Roberts

D=89 km
Center: 71¡N, 186¡E

Count Area: 1509 km^2
Observed Density (@ 1km): 25200 ± 4100
Age: 3.9 ± 0.1 (+sec: 4.0)
Stoffler epoch: Nectarian
Fit Density (@ 1km): 23900 ± 4400
PF fit: good*, large craters shallow, large fraction of secondaries and small area (~ 1500 km2).

Age calculation notes:
good fit (SM).
good fit - large D within error - use as is; adding OSs improves fit for large D, but degrades it for small D - age is slightly older (MRK).

USGS geology: 
Crater material mapped as Pre-Nectarian. Floor material mapped as fill with Imbrian to Nectarian age.

Wilhelms: 
Crater is Pre-Nectarian. (all materials?)

Floor material: 
Best guess is not original (at least in area counted). Flat floor, filled larger craters inside Roberts, no central peak. Material looks softened. Loose?

Geology Observations: 
Ejecta blanket not visible. Rim is severely eroded. No central peak. Most of floor is flat, some hummocky areas to north and south (avoided) that may be original floor or wall collapses or ejecta from nearby craters. Floor has a textured look - possibly due to very degraded small craters. Several secondary chains/clusters with larger craters (up to ~ 2 km) - mostly degraded.

SFD Observations: 
All craters dominated by class 3 and 4. Has flat SFD for all diameters. Slope(Diff)=3.5 ± 0.3.

Discussion notes: 
large fraction of secondaries and small area, difficult crater to use for PF and age. mid sized craters not buried.

Notes from Brian's adjustment:
- Counting boundary:
	Could perhaps be tightened a bit from 1:00 through 6:00?
	Some faint craters missed.

- Secondaries:
	Cluster missed at 3:00, iffy markings at 7:30 and 9:00.

- Removal of large, buried craters:
	The four largest craters could be buried type 4's.

- Size adjustment of large craters:
	The four large craters are poorly sized, and a fifth was missed.

- Explaining unusual cases:
	Roberts is marked in red with comment "narrow-v".

- Future actions:

- Completed actions:
	Fixed boundary (Michelle).
	After discussion, removed each of the large craters (BLE).
	Marked missing craters (BLE).
	Changed secondary markings (BLE).

Last Edited by MRK 12/17/2012