- Origin of name: In Roman mythology,
the god of commerce, travel and thievery; In Greek mythology
Hermes, the messenger of the Gods (in the evening; Apollo in
the morning); Probably received this name because it moves quickly
across the sky.
- Viewing: Best viewed at dawn
or sunset; Always close to the Sun; Has a full cycle of phases;
Can be seen occasionally transiting the Sun [will do so 14 times
in the 21st century, the first one was in 2003, and then parallax
will give us its distance].
- Size and mass: Radius about
40% of Earth's [Mass about 5.5% of Earth's]; From its mass and
volume we know that its average density is high, almost as high
as the Earth's.
- Motion: Orbit 0.4 au from the
Sun, so 1 Mercury year = 88 days, considerably elliptical; Slow
rotation with period = 59 days = 2/3 year, in resonance with
the orbit [so on Mercury, 1 solar day = 2 years!]. Rotation
axis is perpendicular to orbit.
- Past missions: Until
this year, only flybys by Mariner
10 probe in 1974, mostly 10,000 km from the surface; The spacecraft
only took pictures of half of the planet.
- Current missions: NASA's
Messenger mission, launched in 2004, flew by Mercury in 2008 in preparation
to enter into orbit in 2011.
- Future missions: The
ESA together with Japan's ISAS are planning the Bepi-Colombo orbiter
mission for 2011 launch.
- Surface: Mercury's appearance
is similar to the Moon's, but there are fewer craters; Solidified
volcanic lava from past activity fills many craters and plains
[but there are no maria].
- Special features: Very large
Caloris basin [1300 km diameter] due to impact, and "weird/wild
terrain"; Cliffs or scarps (hundreds of km long) due to
tectonic stresses from planet shrinking, after most of the cratering.
Almost none; Gas comes out of the hot planet, but gravity
is too weak to hold much of it near the surface; Even the oldest
remaining craters are mostly sharp and uneroded.
- Consequences: No sounds, rain,
clouds, blue sky; Wide temperature swings [from –150°C, about
–250°F, to 400°C, about 800°F]; One of the hottest
places in the Solar System but may also have ice inside dark
craters (Messenger will look).
Interior and Evolution
It must have a large, heavy, iron-rich core [Mercury is
not as compressed as the Earth, and yet its density is similar],
probably molten but relatively cold, with a thinner rock mantle;
There is a weak magnetic field, but no geological activity in
the mantle and crust for 3 Gyr.
- Formation: Formed about 4.5
billion years ago with the rest of the Solar System; A giant
impact may have vaporized most of the original envelope, made
of lighter material.
- Evolution: Mercury cooled slowly
because of its closeness to the Sun; Intense volcanic activity
erased early craters, but the crust was compressed as the core
cooled and prevented later volcanic flows [after the first 700
million years or so].
[The number of past impacts can be taken to be indirect evidence for asteroid-like
objects near the Sun (Vulcanoids).]
page by luca bombelli <bombelli at olemiss.edu>,
modified 29 sep 2012