ASTRONOMY 161
Introduction to Solar System Astronomy
Class 18
Mercury
Friday, February 23
Mercury: Basic characteristics
Mass = 3.302×1023 kg (0.055 Earth)
Radius = 2,440 km (0.383 Earth)
Density = 5,427 kg/m³
Sidereal rotation period = 58.6462 d
Albedo = 0.11 (Earth = 0.39)
Average distance from Sun = 0.387 A.U.
Mercury: Key Concepts
(1) Mercury has a 3-to-2 spin-orbit coupling
(not synchronous rotation).
(2) Mercury has no permanent atmosphere
because it is too hot.
(3) Like the Moon, Mercury has cratered
highlands and smooth plains.
(4) Mercury has an extremely large iron-rich
core.
(1) Mercury has a 3-to-2 spin-orbit coupling
(not synchronous rotation).
Mercury is hard
to observe from
the Earth (because
it is so close to
the Sun).
Its rotation speed
can be found from
Doppler shift of radar
signals.
Mercury’s unusual orbit
Orbital period = 87.969 days
Rotation period = 58.646 days = (2/3) x 87.969 days
Mercury is NOT in synchronous rotation (1 rotation
per orbit).
Instead, it has 3-to-2 spin-orbit coupling (3 rotations
for 2 orbits).
Synchronous
rotation
(WRONG!)
3-to-2 spin-orbit
coupling
(RIGHT!)
Time between one noon
and the next is
176 days.
Sun is above the horizon
for 88 days at the time.
Daytime temperatures reach as high as:
700 Kelvin (800 degrees F).
Nighttime temperatures drops as low as:
100 Kelvin (-270 degrees F).
(2) Mercury has no permanent atmosphere
because it is too hot (and has low escape speed).
Temperature is a
measure of the
random speed of
atoms (or
molecules).
At a given temperature,
massive atoms move
more slowly than
low-mass atoms.
HOT/COLD
3kT
v=
m
v = typical speed of atom
T = temperature (Kelvin)
m = mass ot atom (molecule)
k = a constant of nature
If an atom’s speed is greater than the escape speed, it
will fly away on a hyperbolic orbit.
Low-mass atoms escape first, then “sluggish”,
massive ones.
vesc
2GM
=
r
v = escape speed
M = mass of planet
r = radius of planet
G = universal constant of gravitation
Daytime on Earth:
T = 300 Kelvin, vesc = 11.2 km/sec
Hydrogen and helium escape.
Daytime on Mercury:
T = 700 Kelvin, vesc = 4.3 km/sec
All gases escape.
Daytime on the Moon:
T = 260 Kelvin (no greenhouse effect)
vesc = 2.4 km/sec
Because vesc is low, all gases escape, despite low T.
Only one spacecraft
has visited
Mercury: Mariner
10 (a fly-by).
It has only imaged
45% of Mercury’s
surface.
(3) Like the Moon, Mercury has cratered
highlands and smooth, lava-covered plains.
The hemisphere of
Mercury that has
been photographed
resembles the Moon:
(Crater walls are
slightly lower.)
Highlands are
heavily cratered.
Low-lying impact
basins, such as
the Caloris
basin, are
covered with
smoother lava
flows.
Unique mercurian feature:
Opposite the Caloris basin is “jumbled terrain”,
filled with closely spaced hills.
“Jumbled terrain” was created when seismic waves
from the Caloris impact converged on the antipodes.
(Planet acts as a giant “lens”).
Another strange mercurian feature:
Mercury has scarps, or cliffs, as much as 3 km high,
500 km long:
They are probably the result of cracking as the crust
cooled.
(4) Mercury has an extremely
large iron-rich core.
Mercury is exceptionally dense:
5400 km/m3.
The Earth is slightly denser, but only because
it is compressed by its own strong gravity.
Mercury must have a huge dense core
(probably mostly iron, like Earth’s).
Radius of
Mercury =
2400 km.
Radius of iron
core =
1800 km.
Why does Mercury have a big metal core?
• Perhaps the solar nebula was so hot that only metals
could condense easily.
• Perhaps an intense solar wind blew away most of
Mercury’s rocky mantle.
• Perhaps an impact with a giant planetesimal stripped
the mantle.
We still have much to learn about Mercury
Head-on impact of
Mercury with a
giant planetesimal:
Most of mantle attains
escape speed.
Return to Mercury:
The MESSENGER
spacecraft
(MErcury Surface,
Space ENviroment,
GEochemistry and
Ranging)
Launched 2004:
Mercury orbit in
2011
Scientific mission of MESSENGER
1) Why is Mercury so dense?
2) What is the geologic history of Mercury?
3) What is the structure of Mercury's core?
4) What is the nature of Mercury's magnetic field?
5) What are the unusual materials at Mercury's
poles?
6) What volatiles are important at Mercury?
Few closing questions:
1) Name the major differences and similarities
between Mercury and the Moon.
2) Name the major differences and similarities
between Mercury and the Earth.
3) How dense is the atmosphere of Mercury?
4) Is there magnetic field on Mercury?
5) What is the name of Mercury’s moon?
6) Are there volcanoes on Mercury?