Spirit is pausing on its long trek to Bonneville crater to look at some soil and an apparently "flaky" rock; Opportunity is staging a little industrial action, refusing to move while its arm is extended, and refusing to shift the position of its arm in the way its operators want. (It’s not a big problem, but it must be irritating, in that it seems to mean a lost day.) Meanwhile, the HRSC camera on Mars Express offers a terrific picture of the caldera of Olympus Mons, the peak of the tallest volcano in the solar system. The scale of this crater is fairly hard to come to grips with. This hole is half as deep again as the Grand Canyon, and you could fit London and its orbital motorway on to the floor of the crater with ease. The back of my envelope (a very fallible source) suggests that in geometric terms someone standing on the rim should be able to see right across the caldera to the other side, though in practice dust in the air might make that hard. (You might think there wouldn’t be much dust in the air at the top of a mountain 22 kilometres high, but martian dust has a way of getting around; Ken Edgett has seen dust devils high up Olympus Mons in pictures taken by MOC, the camera on Mars Global Surveyor).
To stand at the top of those encircling cliffs would be to see a sight that really has no equal on the earth. Or imagine standing on the floor, near the eastern wall, just before dawn. The night sky above you is studded with brilliant stars, shining with a clarity and constancy unknown on earth. Then the first sunlight strikes the cliffs to the north and south of you, creating islands of light in the sky; as the planet turns the two arcs of light spread round the rim like quicksilver until they meet in the west ahead of you, a circle of day around the darkness that still encompasses you.
As well as stirring such musings, the Mars Express picture of Olympus also serves as a reminder that Mars really is a volcanic planet. We get excited about the haematite in Merididani Planum, but that's because all the rest of the planet’s surface seems to be made pretty much entirely of lava (or sediments made of particles derived from lava). The features that jump out at you when you look at the place from space are first the craters and then the volcanoes – they are the marks left behind by the forces from without and within that have done the most to shape the planet’s surface. The rate of volcanism on Mars is low, but it has been going on intermittently for billions of years, and that means even slow-growing volcanoes can end up as vast as countries. There's a great overview of Martian volcanism here
Volcanoes are also the most earthlike of the big Martian features. Earth’s large craters are faded and eroded so much that they no longer look terribly like Mars’s craters; but martian volcanoes really do look like similar volcanoes on earth – the large, softly sloping “shield” volcanoes produced by runny basaltic lava. They're just bigger -- the result of having a thicker, stiffer crust to bear their rate, and of billions of years to grow in. The similarity between Martian volcanoes and the basaltic volcanoes of Hawaii was obvious pretty much as soon as Olympus Mons and the great rounded peaks of Tharsis were seen poking up through a global dust storm by Mariner 9. This was why it made sense for geologists working on Mars to gather in Hawaii for a mapping conference in the mid 1970s -- though it’s said the location was in part also an inducement to people to buckle down and produce the maps they’d said they were going to produce, since only people with work to show would be welcome. In 1980 Mike Carrr and Ron Greeley produce a NASA report entitled “Volcanic Features of Hawaii: A Basis for Comparison with Mars”, and leafing through it with eyes that know more of Mars than of Mauna Kea, a great deal of it looks astonishingly familiar.
Jeffrey Bell, gadfly and curmudgeon (see Dead Dog, last para) is currently combining his experience of living on the earth’s most impressive chain of basaltic volcanoes with a knowledge of lunar geology and a desire to offer an alternative to over-excited marsophiles like me in a series of articles at Space Daily. The gist is that everything on Mars is volcanic until proved otherwise, and that this makes the whole place like a rather dull cross between Bell’s native Hawaii and the moon. First article here, second here. They’re great fun and probably a worthwhile corrective; but in the end they’re also a restatement of the obvious. We know Mars is mainly basalt (and other, related lavas), and that any bits that aren’t are currently covered by basalt in some form or other. It’s the possibility of peeking under the covers – or of seeing how the basalt has interacted with water – that make the current missions such fun. Volcanoes dominate Mars in many ways – but there may still be room for a lot of fun in the watery margins.
PS: a big hello to all the nice people popping over from Neil Gaiman's journal. I hope you find something of interest.