The graph is a bit confusing, because we're not really meant to compare the black and red lines. We're meant to compare the black line to one that's not even on the graph, representing the kinds of rocks that Spirit and Opportunity have seen, and which would have a squiggle that looks slightly different from both the black or the red one. According to Gellert, if you compare the APXS measurements on Jake to those made by Spirit and Opportunity on basalts at Gusev crater and Meridiani planum, you find that Jake "is low in magnesium and iron, [and] high in elements like sodium, aluminum, silicon and potassium, which often are in [alkali] feldspar minerals. It has very low nickel and zinc. The salt-forming elements sulfur, chlorine and bromine are likely in soil or dust grains visible on the surface of the rock."

To a geologist, that string of four elements -- sodium, aluminum, silicon, potassium -- immediately makes one think "feldspar," and a particular flavor of feldspar (rich in alkali elements sodium and potassium) that I don't know if we've ever seen on Mars before. It's the commonest rock-forming mineral on Earth, the pink or opaque white or sometimes gray material in granite countertops. That, combined with the information that the rock is relatively low in magnesium, iron, nickel, and zinc, told every geologist listening to the call that Jake is a rock that formed from a rock melt that evolved, changed, from a straightforward melted Mars rock. (Or, that it was a melted rock made of material that had gone through some such process.) It's a rock that formed through processes that we know in Earth geologic environments, but not one we've ever seen on Mars.

So let's talk about how rock melt compositions can evolve. But we'll begin with something more familiar (particularly to geologists!): liquor. There was a petrologist member of the science team on the panel today, Ed Stolper, who used this simile to try to explain the concept to the listening journalists. Specifically, Stolper talked about making applejack. Applejack is a liquor made by taking a hard cider (usually around 5% alcohol by volume) and storing it at a temperature below freezing. As the cider cools, water ice begins to crystallize. This ice is nearly pure H 2 O, water; alcohol has a lower freezing temperature, and the other stuff in the liquid that give the applejack its flavor stays in solution. Because water has gone in to the solid ice, the remaining liquid is relatively depleted in water, and relatively enriched in alcohol and other stuff. Take the ice out and throw it away, and you've begun concentrating the liquor, increasing its proportion in the liquid. The lower the temperature you take it to, the more water ice freezes out, and the more concentrated the stuff that's not water gets in the remaining solution. I checked last night, and the applejack in my liquor cabinet is 40% alcohol by volume.

This process is called fractional crystallization, and it also operates inside magma chambers. Start with a melted rock and then allow it to cool, and the first mineral that crystallizes out has a different composition than the bulk composition of the melt. What mineral crystallizes out first depends on the composition of the melt in super complicated ways that it takes textbooks and ongoing scientific careers to describe. But when you've got a melt with the composition of the mantle of Mars or Earth, what crystallizes out first are iron- and magnesium-rich minerals that have relatively low amounts of silica: olivine (Mg 2 SiO 4 or Fe 2 SiO 4 ) and then pyroxene (like CaMgSi 2 O 6 ). If those initial crystals go away -- for instance, if they settle out -- what's left behind is a melt containing relatively less iron and magnesium and relatively more silicon, potassium, sodium....all the stuff they found in this rock.

When you take a geology class that's about Earth, you learn about how fractional crystallization can start with a basaltic melt (one that's rich in those dark minerals and iron and magnesium) and work (or evolve) your way up to increasingly silica-rich rocks, from basalt, to andesite, to rhyolite or granite. Jake is not a granite or even an andesite. We're still talking about a very dark, basaltic rock here. But it's a little up the spectrum from any other basalt we've seen before, to the point that it's called an alkali basalt.

APXS wasn't the first instrument to detect this feldspar-like composition in a rock. Roger Wiens, the principal investigator on ChemCam, divulged today that they had been seeing feldspar-like compositions on rocks they've been shooting their laser at since August. That was an "aha!" moment for me; it explained why we have been hearing so little from the ChemCam team, until now. The observation of so much feldspar is so unexpected that they had to be really thorough, checking that their instrument was functioning properly, corroborating its readings with measurements by APXS, and so on, before they were ready to come forward with it.

Wiens had an interesting extra wrinkle to the story. Go back up to the picture of Jake with all the shot points on it and you'll see they shot fourteen different locations. Each of those shot points actually represents 30 laser shots, each of which was analyzed by the ChemCam spectrometer.

He shared this fun spinny cubic graph based on a principal components analysis of the data. Each of the fourteen shot points was assigned a different color. For each shot point, there are 26 dots for the laser shots (they throw out the first four because those are mostly sampling dust coating the rock). You're not meant to be able to read anything specific about the composition of the rock from this graph. The point is that every one of the fourteen shot points had a composition that was distinct from the other thirteen. Each color forms a tight cluster, and there's very little overlap. That's Jake's second surprise: it was a surprisingly heterogeneous rock on the scale of the ChemCam analyses. ChemCam is probably sampling individual crystals of different minerals.