Chapters 7–10: Earth, Moon & Mercury, Venus, Mars

Interior structure

Earth is studied indirectly through seismic (earthquake) waves. P-waves (pressure) and S-waves (shear) travel through the interior and bend as density changes. S-waves are blocked by the liquid outer core, creating a "shadow zone" — the best evidence the outer core is liquid. Faint P-waves reflecting off a solid inner core reveal a core radius of about 3500 km.

Region	Outer radius	Phase	Composition
Inner core	1200 km	Solid	Iron & nickel
Outer core	3500 km	Liquid	Iron & nickel
Mantle	~6400 km (to surface)	Solid (flows slowly)	Iron-rich rock
Crust	8–70 km thick	Solid	Granite & basalt

Central temperature is about 6000 K — hotter than the Sun's surface (5800 K). Heat comes from (1) residual heat from formation/accretion and (2) radioactive decay of elements like uranium, thorium, and plutonium.

Differentiation

Early Earth was largely molten. Dense material (iron, nickel) sank toward the center while lighter material floated up, producing a layered planet: dense core → intermediate mantle → low-density crust. Basalt (3000–3300 kg/m³) forms the ocean floor/upwelled mantle material; granite (2700–3000 kg/m³) is lighter and makes up continents, which is why continents "float" on the denser mantle/basaltic crust.

Plate tectonics

The basaltic crust is broken into a dozen or so rigid tectonic plates that ride on convecting mantle material, driven by escaping internal heat. Plates move ~2–10 cm/year (continental drift) — e.g., it took Africa and South America about 110 million years to reach their present positions. Evidence includes matching magnetic stripes on either side of the Mid-Atlantic Ridge, which record periodic reversals of Earth's magnetic field as new seafloor forms and spreads.

Radiometric dating

Radioactive elements decay into other elements at a fixed, element-specific rate described by a half-life — the time for half of a sample to decay. Uranium's half-life is 4.5 × 10&sup9; years. By measuring the ratio of a radioactive parent element to its decay product (e.g., uranium→lead) trapped in a crystal, geologists calculate the crystal's age.

• Oldest Earth crystals (zircons, found in Canada): ~4.03 × 10&sup9; years
• Oldest Moon crystals: ~4.5 × 10&sup9; years
• Oldest meteorites: 4.5662 × 10&sup9; years (±100,000 yr) — best estimate of the age of the solar system

Earth's oldest rocks have been erased by erosion and plate tectonics, so Moon rocks and meteorites (which lack these processes) give the best age estimate for the whole solar system.

Earth's magnetic field

Earth's magnetic field is generated by dynamo theory: rotation plus convection of molten, electrically conducting material (liquid outer core) generates the field — requires both rapid rotation and a conducting fluid interior. The field creates the magnetosphere, a region that traps charged solar-wind particles in two doughnut-shaped zones called the Van Allen belts. The magnetosphere is compressed on the sunward side and stretched into a long tail on the night side by the solar wind. Occasionally Van Allen particles escape near the poles and produce aurorae. Earth's field also reverses polarity on average roughly every ~500,000 years (recorded in seafloor magnetic striping).

Atmosphere

Earth's original atmosphere was outgassed by volcanoes: mostly CO2, sulfur dioxide, water vapor, methane, and nitrogen — no free oxygen. As the surface cooled below water's boiling point, rain fell for millions of years, forming the oceans, which absorbed most of the CO2 and locked it into rocks, leaving an atmosphere dominated by nitrogen. Almost all atmospheric oxygen was produced later by life; there was little free oxygen until about 2 billion years ago.

Gas	Fraction
Nitrogen (N2)	78%
Oxygen (O2)	21%
Water vapor	0.1–3% (variable)
Argon	0.9%
Carbon dioxide	0.03%

Structure: Troposphere (0–12 km, weather/convection) → Stratosphere (up to 40–50 km, ozone layer converts O3↔O2, absorbing UV) → Mesosphere (up to 50–80 km) → Ionosphere (above 80 km, ionized by UV).

Greenhouse effect

Infrared radiation emitted by the warm surface is partly absorbed by CO2 (and other greenhouse gases) and re-emitted in all directions, including back down — this extra energy makes the surface about 40 K warmer than it would be otherwise. Since 1750, human activity has raised CO2 (~30%, mainly fossil fuel/wood burning), methane (~140%, cattle/rice paddies), and nitrous oxide (~15%, fertilizer/waste/exhaust) — enhancing the natural greenhouse effect.

Tides

Tides are a secondary effect of gravity: because gravity weakens with distance, the near side of an object (facing the other body) is pulled more strongly than the far side. This differential ("tidal") force stretches the object, raising bulges on the near and far sides. Tides couple the rotation and orbits of bodies over time (see also the Moon's synchronous rotation, Ch. 8).

Major properties of the Moon

Quantity	Moon	Earth
Mass	7.3×10²² kg	6×10²&sup4; kg
Radius	1700 km	6400 km
Density	3300 kg/m³	5500 kg/m³
Escape velocity	2.4 km/s	11.2 km/s
Temperature	100–400 K	183–333 K
Atmosphere	No	Yes
Surface gravity	1/6 g	1 g

Dark maria are vast flat basaltic lava-flow plains (Mare Imbrium is largest, ~1100 km across); lighter highlands sit several km higher. Craters cover the whole surface (smallest visible to the naked eye ~200 km; smallest via telescope ~1 km). Highland rock is aluminum-rich, lighter-colored, and older (4–4.4 billion years); maria rock is more iron-rich and younger (3.2–3.9 billion years). Surface regolith (dust from micrometeorite impacts) accumulates at ~5 m per billion years; there is essentially no water (~1 part in 100,000, less than desert sand on Earth) and no organic matter.

Interior structure / lack of metals

The Moon's low average density (3300 kg/m³ vs Earth's 5500) shows its core is not nickel-iron like Earth's — it is deficient in iron and other heavy metals. There is no evidence of a strong lunar magnetic field, consistent with the absence of a large, rapidly rotating liquid core (only a small inner core, ~200 km radius, ~1500 K).

Formation model

The best current model is the Giant Impact ("Impact Theory"): a Mars-sized object collided with the young Earth. Such massive collisions were common in the early solar system. This model explains both the Moon's chemical similarity to Earth's mantle and its deficiency in metals/low mean density (the impactor's and Earth's metallic cores largely stayed with Earth, while rocky mantle material was ejected to form the Moon).

Roche limit

The Roche limit is the minimum distance a large satellite can approach its primary body without being torn apart by tidal forces; for bodies of similar composition it's about 2.5× the primary's radius. Saturn's rings lie inside its Roche limit, possibly debris from a destroyed moon. The Moon likely formed only ~15,000 km from Earth (just outside the Roche limit) and has since migrated outward to its current ~380,000 km distance.

Rotation and orbital evolution

The Moon is in synchronous rotation (tidally locked) — it rotates once per orbit, so we always see the same face; this is common among solar-system moons and is unrelated to lunar phases. Tidal bulges raised when the Earth-Moon system was much closer gradually slowed the Moon's spin until it locked, and ongoing tidal interaction is causing the Moon to slowly spiral outward (and Earth's rotation to gradually slow) over time.

Mercury's eccentric orbit

Mercury has the most eccentric orbit of any planet (e = 0.21), orbital period 88 days. It's visible only briefly near dawn/dusk (max ~2 hours/night) since it never strays far from the Sun.

Quantity	Mercury	Earth
Mass	3.3×10²³ kg	6×10²&sup4; kg
Radius	2450 km	6400 km
Density	5400 kg/m³	5500 kg/m³
Temperature	100–700 K	183–333 K

Measuring Mercury's rotation

Astronomers (Schiaparelli, 19th century) initially assumed Mercury was tidally locked 1:1, like the Moon. 1965 Arecibo radar observations revealed the true values: orbital period 88 days, rotation period 59 days.

3:2 spin-orbit resonance

59 days is exactly 2/3 of 88 days, so Mercury completes 3 rotations for every 2 orbits — a 3:2 spin-orbit resonance rather than the expected 1:1. Because Mercury's orbit is quite eccentric, the varying tidal force at different points in its orbit locked it into this 3:2 ratio instead of a simple synchronous (1:1) lock. Dayside temperatures reach 700 K while the poles (where the Sun sits perpetually on the horizon) stay around 125 K.

Mercury's metallic interior

Mercury's magnetic field (about 1/100 Earth's) is generated by dynamo action from a partially molten core. Its unusually high average density implies a very large iron-rich core (~1800 km) with a comparatively thin mantle (500–600 km) — the highest metal-to-silicate ratio of any terrestrial planet.

Problems with formation models

Explaining Mercury's outsized metallic core has produced 3 competing models: (1) a giant impact stripped away much of the original silicate mantle, (2) the young Sun's heat vaporized surface silicates, or (3) Mercury simply formed from primitive, metal-rich material (similar to certain chondritic meteorites) rather than losing material later. MESSENGER's Gamma-Ray Spectrometer measured surface potassium, thorium, and uranium abundances that rule out the giant-impact and vaporization models, favoring formation from primitive, pre-selected metal-rich material.

Venus as the evening/morning "star"

Venus is the third-brightest object in the sky (after the Sun and Moon). Because its orbit lies inside Earth's, it never strays more than 47° from the Sun, appearing as the "morning star" or "evening star," visible up to about 6 hours a night before dawn or after sunset.

Measuring Venus's very slow rotation

Venus's permanent cloud cover hides its surface from visible light, so early astronomers debated 24-hour or 25-day rotation periods. 1960s Arecibo radar observations settled it: orbital period 225 days, rotation period 243 days — Venus rotates more slowly than it orbits the Sun, and its "day" is longer than its "year."

180-degree axis tilt

Venus's rotation axis is tilted 177.4° — essentially upside down (retrograde), with its south pole "up." This backwards rotation was likely caused by a massive, late collision early in its history (conceptually similar to the Moon-forming impact on Earth).

Surface (volcanoes) and surface conditions

Radar mapping (Magellan) reveals rolling plains with modest highlands/lowlands; two continent-sized highland regions, Ishtar Terra and Aphrodite Terra, cover about 8% of the surface. Maxwell Montes rises 14 km above Venus's lowest terrain. Numerous shield volcanoes (including Sif Mons and Gula Mons) and a general lack of small craters (the dense atmosphere burns up small impactors) are observed. Surface conditions: temperature ~730 K (~860°F, hot enough to melt lead), pressure ~90 bars — no oxygen, no life.

Dense CO2 atmosphere

Venus's atmosphere is about 90× more massive than Earth's, roughly 50 km deep, composed of 97% carbon dioxide and 3% nitrogen, with sulfuric acid clouds and traces of water vapor, CO, and SO2. Surface winds are slow (~4 mph) despite the immense atmospheric mass.

Runaway greenhouse effect

Venus's thick CO2 atmosphere traps far more outgoing infrared radiation than Earth's, producing a much stronger greenhouse effect. Because Venus likely never had oceans to absorb CO2 (its early temperature was already too high for liquid water to persist), CO2 built up in the atmosphere rather than being locked into rock/ocean as on Earth — a runaway feedback that makes Venus, not Mercury, the hottest planet. Any early water underwent photodissociation (UV breaking apart H2O), and the resulting hydrogen escaped to space, explaining Venus's unusually high deuterium-to-hydrogen (D/H) ratio (deuterium being heavier, escapes less easily).

Resurfacing events (no plate tectonics)

Venus's crust has no water in its lithosphere, making it too rigid for Earth-style plate tectonics, yet internal radioactive heat must still escape. Crater counts suggest the surface is relatively young and was likely resurfaced by widespread volcanism roughly 500 million years ago, possibly through periodic, catastrophic episodes of extreme volcanism or lithosphere overturn rather than gradual plate motion. Fluctuating sulfur dioxide levels above the clouds and bursts of radio energy hint that volcanism may still be ongoing today.

Mars in popular culture

Percival Lowell's early-1900s drawings of apparent "canals" fueled speculation about intelligent life and irrigation networks on Mars. Orson Welles's 1938 radio broadcast of The War of the Worlds dramatized a Martian invasion and famously caused public alarm, cementing Mars's cultural status as the likeliest home for alien life.

Orbit and spin

Quantity	Mars	Earth
Semimajor axis	1.52 AU (228×10&sup6; km)	1 AU
Eccentricity	0.093 (perihelion 1.38 AU, aphelion 1.67 AU)	~0.017
Orbital period	687 Earth days	365 days
Axial tilt	25.19°	23.5°
Mass	6.4×10²³ kg	6×10²&sup4; kg
Radius	3390 km	6400 km
Surface gravity	1/3 g	1 g

Mars's fairly high orbital eccentricity causes sunlight intensity to vary by 45% between perihelion and aphelion. Kepler used Mars's orbit primarily to derive his laws of planetary motion. Mars's axial tilt (close to Earth's) gives it Earth-like seasons, but its eccentric orbit makes the two hemispheres' seasons noticeably asymmetric.

Phobos and Deimos

Both small moons resemble asteroids in size and composition (Phobos ~28×20 km, Deimos ~16×10 km) with densities (~2000 kg/m³) far lower than any planet in the inner solar system. They are thought to be either captured asteroids or debris from a massive impact on Mars. Phobos orbits so close (orbital period 7h40m) that it moves faster than Mars rotates — it rises in the west and sets in the east, crossing the sky in about 5.5 hours; its orbit is decaying and it will eventually crash into Mars or break apart. Deimos, farther out, orbits in 30h18m and moves normally (east to west) across the sky, taking about 3 days to cross.

Major space missions to Mars

• Mariner 9 (1971): first probe to orbit Mars; revealed a mix of Moon-like cratered terrain and Earth-like canyons; first hints that liquid water once flowed on the surface.
• Viking 1 & 2 (1970s): first successful landers.
• Mars Pathfinder (1997) with the Sojourner rover (first Mars rover).
• Mars Global Surveyor (1997–2006), Mars Odyssey (2001), Mars Reconnaissance Orbiter (2006), ESA's Mars Express (2003): orbital mapping and atmosphere studies.
• Spirit & Opportunity rovers (2004), Phoenix polar lander (2006).
• Curiosity rover (launched Nov 26, 2011; landed Aug 6, 2012) in Gale Crater — studies climate, geology, and past habitability.
• Perseverance (Mars 2020, landed Feb 18, 2021) with the Ingenuity helicopter (first powered flight on another planet); searches for ancient life, collects samples, tests oxygen production from the atmosphere.
• ESA/Roscosmos Rosalind Franklin rover (2023, in development).

Common goal across missions: search for water, both past (surface) and present (subsurface ice).

Geology and major features

Mars shows a striking north-south asymmetry: the northern hemisphere is lower, younger, crater-poor volcanic plains, while the southern hemisphere is higher, older, and heavily cratered. The Tharsis bulge (equatorial) may result from an upwelling mantle plume; it hosts Olympus Mons, the largest known volcano in the solar system (height 27 km, diameter 600 km, caldera 70 km — no longer active; its great height is possible because of Mars's low surface gravity). Nearby lies the Valles Marineris, a tectonic rift valley (not water-carved) 5000 km long, up to 500 km wide, and 6 km deep. On the opposite side of the planet is the Hellas Basin, formed by a giant early impact.

Thin atmosphere

Mars's atmosphere is about 1/100 as massive as Earth's, roughly 50 km deep (top of troposphere ~30 km), composed of CO2 95.3%, N2 2.7%, Ar 1.6%, O2 0.13%, CO 0.07%, water vapor 0.03%. In winter, CO2 freezes out at the poles (seasonal caps), while a residual water-ice cap remains year-round. Strong winds drive dust storms and dust devils that redistribute surface dust. Mars has an extremely weak magnetic field (~1/800 of Earth's); combined with its rotation rate, this implies its core is nonmetallic, nonliquid, or both — evidence Mars never differentiated as thoroughly as Earth. Its thin atmosphere is slowly leaking away to space today.

Water and climate change

Runoff channels (ancient river remains) and outflow channels (paths of catastrophic flash floods from the southern highlands into the northern plains) provide strong evidence Mars once had abundant liquid surface water, likely during a brief early "Earth-like" warm phase with a thicker atmosphere, rain, and possibly lakes or oceans. Today, most water is locked in the polar caps and in subsurface permafrost; orbiters have detected extensive buried ice deposits, and some crater-wall gullies show evidence of much more recent (possibly ongoing) flows, though whether these involve liquid water or ice remains unclear.

Future exploration / colonization

Current and near-future missions test technologies relevant to eventual human exploration: producing oxygen from the Martian atmosphere (demonstrated by Perseverance/MOXIE), locating accessible subsurface water/ice, improving precision landing, and characterizing weather/dust hazards that would affect astronauts. These efforts are framed as building blocks toward eventual human missions and possible long-term settlement.