AST101 Midterm

Telescope

Instrument that gathers and focuses light to make distant objects appear brighter and larger.

Collecting area

Surface area of a mirror or lens; ∝ D². Larger area = more light = shorter exposure.

Resolution

Ability to distinguish two close objects as separate; improves with larger diameter (D).

Diffraction limit

Smallest angular separation a telescope can resolve; θ ≈ 1.22 λ / D.

Reflecting telescopes

Telescopes that use mirrors (modern).

Refracting telescopes

Telescopes that use lenses (older, smaller).

Atmospheric turbulence

Mixing of air layers causes image blur and twinkling; limits ground-based resolution.

Adaptive optics

Mirrors adjust shape in real time to correct distortion from turbulence.

Mountaintop telescopes

High, dry, dark sites reduce turbulence and light pollution.

Ethical issue with observatories

They are often built on Indigenous land without consent; ethical astronomy requires community involvement.

Space telescopes

No atmosphere → best resolution, full wavelength access; but expensive, hard to repair.

Wavelengths reaching Earth's surface

Radio, visible, and near-infrared pass; UV, X-ray, mid-IR absorbed.

Seeing

Image blur due to turbulence in the atmosphere.

Light pollution

Artificial light brightening the night sky and reducing contrast.

Airborne or space telescopes

To observe IR or UV wavelengths absorbed by the atmosphere.

Ethical concern in modern astronomy

Use of Indigenous lands without consent; need equitable collaboration.

Main patterns in the Solar System

Orderly motion, two planet types, debris structures, and exceptions.

Orderly motion

Planets orbit in same plane and direction with small tilts.

Two planet types

Terrestrial - small, dense, rocky; Jovian - large, gaseous.

Debris belts

Asteroid Belt (between Mars & Jupiter); Kuiper Belt (outside Neptune); Oort Cloud.

Exceptions to Solar System patterns

Venus & Uranus tilts, retrograde moons, Earth's large Moon.

Frost line

Separates inner hot region where only rock/metal condense vs outer region where ices form.

Nebular Theory

Solar System formed from rotating cloud of gas and dust (solar nebula).

Angular momentum conservation

Cloud spins faster and flattens into a disk as it contracts.

Condensation sequence

Hot inner disk → rocks/metals; cool outer → ices → gas giants.

Accretion

Small particles stick together to form planetesimals and planets.

Heavy Bombardment

Leftover planetesimals colliding with young planets ~100 Myr after formation.

Earth's Moon formation

Giant impact between proto-Earth and Mars-sized body.

Moons capture

Irregular orbits indicate capture via gravity or drag (e.g., Triton).

Radiometric dating

Method to determine age from ratios of radioactive (parent) to stable (daughter) isotopes.

Isotope

Same element, different number of neutrons.

Radioactive decay

Spontaneous change of parent atom → daughter + particles + energy.

Half-life

Time for half of radioactive atoms to decay.

Isotopes used in astronomy

Potassium-40 → Argon-40 for old rocks; Carbon-14 → Nitrogen-14 for young materials.

Age of the Solar System

≈ 4.55 billion years (from meteorite dating).

Factors determining a terrestrial planet's fate

Size, distance from Sun, and rotation rate.

Main interior layers of terrestrial planets

Core (Fe, Ni), Mantle (silicates), Crust (basalt/granite).

Differentiation

Separation by density: heavy materials sink, light rise.

Sources of internal heat

Accretion, differentiation, and radioactive decay.

Geological activity of large planets

They cool more slowly (low surface-area-to-volume ratio).

Requirements for a magnetic field

Molten conductive core, convection, and moderate rotation.

Terrestrial planets with magnetic fields

Earth strong, Mercury weak, Venus & Mars none.

Magnetosphere

Region where planet's magnetic field deflects solar wind.

Four major geological processes

Impact cratering, volcanism, tectonics, and erosion.

Crater counts estimate age

More craters → older surface.

Volcanism

Eruption of molten rock; resurfaces old craters.

Tectonics

Surface deformation by internal stress; requires heat.

Erosion

Wind, water, or ice modify surface; requires atmosphere and liquid water.

Comparison of Moon and Mercury geology

Both geologically 'dead'; cratered; Mercury has contraction cliffs.

Evidence of geology on Mars

Olympus Mons (volcano), Valles Marineris (canyon); past erosion by water.

Geological activity of Venus

Few craters, >1600 volcanoes, resurfaced ~750 Myr ago.

Uniqueness of Earth's geology

Active plate tectonics, volcanism, and erosion from water/wind.

Source of terrestrial planet atmospheres

Volcanic outgassing of H₂O, CO₂, N₂ plus comet impacts.

Pressure change with altitude

Decreases exponentially; atmosphere ~1 % of Earth's radius.

Determining a planet's temperature

Distance from Sun, albedo, and greenhouse effect.

Greenhouse gases

H₂O, CO₂, CH₄.

Non-greenhouse gases

N₂, O₂.

Main atmospheric loss processes

Thermal escape, solar-wind stripping, and large impacts.

Mars's atmospheric evolution

Once thicker; lost magnetic field → solar wind stripped gas → thin cold CO₂ atmosphere.

Thermal escape

Hot, low-mass planets lose gas as fast, light molecules exceed escape velocity.

Solar wind and atmosphere loss

Charged particles erode unprotected atmospheres.

Magnetic field protection

It deflects solar wind and shields the atmosphere.

Mars's atmosphere loss reason

Small size → cooled core → lost magnetic field → atmosphere stripped.

Greenhouse effect

Visible light warms surface; IR re-emitted and absorbed by GH gases → surface heating.

Benefits of Earth's greenhouse effect

Keeps average surface temp ~15 °C; allows liquid water.

Venus's extreme heat reason

Thick CO₂ → runaway greenhouse → 460 °C.

Runaway greenhouse effect

Positive feedback where warming increases H₂O vapor → even more warming.

Photodissociation

UV light breaks H₂O → H escapes → O lost; explains Venus's lack of water.

Venus's atmospheric change

Became dry, CO₂-dominated with no oceans.

Earth's CO₂ cycle steps

1 CO₂ dissolves in rain → 2 rain weathers rocks → 3 ions form carbonate rocks → 4 subducted → 5 volcanoes release CO₂.

Feedback type of the CO₂ cycle

Negative feedback → acts as climate thermostat.

CO₂ cycle and climate stabilization

Temp ↑ → faster weathering → less CO₂ → cooling; Temp ↓ → slower weathering → more CO₂ → warming.

Earth's habitability despite Sun brightening

CO₂ cycle compensated for higher solar luminosity.

Feedback driving Venus's runaway greenhouse

Positive feedback with no restoring mechanism.