Spirit Clouds In Latest Pancam Images ? Options

[Nirgal]

among the many new pancams at exploratorium there seem to be interesting
images showing what must be clouds, clearly defined as never seen before
at the Gusev site.

http://qt.exploratorium.edu/mars/spirit/pa...HEP2680L7M1.JPG

Interestingly, those are high-res pancam multi-filter sequences.

Questin for Dan (slinted) refering to our recent discussion about sky color:
would't this be wotrth a try to do interesting sky color composites.
Maybe you could use the brightest pixels
in the clouds as a white point reference ?

[djellison]

You're assuming the clouds would be white.

Why

They're very very rarely white here on earth, let alone on mars

[Nirgal]

You're assuming the clouds would be white.

Why

They're very very rarely white here on earth, let alone on mars

well, at least for the orbiter & telescope images, the clouds provide a good white
reference point: the "natural" color of ice (better than the ice caps, which are often partly coated with pink/red dust)
But you are right: on the ground it's probably different, especially near the
horizon ...

[edstrick]

There's a lot of "twaddle" about martian sky colors, particularly from the "Hoaxland" acolytes and other conspiracy mongers.

The martian sky contains
1.) Colorless molecular gases, primarily carbon dioxide, (no green chlorine, blue methane, etc.)
2.) Reddish-tan "Butterscotch colored" micrometer sized non-spherical, poly-mineralalic, poorly crystalized (not single minerals) dust,
3.) White water ice clouds, and rarely
4.) White dry-ice (CO2) clouds

Aurora/Airglow emissions are low and are entirely negligable during the daytime.

Some back-of-the-envelope arm-waving calculations: The surface pressure on mars is 1/2 of 1% of Earth's. With the lower gravity, the "Column abundance" of gas molecules above the surface is roughly 1% of Earth's. ***IF*** there were no dust-haze and clouds, the sky would be blue, the same color as high altitude sky on Earth. Rayleigh scattering goes up as the inverse-4'th power of wavelength <I think> cut wavelength by half, scattering goes up 16. That's why clear blue sky is strongly blue, not bluish gray or bluish white.

The "blue" of martian clear air is probably almost everywhere, almost all the time essentially totally swamped by scattered light by dust and ice-hazes.

There is a *LOT* of scattered light from haze. When the air is relatively clear (an "optical depth" of say 0.6) and the sun is high, the sky is as bright as or brighter than the surface, except probably at the zenith. The sky gets brighter toward the horizon since there's more haze in the line of sight, and the sunlight is not being attenuated a lot as it penetrates into the atmosphere, so dust at low altitudes is still well illuminated, and the sky gets brighter toward the horizon.

When the air is more opaque, mostly/usually from dust, the optical depth is over 1. Viking landers saw optical depths of well over 2 during major dust storms. Sunlight, even from directly overhead, is significantly blocked by the dust, and the dust at low altitudes is illuminated by a dust-obscured sun, as well as diffuse scattered light from overhead. During such dust storm conditions, the sky actually gets darker toward the horizon.

When the sun is low, things get a lot more complicated and I won't discuss them here, this time, at least.

What *COLOR* is light scattered by dust? That depends.
That depends on the composition of the dust (what color is it if you collect an opaque layer of it?).
That depends on whether the grains are opaque or diffusely transmit light (I'm assuming no clear transmitted rays like from rain drops, spherical cloud particles, or regular ice crystals).
And that depends on the size distribution of the dust grains.

Colorless dust grains that are much smaller than the wavelength of light scatter halfway like gas: Bluish Rayleigh scattering. Reddish sub-micrometer dust will be less blue.

Grains near or a very few times larger than the wavelength of light scatter light 1.) by diffraction around the surface of the grain, 2.) by colorless (unless the grain is a colored metal) reflection from the mineral surface, or 3.) by transmission and internal scattering of light within the dust grain. All three of these depends on the phase angle of the scattering: The angle between light source (the sun), the dust grain, and the observer. (180 degrees is with the dust between you and the sun, 0 degrees is like a full moon.)

The physics is complicated and requires modeling that is *highly* dependent on the properties of the grains, but at an arm waving level, we can do fine.

Diffraction is dominantly forward scattered, diffracted light is not reflected back toward the sun. Hold a razor blade up with a lightbulb almost behind it and at near 180 degrees phase you can see the edge lit from behind by diffraction. Diffraction is the source of the bright zone around the sun in hazy weather that's white or bluish white at high phase angles near the sun.

Surface reflection by mineral grains is relatively colorless unless the grains are opaque, metallic and are strongly colored. Wavelength and angle dependent scattering, "Mie" scattering, can be very complicated, but the reflection tends to be both backwards and forwards, and less to the side (90 deg phase), and has no strong directional peaks or colors (dust grains don't cause rainbows).

Light that enters the mineral grain can be refracted and pass directly through the grain, or it can be scattered once or more times within the grain. The more scatterings, the more random the direction the light takes when it exits the grain. The more scatterings, the longer the path the light takes within the grain (unless the grain is pretty opaque inside and light is not transmitted well), and the more "color" the light can pick up as some wavelengths are absorbed. The *tendency* is for transmitted/internally scattered light to be the most diffusely scattered light from a dust grain, and to be the most strongly colored scattered light, and to be more strongly colored when it is back-scattered (Low phase angle) than when it is forwards-scattered (high phase angle).

Combining diffracted light, surface scattered light, and internally scattered light, and not including light re-scattered from one dust grain to another (we're assuming single-scattering here) ... Reddish mars dust in the air will be most red at low and intermediate phase angles (with the sun behind you or to the side), and will get brighter and less red as you look increasingly toward the sun in the sky. At high phase angles, within 20-30 degrees of the sun, the sky will be off-white, and even closer to the sun, the sky may well be bluish-white.... all due to dust grain scattering.

Water ice and CO2 ice clouds and hazes are going to be pretty much white, maybe bluish white close to the sun. If well defined crystal sizes are present, you could get iridiscent colors next to the sun. That's (maybe) much more likely with CO2 ice grains than water ice grains, which are apparently small and grow/evaporate slowly, probably mostly around dust grains.

Multiple scattering between dust grains, especially when the sun is low or atmospheric opacity is high, smears out angle-dependent scattering and multiplies the color of single-scattering with every scattering event. Dust storm skys will be muddy brownish, except the bluish or off-white region directly around the sun. I know this from direct experience in a 1977 Texas dust storm where the sky turned tanish-blue-to-whitish-tan overhead and darker tan/brown at the horizon, and was a really weird bluish-white toward the sun. It looked very much like Viking Lander pics of the great 1977 global dust storms the landers and orbiters both observed.

[Bill Harris]

Good discussion of the Martian sky, Ed.

Your observation of the Texas duststorm parallels my impression of the Earth skies due to volcanic dust/aerosols in 1982 from el Chichon and Pinatubo in 1991. I was able to also look at the sky in 1991 from an airliner at 35,000 feet, so I had a feel for the appearance of the dust at altitude. A dusky warmish color, increasing in density at the horizon, and a whitish halo around the Sun.

My own personal preference for viewing Mars is a bit less "butterscotch" so things don't look ochre, ochre, ochre, although it's not true color. I grew up in Birmingham, Alabama in the 60's, and our steel-mill polluted skies looked too much like that...

--Bill

[dvandorn]

Good discussion of the Martian sky, Ed.

Your observation of the Texas duststorm parallels my impression of the Earth skies due to volcanic dust/aerosols in 1982 from el Chichon and Pinatubo in 1991.  I was able to also look at the sky in 1991 from an airliner at 35,000 feet, so I had a feel for the appearance of the dust at altitude.  A dusky warmish color, increasing in density at the horizon, and a whitish halo around the Sun.

My own personal preference for viewing Mars is a bit less "butterscotch" so things don't look ochre, ochre, ochre, although it's not true color.  I grew up in Birmingham, Alabama in the 60's, and our steel-mill polluted skies looked too much like that...

--Bill

I have two things to say.

First, we need to remember that Mars is an entire planet, with complex landforms and weather patterns. While its atmosphere is far thinner than ours, it obviously changes in appearance over time and with regard to a number of different conditions, sometimes as frequently and dramatically as our own. So there really isn't such a thing as any one way the Martian sky appears -- its appearance changes during the course of the day, during the change of the seasons, and during changes in weather. Just like on Earth. (Now, as for me, I want to have a feeling not just for how the sky generally looks, I want a feeling for how it changes, and what conditions make it change in given ways... *smile*...)

Second, I think it's really, really fascinating how much most of us want to be able to visualize exactly what it would look and feel like to stand upon the surface of Mars. We want to know what the colors would be like, we want to know what the horizon would look like, and we especially want to know what the sky would look like.

I don't think there is any rational reason for a lot of us to feel so strongly as we do about this desire to *know* what it feels like to have Mars under our feet. (Or the Moon, or Titan... any and all places we can imagine ourselves.) But it's this desire that needs feeding, and it's this desire that drives and justifies the continuing exploration of space.

Rational or not.

IMHO.

-the other Doug

ps -- personally, from what I've seen, in my imagination I see Mars' sky as something that thins out so dramatically overhead and brightens up so dramatically along most of the horizon, with transitions from bright butterscotch to dark, dark violet, that it will give you a visceral sensation of how thin a skin of air surrounds you. If you've ever flown in an airliner at 35,000+ feet, you know how you can just sense the thick portion of the atmosphere, defined by how most of the clouds and haze float atop it? Well, on Mars, I think you'll be able to "feel" that layer as you stand at its bottom -- it will feel like a very shallow pool, indeed. DVD

[Deimos]

Well, on Mars, I think you'll be able to "feel" that layer as you stand at its bottom -- it will feel like a very shallow pool, indeed.  DVD

Perhaps, if that feeling comes from your knowledge of the "thinness" of the Martian atmosphere, as measured by pressure. In that sense the analogy to the view from an airliner helps. However, I'm not sure that feeling could come from your visual perception. There is more light scattering in the Martian atmosphere than there is on a clear day on Earth, at sea level. The dust is not a "thin" layer. Orbiter images show dust extending beyond 30 km above the surface. Yes, it is brighter near the horizon than higher up, as on Earth for similar optical depths. But there is no reason I'd expect more of a visual cue of thinness where Spirit is than you'd find in Death Valley.

[4th rock from th...]

I'd go with this subtle colors from the 2,5,7 filter images. The sky has some processing to enhance the clouds.
Interesting, the horizon looks redish, a little higher the sky starts to look grey/bluish but the clouds look yellow/redish again.

[Nix]

Amazing how a clouded image of the site makes the scene look much more Earthlike. Nice

[edstrick]

The "almost black sky at zenith" idea for Mars partly derives from a characteristic of the Viking Lander cameras. The extreme upper part of the cameras' field of view was vignietted by the optics, possibly <I don't recall> by the ejectible dust-abrasion cover they had on the cameras. (At least one dust cover was ejected to see if there was any dust accumulation on the optics, with minimal evidence for dust accumulated)

The result was in the top 10 degrees of the field of view, the sky rapidly got darker, going pretty nearly black at the top edge of frames that extended up to the top edge. This fooled at least one astronomical artist to paint Mars with a black sky and a bright horizon.

As noted above, the air may be "thin" in terms of not much gas per cubic foot, but the scale height <rate the atmosphere gets thinner with height> is about the same as on Earth. CO2 has about twice the molecular weight as Nitrogen/Oxygen mix, and would reduce the scale height, but the gravity is about 0.4 <very approx numbers here> of one Earth gravity, and the two approximately cancel.

[dvandorn]

The "almost black sky at zenith" idea for Mars partly derives from a characteristic of the Viking Lander cameras.  The extreme upper part of the cameras' field of view was vignietted by the optics, possibly <I don't recall> by the ejectible dust-abrasion cover they had on the cameras.  (At least one dust cover was ejected to see if there was any dust accumulation on the optics, with minimal evidence for dust accumulated)

The result was in the top 10 degrees of the field of view, the sky rapidly got darker, going pretty nearly black at the top edge of frames that extended up to the top edge.  This fooled at least one astronomical artist to paint Mars with a black sky and a bright horizon.

As noted above, the air may be "thin" in terms of not much gas per cubic foot, but the scale height <rate the atmosphere gets thinner with height> is about the same as on Earth.  CO2 has about twice the molecular weight as Nitrogen/Oxygen mix, and would reduce the scale height, but the gravity is about 0.4 <very approx numbers here> of one Earth gravity, and the two approximately cancel.

Ah, but the images taken by Spirit from West Spur looking up at the top of Husband Hill all showed a *very* dark sky beyond the hill crest. That was looking up at, what, 45 to 50 degrees? And the sky was very dark, *much* darker than the hill crest and very much darker than the sky at the horizon of the plains, even in mid-day.

-the other Doug

[Deimos]

Ah, but the images taken by Spirit from West Spur looking up at the top of Husband Hill all showed a *very* dark sky beyond the hill crest.  That was looking up at, what, 45 to 50 degrees?  And the sky was very dark, *much* darker than the hill crest and very much darker than the sky at the horizon of the plains, even in mid-day.

-the other Doug

Yes, but that was when the dust loading in the atmosphere was at its lowest (and close to Earth-like, although of course "Earth-like" varies too), It was also only true in red filters, where the surface itself is quite bright. Blue filter images continued to show the sky brighter than the surface.

Compare http://www.lyle.org/mars/imagery/2P1525614...23R1M1.JPG.html
and http://www.lyle.org/mars/imagery/2P1525615...23R2M1.JPG.html.

Earlier and later in the mission, dust opacity was 3-4 times what it was over winter. This underscores how important seasonal/weather variations are, but even at times of low dust, the skies are far from black.

During Pathfinder, 180-deg sky arc image were taken through the zenith. Markiewicz et al. (JGeophysRes 104, 9099) show results. Variations, not counting the bright area near the Sun, were +- factor of two from the darkest part of the horizon. Considering the logarithmic sensitivity of the eye, you would not perceive bright vs. black--for that dust load.

[slinted]

During Pathfinder, 180-deg sky arc image were taken through the zenith. Markiewicz et al. (JGeophysRes 104, 9099) show results. Variations, not counting the bright area near the Sun, were +- factor of two from the darkest part of the horizon. Considering the logarithmic sensitivity of the eye, you would not perceive bright vs. black--for that dust load.

Another study by Markiewicz and others relating to sky brightness and the diffuse illumination effect on the colors of surface objects, done for a poster session, is available here. It includes a contour map of sky brightness at 670nm (red light) that shows the minimum (located at the same altitude as the sun, opposite azimuth) to be only 1/3rd as bright as the darkest point along the horizon. But, since it is in red light, it is mostly describing the brightness from dust scattering. What we really need to determine color though is a similar countour map, but for all wavelengths.

[edstrick]

"Ah, but the images taken by Spirit from West Spur looking up at the top of Husband Hill all showed a *very* dark sky beyond the hill crest. That was looking up at, what, 45 to 50 degrees? And the sky was very dark, *much* darker than the hill crest and very much darker than the sky at the horizon of the plains, even in mid-day.
"
Doug: Remember... the "raw" images have had an automatic contrast stretch applied. The Zero level and Max level of the 8-bit images are adjusted so everything darker than <say> the 5% darkest pixels are saturated black at 0 graylevel and <say> the brightest 2.5% <I'm making up numbers here> are saturated white at 255 graylevel. That's even if the original data was <for example> 74 darkest and 206 brightest <in 8 bits>... if the sky's the darkest pixels in the image <looking away from the sun and not seeing into dark shadows on the surface>, the darkest pixels in the sky get set to black.

[edstrick]

Oh... regarding the discussion of ice clouds as "white balance" targets...

Viking repeatedly imaged a pattern of extremely high <some 25 kilometer> clouds which were textured with a lot of fine detail in one region southwest of Tharsis. Something like 30 degrees south, 160 or 170 degrees east. (I'm very unsure of the coordinates without checking)

Water ice clouds on Mars tend to be very diffuse, though some imaged from the rovers do have a fair bit of fine detail, but nothing like these. At the low temps <especially at altitude> where ice hazes are present, ice -- even micron-sized -- takes a long time to condense or sublimate, making the clouds diffuse. The combined analysis of limited atmosphere thermal sensing from the Viking Infrared Thermal Mapper, and radio occultation atmosphere profiles, together with the characteristics of the clouds, were that these were CO2 ice clouds, formed by "orographic" waves in the middle atmosphere causing temperatures to persistantly drop below CO2 ice formation temperatures for that altitude.

Someday we'll see them from the surface.