ASTR 120 ยท Lecture 07
Lunar samples and crater chronology anchor much of inner solar-system history.
Definition: Loose fragmented material covering coherent rock on an airless or nearly airless body.
Why it matters and connects: Its grain size, maturity, composition, and thickness record impacts, radiation exposure, and surface mixing.
Boundary or common confusion: Regolith is not ordinary soil: it need not contain biology, water, or chemical weathering products.
Definition: The rate at which projectiles of specified sizes strike a target area over time.
Why it matters and connects: Crater chronology depends on how impact flux changes with epoch and location.
Boundary or common confusion: Flux is not constant through solar-system history and differs among dynamically distinct targets.
Definition: A bright or compositionally distinct streak of ejecta extending outward from a relatively fresh impact crater.
Why it matters and connects: Rays expose and distribute material that has undergone less space weathering, making them useful relative-youth indicators.
Boundary or common confusion: Brightness can depend on composition and illumination as well as age; rays fade rather than marking an exact date.
Definition: Physical and chemical alteration of exposed surface grains by micrometeoroids, solar-wind particles, radiation, and thermal cycling.
Why it matters and connects: It changes spectral slope, brightness, and absorption bands, complicating remote compositional interpretation.
Boundary or common confusion: Its effects differ among silicate, carbonaceous, metallic, and icy surfaces; lunar behavior is not universal.
Definition: A spin state in which a body's rotation period equals its orbital period, keeping nearly the same hemisphere toward its primary.
Why it matters and connects: Tidal evolution explains why the Moon presents a near side and far side and affects surface illumination histories.
Boundary or common confusion: Synchronous rotation does not mean the body does not rotate; it turns once per orbit.
Definition: A substance that vaporizes, sublimates, or migrates readily at the temperatures and pressures of a planetary environment.
Why it matters and connects: Water and other volatiles can survive in permanently shadowed cold traps even on otherwise airless worlds.
Boundary or common confusion: Volatile is environment-dependent; a compound stable as ice in one location may be gas elsewhere.
Airless bodies preserve impact histories and space-weathering records with minimal atmospheric erosion.
Regolith: broken, gardened surface material produced by repeated impacts.
Crater rays: bright ejecta that darkens and matures with exposure.
Space weathering: micrometeorites and solar wind alter color and reflectance.
Both are airless and cratered, but Mercury's higher solar heating, larger metal fraction, and stronger solar environment affect volatile retention, surface chemistry, and tectonic history.
Bright rays mark material excavated and spread across older terrain. The interpretation depends on impact freshness, composition contrast, and how quickly space weathering darkens exposed grains.
For two lunar plains imaged at the same resolution, a higher crater density usually means longer exposure. The clock fails if one surface was buried, saturated, or counted with a different detection threshold.
Airless does not mean volatile-free. Permanently shadowed polar regions can preserve ice if temperatures remain low and delivery exceeds loss.
You observe a permanently shadowed polar crater and a radar-bright deposit. What evidence would distinguish clean water ice, ice mixed with regolith, and a rough rocky surface?
Airless regolith changes with micrometeorite gardening and solar-wind implantation. Maturity indicators help separate a genuinely young crater from an older surface that only looks bright because composition or illumination differs.