Late Heavy Bombardment Craters and Late Heavy Bombardment http://ircamera.as.arizona.edu/NatSci102/NatSci102/movies/

Where do we find craters? – Everywhere! Cratering is the one geologic process that every solid solar system body experiences… Shane Byrne Arizona Univ Mercury Venus Moon Earth Mars Asteroids

Moon

Mercury double basin Rembrandt impact crater 3.9 billion yrs 700 km (430 miles)

Venus has about 1000 impact craters - all rather large, no small ones Venus has about 1000 impact craters - all rather large, no small ones. The craters are distributed randomly all over the planet, unlike Earth or Mars. Impact craters on Earth (about 170) http://aschy.kicks-ass.net/solsys/raw/venus/Golubkin.jpg

Mars Hellas basin ~ 2300 km wide; < 8 km deep Argyre basin 1120 miles wide 3.2 miles deep

What can they tell us about Solar System formation? We have craters! When did they form? How did they form? What can they tell us about Solar System formation?

Typical plot showing that LHB happened LHB factoids: 3.9 - 3.8 By ago lasted ~ 100 My (calc’ed) http://elements.geoscienceworld.org/content/2/4/211/F6.large.jpg

How did the idea of Late Heavy Bombardment come about? (why did someone think of this?) What caused it? What stopped it? How do “they” know? All the Moon’s fault and supported by Apollo missions that landed on the Moon. Observations: Moon has lots of craters Moon has lots of BIG about 45 craters > 300 km ( range: 300 - 1000 km) probably cause by objects 20 - 150 km in size

note: 300 km = 186 mi (SJSU to CalPoly) largest crater: South Pole - Aitken basin (2600 km = 1615 mi, SJSU - Bismark, ND)

Some logical thoughts: # of craters on a planetary surface ~ rate of impactor hits # of craters on a planetary surface ~ the age of the surface  Older surfaces have more craters on them t=0 of the Solar System much more material available than now  frequency of impacts is expected to taper off with time very old surfaces have many more craters than young ones seems to work out on the Moon: The maria, which clearly came later than the highlands, have very few craters compared with the highlands.

How did the Moon get its craters? Two thoughts: thought 1: Moon was bombarded from time it was formed; rate declined drastically from 4.5 to 3.8 By ago (W. Hartmann --- planets formed from swarms of pl’mals) thought 2: impacts declined drastically but increased dramatically for a short time and stopped at 3.8 By ago thought 1 is logical WHAT is so special about 3.8 By ago? Why is thought 2 considered? Observations: Moon maria are younger than 3.8 By Moon rocks brought back by Apollo are dated to be 3.9 By

If there were many more old impacts than young ones, Samples provided by Apollo astronauts of the Moon's crust: mare rocks are 3.3 - 3.6 billion years compared to rocks from the primitive lunar crust are 4.5 billion years the rocks affected by impact events yielded a surprise. If there were many more old impacts than young ones, then there should be many more old impact rocks than young ones. BUT virtually all the impact rocks in the Apollo collection were roughly the same age, 3.9 billion years, and none were older. Fouad Tera & Grenville Turner (Caltech 1975) suggested that an unusual event must have occurred 3.9 billion years ago! a lunar cataclysm created most of the large basins (Imbrium)

George Wetherill he suggested that icy objects orbited the Sun beyond Saturn’s orbit. Uranus and Neptune started to form and their gravitational interaction disturbed this swarm and scattered the unaccreted pl’mals  Late Heavy Bombardment *************************************************************************** Graham Ryder (LPI) he examined melted chunks of rock in meteorites from luner highland samples brought back from Apollo mission andit clearly indicated a sharp cut-off of age to be 3.9 By.