Page 48 - Astronomy - October 2017 USA

Above: Voyager captured Neptune’s three brightest rings after it flew
past the planet. (The dark bar blocks the ice giant’s crescent.) The outer
ring’s clumpy nature doesn’t show up well from this angle. NASA/JPL
Left: Neptune’s dynamic atmosphere surprised scientists, who expected
the planet to resemble its dull cousin, Uranus. The Earth-sized Great Dark
Spot lies at center; several smaller storms also appear. NASA/JPL

detail and character, telling varied stories reasonable proxies for Uranus’ system, More surprises waited on Triton,
of violent pasts. Chancia explains. So he and Hedman Neptune’s biggest moon. Triton was
Two of the new moons, Cordelia and adopted techniques that planetary scientist already a hotbed of intrigue; it’s by far
Ophelia, were identified as shepherd Mark Showalter used to find the moonlet the solar system’s largest retrograde satel-
moons. They orbit on either side of Uranus’ Pan in Voyager 2’s observations of Saturn’s lite, meaning it orbits in the direction
outer Epsilon ring, and their gravitational ring system. opposite to its planet’s rotation. This is
pull herds the small particles in that ring They found distinct patterns in Uranus’ usually a sign of a captured object, but
along their orbital path and keeps them rings consistent with “wakes” carved by most other retrograde moons are small,
from dissipating into space. Uranus’ rings moonlets circling a planet within a ring misshapen asteroids. Triton is three-
are uncommonly narrow; without shep- system. The predicted moonlets are tiny, quarters the size of our Moon, and sur-
herd moons, the small particles would dis- only 2 to 9 miles (4 to 14 kilometers) vived its capture intact. Scientists wanted
perse over long timescales. across. And they are likely dark, like close-up views of the satellite, and since it
Over the years, astronomers have used the rest of the moons and ring system. was the last target, they were free to adjust
imagery, mostly from Hubble, to add more Confirming the moonlets will be a chal- Voyager’s trajectory as needed. So the
moons to Uranus’ count, which currently lenge. But even 30 years later, Voyager is spacecraft swooped only 3,075 miles
stands at 27. But Cordelia and Ophelia still helping to crack Uranus’ secrets. (4,950km) above Neptune’s north pole —
remain the only observed shepherds of the its closest approach to any object during
ring system. Astronomers have long won- Final surprises the mission — and flew toward its encoun-
dered whether more moons are hiding from Voyager’s last planetary encounter came ter with Triton.
view, or whether other forces are at work. August 24, 1989. Far from Uranus’ “fuzzy The last world Voyager 2 visited
Last year, astronomers from the tennis ball,” Neptune was alive with storms stunned scientists. The moon boasted a
University of Idaho revisited the Voyager and bright, quick-moving clouds, delight- thin atmosphere, polar caps, and active
data. Thirty years after the flyby, they ing unsuspecting astronomers. Clouds not geysers that spewed icy material miles
found evidence for two more tiny moonlets only appeared clearly in Voyager images, high. The active cryovolcanism puts Triton
shaping Uranus’ rings. “Nobody — or not but they also cast shadows on deeper cloud in a select group of satellites, in the com-
many people — had looked at this in a very layers, allowing scientists to measure the pany of other dynamic moons such as
long time,” says Robert Chancia, who led planet’s atmosphere in great detail. The Europa and Enceladus.
the investigation. In fact, the Voyager data “Great Dark Spot,” as astronomers termed Voyager also discovered six new moons
were taken before he was born. Chancia the largest tempest, was as big as Earth, orbiting Neptune and delivered clear pic-
and his adviser, Matthew Hedman, usually swirling in Neptune’s southern hemi- tures of its ring system for the first time,
study Saturn’s rings. But recent discoveries sphere and boasting wind speeds as high revealing the rings to be clumpy but com-
by the Cassini spacecraft have added as 750 mph (1,200 km/h). In the decades plete, unlike those at Uranus.
greatly to astronomers’ understanding of since, that storm has died, while new And as it did at Uranus, Voyager dis-
planetary rings. So Chancia and Hedman storms have risen in its place. covered that Neptune’s magnetic pole is
decided to take another look at the Voyager “That was a bit of a surprise to me misaligned from its rotational pole, causing
findings, applying new theories to old data. when you consider the Great Red Spot on extreme variations in its magnetic field as
“There are several narrow ringlets Jupiter has been going on for 400 years,” the planet rotates. Furthermore, both plan-
within the rings of Saturn” that provide Dodd says. ets’ magnetospheres are offset from center

48 ASTRONOMY • OCTOBER 2017