Astro 120 Fall 2005: Lecture 14 page 1
•Relative Ages via Crater Density
•more craters, worn-down craters = older surface
•saturation
•relative ages
•Absolute ages via radiometric dating
•half-life of unstable isotopes
•relative number of parent/daughter - time since formation
•Need to calibrate relative (crater) ages with absolute
•The Moon
•Highlands (old) and Maria (“young”)
•impact history of inner solar system through Apollo
•History of lunar surface: impacts and lava flooding
•Impact history of the inner solar system
•Lots of impacts in 1st Gyr, smaller since
Brief review of last time: Ages of Planetary Surfaces; the
Moon
Reading: Chapter 10, Section 10.3-10.6
Homework 2: Posted on WWW, due 10/14 and 10/17
Astro 120 Fall 2005: Lecture 14 page 2
•4.6 Gyr ago: formation via “Giant Impact”
•impact of a Mars-sized object with primitive Earth
•debris accretes to form molten Moon
•differentiation and solidification
•4.6 – 3.8 Gyr ago: intense bombardment
•continuing impacts obliterate original surface, form terrae
•some huge impacts form basins (i.e. future maria)
•3.8 – 3.2 Gyr ago: lava flows fill maria
•lava wells up from below to flood basins:
•i.e. maria form with denser rock
•3.2 Gyr ago to now: continued cratering
•lava flows ceased 3.2 Gy ago
•further surface changes from impacts
Lunar Surface History
Astro 120 Fall 2005: Lecture 14 page 3
E F G
3.8 Gyr ago 3.2 Gyr ago Today
4.6 Gyr ago
Astro 120 Fall 2005: Lecture 14 page 4
Impact History of the Inner Solar System
(based on the lunar chronology)
1
2
3
4
4.6
cratering
rate
Age [Gyr]
now
maria form
The cratering rate has been the same for the Earth...
then and now
This chronology also applies to Mercury, Venus and Mars.
