Mercury Facts About the Planet
The four planets closest to the Sun—Mercury, Venus, Earth, and Mars—are a diverse group.
Our own planet is the biggest of the foursome, with Venus a near-twin in size. Formed in a cloud of solar dust and gas, battered by collisions, and reduced to rocky balls by heat and gravity, they all began life in the same way. But over time, they became very different. Mercury, the smallest of the inner planets, is closest to the Sun and has little atmosphere to protect it from our star’s searing heat. The craters on Mercury’s hot, dark surface— scars of long bombardment by cosmic material— resemble the craters on the Moon. While all the inner planets are thought to have an iron core, Mercury’s is unusually large, perhaps because the young planet was stripped of its outer layers in a catastrophic collision. Venus, though beautiful in the twilight sky, is obscured by choking clouds of sulfuric acid and may be actively volcanic. The hottest planet in the solar system, it is the victim of a runaway greenhouse effect. Mars is the coldest rocky planet. Once, it may have been warm, with rivers flowing on its surface, but now the planet is an arid wasteland, its remaining water locked infrost and ice. Earth is a world between extremes.
The right distance from the Sun for water to existin liquid form on the surface, our planet has vastoceans, an oxygen-rich atmosphere, and a hugediversity of life forms.
(3 km) high.
THE NEAREST PLANET TO THE SUN, MERCURY CAN BE SEENCLEARLY FROM EARTH ONLY FOR SHORT PERIODS EACH YEAR.
IT IS USUALLY VISIBLE IN SPRING AND FALL AS A BRIGHT GLIMMER JUST ABOVE THE HORIZON AT DUSK AND DAWN.
Mercury is a tiny, dense, deeply crateredworld, so close to the Sun that it is continuallyscorched and blasted by solar emissions.Temperatures during its long daytime period reach a roasting 800°F (430°C)—hot enoughto melt lead. Yet because there is only a thinatmosphere, heat escapes quickly enoughfor nighttime temperatures to drop to–290°F (–180°C). No other planet in the solarsystem experiences such extremes.Mercury spins on its axis very slowly, withone rotation taking almost 59 Earth days. Yet
it is also the fastest-orbiting of all the planets,completing its circuit of the Sun in just 88 days.
By the time the sunny side begins to turn away,the whole planet has been swept around toface the Sun from the opposite side. So oncethe Sun comes up, it takes a long time to setagain—there are 176 days from one sunrise to the next, during which time the planet orbits the Sun more than twice. Despite the long
Mercurial days, Mercury’s sky always looks black due to the incredibly thin atmosphere,which is not thick enough to reflect light.
Mercury has amaximum orbital speed of 30 miles(50 km) per second.
Average diameter 3,032 miles (4,879 km)
Mass (Earth = 1) 0.055
Gravity at equator (Earth = 1) 0.38
Mean distance from Sun (Earth = 1) 0.38
Axial tilt 0.01°
Rotation period (day) 58.6 Earth days
Orbital period (year) 87.97 Earth days
Minimum temperature –290°F (–180°C)
Maximum temperature 800°F (430°C)
Northern hemisphere At the north pole are hugeexpanses of smooth plains
some 1.5 million square miles
(4 million km2
) in area—half
the size of the United States.
A feature known as GoetheBasin contains ghost cratersthat have been flooded andburied by lava flows.
Western hemisphere Until NASA’s MESSENGER made its first flyby in 2008, thishalf of Mercury was unknown.
The flyby revealed that 40 percent of the planet’s surface is covered by smooth, volcanic. Its crust is more like that of Mars than the Moon’s,despite the similar appearance.Southern hemisphere Around Mercury’s poles, in shadowy places that arepermanently shielded from the Sun’s heat, such as in Chao Meng-Fu crater, NASA’s MESSENGER spacecraft found radar-bright patches that could be a mix of frozen water and organic materials.
MERCURY IS ONE OF THE FOUR SOLID, TERRESTRIAL PLANETS MADE OF ROCK AND METAL. IT IS SMALLER THAN SOME MOONS, YET MORE DENSE THAN ANY OTHER PLANET APART FROM EARTH.
For such a small planet to be so dense, Mercury must have a very large iron core. This suggests that it has lost rock from its outer layers. If so,one explanation might be that early in its history Mercury was struck by a planetesimal, one of the many protoplanets that whirled through the solarsystem as it formed. The devastating impact caused by this planetesimal—which was probably about one-sixth the size of the planet itself—blasted away much of Mercury’s rocky exterior.
MERCURY UP CLOSE
MERCURY IS COVERED BY GRAYISH-BROWN DUST AND BEARS COUNTLESS METEORITE CRATERS, FROM VAST, MULTI-RINGED BASINS TO TINY POCKMARKS. ALSO ON THE PLANET ARE TOWERING CLIFFS AND LONG, WINDING RIDGES.
With almost no atmosphere for protection, Mercury is exposed to the
battering of even small meteorites. Impact craters of all sizes pit the
planet’s surface. The largest meteorites have created multi-ring basins
such as the Caloris Basin. The heaviest meteorite bombardment came
early in the planet’s history, punching out most of the largest craters
before quieting down some 3.8 billion years ago. Soon afterward,
lava flows spread across the surface to create smooth plains that
obliterated some craters. Later, the planet’s interior cooled and
shrank, breaking the crust into cracks and ridges. Eventually, about
750 million years ago, Mercury’s mantle shrank so much that lava
ceased to flow out. Since then, the planet’s surface has hardly
changed, though it continues to be scarred by minor impacts.
The naming of Mercury’s surface features follows a system:
craters are named after artists, composers, and authors—for
example, the Tolstoy and Beethoven basins; valleys (valles) are
named after observatories; cliffs (rupes) after ships of discovery;
ridges (dorsa) after scientists; and plains (planitiae) take
international names for Mercury.
MERCURY IS THE LEAST EXPLORED OF THE ROCKY PLANETS,VISITED BY JUST TWO MISSIONS TO DATE: MARINER 10
IN THE MID-1970S AND THE MORE RECENT MESSENGER SPACECRAFT, WHICH STUDIED MERCURY FROM ORBIT.
One reason for the lack of missions to Mercury is the sheer technical difficulty Spacecraft have to travel extremely fast to get to Mercury, and when they reach the planet, they must suddenly slow down enough to get into orbit just as the Sun’s gravity is trying to accelerate them even more. In addition, the Sun’s pull is so strong near Mercury that orbits around the planet are unstable, and proximity to the Sun makes it hard for spacecraft to maintain a stable temperature. Nonetheless, Mariner and MESSENGER have reached the planet successfully and studied its features and properties. A third major mission, the joint European–Japanese BepiColombo, may reveal more about this intriguing planet.
1973 Mariner 10
LAUNCH EARTH ORBIT JOURNEY TO MERCURY
MESSENGER (Mercury surface, space
environment, geochemistry, and ranging)
left Earth in 2004 but took over six years
to achieve orbit around Mercury—the
first craft ever to do so. On March 29,
2011, it sent the first photo from Mercury
orbit. Since then, MESSENGER’s cameras
and other instruments have returned
a flood of data about the planet. Its
investigations have discovered water ice
and organic compounds in shadowed
craters near Mercury’s north pole.
Mariner 10 image of Mercury’s cratered surface Mariner 10
Mariner 10’s first Mercury flyby took place on March 29,
1974. Because getting a craft into orbit around the planet
was so difficult, Mariner 10 was designed to orbit the Sun
instead, enabling it to fly past Mercury three times. These
flybys revealed a highly cratered surface and, to the great
surprise of astronomers, a magnetic field around the planet.
MESSENGER had to circle the Sun seven times to get into its orbit around Mercury. It passed Earth a year after launch and then Venus twice, using both planets’ gravity to slingshot itself onward. It then made three flybys of Mercury to slow down before entering orbit. Its orbit is very eccentric: its lowest point is just 124 miles (200 km) above the surface, while the highest is at an altitude of over 9,300 miles (15,000 km).