[Bernard Marx]

Assuming it is a hole, and not just a shadow, I thought I would toss it out there in hopes a geologist freeper could lend some insight to what kind of natural processes could form such a evenly shaped hole.

I'm not a geologist but I've spent 45 years studying rocks and minerals and know a little about them. Here's one way a square hole could be formed in a rock (and it's basically the same one AppauledAtAppeasementConservat suggested).

A number of minerals crystallize in the cubic (isometric) system and some of them form nearly perfect cubes. One of these is iron pyrite (iron sulphide) -- a likely candidate for this situation because it can be easily dislodged from its host rock matrix or chemically degraded. I've found square holes in rocks where pyrite crystals were formerly located. But there are other cubic minerals that could do the same thing.

If it's a hole left by a crystal the host rock is igneous or metamorphic material brought to Mars' surface by the meteorite impact that made the crater. It's certainly an interesting object and I hope the rover takes a hard look at it and science comes up with an answer. The fuzzy-looking stuff on the top of the larger rock to the left needs some examination too. I've seen rock formtions like that and they generally indicate some sort of hydrothermal action like geysers -- if I'm right it could be a real indication of Martian water, past or present.

[AppauledAtAppeasementConservat]

"A number of minerals crystallize in the cubic (isometric) system and some of them form nearly perfect cubes. One of these is iron pyrite (iron sulphide) -- a likely candidate for this situation because it can be easily dislodged from its host rock matrix or chemically degraded. I've found square holes in rocks where pyrite crystals were formerly located. But there are other cubic minerals that could do the same thing." Interesting that you brought this up. I have been looking into some interesting information that is a theoretical possibility for this hole. First off. Olivine is well known to be present on mars. Now olivine disolves easily in water and if indeed this was a lake bed the it could also explain how a an isometric olivine crystal (obviously megacrystic) that could have formed in the mantel and exposed to water could have disolved away completely or enough to have fallen out of the matrix rock. Here's the articles on the presense of olivine: The first link is good reading on locations of olivine from TES images and the second is the explanation of the formation of isometric olivine in the martian mantle.
http://www.psrd.hawaii.edu/Nov03/olivine.html


http://www.nd.edu/~cneal/eg.101/geology2/geology2.html
Specifically go down to the heading called Expansion of the Martian Mantle to see how isometric olivine occurs on mars. It's possible we could be on to something here... Appearance of this rock look similar to basalt? Certainly darker...

[Orion78]

Thank you very much for your input.

After doing a quick google search, I have discovered that evaporation of salty water produces deposits of the mineral Halite. Halite is salt in it's natural form. Halite has an Isometric structure, (crystals usually look like cubes), and is one of the minerals that NASA scientists are looking for in Gusev Crater. Halite would also erode away very easily and leave a hole in its host rock.

Perhaps "Rectangle Hole Rock" is evidence that Gusev Crater is indeed an ancient lake bed.

[Orion78]

Just a quick note before I hit the hay:

Adirondack, (the name NASA has given to one of the first two target rocks), is also the name of a mountain range in New York that is composed of mostly metamorphic rocks.

http://www.earth.rochester.edu/ees201/GoreckiD/goreckid.html