LJ wrote:
Images taken in August 99 and April 01 show new streaks running down slopes - very distinctive, obviously happening within that time period. What is the medium??

Alex writes:
I'm not sure what you mean by "the medium." The prevailing opinion of these dark streaks is that they represent dust avalanche scars (remnants of dry mass movements) and NOT, contrary to what a few assert, evidence of fluid flow, so clearly the "medium" is subaerial. At any rate, one will note that almost the entire set of dark streaks observed by the MOC NA camera display acute upslope ends, many at rocky outcrops (Sullivan et al., 1999; Sullivan et al., 2000; Edgett et al., 2000). Indeed, Sullivan et al. (2000) noted that the streaks' apexes are almost invariably found below more rugged, slightly steeper terrain and distinct outcrops or knobs, which they interpreted as single-point origins for each individual streak. Moreover, the lack of incised channels with anastomosis and debris aprons argues for a dry downslope mass movement origin (i.e. avalanche scars revealing a lower albedo substrate) and against a fluid formation interpretation.

Just an aside: Rob Sullivan has a fairly detailed paper on this subject coming out pretty soon in JGR-Planets, probably the upcoming MGS Special Issue.

References:

Edgett, K.S., et al., Lunar Planet Sci XXXI, #1058 (2000)
http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1058.pdf

Sullivan, R.J. et al., Lunar Planet Sci XXX, #1809, (1999)
http://www.lpi.usra.edu/meetings/LPSC99/pdf/1809.pdf

Sullivan, R.J. et al., Lunar Planet Sci XXXI, #1911, (2000)
http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1911.pdf

Sullivan, R.J. et al., Lunar Planet Sci XXXII, #2145, (2001)
http://www.lpi.usra.edu/meetings/lpsc2001/pdf/2145.pdf

Alex R. Blackwell
University of Hawaii