Saturday, October 17, 2009

A Plume After All

L-CROSS Centaur Impact Plume
As it turns out, the L-CROSS mission did create a visible plume. Neener neener neener, says the Moon and the L-CROSS science team. I saw the news in the Los Angeles Times, dead tree edition, and it is online at Space.com. Apparently the plume was imaged by a camera aboard L-CROSS itself, just not the imagery everyone was watching in real time. Judging from the stark contrast, they had to tweak up the contrast value to see it.

According to Anthony Colaprete, head of the team, the plume brightness was 'at the low end of our predictions.' (Do'h - that's why you didn't give us our show!)

Because I think of ice crystals as being bright, this does not seem positive for water, but that is probably an extremely naive interpretation. Much more to the point, the L-CROSS team evidently got plenty of good data, and over the next few weeks we may start to hear what they are learning from it.

8 comments:

Anonymous said...

If the image had to be enhanced in order to show the impact effect, then one would have to suspect that if there was water in that crater, it'll be in such small amounts that lunar-based water resources would not be enough to support even a research outpost upon our nearest neightbor, let alone a full blown colony as several classical science fiction novels suggests.

Not exactly a good sign for any potential leap frogging from planet to planet in the near future. At least without bringing some water with us to compensate.

- Sabersonic
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Anonymous said...

You're from one of the warm parts of the world, aren't you Rick? ;) Us northern types know that ice mixed with dirt or gravel isn't bright at all. You don't see it even when you're skidding out of control on it. And a plume of frozen mud (Say, hypothetically speaking, when you're out ice fishing and get creative with the dynamite, not that I even know anyone who would do such a thing) looks like a plume of dirt.

I would have thought that the L-CROSS impact would be hard enough to vapourize ice, but maybe not. I doubt that Lunar dust has enough ice in it to qualify as said frozen mud, but there might be enough to be useful even if it's not visible.

Ian_M

Rick said...

I'm also, alas, from one of the foggy parts of the world. There was a satellite launch from Vandenberg this morning, but the fog rolled in just an hour before launch and I didn't see a thing. :-(

I did think of 'black ice.' Ice in space is usually bright, e.g. when impacts on giant-planet moons have blown off overlying dust. But if the impact energy went more into vaporizing ice than kicking it up, that might have reduced any sort of plume, and the vapor would be too thin to recondense. Turning ice into vapor soaks up a lot of energy.

Jean Remy said...

Well, I think the idea of bright ice mainly comes from images of Callisto and Ganymede (both suspected to have liquid water oceans beneath layers of ice. Both show some very bright markings around tectonic faults or impact craters. It is possible that those bright areas are zones where the underlying oceans have pooled on the surface before freezing. However in general the surface of both moons is more drab that shiny, as the ice is mixed in with a lot of dirt. So the only reason you see bright spots is because reasonable pure liquid water (dirt would have drifted down being denser than the water) has recently been ejected before freezing, recently enough that it hasn't been covered in dust.

On the moon, there is no such ocean of liquid water. If water there is, it is at best frozen solid and mixed in with soil. Therefore punching a hole in that will eject not clean pure water that would freeze into a bright surface but a rather an ugly, muddy mess of mostly dirt with some vapor. It's not so much black ice as a frozen artesian aquifer: basically a porous solid saturated with water. Basically, frozen mud. At best. Unlike Saber, I am not given to pessimism on the matter. We just have to forget about finding a block of ice, and I don't think anyone expected that.

Jean Remy said...

One quick addition.

Black Ice is somewhat of a misnomer. You tend to think of ice as white because when it forms on the surface layer of snow, the white in the ice is actually the boundary between the ice and the air, namely the millions of very small air bubbles that form when surface ice is formed. Black Ice form below the snow: the surface snow melts, seeps between the snow crystals, pools at the bottom and then freeze. Because it's on the bottom, there is far less air being trapped, and the ice is actually solid and clear. It is called "Black" because it's a known danger to drivers in areas that get a lot of snow. Once the wind has blown away the snow, the pure, transparent ice that is left behind looks black because of the asphalt underneath and is therefore difficult to see on the road, leading to very nasty surprises. On concrete sidewalks it becomes a darker shade of the underlying gray, thereby cementing (pun not intended) its reputation as "Black Ice".

Citizen Joe said...

I wanted to point out that the reason they went to the poles and into a crater for this experiment is because in a vacuum, at the moon's distance from the sun, water snaps instantly from solid to gaseous state. A plume of 'water' would only rise to the lip of the crater before being converted to steam. So that won't be terribly visible in visible light spectrum. Essentially, the very light we're using to see it is turning it invisible. But they presumably have all sorts of spectrometers on the mission, so water signatures should show up on other scans.

Anyway, what is visible probably isn't water but instead the various rocks and moon dust that have collected over the millennium.

Rick said...

This makes sense, because of course the ice cubes in my glass are clear.

It also strikes me that if the impact kicked up fairly big chunks (of whatever) it would be visually faint because of much less surface area relative to mass.

Jean Remy said...

Check your ice cubes again. The edges are generally whitish. Water is not actually uniform. At he boundary between water and air odd stuff happens: surface tension. There will be microscopic air bubbles trapped in that surface. One example of that weirdness is the way active sonar works on submarines. The sonar doesn't bounce off the hull, but off the transition between water and air. What the sonar detects is all those air bubbles formed by the passage of the ship and its propellers through the water: that's cavitation. Stealth in a submarine is to generate as little of those bubbles as possible.

You'll see the ice cube clearing as it melts.

Ice, for all that it is common, does really, really weird things.