Mercury
Although it is uncertain when Mercury was discovered, its existence
has been known since antiquity due to its visibility to the naked eye.
Moreover, it is because the ancient Romans had knowledge of Mercury that
it received its name. The explanation is simple. Since Mercury orbits
the Sun faster than any other planet in the solar system, it appears
and disappears more quickly than other objects in the sky. Observing
this phenomenon, the Romans named the planet after the fleet-footed
messenger of the gods found in their mythology—Mercury. What remains intriguing about Mercury is how scant our knowledge has
been regarding some of its key details until recently. For example, it
was only in 1974 that the Mariner 10 spacecraft captured the first
images providing any specific detail of the Mercurial surface. Within
the last several years, unexpected discoveries regarding Mercury's
atmosphere and interior have challenged previously accepted theories.
Now is an exciting time of discovery and new understanding of
Mercury. On March 18,2011, the MESSENGER spacecraft achieved orbit
around Mercury in what will be a year-long mission. Hopefully, many of
our questions will be answered, and, almost certainly, new ones will
arise.
Atmosphere
The Mercurial atmosphere is so thin that it is virtually non-existent. In fact, at approximately 10
15 times less dense than the Earth's atmosphere, Mercury's is closer to a true vacuum than any man-made vacuum ever created.
The
explanation behind the lack of substantial atmosphere is twofold.
Firstly, with a gravity only about 38% that of the Earth's, Mercury is
simply unable to retain much of an atmosphere. Secondly, Mercury's close
proximity to the Sun causes it to be constantly bombarded by solar
winds, which carries off most of what little atmosphere does accumulate.
However,
as meager as its atmosphere is, Mercury does have one. According to
NASA, its chemical composition is thought to be as follows: 42% oxygen
(O
2), 29% sodium, 22% hydrogen (H
2), 6% helium,
.5% potassium, and possibly trace amounts of argon, carbon dioxide,
water, nitrogen, xenon, krypton, neon, calcium (Ca, Ca+), and magnesium.
A
notable result of such a sparse atmosphere is the extreme temperatures
found on the planet's surface. With a low temperature of approximately
-180° C and high of approximately
430° C.
Mercury has the largest range of surface temperatures found on any
planet. The extreme highs present on the side facing the Sun are due to
the insufficient atmosphere, for it is unable to absorb the solar
radiation. As for the extreme colds on the side facing away from the
Sun, without a substantial atmsophere to trap solar radiation, all heat
is lost to space.
Surface
Until 1974, the
Mercurial surface remained, in large part, a mystery to scientists due
to Mercury's close proximity to the Sun. Being so close to the Sun
restricts Mercury's visibility to just before dawn or right after dusk.
At these times, unfortunately, the angle at which we see Mercury from
the ground takes our line of sight through a significant amount of the
Earth's atmosphere, greatly obstructing our view.
However, during
its three fly-bys of Mercury in 1974, the Mariner 10 spacecraft captured
clear and stunning photographs of the planet's surface. Amazingly,
Mariner 10 photographed almost half of the planet's surface during its
mission! The results revealed that Mercury's surface has three
significant features.
The first feature is the huge number of
impact craters that have accrued over billions of years. Caloris Basin
is the largest of these with a diameter of
1,550 km.
The second feature is the plains found between craters. These are
smooth areas of the surface, hypothesized to have been created from
ancient lava flows. The third feature is the cliffs (also known as
scarps), which run anywhere from tens to thousands of
kilometers in length and from
one hundred meters to
two kilometers in height.
The
importance of these two features lies in what they imply. By the
presence of ancient lava fields, it is clear that there was volcanic
activity at one time. However, given the number and age of the craters,
scientists have concluded that Mercury has been geologically inactive
for a significant period.
The third of the surface features tells
us something of great interest as well. Essentially, the scarps found on
the surface are huge cliffs caused by the buckling of the planet's
crust. What is significant about the buckling on Mercury is what it
implies. To understand the significance, we must use a comparison. The
buckling on Earth is due to the shifting of tectonic plates, whereas the
buckling on Mercury is due to the shrinking of its core. The
implication is that because Mercury's core is shrinking so, too, is the
planet as a whole. Recent estimates show that Mercury's diameter has
decreased in excess of
1.5 kilometers.
Interior
Broadly speaking, Mercury's interior is composed of three distinct layers: a crust, mantle, and core.
The planet's crust is estimated to be between
100 and 300 kilometers
in thickness. Mercury's surface is part of the crust, so the presence
of the previously mentioned scarps indicates that the crust is solid and
brittle.
With a thickness of approximately
600 kilometers,
Mercury's mantle is relatively thin. Supposedly, its mantle has not
always been this thin. Mercury used to have a much thicker mantle, but
the leading theory explains that during formation of the Solar System, a
large planetesimal collided with the planet, launching most of the
mantle into space.
Mercury's core has been the focus of much research. With an estimated diameter of
3,600 kilometers,
the core gives the planet some ineteresting properties. The most
obvious of these properties is Mercury's extreme density for its size.
With a planetary diameter of
4,878 kilometers, Mercury is smaller than the Jovian moon Ganymede and the Saturnian moon Titan, whose diameters are
5,270 km and
5,152 km, respectively. However, with a mass of
3.3 x 1023 kg, Mercury's density of
5,400 kg/m3 is almost that of the Earth's
5,540 kg/m3.
One debate that has recently be resolved regarding Mercury's core
is whether or not it is solid or liquid. By measuring how radio waves
bounced off of the planet, scientists were able to determine that the
core is, in fact, liquid. More specificallly, the data gathered help to
measure the amount of wobble in Mercury's rotation. With a solid core
the rotation would be rigid, whereas with a liquid core there are small
amounts of variation in the rotation due to the liquid inside "sloshing"
around.
Orbit & Rotation
With Mercury being
so much closer to the Sun than any other planet, it naturally takes the
shortest time to complete one full orbit. As a result, a Mercurial year
is only about 88 Earth days in length.
A significant feature of
Mercury's orbit is its high eccentricity as compared to other planets.
Furthemore, of all the planetary orbits, Mercury's is the least
circular. What this effectively means is that the difference between
Mercury's perihelion—the closet orbital distance to the Sun—of
4.60 x 107 km and its aphelion—farthest orbital distance from the Sun—of
6.98 x 107 km is rather large when contrasted with that of the other planets.
This
eccentricity, along with the lack of a substantial atmosphere, helps to
explain why Mercury's surface experiences a wider range of temperature
extremes than any other planet in the Solar System. Simply put,
Mercury's surface is much hotter at perihelion than at aphelion due to
the large difference between the two distances. Conversely, the
temperatures of other planets remain relatively stable because their
aphelion and perphelion are virtually the same.
Mercury's orbit is
also significant in that it provides a beautiful example of modern
physics. Over time the orbit of Mercury shifts slightly around the Sun
(See Figure 1). This process is known as precession.
Though Newtonian mechanics (i.e., classical physics) does a fairly good
job of predicting the speed of this precession, it still cannot predict
it precisely. This was a persistent problem for astronomers in the late
nineteenth and early twentieth century. Many theories were introduced
to explain the difference between the actual and theoretical rates. One
theory even suggested the existence of a unknown planet closer to the
Sun than Mercury. However, the truth was finally revealed when Einstein
published his general theory of relativity. As a consequence of this
theory, Mercury's orbital precession was finally described exactly.
Though
it was long believed that Mercury's spin-orbit resonance (the number of
rotations per orbit) was 1:1, it was discovered in the mid-tweentieth
century that it is, in fact, 3:2. This resonance produces what would be
fascinating phenomena for a witness on the planet's surface. For
example, the Sun would appear to rise to the highest point in the sky,
then reverse its course and set in the same direction from where it
rose.
Mercury Facts
Discovery Date: Unknown
Number of Moons: 0
Rings: No
Time it takes light to reach Mercury from the Sun:
3 min 16 sec
Mass:
3.3010 x 1023 kg
Diameter:
4,878 km
Density:
5,400 kg/m3
Length of Day:
58.65 Earth Days
Length of Year:
87.97 Earth Days
Orbital Eccentricity:.206
Average Orbital Distance from the Sun:
5.78 x 107 km
Perihelion:
4.60 x 107 km
Aphelion:
6.98 x 107 km
Axial Tilt: 0.0352°
Geologically Active: No
Atmosphere:
Extremely Thin
Recent Planetary Missions:
MESSENGER
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