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Unmanned Spaceflight.com _ New Horizons _ North Facing Slopes

Posted by: alan Sep 20 2015, 06:57 PM

Some previous discussion:

QUOTE (stevesliva @ Sep 11 2015, 05:39 PM)

So, why are north facing slopes white, while south-facing slopes are dark?

It's not just craters... it's the mountains, too.

QUOTE (Gladstoner @ Sep 11 2015, 05:47 PM)

If it's not just the lighting at the moment, perhaps the 'frost' on north-facing slopes hadn't had time to burn off.

QUOTE (Bjorn Jonsson @ Sep 11 2015, 05:49 PM)

Could be due to sublimation and/or condensation that varies with solar elevation angle (north and east facing slopes in hi-res images of Ganymede's north polar cap are bright due apparently to water ice frost that has been cold-trapped).

EDIT In this case this seems to be mostly or entirely due to the illumination geometry rather than variable albedo.

QUOTE (stevesliva @ Sep 11 2015, 05:59 PM)

I do wonder. Contrast-stretching can do that sort of thing. But it sure looks like a real dichotomy.

QUOTE (Bjorn Jonsson @ Sep 11 2015, 06:10 PM)

My views on this are changing rather frequently ;-). Now I notice this nearby crater where only a part of the north-facing crater wall is bright - hardly an illumination effect:

[attachment=37773:pluto_cr...ight_rim.png]

QUOTE (scalbers @ Sep 11 2015, 06:16 PM)

Trying to keep up with all that is coming in now

A quick thought is that a similar crater wall effect happens on Iapetus.

QUOTE (JRehling @ Sep 11 2015, 08:10 PM)

My read is that Tombaugh Regio was created in an explosion and the blast coated away-from-Tombaugh slopes white and that north-vs-south is a red herring. Of course, that may be wrong.

QUOTE (stevesliva @ Sep 12 2015, 12:19 AM)

Doesn't look radial to me. Before these global mosaics I was thinking it looked like a wind of white deposition could blow out the SSW from Tombaugh Regio, but now it just looks more like Iapetus, as s.c.albers said...

With plenty of local exceptions, the trend in the southern hemisphere is South facing dark, North facing white, and the clincher is that it's not just crater slopes, but also mountain slopes.

Another possibility I would like to add is deposition of frost or snow on the windward slopes of topographical features. With the the sub-solar latitude in the northern hemisphere and the north pole in constant sunlight the sublimation will be in the north and freeze-out primarily in the south with the wind blowing roughly north to south with some trend to the west due to the Coriolis effect.

I wonder how the Pluto-Charon barycenter being outside Pluto affects the Coriolis effect.

Posted by: scalbers Sep 20 2015, 07:05 PM

Earth-moon also has a barycenter offset, though there isn't any tidal locking of the Earth's rotation. I'm leaning toward saying this shouldn't impact the Coriolis force on Pluto, though the challenge is to explain why. One point is that if you're standing on Pluto's surface underneath the barycenter you are still attracted towards Pluto's center instead of the barycenter. Tidal distortion of Pluto's shape by Charon is also pretty small I assume.

Posted by: HSchirmer Sep 21 2015, 04:06 AM

QUOTE (alan @ Sep 20 2015, 07:57 PM)

Could be a new layer of frost or "snow" as sublimated vapor cools and is redeposited locally.

I recall reading (can't remember whether a paper or book paragraph) something that mentioned
a counter-intuitive observation/prediction about dark areas on bodies with exotic ices-
the dark areas get lighter in "spring" when they receive more sunlight.

The suggestion was that the additional heat triggers sublimation of ices below dark material.
That creates vapor, which expands, and triggers evaporative cooling.
That cooling leads to frost/snow, which covers the dark area.
If the cooling vapor creates a haze or fog bank, that might block or scatter sunlight and cool the area.
If the vapor rises and radiates enough heat away, you might get a dusting of "snow" falling back to cover the dark area.