YSM Issue 90.2

astronomy
FEATURE
GRAVITY WAVES SPICE UP
VENUS’ ATMOSPHERE
►BY NOAH KRAVITZ
IMAGE COURTESY OF MAKOTO TAGUCHI
►The Longwave Infrared Camera (LIR) used infrared light to
analyze temperature variation in Venus’ atmosphere. The white
regions are the hottest areas.
Although our immediate neighbor is visible twice a day from
Earth, we know little about the planet Venus. It is very difficult
to keep a spacecraft on its surface—where the temperature is
hot enough to melt lead, and only radar imaging can pierce the
fast-swirling clouds that cover the upper atmosphere. However,
recent data from the Japanese Aerospace Exploration Agency’s
Akatsuki spacecraft has signaled a long-awaited breakthrough in
Venus studies.
The Japanese research team was using Akatsuki’s Longwave
Infrared Camera (LIR) to record heat patterns in Venus’ atmosphere
when the scientists noticed irregularities over some of the
largest Venusian mountain ranges. They attributed these results
to gravity waves, vertical air disturbances that are common on
many planets, including Earth. This finding—which may explain
the existence of the clouds that conceal Venus’ surface from our
view—also has applications to Earth.
“It is hard to imagine anything more different from our planet,”
said Ronald Smith, a professor of Geology & Geophysics at
Yale, regarding the Venusian atmosphere. Venus’ three-layer atmosphere,
composed mostly of heat-trapping carbon dioxide, is
about 100 times as dense as Earth’s—so dense, in fact, that it often
acts as a gas and a liquid at the same time. In the highest layer,
enigmatic super-rotating sulfurous clouds blow constantly westward
faster than Venus spins on its axis.
Gravity waves, not to be confused with gravitational waves, are
common phenomena on Earth. Disturbances caused by triggers
in the lower atmosphere—such as thunderstorms, volcanic eruptions,
wind passing over mountains, and airplanes — travel upwards
into the atmosphere. When a small parcel of air is displaced
upwards, it pushes the next parcel up before it is pulled down by
gravity, and the ensuing chain reaction can propagate for miles.
“These waves oscillate through a balance of buoyancy and gravity,”
said Makoto Taguchi, the principal investigator for LIR.
When Akatsuki passed Venus in December 2015, LIR spotted
a conspicuous bow-shaped hot patch that remained directly
above the prominent Venusian mountain range Aphrodite Terra.
This warm region, as well as the fifteen others that the researchers
found during the subsequent eight months, indicated a slower-moving
air pocket among the otherwise super-rotating clouds.
“The stationary thermal structure reminded me of waves on the
surface of a shallow river in which a big invisible stone on the
bottom prevents the smooth flow of water,” Taguchi said. Since
gravity waves travel vertically, they interrupt horizontal airflow—
exactly as was observed on Venus. “After thorough theoretical
consideration and numerical simulations, we concluded that only
a gravity wave could have created the characteristic bow shape,”
Taguchi confirmed.
This discovery alters the conversation about the role of gravity
waves in maintaining Venus’ unusual atmosphere. “The most
tantalizing idea is that these gravity waves carry momentum, and
when they break down, they release strong winds, which could
explain the mechanism of super-rotation,” said Smith, who investigated
similar phenomena above the New Zealand Alps in 2014.
Because Venus’ atmosphere thins with altitude, gravity waves that
are small at the surface amplify until they finally shatter—like the
crack at the end of a whip—and deliver their stored energy to the
upper atmosphere. The behavior of these gravity waves as they
cross layers of the Venusian atmosphere also provides indirect information
about characteristics such as temperature profile.
Surprisingly, one of the most far-reaching consequences
of the Akatsuki mission may have nothing to do with Venus
specifically. LIR’s camera, an uncooled microbolometer array
(UMBA), represents a breakthrough in space imaging. Standard
cooled semiconductor bolometers, which are common
in meteorology and astronomy for measuring slight changes
in temperature using infrared radiation, are bulky, heavy, and
energy-intensive. UMBA is not dependent upon a cooling apparatus
and is thus better suited for spacecraft where compactness
is a guiding consideration. “This was the first mission to
use UMBA to take a snapshot of a planet,” said Taguchi, who
hopes that UMBA will make infrared imaging accessible to a
wider variety of space missions.
Understanding Venusian gravity waves is important for many
branches of Earth science: the same gravity waves that make for
bumpy airplane landings in Denver also factor into cloud movement
and carry chemicals from Earth’s surface into the upper atmosphere.
Accounting for this variable, however, is still one of the
biggest weaknesses of existing climate change and weather models,
and any step towards filling this gap is substantial.
www.yalescientific.org
March 2017
Yale Scientific Magazine
27