Future Venus Missions Options

[JRehling]

A simple explanation for at least part of the story:

If the light reflected off Venus can cast shadows on Earth, then Earthshine must similarly illuminate the night side of Venus (remember the Clementine Lunar night-side shots, with the Solar corona and various planets in view, and think, if you will, how *dark* the surface really is compared to the white clouds covering Venus). Of course, at closest approach Venus is 100x further away than the Moon, so the effect will always be somewhat less...

Note that a full Earth is roughly 1/4 the luminance of a full Venus... but that's a crescent Venus that we see at its brightest. When the ashen light would be spotted, that would be a large gibbous Earth seen from the venusian cloudtops, so yes, the brightest Earth seen from Venus would be roughly the same luminance as the brightest Venus seen from Earth. Luna would add a tiny smidgen as well.

That said, although some nonzero amount of Earthly light would be shining off of Venus, the question is: is it visible? Again, I think the neighboring effect of that superbright crescent makes it unlikely. If I had $1500 and the goal of investigating this, I would create lab stimuli with Venus-in-telescope appearance and nothing but inky blackness inside, and see if subjects report that the space inside the crescent seems to be filled in. I would guess "optical illusion" before earthshine, or endogenous glow of somesort. I used to seriously research human vision, FWIW, but this sort of situation can not be abstracted from any published results AFAIK.

[Guest_BruceMoomaw_*]

Yep. One would think that -- if the Ashen Light actually existed -- Pioneer 12 would have detected at least some faint indication of it during its 13 straight years in Venus orbit. I think the Ashen Light belongs in the same dustbin of history as Mars' canals and the Moon's transient glows.

[David]

Yep.  One would think that -- if the Ashen Light actually existed -- Pioneer 12 would have detected at least some faint indication of it during its 13 straight years in Venus orbit.  I think the Ashen Light belongs in the same dustbin of history as Mars' canals and the Moon's transient glows.

The Martian canals weren't entirely illusory -- certainly the network of fine lines was fictitious, but in many cases the "canals" were an attempt by the hand and eye to organize real but very small and faint albedo variations. Comparing old sketches of Mars with modern albedo maps is very instructive, both about the limits of human eyesight, and conversely, about the remarkable visual acuity and commitment of many of these early observers.

[Guest_BruceMoomaw_*]

Well, it's a fact that E.E. Barnard -- one of the sharpest-eyed of all astronomers (which is how he discovered Amalthea) -- is also the only naked-eye astronomer to swear that he saw Mars covered with craters.

[Bob Shaw]

There's an empirical test (for once), and that's the relationship between Terrestrial albedo and the (supposed) glow off the darkside of Venus - all that's needed is a small telescope for the Venus data and access to some carefully combined Terrestrial weather satellite images to prove or disprove the assertion.

Sounds like a PhD in waiting...

Bob Shaw

[David]

Well, it's a fact that E.E. Barnard -- one of the sharpest-eyed of all astronomers (which is how he discovered Amalthea) -- is also the only naked-eye astronomer to swear that he saw Mars covered with craters.

I've read that claim attributed to John Mellish. It's a little hard to believe, unless you count the Hellas and Argyre basins as "craters", in which case they've been observed for a long time. Schiaparelli Crater's outline was observed, by Schiaparelli himself, but only because it cuts into the dark outline of Terra Meridiani (and of course he didn't identify it as a crater). It's hard to imagine anything smaller being resolved by early 20th-century telescopes, ideal viewing conditions or no. Even on Hubble images Martian craters are not very noticeable.

[dvandorn]

One reason Martian craters are hard to see from Earth-based telescopes is that, as with most cratered bodies, they aren't very obvious except near the terminator. And we very, very rarely see much in the way of a terminator on Mars. Most of the Mars views from Earth are nearly full-disk, the terminator fuzzed by being viewed through the maximum amount of Mars atmosphere it's possible to have between us and the Martian surface, and by foreshortening.

The human eye has far better naked-eye resolution of the Moon than we had of Mars through telescopes for hundreds of years, and even so, lunar craters weren't really identified as such until people started looking at the Moon through telescopes. And we get very good terminator views of the Moon from Earth. So, even if we were able to see Mars with a terminator crossing mid-disk through cratered terrain, it wouldn't be surprising if we missed craters...

-the other Doug

[Phil Stooke]

I think the whole issue of earth-based identification of craters on Mars is frequently misrepresented.

If I look up at the Moon without a telescope I can see Mare Crisium... it's a dark circular patch which lies in a crater, a big crater which we often call a basin, but a crater nonetheless. But I'm not seeing the crater, the depression, I'm only seeing the dark floor. I simply don't believe that Mellish or Barnard or anyone else ever saw a crater. They only saw - at most - circular albedo markings. The idea that they were craters was pure guesswork, based on the appearance of Plato, Crisium etc. on the Moon. The best proof of this is the case of Nix Olympica, a prominent circular bright spot, trumpeted as a crater when Mariners 6 and 7 appeared to resolve it as a crater with a central peak. But it wasn't.

Phil

[JRehling]

Well, it's a fact that E.E. Barnard -- one of the sharpest-eyed of all astronomers (which is how he discovered Amalthea) -- is also the only naked-eye astronomer to swear that he saw Mars covered with craters.

To detect a dim object and to see fine details are two distinct skills, actually using two different portions of (and cell types on) the retina. I wouldn't know if the two abilities are positively correlated among people with non-troubled vision... they may even be negatively correlated.

[gndonald]

Speaking of balloons, have you ever come across any serious commentary on the VEGA Soviet/French balloons? About all I've found are brief mentions...

Bob Shaw

The following are the best references I have found online:

The Venus-Halley Missions, Don P. Mitchell

The above covers the entire flights and the origins of the ballon plan. The following two pages from Astronautix.com provide additional information into the original VeGa plan, which would have seen four probes launched, two of which would have been dedicated ballon carriers.

Vega 5VS and Vega 5VK

Graham

[Bob Shaw]

If I look up at the Moon without a telescope I can see Mare Crisium...

Phil:

I got new glasses the other week, and can easily persuade myself that I see Aristarchus (well, the plateau) with the not-quite-naked eye. Contrast helps, true - but *knowing* it's there helps a lot more!

Bob Shaw

[ljk4-1]

Well, it's a fact that E.E. Barnard -- one of the sharpest-eyed of all astronomers (which is how he discovered Amalthea) -- is also the only naked-eye astronomer to swear that he saw Mars covered with craters.

There was an article in Sky & Telescope magazine (exact issue I do not recall,
but likely within the last 10 years) that claims the craters he saw were actually
the Tharsis volcanoes.

But why were scientists so surprised when Mariner 4 found so many craters
on Mars? Did they really expect the planet to have more erosion mechanisms?

[Bob Shaw]

There was an article in Sky & Telescope magazine (exact issue I do not recall,
but likely within the last 10 years) that claims the craters he saw were actually
the Tharsis volcanoes.

But why were scientists so surprised when Mariner 4 found so many craters
on Mars? Did they really expect the planet to have more erosion mechanisms?

George Pal put craters on Mars in the 1950s! So they must have been not far below the surface of the semi-technical consciousness...

Bob Shaw

The following are the best references I have found online:

The Venus-Halley Missions, Don P. Mitchell

The above covers the entire flights and the origins of the ballon plan. The following two pages from Astronautix.com provide additional information into the original VeGa plan, which would have seen four probes launched, two of which would have been dedicated ballon carriers.

Vega 5VS and Vega 5VK

Graham

Graham:

Thanks - none of these offer very much in terms of detailed construction, though...

Bob Shaw

[JRehling]

There was an article in Sky & Telescope magazine (exact issue I do not recall,
but likely within the last 10 years) that claims the craters he saw were actually
the Tharsis volcanoes.

We're way off the topic of Venus, but an issue from this past year offers evidence that the relief of Argyre had been observed. The article asserts that some previous analysis (perhaps the one you mention above) goofed by forgetting that telescopes invert images, and reported the southern hemisphere phenomenon as a northern hemisphere phenomenon.

Beyond doubt, the best ground-based telescopic observations in 2003 showed a shadow at Olympus Mons. But that was with adaptive optics.

But why were scientists so surprised when Mariner 4 found so many craters
on Mars? Did they really expect the planet to have more erosion mechanisms?

Sure. It was known that Mars was pretty cold, but warmer than Antarctica. Antarctica is pretty well eroded. Of course, the whole story behind craters, impactors, and the rate thereof over the age of the solar system had not yet been established either. And the seasonal shifts in color on Mars hinted (correctly, but vaguely) as to SOME kind of activity there. It turned out to be dust... but dust that happens not to provide much erosion. Who could have guessed that?

[edstrick]

There was a full set of preliminary science papers from the Venus balloon experiments and I'm pretty sure a followup set of primary science results, published first <I think> in Science, then maybe in JGR <Journal of Geophysical Research, not sure which series>.

Regarding the whole craters on Mars surprise. Remember that essentially up to the Mariner 4 Mission, the best estimates for the martian surface pressure were around 1/10 th atmosphere, not 1/100 th. Improved precision spectroscopy had just shown a pretty solid measure of C02 surface pressure of 1/100 or 1/200 atmosphere, but nitrogen or argon were undetectable. The new figures had gotten attention, but not universal acceptance. So the whole vague arm-waving ideas of Martian geology were implicetly assuming a more active geology and surface environment.

We STILL were caught flat-footed. Sometimes EVERYBODY misses the obvious, including the people who didn't miss it but didn't keep screaming... "Hey.. this is important".