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Posted by: PhilCo126 Sep 28 2008, 11:33 AM

Which is in fact the largest crater in our solar system?

Is it the 4000 km Valhalla multiring basin on Jupiter's moon Callisto ?

Internet searches always come up with the 2000 km Aitken basin on the farside of Earth's Moon

Posted by: Pavel Sep 28 2008, 03:17 PM

The northern hemisphere of Mars might be a "crater".

Posted by: Phil Stooke Sep 28 2008, 06:09 PM

The actual crater inside Valhalla would be quite small. The north Mars lowlands basin - assuming it is impact-generated, which I think it is - is a true crater and a whopper. Next would be South Pole-Aitken on the Moon and Hellas, I suppose.

Incidentally, I am really sick of seeing the SPA basin referred to as "Aitken basin at the South Pole", or similar nonsense.

Phil

Posted by: Vultur Sep 28 2008, 07:25 PM

I didn't know there was a 2000 kilometer crater on the Moon! How big of an impactor does that mean?

Posted by: tasp Sep 28 2008, 09:03 PM

You know, Vultur, that is an interesting question.

Think of all the variables.

How hard and fissure free is the impact zone?

Is impact zone mostly low Z elements, or high Z (lead ore much more power intensive to excavate out of crater) ?

What is the gravity strength of the target world ?

How fast is impactor approaching ??

What is density of impactor ??

Is target world in close proximity to a larger object ?? (consider Jupiter's gravity accelerating an object that winds up smacking Io. On it's own, Io could not generate much accel, but add proximity to Jupiter, look out !!)

Is there already an existing crater in the crosshairs ?? Seems like blasting out an already pulverized area would take less power.

Are there entrained volatile materials in the impact zone?? (seems like disrupting a buried pocket of methane ice might get you a bigger crater than the equivalent amount of buried ice)

Seems like an existing caldera, on the verge of a Krakatoa style cataclysm, taking a major impact might yield quite a devastating zap!

Surely there would be a myriad factors determining the final size of a crater.

Posted by: PhilCo126 Sep 29 2008, 11:12 AM

Indeed tasp, trying to compare with Earth ( KT impactor was probably 12 kilometers in diameter and caused the Chicxulub Crater of about 200 kilometers diameter, which was probably larger ) is also difficult as our Moon doesn't have any atmosphere...
Interesting though is the multiple impact theory which suggests a possibility of near simultaneous impactors of which fragments collided with the Moon

Posted by: john_s Sep 29 2008, 03:29 PM

A rule of thumb is that the crater is about 20x the diameter of the impactor. Despite all the variables mentioned above, I'd guess this probably works to within a factor of two either way in most cases.

John.

Posted by: scalbers Sep 29 2008, 04:33 PM

QUOTE (Phil Stooke @ Sep 28 2008, 06:09 PM)

The actual crater inside Valhalla would be quite small. The north Mars lowlands basin - assuming it is impact-generated, which I think it is - is a true crater and a whopper. Next would be South Pole-Aitken on the Moon and Hellas, I suppose.

Incidentally, I am really sick of seeing the SPA basin referred to as "Aitken basin at the South Pole", or similar nonsense.

Phil

Interesting to consider the finer aspects of the name here. As I gather the edge of the basin is near the south pole - I see via Wikipedia that the Leibnitz mountains on the rim can be seen from Earth. Now I have a better appreciation for those rough edges I've seen on the southern edge of a nearly full moon in a telescope.

Posted by: PhilCo126 Sep 29 2008, 05:29 PM

Well the crater itself in the middle of the extensive system of concentric troughs surrounding the impact site isn't that small after all...
http://www.spacechariots.biz/images/cratertutfig4.png
http://nssdc.gsfc.nasa.gov/imgcat/hires/vg1_1641838.gif

Posted by: Phil Stooke Sep 30 2008, 01:42 AM

"isn't that small after all" ... The Valhalla crater isn't anything like SPA or Hellas, though.

The problem with SPA is that people who know nothing about it but can't be bothered to find out what the name means will insist on butchering the name. It stretches from the South Pole to Aitken, it doesn't mean Aitken is at the South Pole. Ewen Whitaker told me they were thinking of calling it Shoemaker-Aitken instead - that works too.

Phil

Posted by: chuckclark Nov 10 2008, 02:22 PM

Well, given the talk about South Pole Aitkin Basin on the Moon, here is a constant-scale natural boundary map of Moon the uses as its edge a network of fracture lines radiating outward from SPA Basin. (P.E. Clark picked these lines as the map precis.) The dark purple line is the basin edge. Folds up to a pretty good globe, makes a pretty seasonal ornament and, if you need it, you can pretend you hung the moon.
Kidding aside, you can see the SPA Basin-size directly, no distortion (with interruption in map-mode).
Handy cylindrical format shows same data and helps keep you oriented.
Yellow line is something (Don Wilhelm?) called Gargantua Crater.
Not to start any ruckus, but it's a lot bigger than SPA. Wonder how it compares in size to the Northern plains of Mars.

Posted by: Vultur Nov 10 2008, 08:50 PM

QUOTE (john_s @ Sep 29 2008, 04:29 PM)