In 1841, the German
astronomer Johann Franz Encke determined
Mercury’s mass by measuring its gravitational effect on
a comet that now bears his name. This measurement was within 20%
of the best modern measurement of 3.3 x 1023 kilograms,
or about 5.5% of Earth’s mass.
Knowing the mass of Mercury doesn't tell us its exact composition,
but since we know the planet's volume as well, we can estimate
what the interior is made of. We start by dividing the planet's
mass by its volume to find its average density.
Each element or compound has its own characteristic density, and
a planet is made up of many materials of different densities.
From the average density, scientists can guess what materials,
and how much of each, the planet contains.
For example, Earth has an average density of 5.5 g/cm3,
which is between the density of iron (8 g/cm3) and
that of the silicate
compounds found in rocks (3 g/cm3). We believe that
Earth has an interior core of iron making up about 16% of the
Earth’s volume. Mercury has a density similar to Earth,
5.4 g/cm3. Because of Mercury’s small size and
high density, scientists believe that about 70% of Mercury’s
mass is composed of iron, mostly contained in its core.
but true: Mercury is the second smallest planet,
but it is also the second densest body in our solar system.
Only Earth is denser than Mercury. Jupiter, the gas giant,
is the most massive of our planets, but at 1.3 g/cm3
it is denser only than Saturn.
The Mariner 10 mission
sent back more than just pictures of Mercury's surface. On-board
instruments detected a weak magnetic
field - about 100 times weaker than Earth's. This came as
a surprise, since the photographs from this mission showed a Moon-like
surface, and the Moon's magnetic field is about 100 times weaker
yet. A planet's magnetic field shields it from constant bombardment
of charged particles streaming outward from the Sun - the solar
Mercury’s magnetic field and high density both indicate
that the planet has a large, iron-rich core, probably about 3600
kilometers in diameter. This core takes up a large fraction of
the planet's volume, since Mercury's total diameter is only 4880
kilometers. Mercury’s ratio of core size to planet size
is the largest of any planet in the solar system. Mercury probably
has only a thin lunar-like silicate mantle.
but true: Mercury’s iron core is thought to
extend from the planet’s center to nearly three quarters
of the way to the surface, making it about the size of our
Before the Mariner
10 mission in 1974, Mercury’s surface features were little
more than a blur to Earth-bound observers. Mariner 10 made the
of Mercury on March 29, 1974 at a distance of about 700 kilometers.
Its high-resolution photographs of about 50% of Mercury’s
surface allowed scientists to view the planet close-up. On its
surface, Mercury closely resembles our Moon. Impact craters cover
the majority of the planet but unlike the Moon, Mercury’s
cratered upland regions are covered with large areas of smooth
plains. The most distinguishing features on Mercury’s surface
or long cliffs. These wind across Mercury’s surface for
tens to hundreds of kilometers and range from 100 meters to over
1.5 kilometers in height. What makes these cliffs so unique is
that no other planet or moon features such a vast number of them.
They are thought to be thrust faults created when the planet,
as it cooled, shrunk by up to 4 km in diameter.
300 km scarp (above) runs from upper right corner of the
picture to the lower left. (Click on the image to see an
Scarp: This scarp is about 350 kilometers long and transects
two craters. The maximum height is about 3 kilometers. (Click
on the image to see an enlarged version.)
largest surface feature photographed by the Mariner 10 mission
is the Caloris basin. This is a multi-ringed (resembling a bull's-eye)
impact basin 1,340 km across - almost ¼ of the full diameter of
the planet. The basin includes a series of circular mountain ranges
up to 3 km in height - the tallest mountains on Mercury. Caloris
is thought to have been produced when a very large asteroid collided
with the planet about 4 billion years ago. The massive impact
sent seismic waves echoing through the planet. Coming to a focus
on the opposite side of Mercury, these intense waves created there
a region of hilly and broken terrain.
but true: The Caloris Basin is so large that it would
engulf the entire state of Texas. The name Caloris is derived
from the Latin word "calor" or heat.
multi-ringed Caloris Basin, taken from Mariner 10.
half of the basin appears in this photomosaic. (Click on
the image to see an enlarged version.)
and fractures on the floor of the Caloris Basin
and jumbled terrain on the side of Mercury opposite the
Caloris Basin. (Click on the image to see an enlarged version.)
ability to keep an atmosphere depends largely on its mass and
surface temperature, as well as on the composition of the atmosphere
itself. Large planets with strong gravitational fields, such as
Earth, are more likely to retain their atmosphere. Since Mercury
is a small planet compared to Earth it was once thought that any
gases on Mercury would have escaped into space long ago. Yet the
onboard instruments of Mariner 10 showed that Mercury does have
a trace atmosphere. The atmospheric pressure is quite small -
about one trillion times less than that at sea level on Earth.
Elements known to be present in the Mercurian atmosphere include
helium, sodium, and oxygen.
but true: Atmospheric size and composition determine
how the sky will appear to someone on a planet's surface.
For instance, the sky on Earth appears blue while the sky
on Mars would appear to be a pinkish-red. Due to the absence
of a substantial atmosphere on Mercury, the sky would appear
black even during daytime - except for the Sun, which would
appear two to three times larger than we see on the Earth.
With no atmosphere, a visitor to Mercury would hear no sounds.
the most extreme temperature range of any of the planets. When
Mercury is farthest from the Sun (at aphelion)
the predawn temperature reaches a frigid -180°C (-300°F).
As Mercury’s orbit reaches perihelion
the mid-afternoon temperatures can skyrocket to 430°C (800°F).
Mercury’s slow rotational period, its nearness to the Sun,
and its lack of a substantial atmosphere are together responsible
for this enormous variation in temperature. The period from sunrise
to sunset lasts for 88 Earth days allowing the intense solar radiation
to heat the surface. But during the nights (also 88 days long)
the temperature falls dramatically since heat is not trapped within
a thick atmosphere, as it is on Earth or Venus.
The seasons on Earth are caused by the tilt of its axis of rotation.
Mercury’s axis is nearly perpendicular to the plane of it’s
orbit, so it has no significant tilt and the planet has no seasons.
The poles of Mercury never receive sunlight, and the temperature
there remains below -160°C. It is here that scientists believe
that frozen water may be found.
but true: Although Mercury is the closest planet
to the Sun, it is not the hottest. That distinction goes to
Venus, which has a surface temperature of about 460°C.
This is due to the dense atmosphere of carbon dioxide, a very
gas. Solar radiation penetrates Venus' clouds and its
heat becomes trapped.