Page 47 - Astronomy - October 2017 USA

Uranus’ menagerie of major moons

Miranda

Voyager 2 captured the first good photos
of Uranus’ ring system. These dark, faint
rings glow in false color in this computer-
enhanced image, which reveals the nine
rings known before the spacecraft’s arrival.
(The fainter pastel lines are processing
artifacts.) Scientists have since discovered
four more rings. NASA/JPL

After the rich and complex atmospheres
of Jupiter and Saturn, Uranus seemed
pretty bland, Dodd recalls. “You didn’t get
all the great storms you got at the other
planets,” she says. Instead, Uranus “looked
like a fuzzy, blue tennis ball.”
Voyager did reveal a previously unde-
tected magnetic field around Uranus, com-
parable in strength to Earth’s. Due to its
nearly 90° axial tilt, Uranus rolls around its
orbit like a ball. And while Earth’s orbital
and magnetic fields are offset by roughly Ariel Umbriel Oberon Titania
12°, Uranus’ are 60° apart. This results in a The planet’s Frankenstein moon, Miranda, looks like a mad scientist stitched it together from
corkscrewing magnetic field trailing mil- unrelated parts. Some regions feature heavily cratered ancient terrains while others are younger
lions of miles behind the planet. and display canyons, cliffs, and steep ridges. Craters dominate Ariel’s surface, though this moon
What’s more, scientists still aren’t sure also shows some major canyons and ridges. Heavily cratered Umbriel is the planet’s darkest large
moon; the white ring at top resides on a crater’s floor. Bright rays surround several of the big
why the magnetic field exists at all, since
craters that dot the surface of Oberon. Abundant craters also cover Uranus’ largest moon, Titania,
Uranus lacks the standard liquid metallic though several canyons and steep cliffs break up the monotony. MIRANDA: NASA/JPL/USGS; ALL OTHERS: NASA/JPL
inner layer that powers such fields on other
planets. Voyager also revealed intense radi-
ation belts around the planet, similar to
those seen at Saturn. but the star’s repeated appearance and dis- the rings are probably made mostly of ice,
But the highlights, Dodd says, were appearance before it slid out of view behind like Saturn’s, but covered in organic mate-
centered on Uranus’ rings and moons. the planet made astronomers realize that a rial such as methane, and then baked dark
Compared with Saturn’s rings, visible since series of rings surrounded our far-off by the planet’s radiation belts.
the earliest days of the telescope, Uranus’ neighbor. The flyby was a chance to inves- Uranus’ moons, too, camouflage
were still recent discoveries, and scientists tigate them up close. well against the dark of space. When
were eager to learn more. Voyager imaged the ring system for the Voyager left Earth, astronomers knew of
Astronomers at Cornell University dis- first time, informing astronomers of its only five satellites around the planet. From
covered Uranus’ ring system in early 1977, detailed structure. The spacecraft also dis- observations during its brief visit, the
just before Voyager’s launch. The sighting covered two entirely new rings. The close- spacecraft tripled that number, yielding
was a happy accident, when a chance align- up views confirmed that Uranus’ subtle 10 new moons.
ment carried Uranus in front of a distant bands are not like Saturn’s bright icy rings; “I really think the satellites were the
star. Scientists had planned to use the they are dark and reflect little light, mak- highlight of Uranus,” says Dodd. Voyager
occultation to study Uranus’ atmosphere, ing them difficult to see. Scientists think images lent the five larger known moons

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