The orbits of the planets - A clue to the formation of our Solar System

All planets orbit the sun in the same direction.

Most rotate in this direction too.

Except for Pluto, most orbits are in the same plain.

Characteristics of the Solar System

• Disk shape of solar system comes from the disk shape of the nebula

• Revolution and rotation of sun and planets are in pretty much the same direction because they all formed from the same rotating gas cloud.

• Orbits of planets lie in a plane because the solar nebula collapsed in a disk and the planets formed in that disk.

• Strange orbits/rotations of Venus, and Uranus were probably caused by a large collision of planetesimals late in their formation.

Formation of Solar System•The solar system is believed to have formed from a cloud of gas and dust in a process know as accretion.

1. As it collapses its slight rotation increases, due to conservation of angular momentum.

Spins faster as it contracts.Centrifugal force pushes matter outward, and a disk forms.

As the collapsing cloud of dust Spins faster, it contracts, and Centrifugal force pushes matter outward, and a disk forms.

2. Collapsing gas and dust heats through collisions to around 3000 K, so everything is in gaseousform. Hydrogen (about 89%) and Helium(about 10%) make up most of nebula, silicates

and iron compounds about 1% .

Nebula cools

The Sun forms in center, and planets form in outer disk. Outer parts cool off more thanthe inner parts , since the temperature, and density dependsupon the distance fromproto-sun.

In the inner solar system, temperatures are hot (1600 K near the central core). Only metal (iron, nickel) and rocks (silicates) can survive in solid form.

Thus inner planets and asteroids are rocky.

In the outer solar system (beyond the asteroid belt), it is cold enough for gases to condense into solid ices (ammonia, methane, water, etc).

3.Gas molecules and dust grains are in circular orbits.Those in noncircular orbits collide withparticles and eventually dampen noncircular motion.shaped zones and, later planets form.

•Planets will also differentiate later on: heavy metals in core - lighter near surface

Gravity tends to divide nebula into ring-

• Materials moving in the same rotating orbit “rub shoulders” with other materials.

• These materials collide and stick together forming planetesimals, which continue to grow.

Forming Planetesimals by Accretion

Growth of Protoplanets

•Jovian planets began as bits of rock and ice that reached 15 Earth masses, and being so massive allows them to capture hydrogen and helium gas directly.

•Terrestrial planets have very little H & He because their low masses can’t keep these gases from leaking into space.

Clearing the Nebula

• (1) The solar wind, streaming particles from the sun, pushes the small dust, and gas out of the nebula.

• (2) The moons and planets are constantly getting bombarded by meteorites. Heavy bombardment—took place roughly 4 billion years ago.

• (3) Ejection of material from the solar system by close encounters with planets

Icy planetesimals near Jupiter and Saturn were flung just outside of out of solar system, while those near Uranus and Neptune were flung to large orbits, becoming the Oort Cloud .

Characteristics of the Solar System• A planet did not form between Mars and

Jupiter in the asteroid belt area, due to the planet Jupiter.

•Jupiter’s gravitational influence kept disturbing the motions of the planetesimals, breaking them up, and ejecting some from the solar system , or toward the sun.

•Jovian worlds all have ring systems. Their large mass makes it easy for them to hold onto orbiting ring particles.

Characteristics of the Solar System• The comets are just remains of the

icy planetesimals that Jupiter threw out far into the solar system.

Name Dist (AU) Radius

Earth =1

Mass

Earth =1

# of

Moons

DensityG/cm^3

Mercury 0.39 0.38 0.05 0 5.43

Venus 0.72 0.95 0.89 0 5.25

Earth 1.00 1.00 1.00 1 5.52

Mars 1.5 .53 0.11 2 3.95

The Terrestrial Planets

Terrestrial Planets: Have few moons Small and close to the Sun Not very massive , but dense They have rocky outer parts, and iron cores.

All planets in the Solar System revolve around the Sun in a counterclockwise direction when viewed from the north pole of the celestial sphere.

Planetary Configurations

As viewed from Earth, Mercury and Venus never get very far from the Sun.

•The maximum elongation angle for Mercury is 28 degrees, and the maximum elongation angle for Venus is 45 degrees.

•Therefore Mercury and Venus are visible only either shortly after sunset, evening star, or shortly before sunrise, morning star.

Because they lie closer to the Sun than the Earth, Mercury and Venus are called inferior planets.

•When the Mercury or Venus is directly between the Earth and the Sun, we say it is at inferior conjunction.

•When the Mercury or Venus is directly behind the Sun as viewed from Earth, we say it is at superior conjunction.

Inferior Planets

                                                                                                                        

When a superior planet lies directly behind the Sun, we say it is at conjunction.

When a superior planet lies directly opposite the Sun as viewed from the Earth, we say it is at opposition.

The planets lying outside the Earth's orbit and are called superior planets. All of the superior planets can be visible at any time of night.

Superior Planets

The Earth

Diameter - 7,926 miles ( 12,756 km).Mass - 5.98 x 10^24 kgVelocity of escape - 24,840 mi/hr.Period of rotation - 23.93 hours Year- 365.26 days to revolve around the Sun.Axis tilt - 23.45 deg, causing the seasons.Rotation speed - 1040 mi/hr (1670 km/hr) at the equator Temperature range - from -127 to 136 deg F.

93 million miles (149,600,00 km) from the sun.

Composition of the Atmosphere

                    The atmosphere is primarilycomposed of Nitrogen (N2, 78%),

Other components present include, water (H2O,

"greenhouse" gases), Ozone , and Carbon Dioxide.There is so much oxygen due to life and photosynthesis. Atmosphere very thin < 100 km

Oxygen (O2, 21%).

The Earth’s atmosphere was formed by planetary degassing, from the interior of the Earth by way of volcanoes, and other life processes.

Layers of the AtmosphereThe atmosphere of the Earth may be divided into several distinct layers, as the following figure indicates.

Weather takes place here

Reflection of radio waves

The Stratosphere and Ozone Layer

The thin ozone layer in the upper stratosphere has a high concentration of ozone.

This layer is primarily responsible for absorbing the ultraviolet radiation from the Sun, and prevents an intense ultraviolet radiation from reaching the surface, where it is quite hazardous to the evolution of life.

The Troposphere : Lowest level where our weather takes place.

The Ionosphere

The ionosphere is very thin, but it is where aurora take place, and is also responsible for absorbing the most energetic photons from the Sun, and for reflecting radio waves, thereby making long-distance radio communication possible.

The greenhouse effect traps heat in our atmosphere.The atmosphere lets some infrared radiation escape into space; some is reflected back to the planet.

Differentiation When the entire earth was molten, the heavy

elements (iron, nickel) sank to the interior. The lighter materials (granite-type rocks) rose to

the surface. The medium density rocks (basalt-type) wound

up in the middle.

Plate Tectonics

The Earth’s crust is composed of huge moving plates of rock, that float on a soft churning layer. Earthquakes, mountains, volcanoes, occur at plate boundaries.

•The Appalachian Mountains were formed from wrinkling of the Earth's surface produced by the collision of the North American and African plates.

Solid Inner Core 2400 km diameter Iron & Nickel

Liquid Outer Core 2270 km thick Iron & Nickel

Mantle 2900 km thick Basaltic rocks

Crust 20-100 km thick Granitic rocks

Inner Core (kept solid by the immense pressure of all the material on top of it.)

Outer Core (less pressure allows it to be a liquid.)

Interior Structure

The Earth's magnetic field and Van Allen radiation belts

The inner and outer Van Allen belts.The primary source of charged particles is the stream of particles emanating from the Sun that we call the solar wind.

The Earth has a magnet in the center, much like a bar magnet.(dipole) The molten interior, and rotation sustains the field. The poles reverse about every 300,000 years.

The belts surrounding the Earth are called the magnetosphere, and they largely prevents the solar wind from entering.

The Planet Mercury

                               

     

MASS: 0.055 (Earth=1) •DENSITY: 5.43 (g/cm^3) •GRAVITY: 0.376 (Earth=1) •ORBIT PERIOD: 87.97 (Earth days)

• solar ROTATION PERIOD: 176 (Earth days ) 88 consecutive days each of sunshine, and darkness.

Mantle – basalt, but not as thick as moon.

Crust – anorthosite, like the moon

Most iron-rich planet in the solar system.

Very elliptical orbit – more than any other planet except Pluto 46 to 70 million km distances from sun

Temps: +800oF to –280oF. More variation than any other planet ! Some water in deep polar craters that are always in shadow. Detected by Arecibo RT

Difficult to study, because it is always near the sun. Sets or rises within 1 to 2 hours of the sun.

Transits occur about twelve times a century when the sun, Earth and Mercury are aligned

Maria are concentrated on the near side of the Moon.

•Heavily cratered surface Less dense cratering than moon

•Gently rolling plains, Scarps & no evidence of tectonics

Mercury’s Surface

LOTS of craters

•Smoother regions are likely ancient lava flows

Features much like the Moon

Basic Moon FactsBasic Moon Facts PropertyProperty ValueValue

DiameterDiameter 3,470 km (2,160 mi)3,470 km (2,160 mi)

Surface GravitySurface Gravity 0.17 (1/6 of Earth’s)0.17 (1/6 of Earth’s)

DensityDensity 3.34 gm/cm3.34 gm/cm33 (0.6 of Earth) (0.6 of Earth)

Sidereal Rotation 27.3 days Phases 29.5 daysSidereal Rotation 27.3 days Phases 29.5 days

Orbital EccentricityOrbital Eccentricity 0.0550.055

Orbital InclinationOrbital Inclination 5.155.15oo

Distance from EarthDistance from Earth 3.84 x 103.84 x 1055 km (240,000 mi) km (240,000 mi)

Temperature RangeTemperature Range 120-390 K (-240 to +240 120-390 K (-240 to +240 ooF)F)

AtmosphereAtmosphere Almost a Vacuum (Ar gas)Almost a Vacuum (Ar gas)

Only world visited by humansNo global magnetic field

The Moon’s surface was shaped by heavy bombardment followed by lava floods

4.5-3.8 billion years ago: Heavy bombardment by planetesimals, cratering of highlands.

3.8-3.1 billion years ago: Lava flows up through cracks, flooding low-lying maria with basalt.

3.1 billion years ago-NOW: No more lava flows, decreasing bombardment.

Composition of the Moon similar to the Earth’s Mantle, very little metal (iron, nickel), mainly rock

The Structure of the Moon’s Interior

A crust about 65 km thick, A mantle about 1000 km thick, andA core that is about 500 km in radius.The outer core may be molten.

It has no magnetic field to speak of due to little iron and slow rotation.

Thicker crust on the far side explains why there are almost no maria on the far side

Lunar surface featuresLunar surface features

Craters Craters Highlands Highlands Maria Maria Wrinkle ridges Wrinkle ridges Rilles Rilles Weathering & Erosion ProcessesWeathering & Erosion Processes

It’s the expanding vapor, not the impact, that forms the crater. That’s why cratersare round, even if the meteorite impactsfrom a low angle.

Older craters have poorly defined walls with lots of slumping.

Recent craters have sharp walls with little slumping inside.

Some craters have rays, or a blanket ofmaterial thrown out by the impact.

The impact digs through to a deeper, lighter-colored layer.Over time, ultraviolet light from the sun darkens the rays. So, craters with bight rays must be very young.

Maria: Old lava fields created by large meteorite impacts

•Appear:

–charcoal black

–smooth

–few craters

Highlands:

•Surface of the Moon elevated several kilometers above the Maria

•Appear:

–relatively light-colored

–crateredCraters:

•Bowl shaped depression created from impacts with interplanetary debris

Crater rims are raised above the surroundings.Rebound of the rock under the impact can form a central peak in larger craters. Over time, the crater walls canterraces.

slump, creating

Lunar HighlandsLunar HighlandsThe highlands are more heavily cratered The highlands are more heavily cratered

than the other lower regions of the moon. than the other lower regions of the moon. This indicate, according to the “law of This indicate, according to the “law of cratering”, tcratering”, that the highlands are older than the lower, smoother regions. They were formed 4.6 – 3.8 billion years ago.

Rocks of the highlands are made of Anorthosite, a close relative of granite

TheThe lunar maria are just bigger craters, lunar maria are just bigger craters, formed like any other, but they have one formed like any other, but they have one important difference.important difference.

The impacts that formed them were large The impacts that formed them were large enough to punch cracks all the way to the enough to punch cracks all the way to the moon’s still-molten core.moon’s still-molten core.

Over time, runny, dark, molten lava moved up Over time, runny, dark, molten lava moved up through the cracks to fill in the floors of the through the cracks to fill in the floors of the crater basins, giving a smooth, dark surface.crater basins, giving a smooth, dark surface.

Mare – singular Maria - pluralNo plate tectonics,but mountains form at the edgesof the Maria.

As the basalt cooled in the maria basins, it becomes skinned over and then contracted, forming Wrinkle Ridges.

Rilles look remarkably like dry river beds. They are found Rilles look remarkably like dry river beds. They are found on the maria, on the maria, by a flowing material, but it wasn’t water – it was lava.

Erosion on the moonErosion on the moon Erosion does occur on the moon, but Erosion does occur on the moon, but

there’s no running water.there’s no running water.

Two things contribute to erosion

(2) (2) The hot – cold cycle causes rocks The hot – cold cycle causes rocks to break into smaller pieces, and to break into smaller pieces, and can even cause slopes (crater walls) can even cause slopes (crater walls) to slowly slump downhill. to slowly slump downhill.

on the Moon: (1) The drastic changes in temperature, and meteorite impacts.

Main lunar materials

• Highlands – Anorthosite )

• Dark maria– Basalt (Lava rocks)

• Others – impact breccia -made from

smashed/melted remains of other rocks– Regolith -pulverized lunar dirt

Meteorite impacts fracture rocks into lunar Meteorite impacts fracture rocks into lunar soil, a fine powder that builds up at the rate soil, a fine powder that builds up at the rate of about 1 meter depth / billion years.of about 1 meter depth / billion years.

The Moon’s Regolith, or surface layer of powdered and fractured rock, was formed by meteoritic action.

Impacts are still continuing today, but most are micro-meteorites.

The light colored rock in the lunar highlands

are called anorthosite; rich in aluminum and calcium. (similar to Granite)

Lunar Rocks - Highlands

Impacts also melted larger rock fragments

together, forming very rough rocks called “Breccias”

 

The maria were formed from molten rock, mostly of the same minerals that are found in volcanic rocks in Hawaii. This rock is called

basalt. Basalt has more of the heavier elements like iron, manganese, and titanium.

Basalt

Lunar Rocks

Timing of Activity on the Moon• Formation: 4.5 billion years

• Heavy Cratering: 4.5 – 4.0 billion years

• Mare Volcanism: 3.7 – 3.0 billion years

• Later small craters:continually decreasing

Origin of the MoonThe most probableThe most probable : :

A planetary body about the size of Mars A planetary body about the size of Mars collided with the young Earth, and the ejected collided with the young Earth, and the ejected matter coalesced into the Moon.matter coalesced into the Moon.

Apparent motion: The Moon’s orbit:

• The Moon takes about 27 days to go once around the Earth.

• Because of the Earth’s movement in its orbit, the Moon takes 29.5 days to get back to the same place relative to the Sun.

Apollo Missions to the Moon (1969-1972)

Brought back 382 kg of rocks for chemical analysis, radioactive dating.

Tidal Effects

Tidal effects are an important aspect of gravity, the Moon pulls on the Earth - The Earth pulls on the Moon.

The Moon’s pull causes tides (bulging of the

ocean).

The Earth’s pull slows

the rotation of the

Moon (Tidally locked)

and the Moon’s pull

slows the rotation of the

Earth (by about 0.7 seconds/year).The Earth’s pull is also slowing down the Moon.

Spring Tides: Sun, Moon in same alignmentmagnify effect

Neap Tides

Neap Tides: Sun, Moon in opposite alignmentdiminish effect

Spring Tides

The lesser tides occur at First and Last Quarter

The greatest tides occur at New and Full Moons