midterm astronomy #2 ch. 6-9

solar system scale

planets very tiny compared to distances between them

• earth 15 m from grapefruit sun

• pluto 600 m

• closest star a few thousand km

mercury, venus, earth

inner planets; rocky, small, and dense with few if any mons and no rings

mars and jupiter

fairly large distance gap between inner grouping and outer planets

jupitar, saturn, uranus, neptune

outer planets: gaseous large and low density, with lots of rings and moons. Colder

pluto and eris

large axis tilts of uranus and venus rotation period is longer thaan its orbital period

terrestrial planets

small, rocky, higher density, made of rock and metal, solid surface, few (if any) moons and no rings, closer to sun (& closer together), warmer surfaces

jovian planets

large, hydrogen-rich, lower density, mostly hydrogen, helium, hydrogen compounds, no solid surfaces, rings and many moons, farther from sun (further apart) q/ cool temps, at cloud tops

patterns in solar system

• all large bodies orbit at same direction and in nearly same plane

• sun and most planets also rotate in that direction

• orbit nearly circular

• most moons behave in same way

• sun, planets, and large moons (all large bodies) generally orbit and rotate in avery organized way

astroids

rocky bodies orbiting sun - much smaller

found in asteroid belt, between orbits of mars and juptiar

comets

made largely of ices (H2O NH3 ammonia, CH4, methane)

found in kupier belt (beyond neptunes orbit) and Oort cloud (large and far from sun)

exceptions to the rules

• venus rotates backwards

• many small moons orbit different directions w.r.t. planet rotation

• earths moon compared other solar system moons (earth is bigger than the selected moons)

nebular theory

our solar system formed from the gravitational collapse of giant cloud of interstellar gas

orderly patterns of motion

• conservation of angular momentum and energy, and collisions between gas and dust particles that formed the nebula

• gravity pulls inward in all directions

1. heating: temp increases as solar nebula collapsed - consequence of energy conservation

   1. gravitational potential energy

      1. kinetic

      2. higher thermal

spinning: nebula rotates faster as it shrinks - consequences of conservation of angular momentum

1. flattening: nebula flattens into a disk - consequences of collisions between particles that reduce eccentricities or orbits and flatten nebula

formation of spinning disk explains the orderly motion in solar systems

orderly patterns of motion

The disk shape and spin directly produce the same-plane, same-direction orbits.

The frost line

Beyond about 3 AU, it was cold enough for hydrogen compounds (water, methane, ammonia) to freeze into ice.

• Inside it, only metal and rock could condense. These are rare, so planets stayed small.

• Outside it, ice was also available and far more abundant, so planetesimals grew big.

Terrestrial - making planet

Condensation into solid specks, accretion into planetesimals, then collisions built up a few large planets. Too hot for light gases, so they stayed small

Jovian - making planet

ice-rich cores grew to roughly 10 Earth masses, then their gravity captured hydrogen and helium gas from the nebula. Moons and rings formed from the disk of material around each planet.

Giant impact hypothesis

A Mars-sized body hit the young Earth, and the debris ring coalesced into the Moon. This explains the Moon’s low density (little iron), its composition similar to Earth’s crust and mantle, and its size.

Leftover planetesimals

Rocky ones became asteroids and icy ones became comets. Many were flung to the Oort cloud by encounters with the giants, and others bombarded the planets

Radiometric dating

• Unstable (radioactive) isotopes decay at a steady rate. The half-life is the time for half to decay. Compare the parent and daughter isotopes in a rock to get the time since it solidified.

• The oldest Earth rocks are about 4 billion years old. Meteorites and Moon rocks date to about 4.6 billion years, the age of the solar system.

Interior layers

core (iron/nickel, with a solid inner part and liquid outer part), mantle (dense rock), crust (lightest rock). Layers are defined by density.

Lithosphere

The rigid outer layer (crust plus upper mantle) sits above softer, flowing rock. How thick it is decides what geology is possible: a thin lithosphere lets heat, volcanism, and tectonics happen, while a thick one locks the surface

Differentiation

when the planet was molten, dense metals sank to the core and light rock rose to the crust. Earth’s layers formed this way.

Interior heat

Sources are accretion, differentiation, and radioactive decay

Size matters

bigger planets have more volume and less surface area relative to volume, so they stay hot longer. Small planets cool quickly and become geologically dead.

Cratering

Impacts from space. Many craters mean an old, inactive surface

Volcanism

Molten rock reaches the surface

Outgassing

from volcanoes released the gases that formed atmospheres and oceans, so it matters for life

Tectonics

Stress on the lithosphere causes it to crack, fold, and shift (mountains, faults, rifts)

Erosion

Wearing down and moving of surface material by wind, water, ice, and weather

Earth’s atmosphere

• Protects from radiation: the ozone layer absorbs UV, and the atmosphere absorbs X-rays

• Why the sky is blue: Molecules scatter blue light more strongly than red, so blue light reaches you from all directions

• Greenhouse effect: Greenhouse gases (CO₂, H₂O, CH₄) absorb infrared that the surface emits and re-radiate it, warming the planet. Without this, Earth would be below freezing on average

Continental motion and subduction

Plate tectonics moves lithospheric plates over the mantle. At subduction zones, one plate sinks beneath another

Carbon dioxide cycle

A long-term thermostat. CO₂ dissolves in rain and weathers rock, rivers carry it to the ocean, and it forms carbonate rocks. Subduction carries it into the mantle, and volcanoes release it again. If Earth cools, weathering slows, CO₂ builds up, and warming returns. If Earth warms, the reverse happens

Mercury and the Moon

• Both are small, so they cooled quickly. The lithosphere thickened, and volcanism and tectonics ended long ago

• There is evidence of past activity: lava plains (the Moon’s maria) and cliffs on Mercury that formed as the planet shrank

• Heavily cratered, with little or no atmosphere and no erosion

Mars: Evidence for water & What happened to the atmosphere

dry river valleys and channels, outflow channels, minerals that form only in water, sedimentary rock layers, and ice at the poles and underground. Water probably flowed in the past

small, so its interior cooled and its magnetic field faded. It also has low gravity. thinned out (gas escaped to space, and solar wind stripped it). The greenhouse effect weakened, and froze

Venus

has a very thick CO₂ atmosphere and a runaway greenhouse effect. It was too close to the Sun, so oceans evaporated. Water vapor, a greenhouse gas, heated it further, and no carbon cycle removed CO₂. The surface is about 470°C

Jovian worlds

• Dense, icy/rocky core, then a layer of metallic or liquid hydrogen (in Jupiter and Saturn), then a molecular hydrogen atmosphere. Uranus and Neptune have thicker ice layers and less hydrogen

• Jupiter and Saturn are mostly hydrogen and helium because their cores grew big and captured lots of gas. Uranus and Neptune formed farther out, where material was sparser and slower to collect, so their cores grew later and they captured less gas

• Low. Saturn’s is less than water. Uranus and Neptune are denser than Jupiter and Saturn because they contain more ices and less light gas

• Colors come from trace compounds in clouds at different temperatures and altitudes. Jupiter and Saturn show bands (ammonia and other ices, plus tinted compounds), and Uranus and Neptune look blue-green because methane absorbs red light

• Fast rotation and internal heat drive strong winds, bands, storms, and the Great Red Spot (a storm that has lasted centuries). Neptune has surprisingly strong storms

Jovian satellites: Moons and rings

• Galilean moons: Io, Europa, Ganymede, Callisto. They look like a miniature solar system, with the inner ones denser and rockier and the outer ones icier.

• Tidal heating (Io): Io’s orbit is slightly elliptical because of gravitational tugs from the other moons. Jupiter flexes the moon with changing tides, which creates friction and heat. This makes Io the most volcanic body in the solar system.

• Saturn’s moons: Titan has a thick atmosphere with liquid methane lakes. Enceladus shoots out water geysers, which suggests a subsurface ocean.

• Why theses moons are more active than rocky moons of the same size: They contain ice, which melts at low temperature, and tidal heating supplies energy. Rocky moons that small would be frozen and dead.

• Saturn’s rings: Countless icy particles orbiting in a thin flat disk. Gaps are created by moons (orbital resonances) and moonlets that clear paths.

• Ring formation: Ring particles are not leftovers from planet formation. They are debris from moons broken up or eroded by impacts, and the rings must be continually replenished.

Classifying small objects

Why they exist: They are leftover planetesimals that never accreted into planets or were ejected. Jupiter’s gravity prevented a planet from forming in the asteroid belt.

• Asteroids: rocky or metallic, mostly in the inner solar system.

• Comets: icy and rocky, from the outer solar system.

• Dwarf planets: orbit the Sun, massive enough to be round, but have not cleared their orbits (e.g., Pluto, Ceres, Eris)

• Meteors: A meteoroid is a small piece of debris, a meteor is the streak of light as it burns up, and a meteorite is one that lands on the ground.

• Two compositions: Rocky/metal bodies formed inside the frost line, where only rock and metal could condense. Icy bodies formed beyond it.

The asteroid belt

Between Mars and Jupiter at about 2 to 3.5 AU. It is mostly empty space, and the total mass is less than the Moon’s

Gaps in the belt at distances where an asteroid’s orbital period is a simple fraction of Jupiter’s (orbital resonances). Repeated tugs from Jupiter push asteroids out of those orbits

Comets

• Why they have tails: Sunlight heats the icy nucleus, causing gas and dust to escape (sublimation), forming a coma and tails.

• Orientation: The tails always point away from the Sun, not behind the comet, because they are pushed by sunlight and the solar wind. The ion tail is straight and the dust tail is curved.

• Where they come from: The Kuiper belt (short-period comets, near the plane of the planets) and the Oort cloud (long-period comets, any direction).

• Meteor showers: Earth passes through the dust trail a comet left along its orbit, and the particles burn up and appear to radiate from one point in the sky.

Pluto and the Kuiper belt

• Why Pluto is “just” a dwarf planet: It is small, shares its region with many other Kuiper belt objects, and has not cleared its orbital neighborhood. Eris is similar in size, which prompted the 2006 reclassification.

• Kuiper belt objects: Icy bodies beyond Neptune’s orbit (30 to 50 AU), likely leftover planetesimals from the outer disk.

Cosmic collisions

• Likelihood: Small impacts are frequent (meteors every day), and large impacts are rare but have happened (about every few tens of millions of years for really big ones).

• Dinosaur extinction: A roughly 10 km impactor struck about 65 million years ago. Evidence includes a worldwide layer of iridium-rich clay (rare on Earth but common in asteroids), the Chicxulub crater at the Yucatán Peninsula, shocked quartz, and the abrupt change in the fossil record at that boundary.