Australian Sky Telescope MayJune 2017

8 AUSTRALIAN SKY & TELESCOPE May | June 2017

GRAPHIC: YEHUDA HOFFMAN; VENUS: © PLANET-C

Akatsuki spies massive wave on Venus

slow-moving surface of the planet below

— the curious structure seemed to stay

in lockstep with the planet’s rotation,

suggesting a complex interplay between

A MONSTER WAVErecently roiled

Venus’s atmosphere, forming a planet-

spanning, bow-shaped feature. Japan’s

Akatsuki orbiter noted this disturbance

in late December 2015 and early

January 2016, shortly after the craft’s

arrival, Tetsuya Fukuhara (Rikkyo

University, Japan) and colleagues report

in the FebruaryNature Geoscience.

When Akatsuki looked back at the

regionlaterin2016,thewavehad,for

the most part, vanished.

The feature spanned the Venusian

cloudtops from the northern to

southern hemisphere, extending more

than10,000km.Itappearednearthe

eveningterminatoronthedaysideand

wasembeddedintheinterfacebetween

the upper troposphere and lower

stratosphere. Although the cloudtops

whipalongat100metrespersecond

(360kph)—muchfasterthanthe

SLocal cosmic structure (spans 1.7 billion light-years). The arrow is the cosmic dipole’s direction.

the surface and the atmosphere.

The team’s computer models suggest

that air flowing over mountainous

terrain produced a gravity wave

that then propagated upward to

the cloudtops, where the large bow

wave was seen. (A gravity wave is an

undulation triggered in a fluid — such

as the atmosphere — by the interaction

of gravity and other forces.) The

wave’s longitude corresponded with

the western slope of Aphrodite Terra,

the largest of Venus’s three continent-

size highlands, whose surface area is

comparable to Africa’s.

We see similar gravity-wave

phenomena here on Earth, and NASA’s

New Horizons spacecraft chronicled

evidence for gravity waves in the

atmosphere of Pluto during its historic

2015 flyby.

■ DAVID DICKINSON

Void ‘repels’ Milky Way’s galaxy group

ASTRONOMERS HAVE DISCOVERED

a giant cosmic void that explains why

our Local Group of galaxies is moving

throughtheuniverseasfastasitis.

The Local Group lies in a filament

ofamuchlargercosmicstructure.The

galaxy clusters in this cosmic web don’t

stay still but rather gravitate (literally)

toward the largest members. Our Local

Group is moving toward what’s called

theGreatAttractor,adensecollection

in the vicinity of the Centaurus, Norma

and Hydra clusters about 160 million

light-years away. Beyond that and about

four times farther away lies another,

equally influential attractor called the

Shapley Supercluster.

But it turns out that there’s another

player. Using the Cosmicflows-

catalogue of galaxies, Yehuda Hoffman

(Hebrew University, Israel) and

colleagues have mapped the motion

of more than 8,000 galaxies and

confirmed that two titan structures

determine how local galaxies flow

through the cosmic web — Shapley on

one side and a single, as-yet uncharted

void in the opposite direction.

Think of the local cosmic structure

as a gravitational water park — the

twisty slides start high (where the

void is) and end up low (where the

supercluster is), with the natural

motion always being down — that is,

with gravity. Galaxies toboggan along

the gravitational slides.

How fast the galaxies go depends

on how tall the slides are. In the same

sense, the fact that there’s a big void in

December 7, 2015

Dipole

Repeller

Shapley Attractor

Great

Attractor

Local

Group

XInfrared image of Venus’s gravity wave.

The dashed line marks the day-night

terminator.

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