Goodies from Mercury tonight

[mmcirvin]

The mostly unrevealed (until now) face of Mercury.

It looks as Moonlike as ever, though those dark-rimmed craters in the Caloris Basin at upper right are peculiar. You can't tell that Caloris is a huge impact feature nearly as obviously as in the old Mariner 10 photos, where it was right on the edge of the sunlit hemisphere; I guess that's the effect of the high sun phase--the sun's shining down at a higher angle and the relief isn't as clear.

This is still just a single-filter monochrome image, so it doesn't answer a question I have concerning the color of Mercury. I always saw it depicted as Moonlike gray until recently, when there seems to have been a vogue for depicting it as more reddish-brown, but I'm not sure whether this is anything justified or is just people lifting images from each other.

Anyway, off to bed.

Comments

[ikkyu2.livejournal.com]

I always assumed that the reddish-brown depictions of Mercury were due to the explicit association with cinnabar - hydrargum - Mercury; but actually that's a pretty wacky assumption. In that Asimov robot story set on Mercury (in The Rest of the Robots, I think), there are lakes of molten metal; I wonder what color it really is?

[mmcirvin.livejournal.com]

The first place I saw the brown Mercury was, I think, in a texture map adapted from Mariner mosaics that was used in NASA's Solar System Simulator and also adapted for Celestia. All the Mariner 10 Mercury image releases I've actually seen look monochrome, but I don't know if they are.

Mariner 10 took famous color pictures of Venus subtly banded in tan, but those are false-color images with, I think, a UV component included; Venus's banding is only barely visible in visible light.

[mmcirvin.livejournal.com]

Here's a Mariner 10 color composite (http://en.wikipedia.org/wiki/Image:Mercury_Mariner10.jpg) that seems to be somebody's attempt at getting a color image from two channels, a clear-filter image and a blue-filter image. It's brownish.

Anyway, we'll have better color images soon enough.

[mmcirvin.livejournal.com]

In general, I follow the rule that (as stated by a planetary scientist whose identity I have forgotten) you should mistrust highly saturated color images of solar-system bodies.

Viking and Voyager image releases often had artificially jacked-up colors. Lots of ink has been wasted over this in the Viking case because of dumb conspiracy theories, but it's true for Voyager too. Voyager's filter wheel also lacked humanlike RGB channels--it was more like orange-green-violet, I think--so the reconstructions didn't always have the right information to go on, either. And since the pretty "natural-color" releases from space missions are more for PR than for science, there may not have been much attention paid to accuracy.

I've been noticing this in particular for Saturn's moons. In general, except for Titan, they don't really have much color to them at all. Even the dark area on Iapetus is really more black than brown. Cassini icy-moon pictures showing significant color variations often are either compressed-spectrum pictures with IR and/or UV channels in them, or have the saturation artificially increased.

But if you go back to old Voyager images, the colors in them are all over the place. Most of the public releases have odd overall color casts. The wispy hemispheres on Dione and Rhea are yellow or brown in some images and pale blue in others. Enceladus is sometimes greenish. Even Titan looks much redder and more saturated, like a painted supermarket orange.

[mmcirvin.livejournal.com]

...though of course there are exceptions to this rule: I think the color of Neptune really is a pretty intense blue.

[antikythera.livejournal.com]

I'm familiar with this idea as it pertains to deep-sky objects. You can take images of a nebula with four different filters and then apply any colour to any one of them. Sometimes they go for the obvious -- depicting 656.21 nm hydrogen-alpha as red, for example -- but sometimes they mix them up completely in order to make features stand out. This can be scientifically useful but also confusing to the public (especially when they buy telescopes and are disappointed by the view).

[mmcirvin.livejournal.com]

So, how is Mercury not like the Moon? I can see a few outstanding differences:

-Dark-rimmed craters with brighter centers, as mentioned earlier
-More multi-ringed craters, that look like bullseyes
-Near the terminator, you can see some of the famous scarps or rupes, which are those curved cliffs where it looks like the ground was lifted up on one side or lowered on the other. They cut right across most of the craters, so they're probably younger.

I was confused

[notr.livejournal.com]

by the statement that "the same hemisphere was in sunlight during each encounter." I must have had ten-years-obsolete elementary-school science textbooks, because I was thinking, well, yeah, when is it not? But now that I think of it, that 3:2 resonance does sound familiar from one college physics class or another.

Re: I was confused

[mmcirvin.livejournal.com]

Yeah, the truth about Mercury's rotation was discovered in the sixties and famously torpedoed the surprise ending of Larry Niven's story "The Coldest Place" between writing and publication. There was an Asimov murder mystery called something like "The Dying Night" that it also ruined, but I don't remember if the timing was as bad.

There are a few big close-up images on the MESSENGER site now, including one of a big rupes, a close-up of crater Vivaldi, and one of some strange-looking crater chains.