“Motions” in the Heavens

Fixed Stars, Moving Planets

• Stars remain in fixed positions with each other– Constellations (groups of stars)

• Planets “wander” with respect to stars– Retrograde Motion

Aristotelian Geocentrism• Earth is center of Universe– Concentric celestial spheres in which the Sun, Moon, and

stars are imbedded like jewels– Unblemished, symmetrical Universe– People don’t fall of the Earth

as it moves…– Parallax

• Not observed in stars

– Did not explain retrograde motion

Copernicus, 1530

• Heliocentric – Sun, center of Solar System

• Explained retrograde motion

Tycho Brahe, Johannes Kepler

• Brahe: Mapped positions of known bodies– Could predict where any planet would be seen in

near future– Drank himself to death

• Kepler: Planets move in elliptical orbits– Brahe’s student– Mathematical formulae to describe motion

Galileo vs. Aristotle

• “Law of Inertia” and falling off the Earth• The Milky Way as a collection of stars (not a

cloud of light)• Jupiter’s Moons vs. Geocentrism

IoGanymede

Galileo vs. Aristotle

• Sunspots and Lunar Maria– Not “homogenous unblemished bodies”

Isaac Newton

• Popular myth: Isaac Newton invented gravity

• A moving body travels in a straight path unless acted on by an outside force– Gravity between Sun and planets makes for

elliptical orbits

The Earth-Moon System

The Lunar Orbit

Phases of the Moon

Eclipses

VS

Red Shift

• Works for sound or light– If the object is moving

toward you, it will have a higher frequency (waves/s)• Louder (sound)• Bluer (light)

– If moving away (lower frequency):• Softer (sound)• Redder (light)

The Big Bang Theory

• All matter and energy compressed into single inconceivably dense point

• Universe expands and thins to form the galaxies and stars

• 13.7 billion years old “explosion”

Nebular Hypothesis

• Origin of solar system traced to rotating clouds of gas + fine dust– 1755, Immanuel Kant

– Nebulae (“fog” or “cloud”)

– Mostly hydrogen and helium

– These two elements make up all but small fraction of Sun

Gas and very fine dust particles

Temperature rises to millions of degrees

Nuclear fusion begins H atoms combine to form Helium

sunlight!

Some Rocky Bits Are Left

• Most rocky bits have been swept away (accreted onto forming planets)

• Now, rocky bits that are left are basically trapped due to gravity

Asteroid Belt trapped between Mars (4) and Jupiter (5)

Asteroid Belt

The SS at a glance…

• 4.6 billion yrs old• All planets, at first, were composed of gas• Protoplanets closest to sun were hot—gasses

moving fast enough to escape gravitational pull of Pplanet

• Solar wind—a stream of particles originating from the sun—pushed gasses from inner SS to outer

• Terrestrial planets vs. Jovian planets

Terrestrial planets Jovian Planets

Dwarf Planets

Mercury

The Terrestrial Planets…

• Closest planet to the sun

• Smallest planet– radius = 2,400 km

• 1 mercury year = 88 earth days

• Spins slowly on its axis– One day on mercury =

59 earth days– Insane temp. differences

• Side facing sun = 427°C• Far side = - 175°C

– Thin He atmosphere– Magnetic field—possibly

created by liquid core

1. Solid crust (100-300 km thick)2. Silicate Mantle (600 km thick)3. Molten Iron Core (1800 km radius)

Mercury Geology• Mariner 10• Impact craters

• Caloris baisn-1550 km wide—impact so powerful, planetary scale effects—weird terrane

– Intercrater plains—heavily cratered plains between large impact craters

– Smooth plains—similar to lunar mare—lava filled depressions

– Compression folds—long, linear folds caused by partial cooling of interior

Terrestrial Planets:

Venus

• 108 million km from sun• Very similar in size to

Earth—radius 6052 km• Dense atmosphere of

CO2—sulfuric acid clouds• Atmospheric pressure at

surface = 92 times that of Earth

• One year on Venus = 224 earth days

• Spins slowly on axis– One Venusian day = 244

earth days• Mean surface temp =

460°C

Radar image of Venus

True color photo of VenusVenus: UV

Studying Venus• Spacecraft– Russian Venera project

• V1 and 2—flyby • V3—crashed into surface• V4—probe—accurate atm temp, pressure, and comp—ran out of

power• V5 and V6—crushed by atm pressure• V7—crashed• V8—landed and took temp, atm comp and pressure readings for

50 min• V9 and V10—sent first images of surface—survived only 93

minutes• V11-12—detected electrical storms• V13-14—first color pictures• V15-16--orbiters

Studying Venus Cont…• Magellan—1989

– Radar mapped 99% of Venusian surface

• Strange observations– Only 1000 pristine impact

craters—absence of small (<2km) craters

– 167 >Mt. Everest sized shield volcanoes• Pancake domes—caused

by mantle plumes– 60% of surface is flat– No evidence of plate

tectonics—can explain lack of magnetic field

Why is Venus so hot?

• Venus and earth probably had similar atmospheres

• Closer proximity to sun = higher mean temperatures

• Too hot for water to condense• No oceans = no way to remove CO2 from

atmosphere• CO2 + water vapor = runaway greenhouse effect• Water vapor boiled off into space

Terrestrial Planets:

Earth

Everyone should nowknow more about Earth Than any other planet in the Solar system…

Terrestrial Planets:

Mars

• 150 million km from sun• About half the size of

earth (3400 km radius)• Cold– Average temp = -56°C– Temps at poles can drop

below -120°C• Dry– If you were to condense

all water vapor in atmosphere—global layer only 1 mm thick (Earth 3 km)

• Thin, CO2 atmosphere• 2 moons – Phobos and Deimos

Phobos Deimos

Studying Mars• Originally, mars was believed

to have oceans– Poor resolution of early

telescopes– Human eye predisposed to

discerning lines• Viking project—mapped

surface– Viking I and II—landers—

returned first color photos of surface

• Mars Global Surveyor—very successful orbiter—photographed most of the Martian surface

• Spirit and Opportunity landers—still operational

Geology of Mars• Old heavily cratered plains

and young, flat, crater poor volcanic plains

• Tharsis bulge--four huge shield volcanoes– Olympus mons—largest

volcano in SS (25 km tall, 500 km wide)

• Tectonic scheme similar to Venus—dominated by volcanism and mantle plumes– Valles Marineris—a large

tectonic crack in the martian crust—caused by cooling of planet and by uplift in Tharsis region--

Wind and Water on Mars?• Liquid water is unstable

on the martian surface• Ice caps

– Water ice covered by dry ice (CO2)

• Permafrost from poles to 60°Latitude

• Evidence of large floods– Eastern portion of Valles

Marineris• Dust devils• Dust storms

Martian dust devils