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Bridges on the Moon

Post  Copernicus on Sun Jul 31, 2011 10:41 pm

Hey, I was just getting warmed up with the Caves of Copernicus post! ( he-he )

The following post will get us started on lunar bridges... it's quoted directly from this site:

Natural Bridge on the Moon 09.08.10

Just when you think you have seen everything, LROC reveals a natural bridge on the Moon! Who would have thought? Natural bridges on the Earth are typically the result of wind and water erosion -- not a likely scenario on the Moon. So how did this natural bridge form? The most likely answer is dual collapse into a lava tube. From the Apollo era, SELENE, and LROC images, we know that lava tubes did form in the Moon's ancient past. SELENE and LROC images have raised the tantalizing prospects that lava tubes remain intact to this day. However this bridge did not form in mare (basalt), but rather in impact melt from King crater! More astonishingly, the same NAC image revealed two natural bridges - not just one!

Unlabeled image

The bridge is about 7 meters (23 feet) wide on top and perhaps 9 meters (30 feet) on the bottom side, and would be a 20-meter (66-foot) walk to cross from one side to the other. Credit: NASA/Goddard/ASU

How do we know for sure that this feature is truly a bridge? Look closely at the west pit (left side) and you can see a little crescent of light on its floor. That patch of light came from the east, under the bridge. In another lower resolution image (see inset), you can see light passed under the bridge from the west. So there must be a passage. How did this oddity form? The impact melt deposit on the north rim of King crater is over 15 km across and was emplaced in a matter of minutes as the crater grew to its final configuration.

A large (17 km east to west), smooth impact melt "pond" on the northwest rim of King crater (72 km diameter). Credit: NASA/Goddard/ASU

Larger image

The impact melt that was thrown out of the crater pooled on the newly deposited ejecta and must be many tens of meters thick, allowing its interior to stay molten for a long time. As the local terrain readjusted after the shock of the impact, the substrate of this massive pool of melt was jostled to some degree. Local pressures built up and the melt moved around under a deforming crust. You can see that the south end of the bridge extends from a small local rise, shaped something like a blister. Perhaps some melt was locally pushed up forming the rise, then the magma found a path to flow away, leaving a void which the crusted roof partially collapsed? Right now we do not know for certain the details of how the bridge formed, however, the LROC team is processing stereo images into topographic maps to aid scientists in determining exactly what took place on this fascinating melt sheet. There are actually six NAC images in which you can find the bridge under varying lighting (M103725084L, M103732241L, M106088433L, M113168034R, M123785162L, M123791947L). Why so many images? The melt sheet north of King crater is a region of interest -- a high priority for LROC coverage. As the pair of images below vividly illustrate, having a set of images of the same under varying lighting allows scientists to more confidently interpret the local geology and thus better prepare for future exploration.

Left shows the bridge when the Sun is 42 degrees above the horizon and the right is the same area when the Sun is 80 degrees above the horizon (near noon). M113168034R on the left, M123791947L on the right, both are 128 meters across, north is up. Credit: NASA/Goddard/ASU

Explore the entire LROC NAC image and investigate the variety of geologic features in the King crater melt sheet. Can you find the second natural bridge in the full NAC frame (hint - it's fairly close to the one shown above and about half its size)?

I'll post some more info in this thread in about a week or so Smile


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The Big Bridge

Post  Copernicus on Sat Aug 13, 2011 8:30 pm

Okay, I’m back with the lunar bridge information about a week later than planned. The delay was due to the fact that there are not 1 but 2 bridges, and the second bridge was never identified. I’ve also come up with a third bridge candidate, but I can’t positively say it’s a bridge, so I won’t post it at this time.

The first bridge in the article above is fairly easy to find if you know where to look… of course, they never specified where it was.

Here’s a locator image:

Which can be cross-referenced with the next image to find the bridge.

I’m calling this the “Big Bridge” since the second bridge that they challenge us to find in the above article is only half the size… that one will be the “Small Bridge”.

The Big Bridge is in 13 NAC’s with 10 of them being high res!! I will present only a few of the more interesting NAC’s here…

The first one is low res, and is a similar view to the one in the inset in the article:

The rest of the images are high res, and in the next one you can see the sun shining under the bridge from the right…

You can see this also in the next image…

And finally, a look under the bridge span…

Next up, the second bridge (Small Bridge)!


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The Small Bridge

Post  Copernicus on Sun Aug 14, 2011 1:05 pm

Finding the second bridge was no easy task! There are many “dual pits” in the area that are similar to the Big Bridge, so the challenge is in finding a pair that have a hole in the pit wall between the pits making a bridge formation.

Also, the following conditions need to be met for this to be the second bridge they refer to:

1) It’s in NAC M113168034R.
2) It’s fairly close to the Big Bridge.
3) It’s half the size of the Big Bridge.
4) It must be identifiable as a bridge from the NAC’s that were available at the time.

Although these conditions seem straight-forward, they are actually rather vague. First, they never specifically state which NAC both bridges are in. I can only assume that it was the main hi res NAC, M113168034R, featured in their article.

Second, “fairly close” is a pretty vague distance, and third, “half the size” could refer to the size of the pits, the size of the bridge, or both. Finally, fourth, the NAC’s listed in the article as our pool to choose from are for the Big Bridge, and not the Small Bridge, and therefore any NAC’s up to the mid 120’s could be used for this.

So, taking up the challenge, I’ve found a feature that met the conditions, but isn’t anywhere near as obviously a bridge as the Big Bridge. I’m going to stick my neck out here, and present this feature as the second bridge… the Small Bridge.

Here’s a locator image from the same low res NAC that they feature in their inset:

Using only images up to the low 120’s, I show why I think this is a bridge. First, here are 2 images that show these are 2 pits, much better than the low res locator image.

Now we have a look at an image with the sun shining through, from left to right, under the bridge span, illuminating an area in the right pit that should be in shadow.

I’ve tried to illustrate this a bit better here:

You’ll note that the bridge span is quite wide, and it even has a small crater-like depression in the center of it. Take note of the wide shadowing in the pit just to south. This is also how the right hand pit of the pair should look, but the sun shining under the bridge span has lit up the floor so there’s no shadow.

Now, although the bridge span is twice as wide as the Big Bridge, the pits are only about half as big, so this is where the ambiguity in the condition “half the size” comes in.

One other image, not available at the time of their report, shows the sun shining through under the span.

For comparison, here’s an image where the sun is higher and isn’t shining through under the span:

And I’ll leave you with this one:

The Small Bridge is in 12 NAC’s, 3 of which are low res. The Big Bridge is also in all of these NAC’s except for one.

Anyway, one thing that I discovered about this area is that there are a lot of pits here… more than in the Copernicus area that we covered. Since they couldn’t have failed to notice them, but never mentioned this, perhaps these pits aren’t important because this is a “melt” area?

Well, that’s about it for the bridges of King Y… hopefully the new NAC’s in September will show whether my bridge candidate is really a bridge or not! Smile


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Re: Bridges on the Moon

Post  Jackie (Admin) on Sun Aug 14, 2011 4:56 pm

Incredible detective work! I really appreciate how you explain how you look for this feature, but you show us what you did to nudge it to optimal viewing resolution and tone as well. Bravo!
Jackie (Admin)

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Re: Bridges on the Moon

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