•Judge
age of surface by amount of craters:
more
craters Þ more ancient surface
(for
some objects, have radioactive age dates)
–Moon
“dead” after about 1 billion years
–Mercury
“dead” early in its lifetime
–Mars
active through ~1/2 of its lifetime
–Venus
active till “recent” times
–Earth
still active
–
•Big
objects cool off slower
–Amount
of heat (stored or generated) proportional to Volume ( so R3)
–Rate
of heat loss proportional (roughly) to Surface Area
(so R2)
–Heat/(Unit
Area) µ R3/R2 = R so activity roughly
proportional to R
–
•Same
reason that big things taken out of oven cool slower than small things (cake cools slower
than cookies)
•