Astronomy

JunoCam image

North

Visible light

Depth below

cloud tops

0 km

20 km

50 km

90 km

150 km

350 km

Cooler Warmer

Microwave

ASTRONEWS

WWW.ASTRONOMY.COM 21

In July 2017, NASA’s Juno spacecraft made its

first-ever pass over Jupiter’s Great Red Spot,

sending back data that show just how deep

the massive storm extends.

Juno found that the storm stretches

roughly 200 miles (300 kilometers) into

Jupiter’s atmosphere and is nearly as wide as

1.5 Earths, or 10,000 miles (16,000 km) across.

The storm’s depth was measured using Juno’s

Microwave Radiometer instrument, which can

see far below Jupiter’s cloud tops.

The Great Red Spot, which contains clouds

spinning rapidly in a counterclockwise motion

around its oval perimeter, has actually dimin-

ished in size over the past few decades. Its

width has decreased by one-third and its

height by one-eighth since NASA’s Voyager 1

and 2 passed by Jupiter in 1979.

Juno’s pass also brought evidence of a new

radiation zone hovering over Jupiter’s atmo-

sphere, close to its equator. The Jupiter

Energetic Particle Detector Instrument spot-

ted energetic hydrogen, sulfur, and oxygen

ions within the zone, traveling close to light

speed. Scientists believe these particles origi-

nate from fast-moving neutral atoms created

around Io and Europa, two of Jupiter’s moons.

The atoms turn into ions when contact with

Jupiter’s atmosphere strips their electrons.

Juno’s Stellar Reference Unit star camera

also detected energetic heavy ions along the

inner edge of Jupiter’s relativistic electron

radiation belt, an area that hadn’t been stud-

ied by previous spacecraft. However, the ori-

gin of these particles remains a mystery. Juno

continues to orbit Jupiter, with December 16,

2017, marking its ninth close pass to our plane-

tary neighbor. — Amber Jorgenson

Juno charts the Great Red Spot’s depths

A next-generation planet hunt begins

FIRST LIGHT. On December 6, the European Southern Observatory announced that the Echelle

SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO), mounted on the

Very Large Telescope at the Paranal Observatory in Chile, had achieved first light. Echelle spectrographs

separate incoming light into its constituent parts and are used in many astronomical instruments. ESPRESSO

is designed to detect tiny shifts in a star’s motion as planets orbit it; because of its extreme sensitivity,

ESPRESSO should enable astronomers to identify some of the lightest rocky planets ever found. In addition

to its planet-hunting capabilities, ESPRESSO also can be used to test the laws of physics and improve

astronomers’ understanding of the universe and its evolution over time. — A.K.

ESO/ESPRESSO TEAM

MOON MAN. President Donald Trump signed a policy directive

that aims to send humans to the Moon for the first time since 1972.

SLICE IT UP. Juno’s Microwave Radiometer

instrument took vertical “slices” of the Great Red

Spot (above) as it peered below Jupiter’s clouds.

(The topmost slice is a visible-light image from

JunoCam.) The storm’s large-scale structure

remains coherent even at the greatest depth the

instrument observed, meaning it extends deep

into the atmosphere. The Great Red Spot (right) is

a counterclockwise-rotating storm that stands out

in stark contrast to the clouds around it.

ILLUSTRATION: ROEN KELLY AFTER NASA/JPL-CALTECH/S

WRI. GREAT RED SPOT: NASA/JPL-CALTECH/S

WRI/MSSS/GERALD EICHSTADT/JUSTIN COWART