It was amazing how fast it happened. On 4 July 1997 a spacecraft alit on a desolate Martian waste. Within a week, Mars became the world's favorite reality sitcom, with Pathfinder and Sojourner on their familiar set at 19.33º N, 33.55º W.
The lander bobbed its camera up and down and the plucky little rover bumbled around like a pet turtle. The spirit of a great TV scientist was invoked as the lander was dubbed the Carl Sagan Memorial Station.
Introducing Barnacle Bill
The rocks had nicknames, given to them by caffeine-crazed staffers. (This also happened with the Viking landers in 1971—stones got names out of The Wind in the Willows like Mr. Toad, Mr. Badger, and so on.) One week's costar was a 40-centimeter boulder dubbed Barnacle Bill—not because it brought to mind the horny sailor of the old sea chantey, but because it was kind of warty, as if it had barnacles on it.
In an early episode, Sojourner the rover got up close and personal with Barnacle Bill. The wheely wonder pressed its X-ray nose right into the crusty old rock and hilarity ensued! The results had scientists baffled and happy, naturally, and the newswriters muttered something about andesite on Mars.
That made me sit up.
Recipes for Planets
It's not incredible to find earthly rock types on another planet. The laws of chemistry and physics really DO work everywhere in the universe. If we start with a known recipe—a certain assemblage of elements at a given distance from the Sun and a given planetary history—we can calculate pretty closely what kind of world will come out of the oven.
The minerals that form, and the rock types they live in, all fall into place according to well-understood science. Thus we can talk confidently about the iron core of Mercury and the silicate mantle of Venus with the same confidence we have about the deep Earth. And when astronauts brought back stones from the Moon, they matched rock types we had names for—troctolite, norite, anorthosite. Rock types are, in fact, universal. But what these rock types mean in the Martian context is a whole other story, which we must discover from scratch.
So what is Barnacle Bill really made of? And if it's really andesite, then what does that mean?
APXS the Wonder Nose
Let's go back to what Sojourner was doing. It placed its X-ray nose—its spectrometer—against a spot on the rock and bombarded it with alpha particles. The spectrometer measured the alpha particles bouncing back, the protons being emitted, and the X-rays that the bombardment excited. Each type of emission told the Alpha Proton X-ray Spectrometer, or APXS, something about a different set of elements in the target—in fact this elegant instrument can characterize almost every element in one shot.
But the results are a mere list of elements and their abundances. The minerals they reside in, the texture of the grains, and the impurities and peculiarities of their mixture are all, as they say, poorly constrained. The APXS couldn't directly tell us Barnacle Bill's rock type, any more than a list of kitchen ingredients could tell us what a baker will make with them.
The Andesite Hypothesis
Scientists will always make guesses, though, and a guess that this spot on this rock falls into the "andesite" zone is not off-the-wall. Basically, the results seem to show a fairly high percentage of silicon in Barnacle Bill, so high that there may well be significant amounts of free silica—quartz—present. That's "high" compared to Earth's most typical volcanic rock, which is mid-ocean ridge basalt. It's more like Earth's most typical continental volcanic rock. Since the continents are differentiated, that is, multiply-recycled material highly enriched in silica, sure one can speculate that some refining process is at work at Mars too.
But don't take that to the bank. A few instrument readings can't support all that inference.
After all, andesite is an igneous rock—a type of lava—but Mars has sediments too. Two Rutgers geoscientists showed in 1999 that most kinds of sedimentary rocks would also look like andesite if all we had to go on were Sojourner's nose. We're prejudiced on Earth, because the only samples we have of Martian rocks happen to be igneous. They asked the 1999 Lunar and Planetary Science Meeting, "if a meteorite of Martian sandstone hit you on the head would you recognize it?"
Sure enough, the Spirit and Opportunity landers of 2004 showed dramatically that Martian bedrock can indeed be sedimentary. But Barnacle Bill really does look volcanic, and in fact more research has shown that weathered basalt looks like andesite to the satellite eye.
The point is, that one data point was just a data point. The detailed mapping of the Mars Global Surveyor satellite has since shown that felsic (that is, non-basaltic) rocks are extremely rare. Barnacle Bill is probably not one of them.
