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. Actual Surf Temps

Mercury 350° F -333-746° F

Venus 134° F 800-900° F

Earth 44° F 26-80° F

Mars -45° 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 approachAphelion=farthest distance

What is being shown here?

The Sun from Mercury at perihelion and aphelion

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:

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.

Orbital period: 365 days

Rotation period: 1 day

For Earth:

Orbital period: 88 Earth days

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.

Orbital period: 365 days

Rotation period: 1 day

For Earth: For Mercury:

Orbital period: 88 Earth days

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.

Orbital period: 365 days

Rotation period: 1 day

For Earth: For Mercury:

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?

rock

Iron meteorite

(A) Rock

(B) Iron

(C) Both rock and iron

Mercury is small, and composed largely of iron.

Mercury

Earth

1800 km2400 km

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 fly-by mission (it did not orbit the planet). Why do you think this is?

• Heavily cratered terrain

• Intercrater plains• The Caloris Basin• Antipodal terrain• Scarps

Mercury’s main geological features:

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!• 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 never-before-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 rotationdir. of orbital motion

rotating faster than synchronous

rotating slower than synchronous

dir. of orbital motion

dir. of orbital motion

Forces

Forces

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!)