is the innermost and smallest planet in the solar system, orbiting the Sun once
every 88 days. It ranges in brightness from about −2.0 to 5.5 in apparent
magnitude, but is not easily seen as its greatest angular separation from the
Sun (greatest elongation) is only 28.3°. It can only be seen in twilight.
Comparatively little is known about the planet: the only spacecraft to approach
Mercury was Mariner 10 from 1974 to 1975, which mapped only 40%–45% of the
Physically, Mercury is similar in appearance to the Moon as
it is heavily cratered. It has no natural satellites and no substantial
atmosphere. The planet has a large iron core which generates a magnetic field
about 1% as strong as that of the Earth. Surface temperatures on Mercury range
from about 90 to 700 K (-180 to 430°C), with the subsolar point being the
hottest and the bottoms of craters near the poles being the coldest.
The Romans named the planet after the fleet-footed messenger god Mercury,
probably for its fast apparent motion in the twilight sky. The astronomical
symbol for Mercury, displayed at the top of the infobox, is a stylized version
of the god's head and winged hat atop his caduceus, an ancient astrological
symbol. Before the 5th century BC, Greek astronomers believed the planet to be
two separate objects: one visible only at sunrise, the other only at sunset. In
India, the planet was named Budha (बुध), after the son of Chandra
(the Moon). The Chinese, Korean, Japanese, and Vietnamese cultures refer to the
planet as the water star (水星), based on the Five Elements. The Hebrews
named it Kokhav Hamah (כוכב חמה), "the star of the hot one" ("the hot
one" being the Sun).
Mercury is one of the four terrestrial planets, meaning that like the Earth
it is a rocky body. It is the smallest of the four, with a diameter of 4879 km
at its equator. Mercury consists of approximately 70% metallic and 30% silicate
material. The density of the planet is the second-highest in the solar system at
5.43 g/cm³, only slightly less than Earth's density. When corrected for
gravitational compression, Mercury is in fact denser than Earth, with an
uncompressed density of 5.3 g/cm³ versus Earth's 4.4 g/cm³.
Internal structure: core, mantle and crust
Mercury's high density can be used to infer details of its inner structure.
While the Earth's high density results partly from compression at the core,
Mercury is much smaller and its inner regions are not nearly so compressed.
Therefore, for it to have such a high density, its core must be large and rich
in iron. Geologists estimate that
Mercury's core occupies about 42% of its volume. (Earth's core occupies about
17% of its volume.)
Surrounding the core is a 600 km mantle. It is generally thought that early
in Mercury's history, a giant impact with a body several hundred kilometers
across stripped the planet of much of its original mantle material, resulting in
the relatively thin mantle compared to the sizable core
 (alternative theories are
Mercury's crust is thought to be 100–200 km thick. One very distinctive
feature of Mercury's surface is numerous ridges, some extending over several
hundred kilometers. It is believed that these were formed as Mercury's core and
mantle cooled and contracted after the crust had solidified.
Mercury has a higher iron content than any other major planet in our solar
system. Several theories have been proposed to explain Mercury's high
metallicity. The most widely accepted theory is that Mercury originally had a
metal-silicate ratio similar to common chondrite meteors and a mass
approximately 2.25 times its current mass; but that early in the solar system's
history, Mercury was struck by a planetesimal of approximately 1/6 that mass.
The impact would have stripped away much of the original crust and mantle,
leaving the core behind. A
similar theory has been proposed to explain the formation of Earth's Moon (see
giant impact theory).
Alternatively, Mercury may have formed from the solar nebula before the Sun's
energy output had stabilized. The planet would initially have had twice its
present mass. But as the protosun contracted, temperatures near Mercury could
have been between 2500 and 3500 K, and possibly even as high as 10000 K. Much of
Mercury's surface rock could have vaporized at such temperatures, forming an
atmosphere of "rock vapor" which could have been carried away by the solar wind.
A third theory suggests that the solar nebula caused drag on the particles
from which Mercury was accreting, which meant that lighter particles were lost
from the accreting material. Each of
these theories predicts a different surface composition, and two upcoming space
missions, MESSENGER and BepiColombo, both aim to take observations that will
allow the theories to be tested.
Mercury's surface is very similar in appearance to that of the Moon, showing
extensive mare-like plains and heavy cratering, indicating that it has been
geologically inactive for billions of years. The small number of unmanned
missions to Mercury means that its geology is the least well understood of the
terrestrial planets. Surface features are given the following names:
0.466 698 35 AU
0.307 499 51 AU
0.387 098 93 AU
||0.205 630 69
||87.969 34 d
(0.240 846 9 a)
|Avg. orbital speed:
|Max. orbital speed:
|Min. orbital speed:
(3.38° to Sun's equator)
|Longitude of ascending node:
|Argument of perihelion:
|Equatorial surface gravity:
|Sidereal rotation period:
||58.6462 d (58 d 15.5088 h)
|Rotation velocity at equator:
||10.892 km/h (at the equator)
|Right ascension of North pole:
||281.01° (18 h 44 min 2 s) 1
9.5% Atomic Oxygen
5.6% Molecular Oxygen
3.6% Carbon dioxide
- Albedo features — areas of markedly different reflectivity
- Dorsa — ridges (see List of ridges on Mercury)
- Montes — mountains (see List of mountains on Mercury)
- Planitiae — plains (see List of plains on Mercury)
- Rupes — scarps (see List of scarps on Mercury)
- Valles — valleys (see List of valleys on Mercury)
During and shortly following the formation of Mercury, it was heavily
bombarded by comets and asteroids for a period that came to an end 3.8 billion
years ago. During this period of intense crater formation, the planet received
impacts over its entire surface, facilitated by the lack of any atmosphere to
slow impactors down. During this time the planet was volcanically active; basins
such as the Caloris Basin were filled by magma from within the planet, which
produced smooth plains similar to the maria found on the Moon.
Craters on Mercury range in diameter from a few meters to hundreds of
kilometers across. The largest known crater is the enormous Caloris Basin, with
a diameter of 1300 km. The impact which created the Caloris Basin was so
powerful that it caused lava eruptions and left a concentric ring over 2 km tall
surrounding the impact crater. At the antipode of the Caloris Basin is a large
region of unusual, hilly terrain known as the "Weird Terrain". One hypothesis
for the origin of this geomorphologic unit is that shock waves generated during
the impact traveled around the planet, and when they converged at the basin's
antipode (180 degrees away) the high stresses were capable of fracturing the
surface. Alternatively, it has been
suggested that this terrain formed as a result of the convergence of ejecta at
this basin's antipode.
The plains of Mercury have two distinct ages: the younger plains are less
heavily cratered and probably formed when lava flows buried earlier terrain. One
unusual feature of the planet's surface is the numerous compression folds which
crisscross the plains. It is thought that as the planet's interior cooled, it
contracted and its surface began to deform. The folds can be seen on top of
other features, such as craters and smoother plains, indicating that they are
more recent. Mercury's surface is
also flexed by significant tidal bulges raised by the Sun—the Sun's tides on
Mercury are about 17% stronger than the Moon's on Earth.
Like the Moon, the surface of Mercury has likely incurred the effects of
space weathering processes. Solar wind and micrometeorite impacts can darken the
albedo and alter the reflectance properties of the surface.
The mean surface temperature of Mercury is 452 K (353.9°F, 178.9°C), but it
ranges from 90 K (-297.7°F, -183.2°C) to 700 K (800.3°F, 426.9°C), due to the
absence of an atmosphere; by comparison, the temperature on Earth varies by only
about 150 K. The sunlight on Mercury's surface is 6.5 times as intense as it is
on Earth, with a solar constant value of 9.13 kW/m².
Despite the generally extremely high temperature of its surface, observations
strongly suggest that ice exists on Mercury. The floors of some deep craters
near the poles are never exposed to direct sunlight, and temperatures there
remain far lower than the global average. Water ice strongly reflects radar, and
observations reveal that there are patches of very high radar reflection near
the poles. While ice is not the
only possible cause of these reflective regions, astronomers believe it is the
The icy regions are believed to be covered to a depth of only a few meters,
and contain about 1014–1015 kg of ice. By comparison, the
Antarctic ice sheet on Earth weighs about 4×1018 kg, and Mars' south
polar cap contains about 1016 kg of water. The origin of the ice on
Mercury is not yet known, but the two most likely sources are from outgassing of
water from the planet's interior or deposition by impacts of comets.
Mercury is too small for its gravity to retain any significant atmosphere
over long periods of time; it has a tenuous atmosphere containing hydrogen,
helium, oxygen, sodium, calcium and potassium. The atmosphere is not
stable—atoms are continuously lost and replenished, from a variety of sources.
Hydrogen and helium atoms probably come from the solar wind, diffusing into
Mercury's magnetosphere before later escaping back into space. Radioactive decay
of elements within Mercury's crust is another source of helium, as well as
sodium and potassium. Water vapor is probably present, being brought to Mercury
by comets impacting on its surface.
Despite its slow rotation, Mercury has a relatively strong magnetic field,
with a magnetic field strength 1% as strong as the Earth's. It is possible that
this magnetic field is generated in a manner similar to Earth's, by a dynamo of
circulating liquid core material. However, scientists are unsure whether
Mercury's core could still be liquid,
although it could perhaps be kept liquid by tidal effects during periods of high
orbital eccentricity. It is also possible that Mercury's magnetic field is a
remnant of an earlier dynamo effect that has now ceased, with the magnetic field
becoming "frozen" in solidified magnetic materials.
Mercury's magnetic field is strong enough to deflect the solar wind around
the planet, creating a magnetosphere inside which the solar wind does not
penetrate. This is in contrast to the situation on the Moon, which has a
magnetic field too weak to stop the solar wind impacting on its surface and so
lacks a magnetosphere.
Orbit and rotation
The orbit of Mercury is the most eccentric of the major planets, with the
planet's distance from the Sun ranging from 46,000,000 to 70,000,000 kilometers.
It takes 88 days to complete the orbit. The diagram on the left illustrates the
effects of the eccentricity, showing Mercury’s orbit with a circular orbit with
the same semi-major axis. The higher velocity of the planet when it is near
perihelion is clear from the greater distance it covers in each 5-day interval.
The size of the spheres, inversely proportional to their distance from the Sun,
illustrates the varying heliocentric distance. This varying distance to the Sun,
combined with a unique 3:2 resonance of the planet's rotation around its axis,
result in complex variations of the surface temperature.
Mercury's orbit is inclined by 7° to the plane of Earth's orbit (the
ecliptic), as shown in the diagram on the left. As a result, transits of Mercury
across the face of the Sun can only occur when the planet is crossing the plane
of the ecliptic at the time it lies between the Earth and the Sun. This occurs
about every seven years on average.
Mercury's axial tilt is only 0.01 degrees. This is over 300 times smaller
than that of Jupiter, which is the second smallest axial tilt of all planets at
3.1 degrees. This means an observer at Mercury's equator during local noon would
never see the sun more than 1/100 of one degree north or south of the zenith.
At certain points on Mercury's surface, an observer would be able to see the
Sun rise about halfway, then reverse and set before rising again, all within the
same Mercurian day. This is because approximately four days prior to perihelion,
Mercury's angular orbital velocity exactly equals its angular rotational
velocity so that the Sun's apparent motion ceases; at perihelion, Mercury's
angular orbital velocity then exceeds the angular rotational velocity. Thus, the
Sun appears to be retrograde. Four days after perihelion, the Sun's normal
apparent motion resumes.
Advance of perihelion
When it was discovered, the slow precession of Mercury's orbit around the Sun
could not be completely explained by Newtonian mechanics, and for many years it
was hypothesized that another planet might exist in an orbit even closer to the
Sun to account for this perturbation (other explanations considered included a
slight oblateness of the Sun). The success of the search for Neptune based on
its perturbations of Uranus' orbit led astronomers to place great faith in this
explanation, and the hypothetical planet was even named Vulcan. However, in the
early 20th century, Albert Einstein's General Theory of Relativity provided a
full explanation for the observed precession. Mercury's precession showed the
effects of mass dilation, providing a crucial observational confirmation of one
of Einstein's theories—Mercury is slightly heavier at perihelion than it is at
aphelion because it is moving faster, and so it slightly "overshoots" the
perihelion position predicted by Newtonian gravity. The effect is very small:
the Mercurian relativistic perihelion advance excess is just 43 arcseconds per
century. The effect is even smaller for other planets, being 8.6 arcseconds per
century for Venus, 3.8 for Earth, and 1.3 for Mars.
Research indicates that the eccentricity of Mercury's orbit varies
chaotically from 0 (circular) to a very high 0.47 over millions of years. This
is thought to explain Mercury's 3:2 spin-orbit resonance (rather than the more
usual 1:1), since this state is more likely to arise during a period of high
For many years it was thought that Mercury was synchronously tidally locked
with the Sun, rotating once for each orbit and keeping the same face directed
towards the Sun at all times, in the same way that the same side of the Moon
always faces the Earth. However, radar observations in 1965 proved that the
planet has a 3:2 spin-orbit resonance, rotating three times for every two
revolutions around the Sun; the eccentricity of Mercury's orbit makes this
resonance stable — at perihelion, when the solar tide is strongest, the Sun is
nearly still in Mercury's sky. The original reason astronomers thought it was
synchronously locked was because whenever Mercury was best placed for
observation, it was always at the same point in its 3:2 resonance, hence showing
the same face. Due to Mercury's 3:2 spin-orbit resonance, a solar day (the
length between two meridian transits of the Sun) lasts about 176 Earth days. A
sidereal day (the period of rotation) lasts about 58.7 Earth days.
Mercury's apparent magnitude varies between about -2.0 - brighter than Sirius
- and 5.5. Observation of Mercury
is complicated by its proximity to the Sun, as it is lost in the Sun's glare for
much of the time. Mercury can be observed for only a brief period during either
morning or evening twilight. The Hubble Space Telescope cannot observe Mercury
Mercury exhibits moonlike phases as seen from Earth, being "new" at inferior
conjunction and "full" at superior conjunction. The planet is rendered invisible
on both of these occasions by virtue of its rising and setting in concert with
the Sun in each case. The half-moon phase occurs at greatest elongation, when
Mercury rises earliest before the Sun when at greatest elongation west, and
setting latest after the Sun when at greatest elongation east (its separation
from the Sun ranging from 18.5° if it is at perihelion at the time of the
greatest elongation to 28.3° if it is at aphelion).
Mercury attains inferior conjunction every 116 days on average, but this
interval can range from 111 days to 121 days due to the planet's eccentric
orbit. Its period of retrograde motion as seen from Earth can vary from 8 to 15
days on either side of inferior conjunction. This large range also arises from
the planet's high degree of orbital eccentricity.
Mercury is more often easily visible from Earth's Southern Hemisphere than
from its Northern Hemisphere; this is because its maximum possible elongations
west of the Sun always occur when it is early autumn in the Southern Hemisphere,
while its maximum possible eastern elongations happen when the Southern
Hemisphere is having its late winter season. In both of these cases, the angle
Mercury strikes with the ecliptic is maximized, allowing it to rise several
hours before the Sun in the former instance and not set until several hours
after sundown in the latter in countries located at South Temperate Zone
latitudes, such as Argentina and New Zealand. By contrast, at northern temperate
latitudes, Mercury is never above the horizon of a more-or-less fully dark night
sky. Mercury can, like several other planets and the brightest stars, be seen
during a total solar eclipse.
Mercury is brightest as seen from Earth when it is at a gibbous phase,
between half full and full. Although the planet is further away from Earth when
it is gibbous than when it is a crescent, the greater illuminated area visible
more than compensates for the greater distance. The opposite is true for Venus,
which appears brightest when it is a thin crescent.
Studies of Mercury
Mercury has been known since at least the 3rd millennium BC, when it was
known to the Sumerians of Mesopotamia as Ubu-idim-gud-ud, among other
names. The Babylonians (2000–1000 BC) succeeded the Sumerians, and early
Babylonians may have recorded observations of the planet: although no records
have survived, late Babylonian records from the 7th century BC refer to much
earlier records. The Babylonians called the planet Nabu or Nebu
after the messenger to the Gods in their mythology.
The ancient Greeks gave the planet two names: Apollo when it was visible in
the morning sky and Hermes when visible in the evening. However, Greek
astronomers came to understand that the two names referred to the same body,
with Pythagoras being the first to propose the idea.
Ground-based telescopic research
The first telescopic observations of Mercury were made by Galileo in the
early 17th century. Although he observed phases when he looked at Venus, his
telescope was not powerful enough to see the phases of Mercury. In 1631 Pierre
Gassendi made the first observations of the transit of a planet across the Sun
when he saw a transit of Mercury predicted by Johannes Kepler. In 1639 Giovanni
Zupi used a telescope to discover that the planet had orbital phases similar to
Venus and the Moon. The observation demonstrated conclusively that Mercury
orbited around the Sun.
A very rare event in astronomy is the passage of one planet in front of
another (occultation), as seen from Earth. Mercury and Venus occult each other
every few centuries, and the event of May 28, 1737 is the only one historically
observed, having been seen by John Bevis at the Royal Greenwich Observatory.
The next occultation of Mercury by Venus will be in 2133.
The difficulties inherent in observing Mercury mean that it has been far less
studied than the other planets. In 1800 Johann Schröter made observations of
surface features, but erroneously estimated the planet's rotational period at
about 24 hours. In the 1880s Giovanni Schiaparelli mapped the planet more
accurately, and suggested that Mercury's rotational period was 88 days, the same
as its orbital period due to tidal locking.
This phenomenon is known as synchronous rotation and is also shown by Earth's
The theory that Mercury's rotation was synchronous became widely held, and it
was a significant shock to astronomers when radio observations made in the 1960s
questioned this. If Mercury were tidally locked, its dark face would be
extremely cold, but measurements of radio emission revealed that it was much
hotter than expected. Astronomers were reluctant to drop the synchronous
rotation theory and proposed alternative mechanisms such as powerful
heat-distributing winds to explain the observations, but in 1965 radar
observations showed conclusively that the planet's rotational period was about
59 days. Italian astronomer Giuseppe Colombo noted that this value was about
two-thirds of Mercury's orbital period, and proposed that a different form of
tidal locking had occurred in which the planet's orbital and rotational periods
were locked into a 3:2 rather than a 1:1 resonance.
Data from Mariner 10 subsequently confirmed this view.
Ground-based observations did not shed much further light on the innermost
planet, and it was not until space probes visited Mercury that many of its most
fundamental properties became known. However, recent technological advances have
led to improved ground-based observations: in 2000, high-resolution lucky
imaging from the Mount Wilson Observatory 60-inch telescope provided the first
detailed views of the parts of Mercury which were not imaged in the Mariner
Research with space probes
Reaching Mercury from Earth poses significant technical challenges, since the
planet orbits so much closer to the Sun than does the Earth. A Mercury-bound
spacecraft launched from Earth must travel over 91 million kilometers into the
Sun's gravitational potential well. Starting from the Earth's orbital speed of
30 km/s, the change in velocity (delta-v) the spacecraft must make to enter into
a Hohmann transfer orbit that passes near Mercury is large compared to other
The potential energy liberated by moving down the Sun's potential well
becomes kinetic energy; requiring another large delta-v to do anything other
than rapidly pass by Mercury. In order to land safely or enter a stable orbit
since the planet has very little atmosphere, the approaching spacecraft cannot
use aerobraking and must rely on rocket motors. A trip to Mercury actually
requires more rocket fuel than that required to escape the solar system
completely. As a result, only one space probe has visited the planet so far.
The only spacecraft to approach Mercury so far has been NASA's Mariner 10
(1974–75). The spacecraft
used the gravity of Venus to adjust its orbital velocity so that it could
approach Mercury—the first spacecraft to use this gravitational "slingshot"
effect. Mariner 10 provided the first close-up images of Mercury's surface,
which immediately showed its heavily cratered nature, and also revealed many
other types of geological features, such as the giant scarps which were later
ascribed to the effect of the planet shrinking slightly early in its geological
history. Unfortunately, the same face of the planet was lit at each of Mariner
10's close approaches, resulting in less than 45% of the planet's surface being
The spacecraft made three close approaches to Mercury, the closest of which
took it to within 327 km of the surface. At the first close approach,
instruments detected a magnetic field, to the great surprise of planetary
geologists—Mercury's rotation was expected to be much too slow to generate a
significant dynamo effect. The second close approach was primarily used for
imaging, but at the third approach, extensive magnetic data were obtained. The
data revealed that the planet's magnetic field is much like the Earth's, which
deflects the solar wind around the planet. The Moon's magnetic field, on the
other hand, is so weak that the solar wind reaches the surface. However, the
origin of Mercury's magnetic field is still the subject of several competing
Just a few days after its final close approach, Mariner 10 ran out of fuel,
its orbit could no longer be accurately controlled and mission controllers
instructed the probe to shut itself down. Mariner 10 is thought to be still
orbiting the Sun, still passing close to Mercury every few months.
A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space
ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004, from
the Cape Canaveral Air Force Station aboard a Boeing Delta 2 rocket. The
MESSENGER spacecraft will make several close approaches to planets to place it
onto the correct trajectory to reach an orbit around Mercury. It made a close
approach to the Earth in February 2005, and to Venus in October 2006. Another
Venusian encounter will follow in 2007, followed by three close approaches to
Mercury in 2008 and 2009, after which it will enter orbit around the planet in
The mission is designed to shed light on six key issues: Mercury's high
density, its geological history, the nature of its magnetic field, the structure
of its core, whether it really has ice at its poles, and where its tenuous
atmosphere comes from. To this end, the probe is carrying imaging devices which
will gather much higher resolution images of much more of the planet than
Mariner 10, assorted spectrometers to determine abundances of elements in the
crust, and magnetometers and devices to measure velocities of charged particles.
Detailed measurements of tiny changes in the probe's velocity as it orbits will
be used to infer details of the planet's interior structure.
Japan is planning a joint mission with the European Space Agency called
BepiColombo, which will orbit Mercury with two probes: one to map the planet and
the other to study its magnetosphere. An original plan to include a lander has
been shelved. Russian Soyuz rockets will launch the probes in 2013. As with
MESSENGER, the BepiColombo probes will make close approaches to other planets en
route to Mercury, passing the Moon and Venus and making several approaches to
Mercury before entering orbit. The probes will reach Mercury in about 2019,
orbiting and charting its surface and magnetosphere for a year.
The probes will carry a similar array of spectrometers to those on MESSENGER,
and will study the planet at many different wavelengths including infrared,
ultraviolet, X-ray and gamma ray. Apart from intensively studying the planet
itself, mission planners also hope to use the probe's proximity to the Sun to
test the predictions of General Relativity theory with improved accuracy.
The mission is named after Giuseppe (Bepi) Colombo, the scientist who first
determined the nature of Mercury's orbital resonance with the Sun and who was
also involved in the planning of Mariner 10's gravity-assisted trajectory to the
planet in 1974.