[X] My Assistant

[Juramike]

I made a list of 42 putative impact craters on Titan to try to estimate erosion rates of craterform features.

This list has the “typical” craterform features all characterized by an ISS or RADAR circular bright rim with either a RADAR or ISS dark central portion or a RADAR or ISS bright inner portion (dome or peak) and a darker inner circle. Several of these have been previously reported in the literature. Due to the extensive rim erosion, some of these are broken or incomplete circles.

These (broken)circular features have been classified into five groups depending on the level of erosion evident (from most pristine in appearance to the most used and abused):

- Fresh craters - with little to no erosion evident on the crater rim or debris apron.

- Recent craters – with some evidence of fluvial erosion on the rim, some crenellation being present.

- Eroded craters – with a single breach of the rim or with severe erosion of the rim wall.

- Multiple breached craters – with several complete breaks in the rim structure.

- Degraded craters – with collapse or removal of large sections of the rim structure with extensive invasion of new materials (e.g. dune sands).

Here is a map showing the approximate locations of these features on Titan:



-Mike

[Juramike]

By examining the alluvial fans surrounding a crater that is right on the cusp of becoming eroded, it might be possible to obtain an estimation of the erosion rate. This is possible since it is estimated (from above) that a fresh crater will transition to a breached (eroded) crater in 2 Gyr.



Taking the Shangri-La crater as a typical "fresh" crater, we can use the measured values from the ISS image for the rim extension. The measured value is 16 km for the slightly darker area presumed to be the rim and the pedestal beyond the rim. The rim height is estimated to be 1.3 km in analogy with the floor depth of Sinlap. It is assumed that the rim height is 1.3 km above the surface outside the crater as well. A 40 km rim section would thus have a volume of approximately 832 km3.



The T23 crater is an example of a recent crater very close to being breached. The T23 crater rim can be discerned in the image as a semicircular section displaying peak foldover. (This is well beyond the inner dark - bright dark boundary on the left side). Measurement of the width of this rim gives a value of approximately 13.5 km. Again, the rim height is assumed to be the same as for Sinlap, or 1.3 km. A 40 km rim section would thus have a volume of approximately 350 km3.

The second graphic shows the red circular outline of the RADAR blocky crater rim. On the right side of the crater, a nice alluvial fan structure can be seen just beyond the rim, highlighted in light blue. The volume of this alluvial fan can be used to estimate the erosion rate for this crater.

The volume can be calculated based on the 40 km x 40 km approximate dimensions of the fan spread. A deposition slope of 1 degree is used for the fan structure. (This is approximately half the value of 2 degrees described in Radebaugh et al Icarus 2007 Mountains of Titan observed by Cassini RADAR.). This gives an alluvial wedge height estimate 700 m (or half the original rim height). The volume of the entire fan structure is thus 560 km3 or less (a wedge shape was used to make the estimation easier, in reality it would rest against the original rim angle of repose and thus not be a right triangle).




So an original 40 km rim section of 832 km3 volume is estimated to have eroded down to a thinner rim section of 350 km3 and an alluvial structure of 560 km3. The erodible surface of the original rim section was 40 km x 16 km or 640 km2.

Since the projected age of a crater just ready to transition to a breached crater is 2 Gyr, this gives an erosion rate of 560 km3 eroded from a 640 km2 surface over 2 Gyr.

So we estimate an erosion rate of 4.4 E-10 km3 * km-2 * yr -1

(= 4.4 E-10 cubic kilometers per square kilometer per year.)

-Mike

[EDIT 2/12/2008: The T23 crater estimated age should be closer to 2.8 Gyr. This would give a corrected estimated erosion rate of 560 km3/(640km2*2G.8Gyr)= 3E-10 km yr-1 = 0.3 m Myr-1]