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ugordan
post Sep 6 2008, 07:02 PM
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It definitely appears to be atmospheric as the shadow moves below it (click to enlarge):


It seems to have dissipated at the end. Based on the rough 100 km size estimate I get a height in excess of 100 km (around 110 km, but the uncertainty is rather large) if my back-of-the-envelope calculation is right using this 3x magnified bit:


If it's an ordinary cloud, what would make it so circular at that altitude and would it dissipate quickly in sunlight? If it's an impact plume, wouldn't we expect a darker color and much lower and more persistent plume?


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Juramike
post Sep 6 2008, 07:40 PM
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Nice animation, Gordan!

As you've shown, the shadow seems much longer in the earliest images, just what you'd expect as the sun peeped up over the horizon.

-Mike


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ustrax
post Sep 6 2008, 08:11 PM
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QUOTE (ugordan @ Sep 6 2008, 08:02 PM) *
It definitely appears to be atmospheric


I keep my thought after seing your animation...
the shadow seem to me to be originated on an elevation to the right, as the sun decreases in the horizon the shadow decreases and so does the brightness in the circular feature. I keep my guess: they are not related...


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Paolo Amoroso
post Sep 6 2008, 08:12 PM
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QUOTE (ugordan @ Sep 6 2008, 09:02 PM) *
It definitely appears to be atmospheric as the shadow moves below it (click to enlarge):

The movement of the feature suggests that a few hours earlier it was in the night side of Mars beyond the terminator, possibly looking like the bright terminator projections observed telescopically, e.g. the projection seen at Lick Observatory on 5 July 1890 (from Chapter 6 of Sheehan's The Planet Mars).


Paolo Amoroso


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ugordan
post Sep 6 2008, 08:43 PM
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QUOTE (ustrax @ Sep 6 2008, 10:11 PM) *
the shadow seem to me to be originated on an elevation to the right

You do realize it would have to be an awfully big topographic high, don't you? We're talking about objects 100 km in size and the shadow is something like twice that size, even well past the terminator. That would put Olympus Mons to shame, no?

Furthermore, the shadow isn't simply getting shorter, it's actually disappearing at the same time the bright circular object (let's call it 'cloud' for discussion's sake biggrin.gif ) is disappearing.


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TheChemist
post Sep 6 2008, 08:47 PM
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QUOTE (ugordan @ Sep 6 2008, 11:43 PM) *
You do realize it would have to be an awfully big topographic high, don't you? We're talking about objects 100 km in size and the shadow is something like twice that size, even well past the terminator. That would put Olympus Mons to shame, no?


The typical anti-Ustrax that posts lately here. Now he sees an inverse abyss laugh.gif
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ustrax
post Sep 6 2008, 09:21 PM
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QUOTE (TheChemist @ Sep 6 2008, 09:47 PM) *
The typical anti-Ustrax that posts lately here. Now he sees an inverse abyss laugh.gif


laugh.gif Now you got me in my new strategy...to assume there's nothing special in order to be surprised in the end of it all...

I DON'T want Oppy headed towards Ithaca!
(See what I mean?... wink.gif )

QUOTE
You do realize it would have to be an awfully big topographic high, don't you?


No I don't... tongue.gif


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ugordan
post Sep 6 2008, 09:23 PM
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QUOTE (ustrax @ Sep 6 2008, 11:21 PM) *
Now you got me in my new strategy...to assume there's nothing special in order to be surprised in the end of it all...

Congratulations and welcome to the club known as The Pessimist. I hope you'll enjoy your stay!


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ustrax
post Sep 6 2008, 09:27 PM
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QUOTE (ugordan @ Sep 6 2008, 10:23 PM) *
Congratulations and welcome to the club known as The Pessimist. I hope you'll enjoy your stay!


Thanks but I believe it won't be long enough to be enjoyed in its full splendor... rolleyes.gif


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siravan
post Sep 6 2008, 11:08 PM
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QUOTE (ugordan @ Sep 6 2008, 02:02 PM) *
Based on the rough 100 km size estimate I get a height in excess of 100 km (around 110 km, but the uncertainty is rather large) if my back-of-the-envelope calculation is right using this 3x magnified bit:


How do you calculate the height? My back-of-envelop calculation based on the ratio and of the shadow to the object size and its distance from the object is more consistent with an altitude half the object size (50 km if the diameter is 100 km).
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ugordan
post Sep 6 2008, 11:39 PM
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Yep, it appears I messed up the calculation of (at least) illumination incidence angle using the shadow shape, I did an inverse tangent of a/b instead of inverse cosine of b/a. That, by itself, lowers the end result to some 80-ish km, which might still be wrong. 110 km was no doubt suspicious, being at upper fringes of the atmosphere.


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Paolo Amoroso
post Sep 7 2008, 02:40 PM
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QUOTE (Paolo Amoroso @ Sep 6 2008, 10:12 PM) *
The movement of the feature suggests that a few hours earlier it was in the night side of Mars beyond the terminator, possibly looking like the bright terminator projections observed telescopically, e.g. the projection seen at Lick Observatory on 5 July 1890 (from Chapter 6 of Sheehan's The Planet Mars).

The date and time on the drawing can be read clearly only on the printed edition of the book: "1890, July 6d 9h 25m P.S.T."


Paolo Amoroso


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ugordan
post Sep 8 2008, 09:50 PM
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Here's a new version of VMC2RGB, this time implementing the adaptive Homogeneity-Directed interpolation, snatched from dcraw (algorithm was based on the work of Keigo Hirakawa, Thomas Parks, and Paul Lee) as slinted suggested earlier. It's not a groundbreaking improvement and your mileage may vary, but if you take care of how you process the images wou might end up with slightly sharper images. See below for 2x enlarged inset comparing the old and new algorithm, with identical processing in Photoshop:



The target image in question was greatly boosted in saturation and sharpened so it's pretty noisy, but shows more details:
Attached Image


Download it here: Attached File  vmc2rgb_v02b.zip ( 40.37K ) Number of downloads: 382


EDIT: I discovered a bug when using the new algorithm that clips some saturated pixels to zero, it's now fixed.


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stevesliva
post Sep 8 2008, 10:18 PM
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Does fog cast a shadow? Would it be forming above an ice-filled crater in the southern hemisphere?

Or perhaps a leeward cloud that dissipates in the sunlight?
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ormstont
post Sep 11 2008, 10:08 AM
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Ok...more info on our strange cloud...we've got a lot of the Mars Express scientists at ESOC today for planning meetings and I showed the images to our OMEGA team who are the MEX cloud experts! They've only had a quick chance to look so far but they were very excited, hopefully I can chat with them again later in the day. They also said that if the altitude was indeed 80km (I credited you all with doing the analysis for us...and they loved the animations!) then the feature is almost certainly an ice cloud. I'll keep you all posted on any more info I get!


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