[X] My Assistant

[Guest_AlexBlackwell_*]

The T21 Mission Description document is now online (1.2 Mb PDF).

[David]

I'm not quite sure how to read topography out of these radar images: when you have parallel light and dark lines, does the lighter line represent the crest of the dune, and the dark line the trough, or vice versa? Or is the light line one side of the dune and the dark side the other?

[Juramike]

I'm not quite sure how to read topography out of these radar images: when you have parallel light and dark lines, does the lighter line represent the crest of the dune, and the dark line the trough, or vice versa? Or is the light line one side of the dune and the dark side the other?

Sometimes the topography in RADAR images is pretty clear. You get a strong reflection at the angle of RADAR signal, or you get a nice sharp "foldover" pattern.

For example, In T21 you can see this in some mountains (ridges) at the wide end in the overlap zone with the T8 Swath. There is a sharp bright distinct mountain chain surrounded by a brightish mantle.

But in other areas the topography is pretty muddled and is open to speculation. In general, bright = rough (or porous or other effects with dielelectric constant) and dark = smooth. So it is assumed that the brighter areas are rougher ridge tops. And it is assumed that darker broad areas are smooth muddy basins.

But there could be exceptions:
A highland swamp or marsh, although elevated, could appear RADAR-dark because it is smooth.
A rough crusty dried out lowland could appear RADAR bright, even though it is low-lying.
If the tops of the hills are smooth (covered in schizzle) they could appear smooth
If the valleys are covered in small hummocky drifts, not resolvable by RADAR, they could appear bright.

The RADAR-altimetry experiment comes in handy for determining relative altitude. [A recent presentation at DPS is discussing that contrary to initial assumptions, Xanadu is actually low lying. Xanadu is very RADAR-bright and one would assume it is thus high in altitude. ]

So divining altitude by just looking at RADAR images could get you into trouble. Go with caution...

-Mike