Messenger: Mercury
Mercury as seen by the Messenger probe, January 14, 2008. This image shows half of the hemisphere missed by Mariner 10 in 1974?75 and was snapped by Messenger's Wide Angle Camera when it was about 27,000 km (17,000 miles) from the planet.
Mercury
, the innermost
planet
of the
solar system
and the eighth in size and mass. Its closeness to the
Sun
and its smallness make it the most
elusive
of the planets visible to the unaided
eye
. Because its rising or setting is always within about two hours of the Sun’s, it is never observable when the sky is fully dark. Mercury is designated by the symbol ?.
Mercury
Mosaic view of Mercury, showing about half the hemisphere of the planet that was illuminated when Mariner 10 departed the planet during its first flyby in March 1974. The landscape is dominated by large impact basins and craters with extensive intercrater plains. Half of the enormous Caloris impact basin is discernible as a slightly darker region near the terminator (shadow line) just above centre.
The difficulty in seeing it notwithstanding, Mercury was known at least by
Sumerian
times, some 5,000 years ago. In
Classical Greece
it was called
Apollo
when it appeared as a morning star just before sunrise and
Hermes
, the Greek equivalent of the Roman god
Mercury
, when it appeared as an evening star just after sunset. Hermes was the
swift
messenger of the gods, and the planet’s name is thus likely a reference to its rapid motions relative to other objects in the sky. Even in more recent eras, many sky observers passed their entire lifetimes without ever seeing Mercury. It is reputed that
Nicolaus Copernicus
, whose
heliocentric model
of the heavens in the 16th century explained why Mercury and
Venus
always appear in close proximity to the Sun, expressed a deathbed regret that he had never set eyes on the planet Mercury himself.
Messenger: Mercury
Image of Mercury captured by a camera aboard the Messenger spacecraft.
Until the last part of the 20th century, Mercury was one of the least-understood planets, and even now the shortage of information about it leaves many basic questions unsettled. Indeed, the length of its
day
was not determined until the 1960s, and Mercury’s nearness to the Sun gave scientists bound to Earth many observational hurdles, which were overcome only by the
Messenger
(
Me
rcury
S
urface,
S
pace
En
vironment,
Ge
ochemistry, and
R
anging) probe. Messenger was launched in 2004, flew past the planet twice in 2008 and once in 2009, and settled into
orbit
in 2011. It mapped the entire surface of Mercury before crashing into the planet in 2015. Mercury’s proximity to the Sun has also been exploited to confirm predictions made by
relativity theory
about the way
gravity
affects
space and time
.
Planetary data for Mercury
|
*Time required for the planet to return to the same position in the sky relative to the Sun as seen from Earth.
|
mean distance from Sun
|
57,909,227 km (0.39 AU)
|
eccentricity of orbit
|
0.2056
|
inclination of orbit to ecliptic
|
7.0°
|
Mercurian year (sidereal period of revolution)
|
87.97 Earth days
|
maximum visual magnitude
|
?1.9
|
mean synodic period*
|
116 Earth days
|
mean orbital velocity
|
47.36 km/sec
|
radius (mean)
|
2,439.7 km
|
surface area
|
74,797,000 km
2
|
mass
|
3.30 × 10
23
kg
|
mean density
|
5.43 g/cm
3
|
mean surface gravity
|
370 cm/sec
2
|
escape velocity
|
4.25 km/sec
|
rotation period (Mercurian sidereal day)
|
58.646 Earth days
|
Mercurian mean solar day
|
175.9 Earth days
|
inclination of equator to orbit
|
0°
|
magnetic field strength
|
0.003 gauss
|
mean surface temperature
|
440 K (332 °F, 167 °C)
|
surface temperature extremes
|
700 K (800 °F, 430 °C);
|
90 K (?300 °F, ?180 °C)
|
typical surface pressure
|
about 10
?15
bar
|
number of known moons
|
none
|
At first glance the surface of the planet looks similar to the cratered terrain of the
Moon
, an impression reinforced by the roughly comparable size of the two bodies. Mercury is far denser, however, having a metallic core that takes up about 61 percent of its volume (compared with 4 percent for the
Moon
and 16 percent for
Earth
). Moreover, its surface shows significant differences from lunar
terrain
, including a lack of the massive dark-coloured
lava
flows known as
maria
and the presence of buckles and scarps that suggest Mercury is shrinking.
Britannica Quiz
Everything in Space in a 25-Minute Quiz