Tikhov

D=86 km
Center: 62¡N, 172¡E

Count Area: 752 km^2
Observed Density (@ 1km): 20000 ± 5200
Simone's Age: 3.8 ± 0.1
Stoffler epoch: Early Imbrian
Fit density (@ 1km): 18900 ± 5500
PF fit: good*, large craters shallow (12 total), but only need to remove 2 largest to get OK fit, small area < 1000 km2, statistics are poor

Age calculation notes:
- poor fit - anchored at intermediate D. Not improved using Hausen and Holsapple scaling law or layering (SM).
- bad fit - data shallower - could be geology (also having several large craters in a small area) - possible using strength scaling law may help at small D?; adding OSs improves fit slightly, does not change age. Because using strength or layer does not improve fit and fit is bad, it is unlikely we can report a good age for this surface. Geology may be a strong candidate - walls are very subdued indicating burial and collapse - next to a large basin which may or may not be younger (both old anyway) - several large craters which may be basin secondaries (currently not labeled that way) that would produce a shallower distribution. (MRK)

USGS geology: 
Crater rim mapped as Pre-Nectarian. Crater floor mapped as fill with a Nectarian age.

Wilhelms: 
Nectarian? (all materials)

Floor material: 
Best guess is not original (at least where counted). Rim wall not as deep as expected, no central peak. Material looks softened. Loose?

Geology Observations: 
No ejecta blanket visible. Rim looks severely eroded. No central peak. Some possible ejecta/collapses on southern portion (excluded). Floor gently rolling. A few chains/clusters with various sized craters (up to ~ 2.8 km) - mostly degraded. Also note the large craters to the east side were mapped by USGS as basin secondaries. The older, partially buried ones may be, but they have been removed. Not sure about fresher looking craters (currently not secondaries)?

SFD Observations: 
This SFD is unusual in a couple of ways (similar to Millikan). First it has the usual "steep" (is actually a little more flat than steep) then shallow distribution, but it is shifted to larger diameters on the R-plot (to the right) compared to most of the other SFDs. The change point is at ~ 5-6 km rather than 1-1.5. Second, the SFD becomes shallow again for small craters (< 1.5 km). I have been trying to figure out if this is a roll-off due to resolution, but I have picked a fairly conservative value of 800 m for the data cutoff and rechecked the image. Perhaps the production of several large craters has "temporarily reset the smallest craters? Note shallow "right ear" defined by only one crater. Class 3 & 4 craters dominate for smaller diameters (< 5 km), while large craters are class 2. Slope(Diff)=2.2 ± 0.5. 

Discussion notes: 
change boundary to exclude large craters to NW and west (did not help)

Notes from Brian's adjustment:
- Counting boundary:
	Difficult to determine... but I'm thinking a large area near 11:00
should not have been included, and a large area near 5:00 should have been?
	Lots of crater marking issues - this is a difficult case.
	Used topography to get lowest point of floor assuming that is a surface of one age (MRK).
	Moved boundary to exclude largest craters which may or may not be secondaries (MRK).

- Secondaries:
	Could some of the larger craters be secondaries? Doesn't look like it, based on the context image.
	It's possible some of the buried ones are, but those are now gone. (MRK)
	A few minor changes. (MRK)

- Removal of large, buried craters:
	Some of these craters could be removed, and others added.
	Removed a couple more. (MRK)

- Size adjustment of large craters:
	Some can have their sizes adjusted

- Explaining unusual cases:
	Lots of very large craters and lots of resurfacing, hiding small ones.
	Labeled in red, with comment "no shallow".

- Future actions:

- Completed actions:
	Corrected the marking boundary (Michelle).
	Remarked craters of all sizes, and secondaries (BLE).
	Tried to clean up some buried type 4's, but it's hard to tell what is buried and what isn't since there's so much resurfacing?

Last Edited by MRK 12/17/2012