PX282 - J6 - surfaces

moon

image: NASA, lunar orbiter 4

heavily cratered, implying an ancient surface
less cratered regions, mare* (lit. seas) are giant impact basins filled with lava flows until $3$ Gyr ago
this was measured with radioisotope dating* of rock samples
this allows the calibration of cratering history across the solar system
impacts also trace thickness of lunar lithosphere with time
lava flows are traced by rilles*
the lunar surface is covered with soil called regolith*, which is a fine powder from many impacts
rock samples returned by apollo and other missions, also lunar martian meteorites, can be dated with radioisotope dating
the abundance of a isotope A, eg: uranium-235, is:

A (t) = A_{0} \exp (- λ t)

where, $λ = \ln (2) / τ_{1 / 2}$ is the decay constant

A (t) + B (t) = A_{0} + B_{0} = A (t) \exp (λ t) + B_{0}

⟹ B (t) = (\exp (λ t) - 1) A (t) + B_{0}

in special cases, $B_{0} = 0$ can be assumed, eg: lead in zircon crystals, so $t$ is the only unknown
otherwise, it can be calibrated using a different isotope of the same element, B', for which abundance is constant in time

⟹ \frac{B (t)}{B^{'}} = (\exp (λ t) - 1) \frac{A (t)}{B^{'}} + \frac{B_{0}}{B^{'}}

B' is still unknown, but it can be assumed that that ratio is constant for different crystals within a single rock sample because of similar chemical properties of B and B'
the exponential term becomes the gradient of a graph of $B / B^{'}$ vs $A / B^{'}$ measured for multiple crystals in a single rock sample

image: NASA, Messenger

image: NASA, magellan

only large craters(> 3 km) as smaller bodies form up in the atmosphere
cratering rate of the entire planet resurfaced $\sim 1$ Gyr ago
there are thousands of volcanoes, but no sign of plate tectonics
the lithosphere is too thin to support large volcanoes
there are pancake domes and coronae, which is when volcanoes collapse under their own weights
evidence for ancient water erosion (rivers, shorelines, river deltas, superimposed on more recent impact craters)

image: NASA, Magellan

resurfacing until $\sim 2$ Gyr ago
the largest volcanoes because the plates do not move across the mantle plumes (hot spots, eg; hawaii and canary islands)

image: NASA, CORBIS

image: taylor, 2018