All that new Hirise imagery started me thinking about this topic again and I found this link. I was surprised to see the image with what may be three more arches, apparently crying out for some follow-up observation. If this was discussed before then I missed it. Anybody have any news on this?

http://www.jaysplanet.info/nabs/marsform.htm

There's a hint of an arch, but nothing to say there's actually a hole through it. Any passage could be filled with debris. You would need to see a shadow cast by the arch, clearly showing it was pierced, for proof.

Phil

Absolutely, none of them are certainties yet. I suppose an alternative to low angle shadows would be an oblique look showing scattered light getting through. Maybe it's just not important enough geologically but I'd love to see the attempt made to get that clinching evidence. Meanwhile I was wondering if HIRISE has thrown up any more candidates? They might not be headlined by the image team as the first one was, so they could be lurking unremarked in those acres of data.

The article mentioned observing the supposed arch at 1500hrs local time to see a shadow. Does the orbit of MRO allow that or does it have to be adjusted like Modyssey did a while back?

If there are caves on Mars - which we now know there are, on the flanks of the giant Tharsis volcanoes - then IMO there are almost certainly also arches. I really hope HiRISE clinches an unmistakeable example of a Martian arch.