[X] My Assistant

[ngunn]

Good grief, it's the middle of the night here and this appears:
http://ciclops.org/view/5419/Rev_97

[ngunn]

I was intrigued by this statement in the 'looking ahead' article:

QUOTE "Altimetry observations would allow RADAR to confirm the presence of liquid within Ontario."

Can anybody explain how this works? I was of the impression that the lake surface would be invisible to the RADAR, as it is in the imaging mode, meaning any altimetry there would be from the lake bottom. So how exactly will the presence of liquid be confirmed ?

[rlorenz]

I was intrigued by this statement in the 'looking ahead' article:
QUOTE "Altimetry observations would allow RADAR to confirm the presence of liquid within Ontario."
Can anybody explain how this works? I was of the impression that the lake surface would be invisible to the RADAR, as it is in the imaging mode, meaning any altimetry there would be from the lake bottom. So how exactly will the presence of liquid be confirmed ?

This is misleading, and a lesson that you shouldnt read CICLOPS for the plans or capabilities of
other Cassini instruments. (This text wasn't written by the radar team, nor checked by them). The official
mission summary
http://saturn.jpl.nasa.gov/files/20081221_...description.pdf
(which I did check before release) says
RADAR: T49 features altimetry across Ontario Lacus, the first time in the mission RADAR has obtained altimetry across a known or suspected lake. The topography profile will help us understand the slopes driving drainage into Ontario as well as providing evidence about whether it is presently liquid-filled. T49 also includes SAR of the almost completely unmapped southwestern quadrant of Titan, as well as of south polar terrain.

We may (dunno if the data is down yet, and in any case won't get processed until the new year. First
report likely at LPSC...) be able to
- tell if the surface is flat (not a discriminator between a dry playa and a liquid filled lake, but circumstantial
support)
- detect a bottom echo as well as a surface echo - IF the liquid layer is neither too shallow (for the two
echoes to merge within the ~30m range resolution) nor too deep (for the bottom echo to be attenuated)
Again, says that there is a flat layer that is radar-transparent - support, but not total proof of liquid
- through detailed modeling of the echo shape determine the small-scale roughness and dielectric constant
of the surface - which would be somewhat constraining of composition. The microwave brightness from
passive radiometry would come into play here too.

Pathologically, a 'lake' filled with porous, emissive but transparent and dead flat 'foo foo dust' might
fit all of these data, but would be perhaps implausible. Such a scenario would not, I think, be compatible with
the spectral characteristics that were reported by VIMS, albeit on observations acquired a year ago.

So these data could eliminate a number of alternative scenarios, but likely not 'prove' liquid by themselves.