Astronomy - USA (2021-12)

planets

Mercury,

which

is the

smallest and innermost

planet, is constantly cooked

by sunlight. Despite its prox-

imity to our star, it’s not the

hottest world in our solar

system; still, Mercury expe-

riences wild temperature

swings unlike those found on

any other planet. On the day-

side, the Sun penetrates the

planet’s thin, f leeting atmo-

sphere and bakes the surface.

But because there isn’t much

air to distribute that heat, the

temperature on the planet’s

nightside plunges to hundreds

of degrees below zero.

The Sun also helps give

Mercury another peculiar

trait: The world has a faint

cometlike tail. Mercury’s

feeble atmosphere contains

sodium, which glows when

excited by sunlight. Plus, the

diminutive world doesn’t

have much gravity — only

about twice the gravity of

the Moon. This means pres-

sure from sunlight striking

Mercury can liberate sodium

molecules, forcing them

“downwind” of the planet

and creating a dimly

glowing tail.

Thanks to its thin atmo-

sphere, Mercury is also

prone to impacts. This

has left it with a rather

pockmarked appear-

ance. And to make

matters worse, it’s

wrinkled with

age. The world

has steep, cliff like

north-south ridges

that stretch all

over its surface.

Researchers think they

might have formed as

Mercury cooled after its

The heavily cratered surface of

Mercury’s southern hemisphere is

on full display in this mosaic of

images taken by NASA’s Mariner 10

spacecraft, which was the first

mission to fly by and closely

scrutinize our solar system’s

innermost world. NASA/JPL-CALTECH

Mercury’s surface is covered in wrinklelike ridges. This unnamed example of

such a ridge is located in the planet’s northern volcanic plains and was imaged

by NASA’s MESSENGER spacecraft. The feature spans some 87 miles (140 km)

and, like the world’s other wrinkles, is believed to have formed when Mercury’s

core shrank as it cooled, which forced the surface to follow suit. NASA/JHUAPL/CARNEGIE

INSTITUTION OF WASHINGTON

birth,

causing the

planet to shrink and its

crust to slightly crumple.

Mercury has so far been

visited by only two space-

craft: Mariner 10 (launched

in 1973) and MESSENGER

(launched in 2004). The for-

mer was a f lyby mission that

only revealed a partial view

of the tiny planet. However,

the latter was an orbiter,

which not only mapped the

vast majority of Mercury’s

surface, but also discovered

that the planet’s permanently

shadowed polar craters likely

hold abundant water ice.

If you’d like to learn more

MERCURY

about what Mercury has to

offer, you need not wait long.

The European and Japanese

space agencies have teamed

up and sent BepiColumbo to

orbit Earth’s smallest sibling

— and Bepi just made its first

brief pass by the oft-forgotten

planet this October. Stay

tuned for more!

WWW.ASTRONOMY.COM 17

Mass: 0.055 Earth masses

Diameter: 3,030 miles

(4,876 km)

Surface temperature:

805 F (430 C) during the day

–290 F (–180 C) at night

Rotation period (day):

58.8 Earth days

Orbital period (year):

88 Earth days

Moons: None

STATS

With thousands of exotic exoplanets

known, the worlds of our solar system

may seem rather dull. Trust us, they’re not.

BY JAKE PARKS