Craters on the Moon have been filled in with (now solid) lava.
We've brought some of these rocks back.  We can also count the craters
on the site from which a given rock came; the older the rock, the
larger the number of craters.  From studies of the relationship
between number of craters and ages on the Moon, we infer that there
was a period of *very* heavy bombardment early in the history of the
solar system (~4 billion years ago).

  Most of the Moon's surface is very old, > 3.5 billion years old.
  This is because it is geologically dead; it cooled off much faster
  than did the Earth.  Earth's surface is much younger, the vast
  majority is < 2.5 billion years old.  This is because Earth has
  active geology, in particular lava flows and plate tectonics, and
  also has weather to erode things. 

  The oldest fossils known on the Earth (albeit somewhat
controversial) in the rare 3.5 billion year old rocks, are of
single-celled cyanobacteria roughly 3.5 billion years old.  You can
also find fossil stromatolites from that epoch, which are layered
colonies of bacteria.  Stromatolites are rare today, as they are
vulnerable to grazing creatures such as sea snails, and are only found
in extremely salty environments, where these various grazing creatures
can't survive.

  Interestingly, for most of the history of like on Earth (3.5 billion
  years to ~700 millions ago), life was only single-celled.  

The biggest craters on the Moon were caused by asteroids of diameter
~100 km, during the period of Heavy Bombardment.  The Earth is a
bigger target than is the Moon, so it must have received 20 times more
impacts than did the Moon.  The heavy bombardment period in the Solar
System ended about 4 billion years ago.  The earliest life we know of
dates very soon thereafter.  So life seems to have gotten started
almost as soon as it possibly could have. 

How do we know about the history of impacts on Earth? Alvarez found a
planet-wide iridium layer on the Earth, that came at the same
geological time. The layer extrapolates to about 10km object impacting
the Earth 65 million years ago (radioactively dated) -- Cretatious --
Tertiary (KT) boundary event. This is the event that lead to the
extinction of the dinosaurs.  Big impacts have played an important
life in the evolution of life. On the one hand they cause incredible
environmental damage, but on the other they may deliver ingredients
needed for life, such as aminoacids and water. Carbonacious chondrite
asteroids contain 10\% water, comets contain 50% water. Aminoacids
that are extremely rare on Earth but found on meteorites were found in
the KT layer. The crater corresponding to this layer was also found and is
consistent with the size of the asteroid.  Kretatious-Tertiary
boundary event corresponded to 10^14 tonnes of TNT equivalent energy
release due to the impact. Hiroshima Nagasaki weapons -- 20000 tonnes of TNT. 
30 meter asteroid -- 15 Megatons.

There is no evidence that the asteroids brought life to Earth, but
they could have delivered the necessary ingredients.

Debate: contingency vs convergence. 
One line of reasoning: Our existence depends on rare line of events, which all played out just right. 

Large asteroid impact (10 km) will burn 30 meter deep layer throughout
the planet. Could the life exist below this depth, and can recover
later to come back when the planet cools? These are global events,
which throw molten rock and dust into the atmosphere and cover the
planet.  Smaller impacts (Tunguska event) -- explosion above the
surface, in the atmosphere (10 MTons) -- few-tens of meters
object. There was no crater, but burned forest (2000 km^2). These are
also very devastating events (large city size).

Comets (e.g. Halley's commet 10 km), 50% water ice, which sublimes as
the comet gets close to the Sun. Ice goes straight to
vapor. Subliamation causes dust and gases to escape the surface and be
deflected by the radiation pressure (light pressure) from the Sun.

Origin of asteroids and comets: protoplanetary disk (dust and gas),
coalesces into 1-10km range objects due to gravitational
instability. Close to the Sun, the ices evaporate, so the rocky core is
left; this is asteroids. Comets still have ices on them, and they
orbit further out -- Kuiper belt beyond the orbit of Neptune. They are
perturbed by planetary motion, or passage of nearby stars, and are
sent towards the Sun.

Kilometer size asteroids orbit in the inner solar system -- the sensus
is not complete yet, but this is important for knowing the threat to
Earth. Some asteroids are fragments of other asteroids. Some are truly
primitive material from the beginning of the Solar System.

Let's see what determines the temperature of the planet and the
possibility of existence of liquid water and life. Is the temperatur
just right, or is there a feedback that keeps the temperature right?

  Continuing the derivation from last time, we equate the energy per
  unit time received by a planet from its star with the energy
  reradiated by the planet as a blackbody.  
    T_planet = [(1-A) L_star/(4 pi d^2 sigma)]^{1/4}
A = albedo of planet
L_star = luminosity of star
d = distance from star to planet
sigma = Stefan-Boltzmann equation.  

But the star itself is a blackbody, so 
L_star = 4 pi R_star^2 sigma T_star^4. 

Putting this in, then:

T_planet = T_star (1-A)^{1/4} {R_star/2 d}^{1/2}

Plugging in numbers for the Earth, we get 255 C, well below the
freezing point of water.  But we've ignored the greenhouse effect.


  

© Copyright 2008 Christopher Chyba, Michael A. Strauss, Anatoly Spitkovsky