Moon Study Guide

📖 Core Concepts Sidereal vs. Synodic period – Sidereal (27.321 days) is the true orbital time relative to stars; synodic (29.531 days) is the time between identical phases as seen from Earth. Tidal locking – The Moon’s rotation period equals its sidereal orbital period, so the same face always points toward Earth. Mascons – Large positive‑gravity anomalies caused by dense basalt fill in impact basins; they perturb spacecraft orbits. Lunar day – One full cycle of sunrise to sunrise on the Moon equals one synodic month (29.5 days). Supermoon – Full (or new) Moon occurring near perigee; appears up to 30 % brighter and 14 % larger. Crater types – Simple (bowl‑shaped), complex (terraced walls, central peak), peak‑ring, multi‑ring basins. Libration – Combined wobble that lets us see 59 % of the lunar surface over time. 📌 Must Remember Mean orbital distance: 384 399 km (≈ 1 LD). Mean radius: 1 737.4 km. Mass: \(7.35\times10^{22}\) kg = 1/81 Earth’s mass. Surface gravity: \(1.62\ \text{m s}^{-2}\) (0.165 g). Escape velocity: \(v{esc}=2.38\ \text{km s}^{-1}\). Orbital eccentricity: 0.055 (slightly elliptical). Axial tilt: 1.54° to ecliptic, 6.69° to orbital plane. Temperature extremes: +120 °C (sunlit) ↔ ‑171 °C (shadow). Regolith depth: 10–15 m (highlands), 4–5 m (maria). Moon’s recession rate: ≈ 3.8 cm yr⁻¹ → Earth day lengthens  17 µs yr⁻¹. Current water‑ice evidence: Hydrogen enrichment in polar craters (Lunar Prospector), OH/H₂O absorption (Chandrayaan‑1), >100 kg ice from LC‑ROCS impact. 🔄 Key Processes Tidal Evolution Earth’s tidal bulge lags → torque slows Earth rotation → angular momentum transferred → Moon’s orbit expands. Crater Counting Age Dating Map a region → count craters > diameter D per km² → compare to calibrated production curve → infer relative surface age. Mascon Formation Large impact creates basin → basaltic lava fills → dense fill → positive gravity anomaly. Water‑Ice Trapping Permanently shadowed crater → temperatures < 100 K → water molecules from cometary/solar‑wind sources remain stable. 🔍 Key Comparisons Simple vs. Complex Craters – Simple: bowl shape, < 15 km diameter; Complex: terraced walls, central peak, > 15 km. Maria vs. Highlands – Maria: dark, basaltic, younger (3.3–3.7 Ga), cover 31 % of near side; Highlands: bright, anorthositic, oldest (> 4.4 Ga), thicker crust. Perigee vs. Apogee – Perigee ≈ 356 000 km (larger apparent size); Apogee ≈ 406 000 km (smaller). Primary vs. Secondary Craters – Primary: formed by meteoroid impact; Secondary: ejecta from a primary impact, often clustered, can mimic primaries in counts. ⚠️ Common Misunderstandings “The Moon is far enough to have a stable atmosphere.” – The Moon has only an exosphere; any gases quickly escape. “All lunar water is liquid.” – Surface liquid water cannot persist; water exists as ice in permanently shadowed regions or as hydroxyl bound in minerals. “The far side never sees Earth.” – Libration reveals up to 59 % of the far side over time; it’s not permanently hidden. “Mascons are magnetic.” – Mascons are gravitational, not magnetic; they arise from dense mass, not from magnetic fields. 🧠 Mental Models / Intuition “Tide‑push‑orbit‑grow” – Imagine Earth’s tidal bulge as a rubber band pulling the Moon forward; the extra forward push adds orbital energy, slowly lifting the Moon. “Crater‑density clock” – Think of the lunar surface as a sand timer; more craters = older “sand” (time). “Mascon‑well” – Picture a bathtub (basin) filled with heavy water (basalt); the water’s weight makes the bottom dip (gravity high spot). 🚩 Exceptions & Edge Cases Supermoon brightness – Not always 30 % brighter; atmospheric conditions and human perception can reduce the observed effect. Lunar libration – Extreme libration can expose up to 8 % more of the far side, but never the whole far side. Water detection limits – OH/H₂O spectral features can arise from solar‑wind‑implanted hydroxyl, not necessarily bulk ice. 📍 When to Use Which Estimating surface age → Use crater‑density method for relative ages; switch to radiometric dating (Apollo samples) when absolute ages are needed. Predicting spacecraft orbit stability → Include mascon corrections for low‑altitude lunar orbits; use higher circular orbits when mascon effects are undesirable. Assessing water resources → Combine neutron‑spectrometer hydrogen maps (global) with LRO temperature data (local cold traps) to prioritize landing sites. 👀 Patterns to Recognize Circular craters → vertical impact; elongated or polygonal craters → oblique impact or underlying faulting. Higher albedo + magnetic anomaly → lunar swirl (indicates solar‑wind shielding). Bright ray systems → young (Copernican) craters; faded rays → older (Eratosthenian). 🗂️ Exam Traps Confusing sidereal & synodic periods – Sidereal = true orbit (27.3 d); synodic = phase cycle (29.5 d). Assuming the Moon’s orbit is circular – Eccentricity = 0.055; perigee/apogee size differences are exam‑relevant. Mistaking mascons for magnetic fields – Mascons affect gravity, not magnetism. Believing the far side never sees Earth – Libration reveals part of it; a common distractor in “visibility” questions. Over‑estimating lunar water abundance – Ice is confined to permanently shadowed regions; surface hydroxyl is only trace amounts. --- Use this guide for rapid review—focus on the bolded numbers and bullet‑pointed contrasts to lock in the highest‑yield facts before the exam.