By Paul Rincon
Science reporter, BBC News
The chief scientist on Nasa's Mars rover mission is contemplating whether to send Opportunity into a large crater with no means of getting back out.
Rover scientists have not yet found a way out for Opportunity
The decision could commit the rover to spending its final days exploring Victoria Crater, a 60m-deep (200ft) depression on Mars' Meridiani plains.
Steve Squyres said Opportunity would probably be sent in to explore the bowl even if no escape route was found.
But he stressed his team would do its best to find an exit path first.
Opportunity has explored other craters on Meridiani Planum; indeed, it even came to rest in one after descending to the Martian surface in January 2004; but Victoria Crater is a much more challenging proposition.
"You've got to realise this is a big, big crater for a little rover like this. The biggest thing we ever explored with Opportunity was Endurance Crater, which was 150m in diameter.
"This is six times that, so it's huge," Professor Squyres told the BBC News website.
Speaking to me at the Open University in Milton Keynes, where he was due to give a lecture, he explained: "We have found a way in, we haven't found a way out yet. It turns out this rover is better at going downhill than it is at going uphill.
"I don't want to go into this crater until we've either found a way out or sort of convinced ourselves that there probably isn't one. In other words, I want to go in with knowledge of the consequences.
"But even if we find there's no way out, we'll probably go in anyway, because there's just so much to be gained."
The crater has high walls with layers of exposed rock that should reveal significant new information about the planet's geological past.
"The thing Victoria offers is first of all, it is several times deeper than anything we've seen before so we're getting a longer geologic record," the researcher based at Cornell University in Ithaca, US, explained.
"The other thing is that because Victoria is such a large crater with so many points along the crater rim at which we can do geology, we can look not only at vertical variations in the geology, but we can look at horizontal variations, which we couldn't do before."
This should allow scientists to track how features of a particular rock unit change with distance. For instance, it can give information about the extent and shapes of ancient Martian sand dunes.
The rover programme's principal investigator said Opportunity had just begun a partial, clockwise circumnavigation of the 800m-wide (half a mile) bowl to scope out its geology.
Professor Squyres also talked about future exploration of the Red Planet. In addition to surface exploration, he said a mission to return samples of Martian rocks to Earth should be high on the list of priorities for scientists.
"I think it's going to be particularly important to bring rocks back. I'm a big fan of in-situ robotic exploration; that's what I do. But the best scientific instrumentation is always going to exist in laboratories on Earth.
Geologists are keen to get close to the crater's walls
"Return samples are kind of like the gift that keeps on giving. If you don't use them all up when you bring them back, then you preserve samples so they can still be around a generation later when you have a new generation of scientists and a new generation of scientific instruments.
"The best science ever done with lunar samples collected back in the Apollo era is being done today with brand new instrumentation."
This, he said, could be vital for answering the questions of whether life once arose on the Red Planet and also exactly how old Martian rocks are.
Opportunity has been exploring Mars' Meridiani Plains since January 2004. Its "twin", the Spirit rover, continues to explore Gusev Crater on the other side of the Red Planet.
Steve Squyres says sample return should be a science priority
Both robots have continued working far beyond their designed mission lifetimes. Professor Squyres said he had no idea when the rovers were likely to cease functioning: "It could be two years from now, it could be tomorrow," he said.
For the moment they are working well, despite showing some signs of wear and tear.
Opportunity has now driven more than 9km (5.6 miles) across the planet's dusty surface, examining rocks and studying the Martian environment. It has found strong evidence that shallow waters periodically flowed over its region of Mars many millions of years ago.
On Tuesday, Professor Squyres spoke at the Centre for Earth, Planetary, Space & Astronomical Research (Cepsar) Lecture at the Open University in Milton Keynes. He also gave a lecture on Wednesday at the Geological Society's William Smith Meeting 2006 in London.