Astronomy - USA (2019-09)

Europa (Jupiter)

Diameter: 1,940 miles (3,100 km)

Enceladus (Saturn)

Diameter: 313 miles (504 km)

To scale

Titan (Saturn)

Diameter: 3,200 miles (5,150 km)

Triton (Neptune)

Diameter: 1,680 miles (2,700 km)

Ice

Water

Rock

Metal

Pluto

Diameter: 1,430 miles (2,302 km)

Mars

Diameter: 4,212 miles (6,780 km)

Earth

Diameter:

7,918 miles (12,742 km)

WWW.ASTRONOMY.COM 13

PEELING BACK THE ONION. It’s astounding how

far the ancient discipline of astronomy has

progressed in just the past half-century.

As humans continue to send increasingly

sophisticated robotic scouts farther and

farther out into the solar system, we

continue to learn things about distant

worlds that our ancestors would have

thought nearly impossible to know. For

instance, thanks to missions like Voyager,

Galileo, Cassini, New Horizons, and many

more, we now have good, data-based

theories about the interior structures of

many of the solar system’s worlds. And

one surprising feature we see on many

worlds throughout the solar system is the

presence of oceans hidden underground.

These oceans, which are often expected

to be filled with salty, liquid water, offer

renewed hope for finding life on worlds

previously written off as too cold and

uninhabitable to support it. — J.P.

It’s still unclear

whether Mars has

a solid, liquid, or

two-layered core

like Earth.

Fortunately, NASA’s

InSight mission

deployed a

seismometer late

last year to help

researchers find out.

IT’S WHAT’S ON THE INSIDE THAT COUNTS

FAST FACT

Unless noted, all

images are to scale.

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