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Posted by: Nirgal Apr 27 2005, 11:52 AM

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/pancam/2005-04-26/2P167575487EFFA9HEP2680L7M1.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 ?

Posted by: djellison Apr 27 2005, 02:33 PM

You're assuming the clouds would be white.

Why

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

Posted by: Nirgal Apr 27 2005, 02:59 PM

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 ...

Posted by: edstrick Apr 28 2005, 10:28 AM

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.

Posted by: Bill Harris Apr 28 2005, 12:39 PM

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

Posted by: dvandorn Apr 28 2005, 03:35 PM

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

Posted by: Deimos Apr 28 2005, 04:25 PM

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.

Posted by: 4th rock from the sun Apr 28 2005, 05:23 PM

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.

Posted by: Nix Apr 28 2005, 05:47 PM

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

Posted by: edstrick Apr 29 2005, 09:45 AM

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.

Posted by: dvandorn Apr 29 2005, 05:52 PM

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

Posted by: Deimos Apr 29 2005, 06:44 PM

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/2P152561491EFF8992P2423R1M1.JPG.html
and http://www.lyle.org/mars/imagery/2P152561545EFF8992P2423R2M1.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.

Posted by: slinted Apr 29 2005, 07:30 PM

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 http://mpfwww.jpl.nasa.gov/mgs/sci/fifthconf99/6127.pdf 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.

Posted by: edstrick Apr 30 2005, 08:45 AM

"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.

Posted by: edstrick Apr 30 2005, 08:54 AM

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.

Posted by: Nirgal May 2 2005, 08:15 PM

I too have always been fascinated by the question how the martin sky would really look like beacuse none of the spacecraft so far has provided a
real wide-angle view of the sky *including* ground/horizon parts.
Although the MER's have already collected very extensive sky studies with the
pancam, most of it were isolated strips or the parts around the sun.
So, because of the contrast streching, that does not tell us much how the big picture would look like in composition with the ground.

For a long time I was a strong advocate of the "nearly-black-sky-not-far-away-from-horizon"- fraction (it's just hard to imagine else when considering an atmosphere
more than one hundred times thinner than earth's)

In the mean time I have collected quite a few rover images, that do show
a considerable angle of sky away from the horizon, and it's interesting
how much this seems to support the "black-sky"-thesis, e.g.:

http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/20040712a/Nav_clouds_sol153_Opp-B163R1.jpg
http://marsrovers.jpl.nasa.gov/gallery/all/2/n/335/2N156104798EFF9946P0833R0M1.JPG

remarks:
1. as already pointed out, the "absolute blackness-level" is probably exagerated
by the contrast stretching (would be easy to resolve by looking at the raw/
radiance calibrated images)
However, Contrast stretching tends to darken *all* black/dark parts of the image,
i.e. the shadoes of the rocks: so in the images above, the sky appears at least as dark than the shadows of the rocks.

2. Vignetting effects of the camera lens towards the edges of the field of view
can also contribute to the "black-sky" effect. However in the images above it is
obvious that the "blackening" already starts in the middle of the field of view

So in summary, I tend to imagine the sky as bright near horizon, but
getting near black very soon, with a sharp gradient.

However this is probably only so during relatively dust-free periods (low atmospheric tau). In more dusty conditions, the gradient will probably
become much less sharp ...

Qeustion: Are there also navcam-sky-images like the ones above taken during
more dusty (high tau) periods ?

Posted by: Marcel May 4 2005, 08:30 AM

I'd like to know what the nightsky looks like at Mars. Would the atmosphere be more transparent (better seeing ?) than earths ? Would earth look as bright as Venus from here ? Would it be as blue, as Mars is red from here ?

Posted by: Nirgal May 4 2005, 12:18 PM

Hi Marcel,

in another thread,
http://www.unmannedspaceflight.com/index.php?showtopic=884

Bubbinsky posted this wonderful impression
of the earth as the martian "morning star":

http://www.unmannedspaceflight.com/index.php?act=Attach&type=post&id=468

as for the actual earth-color: if I remember correctly there must have been at least one pancam/(multifilter ?) shot of the earth around sol 68 ...
The image published by JPL, however, was a b/w montage of a
wide angle Navcam shot combined with the pancam earth-image.
I don't know how many filters were used for the pancam, but
maybe someone of the multi-filter-experts here could reconstruct
a near-true color image of it ?

Posted by: Marcel May 4 2005, 12:34 PM

Thanks a lot. Didn't notice this image before. Looks pretty blue to me!

Posted by: um3k May 4 2005, 10:01 PM

The Earth wasn't visible through the color filters. It was only visible through the clear filter.

Posted by: slinted May 5 2005, 01:34 AM

Well, I've been working the last couple weeks, trying to come up with a "whole sky image". It seems to me like there are two ways to approach this, both of which have significant problems.

First would be the method most familiar to the what we've been doing with the other MER images, simply mosaicing together enough images taken of the sky on a given sol at a certain time into a single view of the sky. Unfortunately, a full 360 mosaic of the sky would require at least as many frames as the 360 degree mosaics of the ground. Neither rover has come even close to doing this. The greatest coverage of a single time on a specific sol are the sequences similar to what was done by Opportunity on sol 124, namely ~ 8 color images taken over the course of a half hour. Here's a upward looking polar projection of these 8 images (zenith is in the center, north is up, west is left):


As you can see, this isn't nearly enough image coverage to create a complete sky mosaic from just these frames. So, instead of using a single sequence to make the all sky mosaic, I had hoped to combine images taken on different days (hopefully around the same time) into the full sky. Unfortunately, the coverage doesn't exist to do this either, even if time of day were ignored. Here is a projection of *all* the sky images taken by Opportunity, over the first 270 sols. Color wise, it's complete nonsense as it combines images taken at different times of day, but it does show which portions of the sky have been imaged by Pancam and more importantly, which portions have not.

Let me qualify this a bit before I go on. For an image to be included in the above projection, it must have been taken in at least 3 filters all from the same pointing. Some sky images are done in just L2/L7, or just a single filter, and aren't included in that projection, but they don't fill in the gaps completely. Also, navcam coverage will probably fill in the holes in the pancam coverage, but wouldn't do much to help us determine the color at that position.

The holes in the coverage in the southern sky, it seems to me, pose the greatest problem. On sols with relatively little dust in the air, the position at approximately the same altitude as the sun, but opposite azimuth might well be the darkest point in the sky at certain times of the day, and without pancam coverage of that position we don't really know experimentally what color it will be.

So, the mosaicing approach won't be able to come up with a whole sky image, for lack of coverage.

The second approach would be to use a mathematical model of atmospheric scattering and absorption to determine the color and brightness for the whole sky at a given moment in time. As edstrick pointed out in his excellent post in this thread, this is tremendously complicated and requires a VERY thorough understanding of the martian atmosphere in order to render the colors and brightnesses accurately. I had hoped to avoid the models as much as possible, and present a view strictly derived from the images we have of the sky, but it appears that the only way to get the whole sky view will be to use the images (and/or minites data) to come up with the parameters to feed the model, and rely on the model to 'fill in the blanks' in the image coverage. I'm concerned with how consistent the model sky will be vs. those locations which have been directly measured, or even if we have enough data to feed the models for accuracy great enough to put side by side with actual images.

Sadly enough, even with 270 sols of calibrated data, we can't put forward an image and say "here is the whole sky as seen by the rovers", but given the limits of data bandwidth, and the relatively low priority of sky images vs. geologic targets on the ground, I guess we should be happy that we have as many sky images as we do.

Posted by: 4th rock from the sun May 5 2005, 11:33 AM

Not bad at all.

Let me see if I can create a gradient in Photoshop that matches those images and can be used to fill the gaps.

Could you please post some more versions of the bottom mosaic? For example, just using the setting sun or high sun images?

Posted by: tedstryk May 5 2005, 12:40 PM

Would it be possible to reconstruct the missing portions through cloning if that doesn't work?

Not bad at all.

Let me see if I can create a gradient in Photoshop that matches those images and can be used to fill the gaps.

Could you please post some more versions of the bottom mosaic? For example, just using the setting sun or high sun images?

[/quote]

Posted by: djellison May 5 2005, 12:50 PM

I'll see if it can easily be reproduced using Dreamscape in 3ds max

Doug

Posted by: DDAVIS May 5 2005, 01:52 PM

as for the actual earth-color: if I remember correctly there must have been at least one pancam/(multifilter ?) shot of the earth around sol 68 ...
The image published by JPL, however, was a b/w montage of a
wide angle Navcam shot combined with the pancam earth-image.
I don't know how many filters were used for the pancam, but
maybe someone of the multi-filter-experts here could reconstruct
a near-true color image of it ?


This is an attempt to do so, if my attatchment made it. I was hoping such an image would be made from the moment the Rovers made it there.

Don Davis

Posted by: 4th rock from the sun May 5 2005, 04:25 PM

Here are 3 images extrapolating the full sky from Slinted's images:





I had to use cloning and rotation, as the gradients alone couldn't account for all of the brighness changes.

The 2 last ones are diferent versions of the "all images" information, one with the sun at Zenith, the other with a lower sun and some darkening in the south(?).
It looks as though the haze might make the sky brighter close to the horizon in the opposite direction from the Sun. But no indication of a dark zenith from this data!

Posted by: odave May 6 2005, 04:58 PM

I've been fascinated by the amateur astronomy done so far by the MERs - I say "amateur" because they are geologists (aerologists?) by trade

I would think the seeing may be worse with all the dust blowing around, but then the atmosphere is a lot thinner. I wonder, too, what kind of details one could observe on the Earth from Mars? Would my 8" f/10 SCT at 200x show clouds and ocean? Any hints of land masses? We can certainly see surface features on Mars, so I'd assume the view of Earth would be at least as detailed. How about night-side light pollution from cities at that distance? I'd think not - probably too small and too dim as compared to the reflected sunlight. (You can see light pollution from orbit, how about from the Moon?)

It would be quite an observing session - and you don't need to put up with that pesky dew!

Posted by: dvandorn May 6 2005, 09:04 PM

Observing Earth from Mars would manifest the same issues we have observing Venus from Earth, to wit:

1 - When Earth is closest to Mars (and therefore appears larger and, you would think, brighter), it's always going to be in a crescent phase.

2 - It's also going to be relatively close to the Sun, appearing (like Venus does to us) as a morning or evening star. So it will never be as bright as it would be if it were full-disk-illuminated at closest approach.

Because Earth at its closest (and therefore at "best seeing") shows a crescent, you'd have just a small, tantalizing slice of illuminated area to observe. Within that slice, I would think you would see a mottled blue-and-white appearance, with occasional brownish-greenish land masses rotating through.

It would be exceedingly difficult, starting from first principles, to separate the transient cloud formations from the static land mass formations with anything less powerful than a major observatory. The polar ice caps would be pretty easy to pick out, though. So, mapping Earth's surface using telescopic images from Mars would be a challenging exercise.

As for man-made lights on the Earth's unlit side, they would be detectable only by long-period exposures, I think, not visible to the naked eye. Even at its slimmest crescent, the illuminated portion of the Earth would be *so* much brighter that, as long as it was in view, I doubt you'd see any of the relatively dim stretches of man-made light from the cities and towns experiencing planetary night. But a really good telescope and good, discriminating astrophotography software would probably be able to see night-time ights.

All in all, though, trying to characterize the Earth's surface, atmosphere and climate telescopically from Mars would be a challenging and confusing exercise. I think the Martians would probably be highly tempted to send a few probes in, to get better resolution and a better view of the daylit side of the planet.

-the other Doug

Posted by: ilbasso May 6 2005, 09:21 PM

Playing around with Starry Night Pro and looking at Earth from Mars, I came up with a range of brightnesses of about magnitude -1.1 (a little dimmer than Sirius) to -3.8 (almost as bright as Venus gets at its brightest), depending on Earth's distance and phase.

The angular diameter of the disk varies by distance, of course. At its closest in August 2003, the Earth was about 45" of arc in diameter, more or less, as seen from Mars. That's a little larger than Jupiter's equatorial diameter as seen from Earth right now. Of course, at that close approach, Earth was at inferior conjunction relative to Mars, so there was very little of the disk actually visible from Mars. (I wonder, would you be able to see any city lights?)

The only time you would see Earth's full disk is when it is on the other side of the Sun from Mars, at which point it will be as small as it gets as seen from Mars. At that point Earth's angular diameter is more on the order of 7" of arc.

We can see Martian features like Syrtis Major from Earth, even with small telescopes, and they're much smaller relative to Mars' disk size than Africa would be relative to Earth's disk seen from Mars. Plus, Earth is nearly twice as large as Mars, so I imagine Africa would be easy to pick out. However, you'd have to look at a time when the Earth was farther away from Mars so you could see more of Earth's disk, making the features smaller.

Posted by: fredk May 7 2005, 12:47 AM

It's certainly fun thinking about what I could see if I could haul my 'scope out on the summit of Husband Hill. I agree views of Earth would be dominated by cloud systems and polar caps with some landmasses showing through with more difficulty. It would be a nice sight to see crescent Earth and moon in the same field, though. Didn't mgs image them?

Also, infrared imaging would open up the entire dark side of the Earth during inferior conjunction, and it shouldn't be hard to map out the continents that way.

Posted by: ilbasso May 7 2005, 12:06 PM

The other cool thing is that Earth's Moon would be visible as a naked eye object from Mars. Today from Gusev, the Earth is magnitude -2.73 (as bright as Jupiter is here right now), and the Moon would be seen as a magnitude 1.3 'star' about 5.5 minutes of arc from the Earth, or about 1/6th the diameter of the full Moon as seen from Earth.

In a telescope, the Earth would be a crescent about 14" of arc in diameter. With small amateur telescopes, you would just barely be able to make out the Moon as anything other than a point of light.

Posted by: David May 8 2005, 05:12 PM

There's no point in sending probes to Earth -- everybody knows that with the runaway greenhouse effect of its thick atmosphere, the surface is far too hot for life to develop. Three quarters of its surface is covered with noxious molten water, and showers of molten water precipitate over most of the surface area, effectively sterilizing the solid areas. If life on Earth were even remotely possible, you'd have to look for it on the relatively hospitable (if scorchingly hot) southern polar cap.

Posted by: djellison May 8 2005, 05:35 PM

If you had a 12 inch telescope with a 2048 x 2048 CCD installed - you might get a view a little like this



http://www.msss.com/moc_gallery/r03_r09/images/R05/R0500763.html for the raw data

Doug

Posted by: Edward Schmitz May 8 2005, 06:36 PM

I like that! We get so stuck on oursevles, that we can forget this isn't the one true perspective.

Posted by: djellison May 8 2005, 06:39 PM

I think every probe that leaves the earth and is so-equiped should image the earth on departure - simply as a reminder of where and what we are. It may be only a few pixels, or even just a pale blue dot - but it speaks volumes

Doug

Posted by: Stu May 8 2005, 07:43 PM

Talking about looking back at Earth, there will be an amazing event visible in Mars's sky in a few decades time...

http://www.newmars.com/archives/000130.shtml

Posted by: wyogold May 8 2005, 09:39 PM

ha ha ha.
that is great.

dream what could be not what is.

scott

Posted by: wyogold May 8 2005, 09:52 PM

that wrinkles my brain.....I live in that photo somewhere. I have a much clearer view of the moon. We sent a probe from here (or there depending on which perspective you looking at it) And it took a picture of us.

As I sit in my house thinking I'm on that blue dot and it would take me several days to go around the entire thing. Oh yahhh, there is a LOT of space out there.


scott

Posted by: ilbasso May 9 2005, 02:40 AM

This event will happen November 10, 2084, for those who are making travel plans.

Posted by: dvandorn May 9 2005, 04:48 AM

Well, why not? I'll only be 129 years old by then...

-the other Doug

Posted by: Stu May 9 2005, 07:13 AM

I know WE won't see it, but it will be a heck of a thing for the settlers on Mars at that time to witness.

Posted by: Bob Shaw May 9 2005, 10:28 AM

David:

Your interpretation of the life-bearing possibilities of Earth are just so Mars-centric! We Jovians know full well that water per se isn't a poison, it's full of nice fresh Hydrogen, just waiting to be cracked out of that useless Oxygen stuff! Yummy!

Earth *could* support life, but only where the pressure is halfway bearable, and first indications indicate that there may be one or two spots in the lowest portions of the atmosphere which could be barely habitable (if rather hot). As for the semi-vacuum above the atmosphere, those rock surfaces may be a valuable source of Silicon Dioxide and Aluminium so don't write them off. Perhaps we could develop rovers to perform some initial sample collections...

Yours,

A Kraken

(With apologies to John Wyndham)

Posted by: um3k May 9 2005, 02:45 PM

I'll only be 95!

Posted by: tedstryk May 9 2005, 03:28 PM

I will be 105! Hmmmm.....I was thinking about going for some fast-food for lunch, but I think I'll pass.....

Posted by: ustrax May 9 2005, 04:05 PM

110...And then I'll tell all my grand children about those glory days back in the beggining of the century when even this old rug could see an abyss where others saw a hole...
One of those grand children did even stay at the Ultreya Inn the night before a spelunking tour on the site...

Posted by: ustrax May 9 2005, 04:06 PM

110...My head is not definitely the same...Where you read 'hole' read 'shadow'...

Posted by: dvandorn May 9 2005, 06:32 PM

"How do you know it's an abyss and not a shadow? Are you an areologist?"

"No... but I did stay at an Ultreya Inn Express last night."



-the other Doug

Posted by: wyogold May 10 2005, 02:47 AM

Posted by: ilbasso May 10 2005, 11:40 AM

Oh, and one final thing on Earth transits. Arthur C. Clarke wrote a story about a marooned astronaut on Mars watching a transit of Earth across the sun. Problem was, he wrote the story in 1971...and the transit in question occurred in 1984.

Posted by: TheChemist May 11 2005, 12:19 PM

Since the color of martian sky was discussed early in this thread :

http://photojournal.jpl.nasa.gov/catalog/PIA07930

"Here is the martian twilight sky at Gusev crater, as imaged by the panoramic camera on NASA's Mars Exploration Rover Spirit around 6:20 in the evening of the rover's 464th martian day, or sol (April 23, 2005)."

"...This small panorama of the western sky was obtained using camera's 750-nanometer, 530-nanometer and 430-nanometer color filters. This filter combination allows false color images to be generated that are similar to what a human would see, but with the colors exaggerated. In this image, the bluish glow in the sky above where the Sun had just set would be visible to us if we were there, but the redness of the sky farther from the sunset is exaggerated compared to the daytime colors of the martian sky...."

Posted by: Tman May 11 2005, 03:58 PM

Wooow, fantastic!

It shows exactly the blue sky around the sun. To me it looks like using somewhat excessive colors, though it's wonderful.

It seems recently on sol476 and sol480 was a new "sun in the west" campaign. Following a L456 raw picture shows Gusev's west rim near THE crater as sharp as never before:

http://www.greuti.ch/spirit/spirit_sol480_panL456.jpg

Posted by: alan May 11 2005, 04:50 PM

Did you use this in that image?
http://qt.exploratorium.edu/mars/spirit/pancam/2005-05-10/2P168996464EFFAAB2P2681L5M1.JPG
According to the timestamp it was taken after sunset
http://qt.exploratorium.edu/mars/spirit/pancam/2005-05-10/2P168995073ESFAAB2P2689L6M1.JPG
perhaps thats why the Gusev's rim is so sharp.

Posted by: Tman May 11 2005, 05:19 PM

Yes these pics are from sol480. It's that was I've assumed for such a sharper view of this rim, that sun must been behind the rim at least.