Page 36 - Ultimate Visual Dictionary (DK)

THE UNIVERSE

Mercury TILT AND ROTATION OF MERCURY
Axis of
to orbital plane
rotation Perpendicular
MERCURY IS THE NEAREST PLANET to the Sun, orbiting at an Axial tilt of 2°
average distance of about 36 million miles (58 million km). North
Pole
Because Mercury is the closest planet to the Sun, it moves
faster than any other planet, travelling at an average speed Orbital
of nearly 30 miles (48 km) per second and completing an plane
orbit in just under 88 days. Mercury is very small (only
40 percent bigger than the Moon) and rocky. Most of the
MERCURY
surface has been heavily cratered by the impact of meteorites,
although there are also smooth, sparsely cratered lava-covered plains. The Caloris
Basin is the largest crater, measuring about 800 miles (1,300 km) across. One rotation
It is thought to have been formed when a 38-mile- (60-km-) diameter takes 58 days South Pole
and 16 hours
asteroid hit the planet, and is surrounded by concentric rings of
mountains thrown up by the impact. The surface also has many clifflike DEGAS AND BRONTË (RAY CRATERS)
ridges (called rupes) that are thought to have been formed when the
hot core of the young planet cooled and shrank about four billion years Bright
ago, buckling the planet’s surface in the process. The planet rotates ray of
ejecta
about its axis very slowly, taking nearly 59 Earth days to complete (ejected
one rotation. As a result, a solar day (sunrise to sunrise) on Mercury material)
is about 176 Earth days—twice as long as the 88-day Mercurian year.
Mercury has extreme surface temperatures, ranging from a Brontë
maximum of 800°F (430°C) on the sunlit side to -270°F (-170°C)
Unmapped
on the dark side. At nightfall, the temperature drops very quickly
region
because the planet’s atmosphere is almost nonexistent. It
consists only of minute amounts of helium and hydrogen
captured from the solar wind, plus traces of other gases.
Degas with
FORMATION OF A RAY CRATER Path of rocky ejecta central peak
Path of meteorite
Debris thrown colliding with planet (ejected material)
out by impact Ejecta forms
secondary craters
Wall of rock
thrown up
around crater
Impact forms
saucer-shaped Loose debris
crater on crater floor
Fractured rock
SECONDARY CRATERING
METEORITE IMPACT
Wall of rock forms Ray of ejecta
ring of mountains (ejected material)
Small secondary
crater
Loose ejected rock

Central mountain Falling debris
rings form if floor forms ridges on
of large crater side of wall
recoils from
meteorite impact

RAY CRATER

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