There is a new paper in press in Icarus on Triton:

On the Negligible Surface Age of Triton
Paul M. Schenk and Kevin Zahnle

The article was published online on Sunday. I just got my hands on it. I haven't ready through all of it yet, but it is very intriguing. The authors preformed a new crater counting analysis of Voyager 2 images of Triton. The counted only 100 craters larger than 5 km across, and 21 craters larger than 10 km across. Their analysis allowed them to differentiate between craters and circular diapir features in the cantaloupe terrain.

What is particularly striking isn't so much the low number of observed impact craters. Smith et al. (1990) suggested a surface of less than 500 My based on the low number of crater they counted. What makes Schenk and Zahnle's analysis striking is the complete and utter lack of impact craters on the trailing hemisphere. Every single one of the measured impact craters are on the leading hemisphere. There are a couple of possible impact features on the trailing hemisphere, but all the definitive impact craters are on the leading hemisphere. The authors suggest that this indicates that all the craters likely formed from planetocentric sources (e.g. impact spalls from one of the other Neptunian satellites or disrupted satellites). Calculations of the impactor flux at Triton from planetocentric and heliocentric sources suggest a surface age of 50 My for the "heavily" cratered terrain and 6 My for the cantaloupe terrain. This suggest that Triton has a younger surface than everyone's darling, Europa.

Once you start looking at surface ages like this, the authors suggest that Triton (EDIT: not Neptune, silly) is likely still experiencing cryovolcanism, in the form of diapirs and cryoflows. They further suggest, as Stern and McKinnon did back in 2000, that this level of activity could be evidence that the south polar plumes are driven by internal heat rather than solar heating.

This is *very* interesting, but...



...I don't buy this.

Impacts from Neptunian satellite fragments could only concentrate on one hemisphere of Triton if (1) Triton's rotational and orbital periods were synchronized right from the get-go after capture, or (2) the impacts occurred a long time after Triton's capture.

It's hard to imagine either of these being very likely.

(2) seems to be the best bet. These craters formed very recently.

Hmm. How much tidal heating does Triton get, VP? Wondering if it got an early boost (if not a pretty thorough melting) from the capture...maybe it's only been with Neptune for 50 My?

As an added consideration, Triton revolves in retrograde. Its leading face is doing the equivalent of driving north in the southbound lanes of traffic. Material spalling off of another Neptunian satellite would have to REALLY be moving fast to lose all of its orbital velocity, and also catch up to Triton's trailing face. In fact, those speeds are certainly impossible for material with that sort of origin. An event like that would essentially only crater the leading face.

Would that curious 'finger' imaged projecting poleward into the cantelope terrain possibly be a nascent 'tiger stripe' ??

Is it greatly difficult to do a Monte Carlo type simulation of a large number of objects randomly encountering the Neptune vicinity and doing stats on the resulting crater distribution on Triton ??


{Triton's orbital velocity around Neptune is rather large, is it really that surprising the surface evidences a 'wake shielding' effect ??}

{if someone would like to post a link to a nice Voyager II Triton image, feel free . . . }

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I wondered that too...but then again, I also wonder if we have a good understanding of meteoroid density in the outer Solar System. Accurately determining the age of Triton's surface plus considering both the 'driving north in the southbound lane' and gravitational focusing effects of Neptune on the impact rate might yield some very valuable insight with respect to the distribution of debris out there.

The finger might be a cycloid (which could also be the origin of the "tiger stripes" on Enceladus).

Check out the most poleward ridge in ElkGroveDan's posted image. It definitely has the "cycloid look". The finger may actually be a tailcrack off the cycloid.

Here is an article on proposed ridge formation on Triton:
Procktor, L.M; Nimmo, F., Pappalardo, R.T. LPS 36 (2005) Abstract 1722. "A Shear Heating Theory for Ridges on Triton."
(free abstract. but this link is for the whole shebang of abstracts, it may take a while to download: ftp://www.lpi.usra.edu/pub/outgoing/lpsc2005/full96.pdf

In the abstract the authors state:

"Since diurnal stressing was important when Triton began to circularize its orbit (not too long after capture) and the surface is young, capture must be relatively recent IF diurnal stresses are an important mechanism for ridge formation."

Recent stress cracking, recent resurfacing, and recent cratering all seem to add up to a recent capture.

-Mike

Mike, your link doesn't work unfortunately...
Click to view attachment

As far as I know there was some speculation of sublimated N2 migrating from the sunlighted into the dark hemisphere and freezing there again.
Hence Triton's surface should be refreshed in a short time.
As Triton has a 158° retrograde orbit and Neptune's axis tilt is about 23°, the illumation conditions there are quite chaotic.
I remember this french article dealing with the question of latitude changes of Triton's subsolar point over historical time showing some tables:
http://bugle.imcce.fr/fr/observateur/support/Triton/
There seems to be some disagreement on the calculation of those variations... ?
Anyhow, Triton's seasons don't look that simply predictable as Earth's...

Combinating both aspects should cause a very differentiated and complex terrain structure.
Sadly we know only 30-40% of Triton in detail...

Hopefully Neptune and Triton will be the next target for a major orbiter project in not so far away future...

Bye.

Remove the closing bracket ")" from the URL and it should work.

pele is correct, these craters are relatively recent, and so is the event that produced the impacting bodies.
the surface is extremely young, given that there are essentially no impacts on the trailing side. hence we see no record at all of Tritons earlier geologic epochs.
paul