Astronomy

ASTRONEWS

Earth’s

axial tilt

23.5°

Jan. 2015July 2015Jan. 2016July 2016Jan. 2017July 2017Jan. 2018July 2018Jan. 2019July 2019Jan. 2020July 2020Jan. 2021July 2021

0.0°

• 0.5°

2.0°

2.5°

South latitude at center of Jupiter’s disk

0.5°

1.0°

1.5°

3.0°

3.5°

Date

Jupiter’s

axial tilt

3°

3°

Io

Jupiter’s equatorial plane

Sub-Earth point

Subsolar point

Europa

CallistoGanymede

Jupiter

WWW.ASTRONOMY.COM 19

EARLY EARTH. Scientists recently modeled an extended period of bombardment that took place on Earth after the

Moon formed, finding that Earth accreted two to five times more material during this time than expected.

Saturn is known for its stunning — and mysterious

— ring system. The age of the rings in particular

has intrigued astronomers for centuries: Are they as

old as the planet itself, or a relatively new feature?

New evidence from the final months of the Cassini

spacecraft’s operation has revealed that Saturn’s

rings are just a few hundred million years old.

That evidence includes the mass of the rings,

which Cassini measured during its Grand Finale

passes between the planet and the rings. The rings

are light, roughly equivalent to the mass of Saturn’s

moon Mimas. And according to planetary scientists’

best theories connecting the age and mass of the

rings, “this is a clear indication that the rings did

not form together with Saturn,” said Luciano Iess of

the Sapienza University of Rome and leader of

Cassini’s radio experiment team, at the American

Geophysical Union Fall 2017 meeting.

Additional evidence comes from 12 years of

measurements from the Cosmic Dust Analyzer on

Cassini. The experiment measured the rate at which

micrometeorites accrete onto and darken the rings;

darker rings indicate an older age. This flux is actu-

ally about 10 times greater than previously

believed, darkening the rings faster than predicted

and indicating their age is only about 150 million to

300 million years.

This new evidence may finally resolve the ques-

tion of the age of Saturn’s rings. NASA’s Voyager

spacecraft returned data that indicated the rings

were young in the 1980s, but astronomers strug-

gled to explain why. Now that the Voyager data are

confirmed, only one question remains: How did the

rings form in the first place? — A.K.

Saturn’s rings are

a recent addition

JUPITER GETS TIPSY

YOUTH AND BEAUTY. Evidence from Cassini has finally

helped scientists pin down the age of Saturn’s rings to a

few hundred million years. If you traveled to the Triassic

Period and viewed Saturn through a telescope, you might

be sorely disappointed. NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE

ASTRONOMY

: ROEN KELLY

TAKE IT TO THE LIMIT. Earthbound observers always

see an equatorial view of Jupiter because the gas giant’s

axis tilts just 3° from perpendicular to the planet’s orbit

around the Sun. From the Sun’s perspective, the latitude

of the subsolar point — the spot at the center of the jovian

disk — can extend up to 3.56°. From Earth, which adds its own

orbital motion to the equation, the latitude of the sub-Earth

point can reach 3.88°. Both values hit their maximums south

of Jupiter’s equator in April 2018. — Richard Talcott

The last time

Jupiter’s south

pole tipped

closest to Earth,

in April 2006, the

moon Ganymede

appeared well out

of line with the

planet’s equator.

ALAN FRIEDMAN

Io, Europa, and

Ganymede (left to

right) lined up with

Jupiter’s equator

in early 2015, the

most recent time

the jovian system

appeared edge-on

to Earth. JOHN CHUMACK

Jupiter’s

four large

moons orbit

in the planet’s

equatorial

plane, so they

do not currently

line up.

The time it would have taken

to digitize the Apollo 11

mission recordings on existing

equipment, before a University of Texas at

Dallas team recently developed a faster method.

170

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