Last night I was looking through my telescope. Venus was in it's familiar phase but I was surprised I could see the dark side of it as well. It looked similar to how you can see the dark area of the moon because of reflected earthshine. Now Venus has no moon, or anything else for light to reflect off of so how come I am able to see the dark side of it? Is it because the light from the sun bends around the planet through its thick atmosphere? Or is it because Venus is so hot that it actually radiates its own blackbody radiation in visible light like a hot glowing piece of metal?

The atmosphere doesn't have to be thick to do that--it's the reason for the red color of full lunar eclipses. The earth's atmosphere is showing sunsets all around its edges, which would probably be quite a sight from the moon.

There are calculations to suggest that what is seen during this phenomenon (the "ashen light") is lightning.

Grant Hutchison

It's known as "ashen light". I've never seen it, but I've heard of it, and I believe its cause is not understood.

Although I've never heard it suggested, I wonder if Earthshine may be the answer. Venus is bright enough that objects here on Earth can cast shadows in its light, meaning Venus is illuminating the night side of Earth. Why can't Earth illuminate Venus' bright cloudtops? Usually Earth should be brighter in Venus' sky than Venus is in Earth's sky, since during close approaches Venus displays a crescent to Earth while Earth displays a gibbous to Venus. And I believe that the Ashen light is most obvious when Venus presents a thin crescent to Earth.

Venus is currently about 100,000,000 km from Earth. This is about 260 times farther than the Moon. Venus' albedo is 10x that of the Moon's. So Earthshine on Venus should be 260^2 * 10 = 6780 times fainter than Earthshine on the Moon at the current Earth/Venus distance. This number will peak at 1700 times fainter as Venus gets closer to Earth. Add to this Moonshine. Earth's moon should be very bright in Venus' sky as well. Earthshine on the Moon is easily observable with the naked eye. Is Earthshine that is thousands of times fainter detectable in a telescope? Maybe. Earthshine on the Moon is so obvious that it could probably be 10 times fainter and still be obvious to the naked eye. An 8 inch mirror has 2500 times the light-gathering area as the naked eye pupil. (200mm)^2 / (4mm)^2.

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Neat, I've never heard the lightning theory.

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I saw it last night at about 9 to 9:30PM PST off the coast of Southern California. That website says that this "Ashen Light" is best visible when the terminator of venus faces the earth. What does that mean anyway? I loaded up my planetarium program and last night was about when exactly half of Venus was lit up and half was dark. I was using a 4.5 inch 'scope. Would earth and moon shine reflected off of Venus even be visible in a telescope as small as mine? Lightning would be a very interesting explanation if it turns out to be true.

Probably not. My idea probably isn't the right answer anyway, or I imagine others would have suggested it centuries ago.

4.5 inches is 114 mm. (114mm)^2 / (4mm)^2 = 812. Your mirror has 812 times more light gathering area than your pupil. But keep in mind that 4mm is the diameter of a dark-adapted pupil in a young person. Even without dark-adapted eyes, (I'm guessing 2mm diameter pupil), Earthshine is easy to see on the Moon, so your scope actually has 1600 times the light gathering area of a non-dark adapted pupil. But since your telescope also magnifies Venus, hence spreading its light out, that negates some of this.

I also wonder how bright Earth's shiny spot is. The oceans would look dark, except for the point where the image of the sun was reflected. I imagine this small area of Earth's surface would outshine the rest of Earth combined, but I don't know by how much. I have noticed that Earthshine on the Moon is not consistent. Sometimes it is very bright, and 20 minutes later it is not nearly as bright. I've often wondered if this was due to the "shiny spot" moving over land, or being covered by clouds.

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Nice, another interesting phenomenom I don't recall hearing about.

The paper is curious, too. Is it saying there is a much greater chance of seeing the ashen light when Venus is approaching inferior conjuction vs. post inferior conjunction? If so, why?

I found that the Cassini visits to Venus showed no evidence for lightning, contrary to their explanation; thus, adding to the intrigue.

[Added: Here is one site.
Tony's explanation is what I favor for the moment. Check your math again Tony, however. Light diminishes to the inverse square, but in one stretch not both ways; both ways makes it an inverse fourth power reduction in brightness. Also, the albedo will help (not reduce) the brightness.

Then we can add other dim light sources such as the Moon, Mars, Jupiter, Zodiac lighting, numerous nighttime golf driving ranges, Vegas, etc. Hmmm, the goofy latter ones would explain the pre-inferior conjuction oddity - golf driving ranges turn their lights off after 10pm or so.

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I'd never heard of this either. Fascinating. And since the ESA has a probe orbiting Venus as we speak, aren't there any results from that? I think that one of the instruments is supposed to measure that.

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As above, so below

Yes, that's what it says. The observational evidence they cite suggests that ashen light is reported more often when observing Venus just after sunset (on the approach to inferior conjunction) than when Venus is visible pre-sunrise (after inferior conjunction). Because of Venus' retrograde rotation, we're looking at the sunset terminator when we see it before inferior conjunction. And they cite evidence that the radio bursts they associate with lightning are more common around Venusian dusk. So they're suggesting that that this temporal association (Venusian sunset) may imply a causal link.

What the mechanism for sunset lightning would be on Venus, I couldn't guess. I'm not sure such a big, soupy, uniformly heated atmosphere would reproduce the afternoon and evening convective instability we often observe on Earth. But I'm also not sure it wouldn't.

Grant Hutchison

Ah, that is interesting. Venus receives almost twice the radiant wattage as Earth [ (1/0.72au)^2 = 1.92], that might stir-up some lightning activity. Perhaps the atmosphere of Venus dampens or reflects AM band signals, minimizing Cassini's reception of it during its brief fly-by.

Did I miss the [/facetous] code at the end of that one? I assume it would be even more uniform on Mercury, and the light would all be from ash, too, right?

It shouldn't be too hard to figure the actual illumination of Venus by its neighbors, following Tony's lead. [I hope I can find those contrasting requirements you've given (night sky, cd/m^2) in order to detect illumination of objects.]

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Lighten up! This is a stellar board!

There was some recent research on this, as I recall reported in New Scientist, that the Venusian atmosphere actually glows. A useful rule of thumb is that if something is glowing a dull red, it's at about 1000 degrees, for practical purposes regardless of material.

But why would it only be visible at those particular times?

Ug, compared to Earth's illumination, the Earth shine we would see off Venus at inferior conjunction would be reduced by 1.3x10^16. So, not the source of the Ashen light.

[Earth shine off Venus = (Ie)(Ae)(Av)/(d/r)^4 ;
Ie, illumination at Earth
Ae, Earth's albedo (.367)
Av, Venus's albedo (.65)
d, distance from Earth to Venus
r, radius of Earth]

My intuition was wrong, again.

[Edit: But wait, there is hope in further cacluations here.]

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Lighten up! This is a stellar board!

The Moon & Venus were hanging in the west last night, and I saw exactly what you mean!

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PW -- Plant Whisperer

There is an other cause for the ashen light : Radioactivity.

Since it is fully ATM I will stop here.

May be i will open a thread in ATM . Don't know what i can say or not , if I am right this is serious stuff.

Galacsi.

Well, only the lower portion of Venus's atmosphere is big, soupy, and uniformly heated.
The clouds are in temp. range 20-70 deg. Celsius and there are raging winds >300 kph in the cloud layer, so the lighting could get even 100x powerful as on Earth.
IMHO the lighting is the most logical explanation.

If the lightening is that powerful could there be an associated plasma effect. The new plasma television sets shift the picture so slightly to prevent burn in. This is done in a way that the human eye does not see the movement.

I don't want to go ATM here either so I hope someone like tusenfem might have a take on this.

But that glow is not caused by hot temperature, it is caused by some kind of chemoluminiscence involving atomic oxygen from photodissociation of CO2.
The top of Venus's atmosphere is actually freezingly cold.

I suppose somene has studied electric arc in carbon dioxide at 89 atmosphere pressure. Perhaps the radio noise is at a much higher or lower frequency than one atmosphere lightning. If I recall correctly an earlier probe did detect the radio noise of Venus lightning. Perhaps ionized layers in Venus atmosphere sporatically reflect (absorb, refract) radio noise from the lightning. If the lightning occurs mostly in the lowest kilometer of the Venus atmosphere, the light would be greatly defused after passing though the very thick Venus atmosphere and clouds. Perhaps Venus produces lots of lightening on rare and brief occasions? Neil

Well, when Venera probes detected signs of lightning they also measured higher concentration of SO2 than later probes.SO2 is a volcanic gas and particulate matter from volcanos can start lightning.
And why that lightning shouldn't be in the upper atmosphere?There are no clouds in the lower atmosphere and so no lightning, no?
The conditions in the middle to upper Venus's atmosphere are similar to conditions in the Earth's atmosphere, well, expect for clouds being H2SO4 and "air" almost pure CO2, much higher wind speed, a bit higher temperature and much the cloud layer is much thicker.
I do not see any reason why lightning on Venus should be some different process than here.

There is even rain of H2SO4, that evaporates in height of 25 km because of temperature and going through very dry air, it is called virga and it is frequent in some deserts here on Earth (Of course that rain is normal water on Earth and it evaporates in much lower height).
Venus is actually quite similar in Earth, just the high surface temperatures are "scary".
But even candle flame is 500 degress hotter and the pressure is only about 1/11 of the pressure on the bottom of Marian Deep so it is not so hellish as it is commonly tought.

I've seen it, too, and believe it's a combination of Earthshine, Moonshine, and something else without an ATF label...

Just kidding.

Seriously, I doubt it's just lightening, as that would be flickering, sporadic, etc., and that's not at all what I've observed. Rather, it's consistant, which indicates some sort of extra-planetary illumination.

I suspect it really is Earthshine, Moonshine, and a general bending of solar radiation in Venus' upper atmosphere. If the surface pressure is 91 times our own atmosphere, then considerable bending would occur above the cloud layer, at least more so than our own.

Here's a link to a thermal glowing reference. I would have quoted them directly, but I'm edgy about copyrighted material.

http://www.redorbit.com/modules/news...print&id=63857

It's a hot planet, folks. We can detect dull red glowing, especially with dark adapted vision, very well. Has anybody checked spectrographically at the appropriate wavelengths?

This is also a wonderfully simple answer, Occam would be proud.

You are looking at (half) an entire world's lightning output, however, and lightning can be very common on that sort of scale.
On Earth, it's estimated that lightning hits the surface, somewhere, 30-100 times per second. It's also known that slow intracloud lightning (which produces illumination lasting several tenths of a second) is ten times more common than ground strikes.
If the same thing were happening on Venus (big if), then you'd be looking at 150-500 lightning events per second, each of them lasting a significant fraction of a second, appearing at the limit of visibility on a small area of your retina.

Grant Hutchison

AFAIK visible light excitation start at around 1000 degress Celsius.
Maybe some little glow, but surely undetectable without sensitive equipement.

http://www.space.com/scienceastronom...ts_010122.html

Maybe no lightning, but a dull green glow.