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This week saw the release of an ethereal image of Neptune, the most distant planet from the sun, taken by the James Webb Space Telescope. The image captured seven moons and the planet’s thin rings, which are very difficult to photograph from Earth.
In the mid-1980s, Earth-based images hinted at their existence, but the rings weren’t fully captured until 1989 when they were photographed by the Voyager 2 spacecraft during the one and only close encounter with the planet.
Neptune appears quite different through Webb’s eyes than in the Voyager images — like a glowing crystal ball with the ghostly rings wrapped around it. That’s because the telescope sees in near infrared, which is reflected by clouds high in the planet’s atmosphere.
Voyager, which photographed the planet in visible light, saw a beautiful blue ball streaked with white clouds and a dark storm the size of the entire Earth.
I was fortunate to be at the NASA Jet Propulsion Laboratory in August 1989 when Voyager flew past Neptune. Early observations from telescopes on Earth had suggested that Neptune’s rings were incomplete arcs, rather than true rings reaching all the way around the planet.
During occultation events — where a planet passes in front of a distant star, becoming backlit — the starlight would wink off and on as it passed one side of the planet, suggesting the presence of rings obscuring the starlight. But the same didn’t happen on the other side of the planet. This led to the idea that Neptune was surrounded by separate arcs rather than complete rings.
Photographing the rings turned out to be a challenge because at a distance 30 times farther from the sun than Earth, the light out at the edge of the solar system is only one thousandth as bright. That meant Voyager’s cameras had to take long exposures while flying past the planet at 90,000 km/h. On top of that, unlike the bright icy rings of Saturn, Neptune’s rings are made of very dark and hard to photograph material.
During a press conference back in 1989, one of the scientists said that photographing the rings of Neptune was like trying to take a snapshot of a black race car speeding by at night without using a flash.
A day or two before the close encounter, a friendly betting game emerged among the scientists who were debating on whether or not the planet had a ring system. As Voyager drew close to Neptune, an early, low-resolution image showed what appeared to be the sought after ring arcs.
The “Arc-ers” rejoiced. But the “Ringers” were still hopeful, believing that the images would improve as the spacecraft got closer and the arcs would grow longer, joining together into rings.
It was not until Voyager flew past Neptune, entered the planet’s shadow and looked back towards the sun that the rings finally appeared in their full glory and the Ringers were victorious.
I don’t know exactly what the wagers were or how much was won or lost.
The rings turned out to be made of fine dust that is best seen when light shines through it, like dust in your home that shows up floating before a window when a sunbeam is shining through. Voyager discovered four rings, some with clumps made of larger particles that have been grouped together by the gravity of small moons embedded within the rings. Those clumps are what looked like arcs from a distance.
Thanks to the Voyager mission, we now know that all four of the largest planets in our solar system have rings and they are all different from each other.
Saturn is most famous for its brilliant icy rings first seen by Galileo in 1610. A new study has suggested that they might be the remains of a hypothesized moon known as Chrysalis. The idea is that Chrysalis was drawn too close to the giant planet around 160 million years ago and was torn apart by powerful tidal forces.
Jupiter, by contrast, is surrounded by a gossamer-thin ring system made of particles as fine as smoke and tinted red, created by dust blown off of its closest moons.
Uranus has dark rings forming thin lines with dust between them and Neptune has clumpy rings believed to be the results of collisions between small moons.
Even Earth may have had a ring in its early days according to the giant impact hypothesis, in which a Mars-sized object collided with the protoplanetary Earth and formed a ring of debris that eventually coalesced into our moon.
Rings are dynamic structures that are continually influenced by the gravity of orbiting moons and the planet itself, but how they change over time is unclear.
Voyager only saw the rings of Neptune once and it took twelve years to get there. No spacecraft has visited the planet since.
But now that the James Webb telescope can see them clearly at any time, we can watch for the up to two decades the telescope is expected to last to see how those mysterious rings evolve.
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