Lecture 17: Mercury- The Winged Messenger with a 3:2 Spin-Orbit Resonance

"A hot time in the old town tonight."

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Date: November 8, 1994
Reading Assignment: pp. 202-216

Description : geology, atmosphere, interior, magnetosphere, and orbit of Mercury

Objectives

Be able to describe what the spin orbit coupling of Mercury is and why it results.

Be able to describe the inferred internal structure of Mercury.

Be able to describe why we think Mercury has this internal structure.

Be able to compare the internal structure of Mercury to the Earth.

Be able to describe the basic surface features of the planet Mercury and how they are similar and different from the Moon.

Be able to describe scarp and why they have formed on Mercury.

Be able to describe why atmospheres form and escape from planets.

Describe how Caloris Basin was formed and how this created "Weird Terrain".

Lecture Outline

Slide # 1: Lecture 17: Mercury- The Winged Messenger with a 3:2 Spin-Orbit Resonance

Slide # 2: Mercury

orbit and rotation

composition and internal structure

surface features and history

atmosphere

magnetosphere

Slide # 3: Lecture 16: The Solar System- The Big Picture

sizes and scale

a tour of the neighborhood

atmospheres, gravity, and heat

young and old surfaces

Slide # 4: Planetary Structure

core

mantle

crust

hydrosphere

atmosphere

magnetosphere

Slide # 5: Terrestrial Planets and Moons

smaller planets and moons

Slide # 6: Planetary Orbits

inner solar system

Slide # 7: Planets and Moons

Terrestrial- rocky

Earth, Venus, Mars, Mercury

Jovian- Gas Giants

Jupiter, Saturn, Neptune, Uranus

Rocky Moons

the Moon, Io, Ceres

Icy Moons

Pluto, Europa, Callisto

Slide # 8: Moons

Mercury, Venus - none

Earth - 1

Mars - 2

Jupiter - 16

Saturn - 18

Uranus - 15

Neptune - 8

Pluto - 1

Slide # 9: Planetary Masses

Mercury = 1/20

Venus = 4/5

Earth = 1

Mars = 1/9

Jupiter = 318

Saturn = 95

Uranus = 15

Neptune = 17

Pluto = 0.002

Slide # 10: Mercury- Revolution

semimajor axis = 0.387 AU

elliptical orbit

mimimum radius = 0.31AU

maximum radius = 0.47 AU

orbital period = 88 days

Slide # 11: Mercury Seen from Earth

Mercury can never be more than 28 degrees from the Sun

Slide # 12: MercuryÕs Rotation

the Mercury rotates 3 times for every two orbits

two Mercury years is the same as three sidereal days on Mercury

Slide # 13: MercuryÕs Rotation

after 1/2 a rotation

Slide # 14: MercuryÕs Orbit

day 0

Slide # 15: MercuryÕs Orbit

day 15

Slide # 16: MercuryÕs Orbit

day 22

Slide # 17: MercuryÕs Orbit

day 30

Slide # 18: MercuryÕs Orbit

day 44

Slide # 19: MercuryÕs Orbit

day 59

Slide # 20: MercuryÕs Orbit

day 67

Slide # 21: MercuryÕs Orbit

day 74

Slide # 22: MercuryÕs Orbit

day 88

Slide # 23: MercuryÕs Orbit

day 103

Slide # 24: MercuryÕs Orbit

day 110

Slide # 25: MercuryÕs Orbit

day 118

Slide # 26: MercuryÕs Orbit

day 132

Slide # 27: MercuryÕs Orbit

day 147

Slide # 28: MercuryÕs Orbit

day 155

Slide # 29: MercuryÕs Orbit

day 162

Slide # 30: MercuryÕs Orbit

day 176

Slide # 31: Spin-Orbit Resonance

the 3:2 spin-orbit relationship is a tidal resonance

occurs because of tidal effects from the Sun

3:2 ratio occurs from

elliptical orbit of Mercury

orbital speed at perihelion determines rotation rate

Slide # 32: Two Types of Density

planetary average

entire planet

mass / volume

atmospheric density

density of gases in the atmosphere

is the atmosphere thin or thick?

Slide # 33: Planetary Density

density = mass / volume

average density depends on two things

composition

iron and nickel have high densities

hydrogen has a low density

compression from gravity

more mass means more compression

Saturn and Jupiter have the similar composition, but different densities

Slide # 34: MercuryÕs Composition

average density = 5.4 gm/cm3

very similar to Earth (5.5 gm/cm3)

much less total mass (0.055 Earth masses)

a large fraction of the planet must be an iron core

the fractional size of the core must be much larger than Earth

Slide # 35: MercuryÕs Interior

EarthÕs core vs MercuryÕs core

Slide # 36: MercuryÕs Atmosphere

no real atmosphere

some hydrogen and helium from the solar wind becomes temporarily trapped

some traces of potassium and sodium have also been detected

Slide # 37: Atmospheres

Why does the Earth have an atmosphere?

Why doesnÕt Mercury have an atmosphere?

Slide # 38: Atmospheres- Gravity

determines the escape velocity

speed needed to escape the planetÕs gravity

high mass planets have high escape velocities

Slide # 39: Atmospheres- Temperature

determines the average molecular energy

high temperature means faster molecular motion

Slide # 40: Atmospheres- Molecular Weight

determines the average molecular speed

low mass molecules have high speeds

Slide # 41: Atmospheres- Earth

medium mass planet

escape velocity = 11 km/s

medium temperature

300 K

gas composition

Nitrogen and Oxygen cannot escape

Hydrogen and Helium escape

Slide # 42: Atmosphere- The Moon

low mass planet

2 km/s escape velocity

medium temperature

260 K (average)

gas composition

all gases escape

Slide # 43: Atmosphere- Jupiter

very massive planet

escape velocity high

low temperature

125 K

gas composition

No gases escape

Slide # 44: Atmosphere- Mercury

low mass planet

escape velocity low

high temperature

700K daytime (100K night)

all gases escape

Slide # 45: Atmospheres

molecular speed and escape velocity

Slide # 46: Surface of Mercury

images from Mariner 10

1974 planetary flyby

approached within 10,000 km

Slide # 47: Planetary Surfaces

crater density tells us surface ages

can be used without landing on the planet

Slide # 48: Geological Features

heavily cratered surface

intercrater plains

lower crater density than the moon

scarp

places where the crust wrinkled when the planet cooled

weird terrain

rippled terrain opposite to Caloris basin

produced by seismic waves during the Caloris impact

Slide # 49: Geological Features

no active volcanos

no true plate tectonics

evidence of old lava flows

Slide # 50: Surface of Mercury

heavily cratered

Slide # 51: Intercrater Plains

quadrangle of Mercury

Slide # 52: Caloris Basin

subsolar point at aphelion

Slide # 53: Weird Terrain

impact created strong seismic waves

waves were refocused opposite to Caloris

Slide # 54: Weird Terrain

unusual ripples caused by the Caloris impact

Slide # 55: Magnetic Fields

two conditions needed to form magnetic fields

liquid metal core

rapid rotation

weak magnetic field has been detect

Slide # 56: Mercury

weak magnetic field has been detected

large liquid core

VERY slow rotation

we donÕt understand why it has a magnetic field

Slide # 57: Comparative Planetology

helps us understand the origin of the solar system

helps us understand Earth