Sky & Telescope - USA (2020-06)

SOLAR ORBITER: ESA / ATG-MEDIA LAB; PLUTO: NASA / JHU APL / SWRI / ALEX PARKER

skyandtelescope.org • JUNE 2020 9

SPUTNIK PLANITIA, a basin that makes

up the left lobe of Pluto’s “heart,”

may drive subtle winds over the entire

dwarf planet, Tanguy Bertrand (NASA

Ames Research Center) and colleagues

suggest. They published their fi ndings

in the February Journal of Geophysical

Research: Planets.

Among the many surprises Pluto pre-

sented to scientists was a global atmo-

sphere. Hints on the surface, such as

probable wind streaks west of Sputnik

Planitia, reveal the thin atmosphere’s

impact. Bertrand and colleagues exam-

ined what could drive these winds using

high-resolution climate simulations.

In what Bertrand’s team nicknames

Pluto’s “heartbeat,” nitrogen ice in

Sputnik Planitia sublimates in Pluto’s

daytime and condenses back into ice

at nighttime. However, because Pluto’s

spin is tilted with respect to its orbit

around the Sun so that it’s almost on

its side, the northern edge of Sputnik

Planitia is in constant daylight. As a

result, the nitrogen there is constantly

SOLAR SYSTEM

Pluto’s Icy Heart “Beats,” Driving Planet-Scale Winds

sublimating, inducing a southerly wind.

Pluto’s rotation defl ects that wind

slightly westward, until it nears the

basin’s mountainous western bound-

ary, where it returns southward. At the

southeastern edge of Sputnik Planitia,

which lies closer to the part of Pluto

under polar night, nitrogen condenses

back into ice. Here, the winds turn

northerly again, completing the coun-

terclockwise spiral.

The simulations show that from

Sputnik Planitia, nitrogen gas rises to

higher altitudes, fl owing from northern

summer to southern winter. But just

as an ice skater’s spin slows when they

spread their arms, Bertrand explains,

the nitrogen molecules slow down

as they move from north to south,

because they’re moving farther from

the planet’s rotational axis. As a result,

high-altitude nitrogen ends up mov-

ing more slowly than the dwarf planet

below it. The consequence is that the

winds on Pluto blow westward, opposite

the planet’s rotation — a pattern unique

pSputnik Planitia, a 3-km-deep basin, makes

up the left lobe of Pluto’s “heart.”

among solar system worlds (except

maybe Neptune’s moon Triton).

“Sputnik Planitia may be as impor-

tant for Pluto’s climate as the ocean

is for Earth’s climate,” Bertrand said.

William McKinnon (Washington

University in St. Louis), who was not

involved with the study, agrees that

Sputnik Planitia is the “source and

sink” of Pluto’s atmosphere.

■JULIE FREYDLIN

• Watch simulations of Pluto’s winds at
  https://is.gd/plutowinds.

THE EUROPEAN SPACE AGENCY’S

Solar Orbiter took to the skies on Febru-

ary 9th, the fi rst space-science mission

to launch in 2020. The orbiter joins a

growing armada of solar telescopes,

including the Parker Solar Probe

(S&T: Apr. 2020, p. 10) and the ground-

based Daniel K. Inouye Solar Telescope,

which saw fi rst light in January. Solar

Orbiter will begin collecting data in

May as it cruises toward the Sun, but

full science operations won’t begin until

November 2021.

Solar Orbiter will be the fi rst mission

to image the Sun’s poles. (Ulysses, a

joint ESA/NASA mission, launched in

1990 to study the Sun’s polar regions

but didn’t carry cameras.) With the

help of three gravitational assists, one

SUN

European Solar Orbiter

Launches

pAn artist’s impression shows the Solar Orbiter

spacecraft approaching the Sun.

maneuver past Earth and two past

Venus, the spacecraft will enter an

initial orbit inclined 17° to the eclip-

tic plane. From this vantage point,

Solar Orbiter will study the the Sun’s

extended atmosphere and the birthplace

of the solar wind.

Several further gravity assists past

Venus will gradually steepen the incli-

nation, putting it at 33° by 2029. The

closest pass by the Sun, at 0.28 astro-

nomical unit, will take the spacecraft

well inside Mercury’s orbit.

Ten instruments are hitching a ride,

including the NASA-contributed Helio-

spheric Imager, which will take the fi rst

high-resolution images of the Sun from

up close. Other instruments include an

X-ray spectrometer and telescope, two

ultraviolet detectors, and an instrument

to survey the Sun’s magnetic fi elds. A

Sun-blocking coronagraph named Metis

will also make key observations of the

inner corona.

This new mission overlaps with —

and is complementary to — NASA’s

Parker Solar Probe. Parker studies the

Sun from close up, but it can’t image the

Sun directly. Solar Orbiter will image

the Sun from an inclination that allows

study of the poles, offering a compre-

hensive view of the Sun’s activity.

■DAVID DICKINSON