barely affects orbits; the Sun's pull dominates, so the grains settle around the plane of Saturn's orbit around the Sun. The planet's equator is tilted nearly 27° to that plane (see Saturn's seasons).
Phoebe, the source
The moon Phoebe, about 213 km across, orbits some 13 million km from Saturn, in the heart of the ring. Discovered in 1899 by William Pickering, it was the first moon found photographically. Its orbit is retrograde, opposite to Saturn's rotation, a strong hint that it is a captured body, like the other irregular moons.
Phoebe is heavily cratered. The micrometeoroids constantly striking it blast out dust that escapes its weak gravity and spreads along its orbit. Other irregular moons may add to the ring, but Phoebe, by far the largest, is thought to be its main source.
The key to the Iapetus mystery
The Phoebe ring helped solve a puzzle more than three centuries old. In 1671 Giovanni Domenico Cassini, who had just discovered Iapetus, noticed it was visible only on one side of Saturn. He correctly concluded that the moon had one bright face and one dark face. The dark region, covering Iapetus's leading hemisphere, is now named Cassini Regio.
The proposed sequence runs like this:
- Radiation pressure and the drag caused by absorbing and re-emitting sunlight (the Poynting-Robertson effect) make the ring's grains slowly spiral in toward Saturn.
- Iapetus, orbiting prograde about 3.56 million km out, lies in their path. Because the grains travel in the opposite direction, they hit its leading hemisphere head-on, like bugs on a windshield.
- The dark dust absorbs more sunlight and warms the surface. Ice there sublimates and recondenses on colder, brighter areas. This runaway thermal process, described by John Spencer and Tilmann Denk in 2010, sharpens the contrast and explains why the boundary between the two faces is so abrupt.
The mechanism accounts for where the dark material lies, but not every detail: the color of Iapetus's dark side differs slightly from Phoebe's, and the exact chemistry of the deposit is still studied. Some dust also reaches Hyperion, whose hollows are lined with dark material.
A ring in a class of its own
The Phoebe ring shows how different "rings" can be. The main rings are dense disks of ice shaped by shepherd moons and resonances. The E ring is a cloud of fine ice fed by Enceladus. The Phoebe ring is a tenuous debris cloud governed by the Sun as much as by Saturn. For the full lineup, see ring names.
FAQ
How big is the Phoebe ring?
It extends from about 6 to 16 million km from Saturn and is millions of kilometers thick. Its volume could hold about a billion Earths.
Can you see the Phoebe ring?
No. It is too dark and too sparse to see in visible light, even from a spacecraft. It is detected only through its faint thermal infrared glow.
Who discovered the Phoebe ring?
Anne Verbiscer, Michael Skrutskie and Douglas Hamilton, using the Spitzer Space Telescope. The discovery was published in Nature in October 2009.
How is the Phoebe ring linked to Iapetus?
Dust from the ring spirals toward Saturn and strikes the leading hemisphere of Iapetus. It largely explains that moon's dark face, first noticed by Cassini in 1671.
Sources
- A. J. Verbiscer, M. F. Skrutskie and D. P. Hamilton, "Saturn's largest ring," Nature, 2009
- D. P. Hamilton et al., "Small particles dominate Saturn's Phoebe ring to surprisingly large distances," Nature, 2015
- J. R. Spencer and T. Denk, "Formation of Iapetus' extreme albedo dichotomy by exogenically triggered thermal ice migration," Science, 2010
- NASA Science, Saturn