Mercury
Mercury
Mercury
Mercury is always close to the sun!
What is being shown here?
The sun looks bigger from Mercury than from Earth
Effective Temperature :
An estimate of the surface temperature of a planet,
based only on its distance from the sun
Assumptions:
Energy hitting surface of planet= Energy radiated by planet
Planet has no atmosphere
Planet absorbs all of the light that hits it
Effective Temperature :
An estimate of the surface temperature of a planet,
based only on its distance from the sun
Planet
Eff. Temp.
Mercury
350° F
Venus
134° F
Earth
44° F
Mars
-45° F
Actual Surf
Temps
-333-746°
F
800-900°
F
26-80° F
-190- -10°
F
What’s going on? Why is Mercury not
uniformly hot?
Does Mercury have seasons like on Earth?
Winter on Earth
Summer on Earth
Any time on Earth!
One of these is a circle. One is Earth’s (elliptical) orbit.
Does Mercury have seasons like on Earth?
Does Mercury have seasons like on Earth?
Mercury’s orbital path around the sun
Mercury’s orbit has a higher eccentricity.
Perihelion=closest approach
Aphelion=farthest distance
What is being shown here?
The Sun from Mercury at perihelion and aphelion
Mercury has a 2:3 resonance between its spin and
orbital periods. So every time Mercury goes ~2/3 of the
way around the sun, it has rotated once.
For Earth:
Orbital period:?
Rotation period: ?
Mercury has a 2:3 resonance between its spin and
orbital periods. So every time Mercury goes ~2/3 of the
way around the sun, it has rotated once.
For Earth:
Orbital period: 365 days
Rotation period: 1 day
Mercury has a 2:3 resonance between its spin and
orbital periods. So every time Mercury goes ~2/3 of the
way around the sun, it has rotated once.
For Earth:
For Mercury:
Orbital period: 365 days
Orbital period: 88 Earth days
Rotation period: 1 day
Rotation period: 59 Earth days
Mercury has a 2:3 resonance between its spin and
orbital periods. So every time Mercury goes ~2/3 of the
way around the sun, it has rotated once.
For Earth:
For Mercury:
Orbital period: 365 days
Orbital period: 88 Earth days
Rotation period: 1 day
Rotation period: 59 Earth days
Orbital period: 1 1/2 Mercury days
Rotation period: 1 Mercury day
Class Action: Terrestrial planets: Length of Mercury night
Messenger Animation: A day on Mercury
Also, the poles on Mercury never see sunlight!
Images of north polar region colored by amount of
sunlight received
Summary of Mercury’s orbital characteristics:
-Mercury is much closer to the sun than Earth
-Mercury’s tilt is very low, so it does not have seasons caused by the tilt
-Its rotation period is almost equal to its orbital period, so there are long
days, long nights
-Mercury’s orbit is eccentric so the sun changes size throughout the
year
-Both the long days/long nights and eccentricity cause extreme
temperature swings
-The poles of Mercury never see sunlight and can be very cold, so
different parts of the surface of Mercury can be different temperatures
Now let’s talk about Mercury’s interior and its geology
Density is a measure of compactness.
Density is a measure of compactness.
The density of rock is ~2.5 g/cm3.
The density of iron is ~8 g/cm3.
The density of terrestrial planets is
~5 g/cm3. Given this information,
what are terrestrial planets most
likely made of?
(A) Rock
rock
(B) Iron
(C) Both rock and iron
Iron meteorit
Mercury is small, and composed largely of iron.
1800 km
2400 km
Mercury
Earth
How did Mercury get this dense?
How did Mercury get this dense?
Two ideas:
Heat from the early sun vaporized many of the rocky materials
when Mercury was forming.
A giant impact stripped off the original crust and mantle.
Really, no one knows.
How might you test these hypotheses?
Does Mercury have an atmosphere?
Sort of. Hydrogen, Helium, Oxygen, Sodium, Potassium and
Calcium have been detected.
But atmospheric pressures are very low
Why is this?
What is Mercury’s surface like?
Mariner 10 (c. 1974) was the first and only spacecraft
to visit Mercury before 2008.
Mariner 10 was a flyby mission (it did not
orbit the planet).
Why do you think
this is?
Mercury’s main
geological features:
• Heavily cratered
terrain
• Intercrater plains
• The Caloris Basin
• Antipodal terrain
• Scarps
Heavily cratered highlands and
intercrater plains
What caused
the intercrater
plains?
The Caloris Basin
Caloris
Antipode
Scarps and Ridges
These scarps are fault
scarps.
What caused them?
Here is a fault scarp on the Earth (in Idaho)
http://skywalker.cochise.edu/wellerr/pglnx/chapter17x.htm
Ice on Mercury??
Messenger mission to Mercury.
Launched in 2004.
Now orbiting Mercury!
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Why is Mercury so dense?
What is the geologic history of
Mercury?
What is the structure of
Mercury's core?
What is the nature of Mercury's
magnetic field?
What are the unusual materials
at Mercury's poles?
What volatiles are important
at Mercury?
The Caloris Basin
The Caloris Basin
(a false color image)
The Caloris Antipode
Strange “blue hollows” on
Mercury represent neverbefore-seen minerals
Images of north polar region shows very reflective
materials in shadowed regions (like craters)
Images of north polar region shows very reflective
materials in shadowed regions (like craters)
The highest-resolution radar image of Mercury's south polar region
made from the Arecibo Observatory (Harmon et al., Icarus, 211, 37-50,
2011) is shown in white on MESSENGER orbital images colorized by
the faction of time the surface is illuminated. (Text: Marc Boucher)
Crater counting lecture tutorial
Why did Mercury end up in this state?
Planets like to be in ‘synchronous’ rotation
because of tides. The closer a planet is to the
sun, the stronger the tides.
in synchronous rotation
dir. of orbital
motion
Forces
rotating faster than synchronous
dir. of orbital
motion
Forces
rotating slower than
synchronous
dir. of orbital
motion
Why did Mercury end up in this state?
Mercury’s orbit is eccentric.
During which part of an orbit is the tidal force greatest?
At this point of the orbit, does the planet move slower or faster than
average?
To keep its bulges aligned while near this point, will it need to rotate
faster or slower?
(For many years, scientists assumed Mercury was in synchronous rotation!)