Erosion on Titan Options

[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]

Here is a gallery of Fresh Craterform features from the identified list on Titan.

[In each of the sets of images that follow, the least eroded set is first in the series, the most eroded set is last]

Sinlap is a typical example of a fresh crater, it has a nice bright debris apron, a clear complete circular rim with little crenellation. Alluvial fans or erosion in the rim is not evident. There is a slight hint of streams beginning to form in the debris apron.





-Mike

[Juramike]

Here is a gallery of images of Recent Craterform features from the identified list on Titan.

Ksa is the least eroded example of a recent crater. There is some initial incision of the rim present. Some alluvial structure with bright streambeds (rapid erosion) is visible beyond the rim either on the outside or on the inside portion of the rim. More typical examples include the T23 crater and Menrva. In addition, there can be some burying of the structure evident in higher latitudes (more evidence they've been around a while).








-Mike

[Juramike]

Here is a gallery of images of Eroded Craterform features from the identified list on Titan.

The Shikoku crater is a good example. The crater rim is now showing definite signs of degradation. There is at least one crack or breach of the rim. Definite RADAR dark streambeds cut into the apron.





-Mike

[Juramike]

Here is a gallery of images of Multiple-Breached craterform structures from the identified list on Titan.

Guabonito is a typical example. It shows multiple cracks or narrow breaches around the crater rim. Severe crenellation is evident in the remaining sections of the rim. In some features, streambeds or dune sands have breached the wall and penetrated the interior section of the craterform.

The pattern of erosion around appears to be uniform. Crack formation does not appear dependant on local tectonics or wind direction as evidenced by the random vectors of the cracks around the rim. This supports that crenellation and erosion due to rainfall-created streambeds drives the initial crack formation around the crater rim.










-Mike

[Juramike]

Here is a gallery of images of Degraded craterform structures from the identified list on Titan.

These are the most difficult to identify. Several of these are only partially complete rims and are thus more open to speculation. Some identifications are based on a combination of ISS and RADAR images. ISS imaging is assumed to indicate elevation. In extreme cases, (e.g. entry 42), the ghost image of the rim is evident from a change in the dune sand pattern.

A good example is the crater to the West of the Huygens landing site (entry 29). In this example, the cracks have widened and some of the remaining crater sections have collapsed and been eroded away. Only a few rim section remain surrounded by an ice mantle or eroded deposits presumably from the rim. Dune sands have invaded and pass through the central section.












-Mike

[Juramike]

Here is an EXCEL file showing the entry number, latitude/longitude coordinates, and diameter of the crater, it’s identified level of erosion, the source where the image was plucked from, and the literature reference, if applicable of the list of 42 craterform features I identified on Titan.

 Crater_Erosion_Table_20071024_as_EXCEL_97.xls ( 29K ) Number of downloads: 898

Here is another map showing the numbered locations of the craters above on a combined ISS/RADAR map of Titan. (numbers on map correspond to entry number in EXCEL file and image numbers above):




-Mike

[MahFL]

Wow...I did not realise there were so many craters on Titan.

[ustrax]

Wow...I did not realise there were so many craters on Titan.

How does Juramike manages to do all this extraordinary work?! Amazing!

[tty]

Wow...I did not realise there were so many craters on Titan.

It is actually remarkably few, compared to most other bodies with solid surfaces in the solar system. It seems that a 50-100 km scale impact crater doesn't last more than a few hundred million years on Titan.

[Juramike]

With 42 craters sprinkled among different erosion states, it should be possible to calculate the erosion rate of the crater rims. (That's kinda where I'm going with all this: using emprical data to calculate the observed erosion rate].


The overall erosion sequence for the observed craters is:

Fresh --> Recent --> Eroded --> Multiple Breached --> Degraded


Each step should have it's own k value (rate constant).

Since there are not so many eroded craters in the list, it seems likely that k(Eroded --> Multiple breached) is faster than the other rates.

Qualitatively, once one crack happens in a crater rim, other breaches happen rapidly.

I'm not sure of the exact mathematical treatment to use to calculate erosion rate. Unlike radionuclei or molecules bouncing into each other, degradation is not a random event, so a T1/2 logrhythmic decay doesn't seem the right way to go.

[For example, if a bunch of craters of y km rim thickness are all being eroded at x km rim/yr, then they should all start to decay after y/x yr. That is, they should all start to go at once. So it would look more like a step function rather than a smooth logrhythmic decay]

-Mike

[Juramike]

For grins and giggles, I made a combo/hybrid of the T23 crater and Menrva. They are both at the same level of erosive degradation.

Here it is:



(I think they fit pretty good. What do you think?)

-Mike

[Juramike]

Binning the total list in log 2 sizes and making a log 10/log 2 Hartmann Plot, the observed data (blue dots and line) seem pretty consistent with a 4.5 Gyr old surface:



Craters > 100 km fit the projected rates for 4.5 Gyr.

Craters < 100 km seem to fit a lower rate.
(Some Possible reasons for a paucity of smaller craters: some atmospheric filtering of smaller impactors, as well as smaller craters being difficult to observe by only ISS. RADAR seem better at identifying smaller craters, and RADAR coverage is still not complete.)

-Mike

[EDIT: These rates were calculated based on the ratio of binned observed craters over the ENTIRE surface of Titan (8E7 km2)]

[Juramike]

To try to figure the erosion rate I’ll focus on one question:

On average, how long does it take to breach a crater rim on Titan?

Using a Hartmann Plot, I plotted combined crater rates of craters that had intact rims (“fresh” + “recent” craters as defined in the list) in the same graphic with total crater rates on Titan.

In the graphic, the blue line is total crater rates on Titan, the purple line is the rate for craters with intact rims (fresh and recent) calculated over the entire surface of Titan:



Two things can be gleaned from the graphic: - Even “fresh” + “recent” craters are enough imply an older surface (>1 Gyr).

- Craters with intact rims are approximately 50% of the total crater count.

Here is the breakdown by size:

Crater bin size (diameter in km, %intact craters of total):
16 (70%)
32 (33%)
64 (0%)
128 (20%)
256 (50%)
512 (50%)
1024 (0%) [The degraded sliced carrot feature is the only crater in this bin]


Conclusion: Even with a limited sample, assuming the observed total crater count implies an 4.5 Gyr surface, it seems to take about 2 billion years to degrade the rim of a crater on Titan. This rate appears independent of crater size.

-Mike

[tty]

I don't quite agree with your interpretation. While the relative shortage of small craters could to some extent be due to the thick atmosphere and the difficulties of detection it seems to be quite likely that it is also due to many smaller craters having eroded until they are undetectable. After all SAP radar is pretty good at detecting heavily eroded craters here on Earth. Your # 37 is a beautiful example of a (probable) crater that has eroded almost into invisiblity. On Earth (with a vastly higher erosion rate) I would guess that it is from tens to hundreds of million years old, depending on geological context. Where there are craters that have eroded to that point it seems almost certain that others have disappeared completely.