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Mercury Facts
Mean distance from Sun:
0.3871 AU (57,910,000 km/35,980,000 mi)
Length of Year:
88 days
Rotation period:
58.65 days
Mean orbital velocity:
48 km/s (30 mi/s)
Inclination of axis:
Average temperature:
800° F (427° C) day
-300° F (-183° C) night
4,878 km (3,031 mi)
Number of observed satellites:
Magnetic field
•Mercury is one of the five planets known to the
ancients. They called these planets "wandering stars".
•Mercury may be seen as an evening "star" near where
the sun has set, or as a morning "star" near where the
sun will rise.
•The ancient Greeks called the evening star Hermes and
the morning star Apollo, believing them to be different
•The planet is named for Mercury, the Roman
messenger of the gods.
Mercury is not much bigger
than our Moon. Of the nine
planets orbiting the Sun, only
Pluto is smaller.
It is so close to the Sun that
it can be seen only in the
twilight sky of the Earth.
The hazy atmosphere near the
horizon on Earth spoils the view
of Mercury for ground-based
Earth-based telescopic photo of Mercury taken from
Catalina Observatory 1.5 meter (61 inch) telescope.
Mercury Orbital Facts
Until 1965, scientists thought that the same side of Mercury always faced
the Sun.
Mercury takes only 88
Earth days to complete
one orbit, but it rotates
slowly about its own
axis, only once every 59
Earth days.
This means that it
rotates precisely three
times for every two
orbits, known as a 3:2
orbital resonance.
More Orbital Facts…
It orbits the Sun in a highly elliptical orbit at about one third of the Sun-Earth
distance. Only the orbit of Pluto, the outermost planet, is more elliptical.
At perihelion (closest approach to the Sun), it is only 46 million kilometers from
the Sun, but at aphelion (furthest approach) it is 70 million kilometers away.
The axis of rotation is almost perpendicular to the orbital plane,
so Mercury does not have opposite seasons in each
hemisphere, as Earth has.
Instead, the temperature at the equator varies as the planet's
distance from the Sun changes during its elliptical orbit.
The maximum temperature is 700K (427oC), but the minimum
only 90K (-183oC). Mercury has very little atmosphere so the
surface cools down rapidly on the night side.
Transit of Mercury
Visible from
Earth about
every 8 years.
Image of May 7, 2003 Mercury Transit was taken by NASA/ESA's SOHO (Solar & Heliospheric Observatory)
Transit of Mercury on
November 8,2006. Sunspot
#923, which is just below the
equator at the left-hand side, is
much bigger than Mercury is.
Two more sunspots at the righthand side at the equator.
Mercury appears as a small
black dot in the lower middle of
the solar disk.
Next transit: May 9, 2016
Mariner 10
3 Flybys 1974-1975
Mariner 10 was the seventh successful
launch in the Mariner spacecraft series, and
the first to use the gravitational pull of one
planet (Venus) to reach another (Mercury).
Instruments on board the spacecraft were designed to measure the
atmospheric, surface, and physical characteristics of Mercury and Venus.
Experiments included television photography, magnetic field, plasma, infrared
radiometry, ultraviolet spectroscopy, and radio science detectors. An
experimental X-band, high-frequency transmitter was flown for the first time on
the spacecraft.
10,000 pictures with 57% planet coverage reveal an intensely cratered, Moonlike surface and a faint atmosphere of mostly helium, resulting from solar wind
Incoming View of
This photomosaic of
Mercury was
constructed from
photos taken by
Mariner 10 six hours
before the spacecraft
flew past the planet on
March 29, 1974. These
images were taken
from a distance of
5,380,000 kilometers
(3,340,000 miles).
This two-image
mosaic of Mercury
was constructed
from photos taken
by Mariner a few
hours before the
closest and first
encounter with the
planet on March
29, 1974.
Outgoing View
of Mercury
This mosaic of
Mercury was created
from more than 140
images taken by the
Mariner 10
spacecraft on March
29, 1974. The
images were
acquired after the
spacecraft exited
Mercury's shadow.
Mercury’s Atmosphere
Like the Earth’s Moon, Mercury has a very volatile atmosphere. What
little atmosphere exists is made up of atoms or ions blasted off its
surface by the solar wind and has less than a million-billionths the
pressure of Earth's atmosphere at sea level. It is composed chiefly of
oxygen, sodium, and helium.
Mercury's extreme surface temperature enhances the escape of these
volatile atoms into space.
With no atmosphere or hydrosphere, there has been no erosion from
wind or water.
Mercury may have water ice at its north and south poles. The ice exists
inside deep craters. The floors of these craters remain in perpetual
shadow, so the Sun cannot melt the ice.
Meteorites do not burn up due to friction as they do in other planetary
Mercury’s Surface
Highly cratered with smooth
Relatively ancient, volcanic
Similar to Earth’s Moon, but
fewer craters and more
Geologically longer active
than Moon?
Plains mostly lava or impact
Caloris Basin
Largest structure on
Mercury ~1300 km
Asteroid-size impactor
early in solar system
Basin contains smooth
plains but is highly
ridged and fractured.
Antipodal from Caloris
"Weird terrain“, hilly,
lineated region.
The shock wave
produced by the Caloris
impact was reflected
and focused to this
antipodal point, thus
jumbling the crust and
breaking it into a series
of complex blocks.
The area covered is
about 100 kilometers
(62 miles) on a side.
Discovery Rupes
Sinuous feature may be a
thrust fault.
This feature and many
similar ones on Mercury
suggest compressional
forces and 1-2 km radial
shrinkage planet-wide.
200 km
Cooling, contraction od
Mercury at end of
volcanically active early
Mercury’s Interior
Mercury’s density is
similar to Earth’s, but
planet is only ~1/3 the
size of Earth.
Large iron core, 75% of
radius (~1850 km),
Silicate mantle only ~550
km thick.
Origin of Mercury’s Large Core
Unknown, but
1. Radial
(iron) zonation of
the inner solar
2. Young Mercury
was a larger
planet, but giant
impact removed
much of mantle.
Messenger Discovery Mission
MErcury Surface, Space
ENvironment, GEochemistry,
and Ranging mission.
MESSENGER will enter
Mercury orbit in March 2011
and carry out comprehensive
measurements for one Earth
$286 million total mission
Principal Investigator:
Dr. Sean C. Solomon
Carnegie Inst. of Washington
Project Management:
Johns Hopkins University Applied
Physics Laboratory
JHU/APL, GSFC, Univ. Colorado,
Univ. Michigan
Composite Optics, Inc.
GenCorp Aerojet
Jet Propulsion Laboratory
Messenger Mission Summary
Launch dates:
August 3, 2004
Launch Vehicle:
Delta II 7925H
Earth flyby (1) August 2005
Venus flybys (2):
2006, 2007
Mercury flybys (3): 2008, 2009
Mercury orbit:
March 2011
Figure 2. MESSENGER mission timeline featuring major trajectory adjustments (DSM dates may change)
Science Payload
Messenger “Science Drivers”:
•What planetary formation processes led to the high
metal/silicate ratio in Mercury?
•What is the geological history of Mercury?
•What is the nature and origin of Mercury's magnetic field?
•What is the structure and state of Mercury's core?
•What are the radar-reflective materials at Mercury's poles?
•What are the important volatile species and their sources
and sinks on and near Mercury?
MESSENGER’s WAC multi-spectral
images to study compositional
variations across the surface of
Mercury. The white arrows identify
areas of Mercury’s surface that are
interpreted to be relatively young
volcanic plains, and the black arrows
point to reddish areas interpreted to be
volcanoes. Most of the color
differences studied here are believed to
indicate variations in the mineral
composition and physical state of the
rocks at different places on Mercury.
Capturing Mercury
through MESSENGER's
Dual Cameras
Date Acquired: September 29, 2009
Image Mission Elapsed Time (MET): 162709161,
Instrument: Narrow Angle Camera (NAC) and Wide Angle
Camera (WAC) of the Mercury Dual Imaging System
Resolution: Top WAC image: 24.5 kilometers/pixel (15.2
miles/pixel). Bottom NAC image: 3.5 kilometers/pixel (2.2
Scale: Mercury's diameter is 4880 kilometers (3030 miles)
Spacecraft Altitude: 137,000 kilometers (85,000 miles)
Rupes, Rupes, Every
Giant scarps (cliffs), called
rupes, are believed to have
formed when Mercury’s
interior cooled and the entire
planet shrank slightly as a
result. This figure, recently
published in Science magazine,
shows one of these scarps
(white arrows) that is about 270
kilometers (170 miles) long.
Date Acquired: September 29, 2009
Image Mission Elapsed Time (MET): 162744001
Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
Resolution: 410 meters/pixel (0.25 miles/pixel) in the lower right corner of this image
Scale: This image is about 420 kilometers (260 miles) across
Spacecraft Altitude: 16,200 kilometers (10,100 miles)
ESA-ISAS Mission to
Launched 2018,
1 Lander and 2 Orbiters
Will start working 2025
The mission is named in honor of
Giuseppe (Bepi) Columbo, an Italian
scientist who explained Mercury's
unusual rotation (three turns for every
two trips around the Sun). He also
suggested to NASA the way to make
multiple fly-bys of Mercury during the
Mariner 10 mission.