Global True Color View Of Venus? Options

[elakdawalla]

I'm creating a website with views of the worlds of the solar system, to scale with each other (it'll march up and down the few orders of magnitude necessary), and I am having a terrible time finding a global view of Venus to include in it that fits the criteria I'm trying to apply. To the extent possible, I am searching for:

- Full-disk, global view
- Minimum phase angle available
- Approximate true color, as would be perceived by a human observing the globe from space

For Venus, the only global views I am finding are either based on Magellan data (radar views, nothing like what a human would see) or are colorized ultraviolet views (which greatly overemphasize the visibility of cloud patterns in the Venusian atmosphere). I've seen the lovely partial global view of Venus on Don Mitchell's website -- that's the sort of thing I'm looking for, but I need a full disk. Does anybody have any suggestions? Anybody done any work with Mariner 10 or Galileo data that produces a nice, realistic view?

[JRehling]

A long-dormant thread awakens!

I think one thing is clear: If a person orbiting Venus looked down at the clouds, the sensation would probably be pain. It hurts to look at white snow on a sunny day and Venus is similar but twice as bright. It would have an overwhelming brilliance that would require some sort of filtering – sunglasses – for a person to have a chance at perceiving it without saturation of the cones. And once you're filtering, "real" goes out the window.

We could employ neutral filters that damp down every wavelength equally, but the psychophysical research says that the degree of damping affects the perceived color (i.e., the Purkinje Effect). So now we're once again into the realm of the subjective.

Over the past few months, I've looked at Venus more often than any other year. Sometimes at night, but often in the day, seeing its gibbous and crescent forms in a blue daytime sky, including yesterday when I saw it at inferior conjunction – north of the Sun, appearing neither in the evening nor morning, with its southern crescent lit. I've looked at it at least ten times since early December. In the daytime blue sky, Venus looks white – the same white as a front lit cumulus cloud. In a dark sky, I have felt like there are hints of color, but they're right at the threshold of perceptibility and, moreover, the atmospheric conditions on Earth may introduce a bigger color signal than whatever is on Venus itself.

Here's an interesting look at the empirical facts, from Venus Express:

https://www.researchgate.net/profile/Santia...spectrum-in.png

The top panel shows that Venus' spectrum is sloped very slightly towards the blue over most of the visible range, but then it begins to fall off steeply in reflectance into the violet. In essence, Venus is not spectrally neutral but the short, steep decline in violet is offset by the long, gradual excess blue relative to the longer wavelengths. But this empirical data is only as meaningful as our ability to translate it in terms of perception and there we get into the murkiness of human color vision, phase angle, etc.

In summary, Venus appears almost painfully, brilliantly white. In conditions when lenses, mirrors, or filters decrease the intensity, it's still very close to white with an excess of blue and a deficit of violet perhaps canceling out, but what is perceived will depend upon context.