ASTR 120 ยท Lecture 05
The Moon became a calibration surface after Apollo; Mars orbiters and rovers made comparative geology quantitative.
Definition: A roughly circular depression excavated when a projectile transfers energy and momentum to a planetary surface.
Why it matters and connects: Crater size, morphology, ejecta, and abundance record impact conditions, target properties, and surface history.
Boundary or common confusion: Crater diameter is not the projectile diameter, and later erosion or burial can erase the record.
Definition: The number of craters above a stated size per unit surface area.
Why it matters and connects: When impact flux and preservation are comparable, higher crater density generally indicates longer exposure and an older surface.
Boundary or common confusion: It is a relative clock unless calibrated, and resurfacing, saturation, and image resolution can bias counts.
Definition: Any process that buries, erodes, deforms, or replaces an older surface and resets part of its visible crater record.
Why it matters and connects: Volcanism, tectonics, sedimentation, glacial flow, and impacts can create young-looking terrain on an old world.
Boundary or common confusion: Few craters date the exposed surface, not necessarily the planet or underlying rock.
Definition: The ordering of geologic units and events using superposition, cross-cutting, embayment, and other spatial relationships.
Why it matters and connects: It reconstructs relative history even when absolute ages or samples are unavailable.
Boundary or common confusion: Local stratigraphic order may not transfer globally without correlation evidence.
Definition: The transport and eruption of molten or mobilized interior material onto or near a surface.
Why it matters and connects: Volcanic plains, shields, domes, and gases reveal internal heat, composition, and pathways through the lithosphere.
Boundary or common confusion: Smooth terrain is not automatically volcanic; morphology, spectra, and context must rule out sedimentary or impact deposits.
Definition: Deformation of a planetary lithosphere by extension, compression, shear, loading, contraction, or convection.
Why it matters and connects: Faults, folds, rifts, ridges, and scarps reveal stress and interior evolution.
Boundary or common confusion: Tectonics does not imply Earth-like plate tectonics; most worlds deform without mobile global plates.
Craters, volcanoes, tectonics, and erosion make planetary surfaces historical records.
Impacts: add craters and expose subsurface material.
Volcanism and tectonics: erase, deform, or bury older terrain.
Weathering: changes slopes, brightness, and small-scale texture over time.
When you inspect a planetary surface, ask which process made the feature, which process modified it, and whether the observed terrain is old enough to preserve the evidence.
Compare the two terrains only after checking the mapped area and resolution. A younger volcanic plain can have fewer craters because lava buried the older record.
If one 10,000 km2 region has 80 counted craters and another has 8 at the same detection threshold, the first has ten times the crater density. That supports an older exposure age unless resurfacing or preservation differs.
A channel-shaped feature suggests flow, but context determines the fluid or process: lava channels, water-carved valleys, collapse troughs, and tectonic rifts can all produce elongated forms.
Crater counts give relative ages most cleanly when impact flux, target properties, erosion, resurfacing, and detection threshold are comparable. Absolute ages require calibration, not just a crater tally.
You are given two equal-area image tiles: one densely cratered and rough, one smooth with lobate margins and wrinkle ridges. Build a two-step argument for which is older and name one observation that could overturn your interpretation.