Surface exposure and space weathering (Apollo 11 samples)
With no atmosphere or magnetic field, the lunar surface is continuously bombarded and irradiated, and the Apollo 11 samples carry a detailed record of it — compiled sample-by-sample in the Lunar Sample Compendium. Three overlapping processes leave measurable traces.
Cosmic-ray and solar-flare irradiation. Galactic cosmic rays and solar-flare particles produce cosmogenic radionuclides (e.g., 10002 measured ²⁶Al 97, ²²Na 44, ⁵⁴Mn 28, ⁵⁶Co 27 dpm/kg) and leave depth-dependent nuclear tracks. These yield cosmic-ray exposure ages — how long material stayed within ~1 m of the surface — of tens to hundreds of Myr for rocks (10044 ~80 Myr, 10017 480 Myr) and ~2.3 Gyr for the ancient soil breccia 10060. Because dose and track density fall off with depth, the data even recover a rock’s lunar orientation: 10017 had “tumbled” on the surface, and its true up-face was identified from ⁵⁶Co activity and top-versus-bottom track gradients (its pre-flight photographs turned out to be inverted).
Micrometeorite bombardment. Exposed faces are studded with glass-lined microcraters (“zap pits”) and develop a thin patina; tops are rounded by differential erosion, and the gradient of solar-flare tracks with depth lets erosion rates be estimated (“solar-flare paleontology,” studied in 10017, 10084, 10072, 10003).
Impact gardening and soil maturity. Repeated small impacts weld soil into glassy agglutinates — 10084 is roughly half agglutinate (~52%) — and stir in implanted solar-wind gases. The accumulation is tracked by the magnetic maturity index Iₛ/FeO (~75 for Apollo 11 soils = mature), marking long residence at the very top of the regolith.
Together these make the regolith a slowly overturned, thoroughly irradiated layer, and they are why the astronauts’ disturbed-soil trails still read dark in orbital images decades later — fresh, unweathered material exposed at the surface.
Related
Section titled “Related”- Lunar regolith and soil mechanics
- Apollo 11 basalt suites (high-K and low-K)
- Lunar field geology (Experiment S-059)
- Solar Wind Composition experiment (S‑080)