QUOTE (centsworth_II @ Apr 11 2009, 09:43 AM)
How can you tell secondary craters from a large impact vs. small craters from a fragmented meteor? My inexpert guess is that a small, heavily pocked area indicates the later.
I can try to offer a qualitative answer (I'd need lots of rust-remover to give a quantitative answer...).
The size of a crater is a function of the energy of the impinging object (diam ~ Energy**3 ?). E ~ M*V**2.
So a small object smashing in really fast makes the same size crater as a larger object coming in slower.
There is a dichotomy of approach velocities, with primary objects coming in at or above the Martian escape velocity (5km/sec), and secondary objects being much slower (~2-3 km/sec and under).
Atmospheric drag goes in proportion to the surface area of the impinging object, so smaller objects are more easily slowed down and/or evaporated that large objects. The effect of this is to produce a threshold of size below which crater-producing impacts become unlikely (for given velocity/density/strength/trajectory-angle).
With a large field of meter-ish sized craters, we would expect the group of crater-producing objects to be all within a factor of about 100 in mass (assuming nearly-equal velocities).
Now, I am not sure of the actual numbers, but the Martian atmosphere would essentially filter out smaller impactors. The question to ask here is whether primary impactors that would produce a one-or-two-meter-sized crater are below this threshold, and therefore much less likely?
A primary impactor group would need to split into a number of similar-sized fragments, and the effect of atmospheric drag would tend to scatter their V-squared energy for impact crater size even more.
A large impact producing showers of secondary debris, however, could easily produce a number of similar fragments, since the parent material and the launching shock forces in a given area of the source rock are similar. The ballistic launch of these fragments then has a decent likeleyhood of producing a group of similar-sizes fragments well above the threshold for significant atmospheric drag, but that produce fairly small craters.
(Note that a grazing-impact object might produce a group like this if it breaks up and falls just ballistically...., but that is also a chancey scenario.)
Thus, the secondary impact scenario at this scale seems more probable. Probabilities are the best we can do without samples to analyze. Field trip, anyone?
'Nuff said?
-- MarkG
