Astronomy Midterm Exam Review
Solar System — overview & key objects
Definition: The solar system = the Sun plus everything gravitationally bound to it: 8 planets, dwarf planets (Pluto, Eris...), moons, asteroids, comets, Kuiper Belt, Oort Cloud, dust and gas.
Main components & examples
Sun — G-type main-sequence star; ~99.8% of system mass.
Planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Dwarf planets — Pluto, Eris, Haumea, Makemake, Ceres (in asteroid belt).
Asteroids — rocky/metallic bodies mostly in asteroid belt; sizes mm → 100s km.
Comets — icy bodies from outer solar system; develop comae/tails when near Sun.
Kuiper Belt — disk beyond Neptune (30–50 AU); source of short-period comets.
Oort Cloud — hypothesized spherical cloud far beyond ~5,000–100,000 AU; reservoir of long-period comets.
Interplanetary dust and meteoroids — small particles; produce meteors when entering atmospheres.
Terrestrial vs Jovian planets
Terrestrial planets (inner): Mercury, Venus, Earth, Mars
Small, rocky, higher density (~3.9–5.5 g/cm³), solid surfaces, few/no rings, closer to Sun, thin atmospheres (except Venus/Earth).
Typical composition: metal core + silicate mantle/crust.
Jovian (giant) planets: Jupiter, Saturn, Uranus, Neptune
Large, low average density (Jupiter ~1.33 g/cm³), mostly H/He or ices, thick atmospheres, many moons, rings, no solid “surface” (except possible core).
Subtypes: Gas giants (Jupiter, Saturn) vs Ice giants (Uranus, Neptune — more water/ammonia/methane ices).
Asteroids vs Comets
Asteroids
Composition: rock + metal.
Location: mostly asteroid belt (2–3.5 AU), also near-Earth asteroids, Trojan asteroids.
Appearance: do not form comae/tails.
Evidence: spectra, meteorites on Earth (direct samples).
Comets
Composition: ices (water, CO₂, CO, CH₄), dust, organic compounds.
Behavior: develop coma and tail when heated by Sun; tails point away from Sun (ion and dust tails differ).
Orbits: often highly elliptical; short-period (from Kuiper Belt) vs long-period (from Oort Cloud).
Asteroid Belt vs Kuiper Belt vs Oort Cloud
Asteroid Belt: between Mars & Jupiter (~2.1–3.3 AU). Source of many meteorites. Dominant object: Ceres.
Kuiper Belt: 30–50 AU beyond Neptune; contains Pluto, many small icy bodies; source of short-period comets.
Oort Cloud: ~thousands–100,000 AU, spherical reservoir of long-period comets, only inferred from comet orbits.
Simple timeline (formation context):
Solar nebula collapses → protoplanetary disk forms.
Inside the frost line: rocky planetesimals → terrestrial planets.
Outside frost line: ices + rock → planetesimals → gas/ice giants and Kuiper Belt objects.
Jupiter’s formation/scattering affects asteroid belt; Neptune’s migration populates Kuiper/Oort.
Methods of studying the solar system
Telescopic observations (ground & space): imaging, spectroscopy, photometry.
Spacecraft & probes: flybys, orbiters, landers, rovers (direct measurements).
Remote sensing techniques: radar (e.g., measure surface roughness/rotation), radiometry, gravimetry.
Sample return & meteorites: direct compositional analysis (e.g., Apollo lunar samples, meteorites).
Seismology: seismic waves on planets (Earth, Moon by Apollo seismometers).
Astronomical timing/occultations: measure sizes/atmospheres.
Laboratory experiments & modeling: recreate conditions (high P/T chemistry, impacts).
Solar system formation — nebular contraction & planetesimals
Starting point: Giant molecular cloud → collapse triggered by shock (nearby supernova, density fluctuation).
Key force: Gravity (dominant), with conservation of angular momentum creating a rotating disk.
Steps (classic nebular hypothesis)
Molecular cloud collapse → proto-Sun forms at center; disk of gas/dust (protoplanetary disk).
Cooling and condensation: solids condense from gas (metals, silicates inside frost line; ices outside).
Dust → grains → pebbles (sticking via electrostatic forces).
Planetesimals form by coagulation and perhaps streaming instability (1–100 km bodies).
Accretion: planetesimals collide and stick → planetary embryos (Mars-size).
Runaway & oligarchic growth: embryos sweep up material; gas giants accrete gas if core grows quickly.
Late heavy bombardment & clearing: remaining planetesimals are scattered, forming belts and Oort Cloud.
Forces: Gravity for collapse and accretion; pressure/temperature gradients; collisions; angular momentum conservation.
Gravity assist (slingshot)
Definition: Using a planet’s motion & gravity to change a spacecraft’s speed and direction relative to the Sun without using extra fuel.
Mechanism: spacecraft approaches a moving planet; in the planet’s frame it exchanges momentum; in the Sun’s frame it gains or loses heliocentric velocity.
Example missions: Voyager flybys (used multiple assists), Cassini, Galileo.
Diagram (sketch): incoming trajectory → close encounter around planet → outgoing trajectory altered and faster/slower.
Structure of the Earth — composition & differentiation
Layers (by composition & mechanical behavior)
Crust: continental (granitic, thicker ~30–70 km) & oceanic (basaltic, ~5–10 km).
Mantle: solid but slowly flowing; upper mantle + asthenosphere (partially ductile), lower mantle.
Core: outer core (liquid iron-nickel, creates magnetic field via dynamo), inner core (solid iron-nickel).
Differentiation: Early Earth molten → heavy metals sank to form core; lighter silicates rose → crust/mantle.
Atmosphere, convection, troposphere
Troposphere: lowest atmospheric layer; temperature generally decreases with altitude; weather occurs here; convection drives vertical motion.
Convection: warm air rises, cools, sinks — transfers heat; driven by temperature gradients (solar heating).
Atmospheric composition (Earth): ~78% N₂, 21% O₂, trace gases (CO₂, CH₄, H₂O vapor).
Pressure profile: decreases exponentially with altitude.
Greenhouse effect & climate change
Greenhouse effect (natural): greenhouse gases (CO₂, H₂O, CH₄, N₂O) absorb outgoing IR radiation, warming the surface above the no-atmosphere equilibrium.
Runaway greenhouse: extreme case (e.g., Venus) where positive feedback causes oceans to evaporate and surface temp to rise uncontrollably.
Climate change (current):
Cause: anthropogenic greenhouse gas emissions (CO₂ from fossil fuels, deforestation; CH₄ from agriculture; N₂O).
Evidence: rising global temps, ice melt, sea-level rise, changing precipitation patterns, direct measurements of atmospheric CO₂, isotopic fingerprinting of fossil carbon.
Effects: more extreme weather, ecosystem shifts, sea-level rise, socioeconomic impacts.
What you can do: reduce emissions (energy efficiency, renewables), lifestyle changes, policy advocacy, carbon sequestration, conservation.
P-waves, S-waves, shadow zones
P-waves (primary): compressional; travel through solids & liquids; fastest.
S-waves (secondary): shear; travel only through solids; slower.
Shadow zones: S-waves disappear beyond ~104° from earthquake epicenter (S cannot travel through liquid outer core). P-wave refraction at core-mantle boundary creates P-wave shadow zone (between ~104°–140°) used to infer liquid outer core and size.
Plate boundary types & subduction
Divergent: plates move apart (mid-ocean ridges); new crust forms by upwelling magma.
Convergent: plates collide — subduction (oceanic under continental or oceanic) or continental collision (mountain building, e.g., Himalayas).
Transform: plates slide past (San Andreas Fault).
Subduction: oceanic plate sinks into mantle; creates volcanic arcs, deep ocean trenches, earthquakes.
Magnetosphere & auroras
Magnetosphere: region around Earth dominated by magnetic field; deflects solar wind; bow shock, magnetotail.
Auroras: charged particles from solar wind guided by field lines into polar upper atmosphere, excite atoms (oxygen, nitrogen) → visible light (green/red/blue/purple).
Evidence: satellite measurements, ground auroral observations, magnetometer data.
Tides
Cause: gravitational pull of Moon (primary) and Sun (secondary) plus centrifugal forces from Earth–Moon system rotation.
Effects: two high tides and two low tides daily at many places; tidal bulges; tidal locking (e.g., Moon always shows same face to Earth) and tidal heating in some moons (Io).
Diagrams: sketch Earth with tidal bulges toward and opposite Moon.
Impact craters, types, surface age
Types: simple (small bowl-shaped), complex (central peak, terraced walls), multi-ring basins (largest impacts).
Surface age inference: crater counting (more craters = older surface), radiometric dating of returned samples.
Why/How: impacts are frequent in early solar system (Late Heavy Bombardment) — resurfacing processes (volcanism, erosion) erase craters.
Formation of the Moon
Giant-impact hypothesis (accepted): Mars-sized body (Theia) struck proto-Earth ~4.5 Gyr ago; ejected debris orbiting Earth accreted to form Moon.
Evidence: Moon’s low iron core, isotopic similarity of Earth and Moon rocks, angular momentum of system.
Lunar maria; near side vs far side; regolith
Lunar maria: dark basaltic plains from ancient volcanic floods (e.g., Mare Imbrium). Fewer craters (younger) than highlands.
Near side vs far side: near side has more maria (thinner crust favored basalt flooding); far side heavily cratered and thicker crust.
Regolith: powdery broken rock on Moon/Mercury surfaces formed by micrometeorite impacts and space weathering.
South Pole–Aitken Basin (Moon)
One of the largest and oldest impact basins (~2,500 km diameter) on the Moon’s far side — key target for studying deep crust/mantle material.
Caloris Basin & weird terrain (Mercury)
Caloris Basin: huge impact basin (~1,550 km) on Mercury; opposite hemisphere shows “weird terrain” (hummocky, disrupted) due to seismic focusing.
Scarps & hollows (Mercury): scarps = cliffs from planetary contraction; hollows = irregular shallow depressions likely due to volatile loss.
Mercury & Moon interiors
Mercury: large metallic core relative to size; thin silicate mantle; surface heavily cratered; some evidence of past volcanism.
Moon: small iron core, thick mantle & crust; low-density basaltic maria overlay older highlands.
Phases of Venus & retrograde rotation
Phases: Venus shows phases like Moon (crescent → full) when viewed from Earth (proved it orbits Sun inside Earth’s orbit).
Retrograde rotation: Venus rotates slowly in the opposite direction to most planets (day longer than year); consequence likely from giant impact(s) or tidal interactions.
Venusian atmosphere & polar vortex; runaway greenhouse
Atmosphere: extremely dense (~92 bar), mostly CO₂, clouds of sulfuric acid; surface temps ~460°C.
Polar vortex: double-eyed vortices at poles due to atmospheric super-rotation; complex cloud dynamics.
Runaway greenhouse: Venus likely had water; greenhouse heating evaporated oceans → photodissociation of water → loss of hydrogen to space → permanent hot CO₂ atmosphere.
Volcanoes, lava domes, impact craters (general)
Volcanoes: surface vents from magma; shield (broad), stratovolcanoes (steep), lava domes (thick viscous lava).
Lava domes: viscous lava piles near vent (commonly on Earth and Venus).
Impact craters: ubiquitous on airless/older surfaces.
Example planetary volcanoes: Olympus Mons (Mars) — shield volcano, largest in solar system.
Ishtar Terra & Aphrodite Terra (Venus)
High-elevation continental-sized plateaus on Venus containing tesserae, volcanoes, and complex tectonic features.
Mars — topography & major features
Tharsis region: immense volcanic plateau with large volcanoes.
Olympus Mons: tallest volcano (~22 km high).
Valles Marineris: massive canyon system (~4,000 km long).
Hellas Basin: large impact basin in southern hemisphere.
Sketch: volcanoes (Tharsis) on one side, Valles Marineris nearby — correlate with crustal stresses.
Mars missions (key)
Viking (1970s): landers & orbiters; first landers to do experiments.
Pathfinder (1997): Sojourner rover; technology demonstration.
Spirit & Opportunity (2004): twin rovers; evidence for past water.
Curiosity (2012): Gale Crater rover; found organic chemistry, ancient habitable conditions.
Perseverance (2021): Jezero Crater; sampling for return, astrobiology focus.
Ingenuity: helicopter demonstrator (first powered flight on another planet).
Phoenix: polar lander (2008) found water ice near surface.
MRO (Mars Reconnaissance Orbiter): high-res imaging, climate and geology studies.
Evidence of past water on Mars
Runoff channels & valley networks: indicate sustained surface water flow.
Teardrop-shaped islands: formed by erosion around obstacles in flowing water.
Deltas & layered sediments: indicate standing bodies of water (lakes) with sustained inflow.
Gullies & recurring slope lineae (RSL) debate: may be dry granular flows, brine activity, or seasonal CO₂ frost effects.
Liquid impact crater ejecta morphologies: lobate flows imply fluidized ejecta (possible subsurface ice presence).
Mineral evidence: clays (phyllosilicates), sulfates — form in presence of water.
Martian polar ice caps / Swiss cheese terrain
Polar caps: layered deposits of water ice and CO₂ ice seasonally; layered terrains record climate cycles.
Swiss-cheese terrain: in southern polar cap — pits formed by sublimation of CO₂ ice, looks like holes.
Martian atmosphere, dust devils, frost, dust storms
Atmosphere: thin (~0.006 bar), mostly CO₂; low pressure means liquid water unstable at surface except transient brines.
Dust devils: common, clean solar panels, contribute to atmospheric mixing.
Frost: CO₂ frost in winter; water frost in some locations.
Dust storms: regional to global; influence temperature and solar power for landers.
Life on Mars — current thinking
Possibility: no confirmed life yet. Habitability potential in past (liquid water), subsurface today (protected from radiation, possible brines).
Search methods: rover instruments, returned samples, biosignature detection, methane monitoring (seasonal methane spikes debated).
Constraints: surface radiation, thin atmosphere, oxidizing regolith — subsurface niches are most plausible.
Planetary surface features you should remember (Name That Feature style)
Crater: circular depression from impacts.
Rille/valley: possible collapsed lava tube or channel.
Mare (Moon): basaltic plain from volcanic flooding.
Lobe/flow: ejecta or lava morphology.
Butte/mesa: isolated elevated landforms.
Scarp (Mercury): cliff from planetary cooling/contraction.
Dune fields (Mars): aeolian deposits shaped by wind.
Seismic/structure evidence & how we know things
Seismology: P & S wave travel times → interior layering (Earth & Moon via Apollo).
Gravity mapping: spacecraft track gravitational anomalies → infer mass anomalies and internal structure.
Magnetometers: detect intrinsic or remanent magnetic fields (e.g., Earth strong, Mars weak remanent).
Spectroscopy & sample analysis: surface mineralogy & isotopes.
Crater counts & radiometric dating: relative and absolute ages.
Impact ages & surface dating (how to tell old vs new)
Crater counting: density → older = more craters.
Ejecta overlap relationships: stratigraphy (which ejecta sits over another).
Radiometric dating: ages from returned samples (Moon) calibrate crater-count chronology.
Mercury specifics (interior & surface)
Interior: very large core (70%+ of planetary radius), thin mantle; suggests mantle-stripping or early high-temperature processes.
Surface: heavily cratered, lobate scarps, hollows, Caloris Basin.
Moon specifics (formation & features)
Formation: giant-impact hypothesis (see above).
Maria: basaltic flows on near side.
Near vs far side: mare distribution asymmetry (thinner near-side crust).
South Pole–Aitken Basin: deep, ancient basin important for sampling deep materials.
Regolith: thick, important for thermal and mechanical surface properties.
Phases (Venus example) & observational evidence
Phases of inferior planets (Mercury & Venus): demonstrate interior orbits; observed by Galileo in 17th century to support heliocentrism.
Diagrams you should be able to draw (simple sketches)
Solar system layout: Sun → terrestrial planets → asteroid belt → gas giants → Kuiper Belt → Oort Cloud (spherical).
Planet cross-section: crust → mantle → outer core (liquid) → inner core (solid).
Tidal bulges on Earth due to Moon.
Gravity assist geometry around a moving planet.
Impact crater cross-section: ejecta blanket, central peak (for complex crater).
Planet formation steps from disk to planetesimals to embryos → planets.
Quick timelines (very high-level)
~4.6 billion years ago: Solar nebula collapse → formation of Sun & protoplanetary disk.
~4.5–4.4 Ga: Planetary accretion; Moon forming impact.
~4.1–3.8 Ga: Late Heavy Bombardment (hypothesized) — heavy cratering.
After ~3.8 Ga: Surfaces cool; volcanic activity and erosion alter landscapes; life appears on Earth (∼3.5–3.8 Ga earliest evidence).