[X] My Assistant

[prometheus]

Is that wet soil on those gully slopes?

[Guest_Edward Schmitz_*]

This is the color swath that was taken at the same time.

http://hiroc.lpl.arizona.edu/images/TRA/TR...1410/color.html

It would appear that the darker areas are frost free and not necessarily wet. Although they might be. Probably not though.

[Stu]

It would appear that the darker areas are frost free and not necessarily wet. Although they might be. Probably not though.

Thanks for clearing that up...

[prometheus]

Maybe not wet but maybe rich in salts from past weeps and as such acting like an anti-icing agent for the frost. So maybe just a clue that at some time in the past those darker areas were where salt rich water weeps actually occured.

[Tesheiner]

Could anybody explain the reason for those discontinuities between the "strips" which make a HiRISE image?
See, for instance, the image from Terra Sirenium Crater as posted by prometeus.

I could understand that effect if each strip was related to an individual pass but I'm pretty sure that's not the case. I would say each strip is related to one of the 10 CCDs making the whole sensor but why those gaps?

[edstrick]

Data is progressively transferred off the CCD chips as the electrons are bucket-brigade transfered across individual pixels in the chips' fields of view and track the moving planetary surface across the chip. I think we're seeing the data transfer off chips as a data-take ends, but I don't have a clear idea why it causes the sawtooth effect.

[djellison]

Distortion induced by off nadir imaging?

Doug

[helvick]

Since Tuvas said on the HiBlog (stage 12) that they geometrically reproject the images so that the result is an image that distances can be read from I suspect that this is an artifact of a sunusoidal equidistant map projection.

[Tesheiner]

> I suspect that this is an artifact of a sunusoidal equidistant map projection.

I thought about something related to the map projection too, but look that we are talking about images which cover a quite small space (e.g. Terra Sirenium Crater's picture covers only 5km x 5km). We shouldn't have projection artifacts on this map scale.

[Phil Stooke]

Tesheiner's right - over such a small area map projection effects are insignificant. Helvick's idea could apply for images spanning several tens of degrees (individual strips made from Map-a-Planet images in sinusoidal projection and fitted together) - but that's clearly not the case here.

Phil

[Tesheiner]

Ok, here is what I've got by overlaping part of the huge B&W VC image (TRA_000873_1780_RED) on top of the same region of it's color version (TRA_000873_1780_RGBcolor). Remember that the first *is* map projected while the second isn't; I had to rescale and rotate the color part to match with the map-projected overlay.
What we see is that the sawtooth gaps are not distortions but actually missing data.



It seems this data may have been lost during the map-projection process because it is actually there on the RGB version.

[tuvas]

Tesheiner's right - over such a small area map projection effects are insignificant. Helvick's idea could apply for images spanning several tens of degrees (individual strips made from Map-a-Planet images in sinusoidal projection and fitted together) - but that's clearly not the case here.

Phil

I don't fully understand the sawtooth affect myself, but I can tell you it's introduced in the Geometrically projected image stage. As to what's causing it, well, your guess is as good as mine...

[ugordan]

A bug in the code?

[nprev]

Almost looks as if it's a spherical projection artifact, but the scale's so small... Did someone change Mars' radius to that of Phobos by mistake?

Wait a minute...just thinking out loud, here, but is the HIRISE FOV so small that spherical distortion does in fact take place at the edges of frames? If, so, then the sawtooth effect might be an attempt to correct this distortion.

BTW, if that is in fact wet soil, then I propose a future lander with a garden shovel to dig for worms!

[ugordan]

but is the HIRISE FOV so small that spherical distortion does in fact take place at the edges of frames?

On the contrary, the very small FOV means the curvature of the surface at that scale is so small it can effectively be regarded as flat. Just as Phil said, you need several degrees wide footprints to end up with significant map projection distortions.
The sawtooth appears to occur only in the upper part of the swaths which is also inconsistent with map projections.