Page 32 - Astronomy - October 2017 USA

11
Measuring a day

Jupiter
3.1° Rotation axis Magnetic axis
10°
NORTH Saturn 26.7° 47° 28.3°
Uranus Neptune

Ecliptic
plane –97.8°

60°

The outer planets have no solid surface to track as they rotate in order to measure the length
12 of their day. Instead, astronomers track the rotation of the planets’ magnetic poles, which
typically are offset from the axis of rotation. Jupiter’s magnetic pole is offset from its rotational
axis by about 10°, while Uranus’ axes are offset by 60° and Neptune’s by 47°. Saturn’s magnetic
and rotational axes are almost perfectly aligned, with an offset of less than 1°, making
measurements of the length of its day challenging. Both Voyager spacecraft measured the
length of Saturn’s day to be about 10.7 hours, but the Cassini spacecraft has measured the
length to be slightly shorter (10.6 hours) or longer (10.8 hours), depending on the hemisphere
in which measurements are taken. ASTRONOMY: ROEN KELLY

13 moon somehow feeds the ring. Could there
be ice volcanoes on Enceladus, providing
the sloshy material that would fill in impact
craters? If so, some of that gushing material
perhaps could escape the surface and orbit
Saturn as part of the E ring. Those first
detailed observations from Voyager trig-
gered an ongoing fascination with this
small, reflective moon, adds Ingersoll.
Scientists now know from the Cassini
mission that an underground water ocean
11. Enhanced color image processing hydrocarbons perhaps in liquid form at the feeds geysers at Enceladus’ south pole.
increases the ability to see clouds and surface — made Titan even more intrigu- They’ve also discovered likely
storms in Saturn’s atmosphere, such
as the convective clouds in this image ing for further study. In fact, says Spilker, hydrothermal activity at the ocean floor.
taken November 5, 1980, from a range of “it was really the Voyager flyby of Titan, On Earth, biological ecosystems thrive in
5 million miles (8 million km). NASA/JPL-CALTECH and what we learned and what we didn’t such environments. Could they do the
learn, that led to this strong desire to go same on Enceladus? That’s a question a
12. The spacecraft revealed oval-shaped
storms in Saturn’s atmosphere, similar back.” And ultimately, it was that flyby that future dedicated mission to the small
to those seen on Jupiter. Imaged sparked the Cassini mission. moon might answer.
November 7, 1980, from a distance The Titan discoveries were just the
of 4.6 million miles (7.5 million km), The weather out there
these brown ovals in Saturn’s northern beginning of revealing the saturnian satel-
hemisphere span about 6,000 miles lites. From afar, two of the planet’s closer- Scientists didn’t send Voyager to Saturn to
(10,000km) in diameter. NASA/JPL in moons, Mimas and Enceladus, had been study only its rings and moons. The planet
considered twins due to their similar sizes. and its atmosphere were also a science
13. Winds in Saturn’s upper atmosphere
are estimated to reach speeds of But on November 12, 1980, Voyager found focus. Like that of its sister giant planet,
1,118 mph (1,800km/h), faster than the that their surfaces look vastly different. Jupiter, Saturn’s atmosphere hosts incred-
winds on Jupiter. This ribbonlike feature While Mimas was pockmarked with the ible storms and enormous jet streams, and
is the result of a westward jet stream evidence of billions of years of collisions the Voyager twins were the first spacecraft
imaged on November 10, 1980, from a
distance of 2.2 million miles (3.5 million from space debris, Enceladus, oddly, was to photograph the details in those cloud
km). It was still visible to Cassini in 2012, not. Scientists had expected a heavily cra- tops up close.
though its shape had changed in the
tered surface, “and yet there were parts of While reanalyzing Voyager images of
intervening years. NASA/JPL
Enceladus that were smooth,” says Saturn’s poles, astronomer David Godfrey
14. Saturn’s polar hexagon spans a Caltech’s Andrew Ingersoll, who was a discovered a hexagonal shape surrounding
diameter of 20,000 miles (32,000km) and member of the Voyager imaging team. The the northern pole. That surprising shape
reaches depths of 60 miles (100km) in the world was also very reflective, similar to comes from jet streams moving at differ-
planet’s atmosphere. The bizarre feature
was first identified in Voyager images and the brightness of freshly fallen snow. ent speeds. “It’s really just a meandering
later confirmed by the Cassini spacecraft, Observations from Earth of a ring current flowing east a little faster than the
which snapped this image June 14, 2013. around Saturn at the distance of Enceladus’ rest of its neighbors,” explains Ingersoll,
NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE
orbit already hinted that perhaps that small who was also the head of the Voyager
32 ASTRONOMY • OCTOBER 2017