NASA has been holding back results of rover Curiosity's examinations of Mars rock 'Jake Matijevic' for several weeks. Now we know why. The ChemCam and APXS analyzers came up with unexpected results that had to be verified. The NASA JPL team intended to study a typical basalt (a common lava rock on earth) because it would have a fine-grained uniform composition good for cross calibrating their instruments. They found out the rock 'Jake Matijevic' has a geologically "evolved" mineralogy never before seen on rocks from Mars. Rocks like this are found on earth on oceanic islands like Hawaii, but haven't been found before on Mars. Tectonic processes on Mars may have been more like those on earth than we previously recognized and, perhaps, conditions on Mars were a little more amenable to the existence of primitive forms of life. Amazingly, there are groups of basaltic meteorites found on earth known to have originated from Mars, but none of them have this chemistry and mineralogy.
(Note: I was involved in research studying meteorites in my first year of grad school at UCLA.)
Curiosity Rover's Tests of Mars rock 'Jake Matijevic' Surprised Scientists
This image shows where NASA's Curiosity rover aimed two different instruments to study a basaltic rock known as "Jake Matijevic."
NASA's press release and media reports on these results were a series of teasers spoken by scientists in a swamp of words written by journalists. Fortunately, Emily Lakdawalla's blog for the Planetary Society presented the information essential to understanding what Curiosity discovered. Please read her informative blog post for discussion of basic geological definitions and concepts, that I won't repeat here, which NASA scientists used in interpreting the results of the chemical analyzers.
The Curiosity rover landed in an area with a landscape similar to areas of Death Valley on earth.
A mosaic of three Mastcam-100 images taken on sol 50 facing northeast. There is no sky visible in this view; occupying the distance is Gale's crater rim. This stunning NASA/JPL Mars image was processed and color enhanced by Emily Lakdawalla. For a larger image click here.
Emily Lakdawalla describes Curiosity rover's scientific instrumentation.
Most of Curiosity's instruments and tools are designed to make the observations necessary to read the life stories of rocks. MAHLI -- the hand lens imager -- can see the structure of the grains (if they're not microscopic) and how they're held together. APXS and ChemCam both measure the elemental composition of the rock. They can't see minerals; they just see elements. It's SAM and CheMin, the analytical laboratory instruments in the belly of the rover, that will eventually be used to perform mineral analyses.
Today's results came from ChemCam and APXS, the elemental analyzers, in measurements they performed on the rock dubbed Jake Matijevic at their first science stop, beginning sol 43. The team selected the rock for their first in-situ analysis because it looked like a homogeneous basalt, a type of igneous rock common on all planetary bodies (think Hawaii, the lunar maria, the Martian meteorites…). But the chemistry of this particular basaltic rock surprised them.
APXS measures chemistry from a larger single area than ChemCam, so it reads something closer to the bulk elemental composition of the rock. The APXS has a little lump of radioactive curium-244 that emits X-rays. When APXS is held close to a rock, the X-rays strike the atoms at the surface of the rock, which emit or fluoresce X-rays in response. Each chemical element emits X-rays with different, diagnostic energies. APXS detects these emitted X-rays, counting them and recording their energy. APXS benefits from long exposures: the longer it's held against a rock, the more X-rays it counts, and the more sensitive it is to trace elements in the rock. Curiosity's APXS improves on Spirit and Opportunity's by working much faster: it counts as many X-rays in one hour as Opportunity counts in 17 hours. As far as I understand it, this is simply because Curiosity's APXS can get closer to the rock than Opportunity's can!
Here's the graph that Ralf Gellert, APXS principal investigator, showed today. This graph contains two squiggles. The black one is that measured on Jake Matijevic. The red one was measured on the calibration target, a bit of Earth basalt that Curiosity brought with it, whose composition is extremely well understood.
The more massive the nucleus of the element, the greater the energy of the X-ray observed by the AXPS. The x-ray energy is directly related to the binding energy of the inner shell of electrons in an element. The relative amount of an element is measured by the height of the spectral peak at the energy level of each element. The absolute amount is determined by comparison to a known standard basalt from earth.
Because the y-axis is on a log scale the relative enrichment in K (potassium) and depletion in Mg (magnesium) and Al (aluminium) is mineralogically significant. It indicates the presence of the mineral feldspar, the most common mineral in igneous rocks in the earth's crust. The slight depletion in Ca (calcium) indicates that the feldspar is not dense calcium feldspar (anorthite) that forms at very high temperatures. However, a lighter and lower temperature form of the feldspar plagioclase containing a mixture of sodium and calcium may be present. Low density potassium-sodium feldspar (orthoclase) that is common in the less dense rock that makes up the earth's crust is clearly present. Depletion of magnesium and aluminum indicate less of the dense mineral olivine, that crystallizes at very high temperatures, (gem quality olivine is peridot) in the basalt, than NASA/JPL apparently expected.
Unfortunately, there's a problem with analyzing the data based on the red and black curves. The red curve, representing the standard basalt, is not precisely a standard Mars basalt. This leaves us with a serious problem in analyzing the results because there is an apparent contradiction concerning the element zinc.
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."
The principal investigator says that zinc is low, but in the figure it's clearly much higher in the rock sample than the standard. Zinc levels are important because zinc enrichment relative to iron indicates the presence of specific minerals in the Mars mantle rocks that were in equilibrium with the magma melt that later solidified to form the rock 'Jake Matijevic'.
Garnet strongly prefers iron over zinc and the mineral clinopyroxene also prefers iron to zinc, but the common mantle minerals olivine and orthopyroxene do not discriminate. The apparent enrichment of zinc to iron in the figure may indicate the possibility of the presence of garnet in the mantle of Mars. This would be an exciting discovery because garnet in the mantle is often associated with rocks called eclogites which are formed at high pressure by subduction of oceanic crust on earth. Possibly, plate tectonics on Mars has been more active than scientists realized. On the other hand, if zinc is not enriched compared to iron, this rock is probably not associated with eclogites, which form when sea floor basalt dives down into the mantle. Very high pressure conditions transform the lighter basaltic minerals into the denser eclogitic minerals while the bulk chemistry stays constant. Garnet is a key high density mineral that forms in eclogites, enhancing the depth and rate of subduction of oceanic crust on earth.
What we know without dispute is that this rock is more geologically evolved than other Mars rocks, and meteorites from Mars, that we have examined to date. The blog post explains one process, fractional crystallization, that produces a more evolved basalt. In the Hawaiian islands similar enrichment in light, alkali rich, minerals happens in the late stages of volcanism when low volumes of magma are generated from deeper in the earth's mantle than the main, high volume, eruptions. These late stage eruptions concentrate the elements that are most incompatible with mantle minerals.
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 H2O, 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 (Mg2SiO4 or Fe2SiO4) and then pyroxene (like CaMgSi2O6). 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.
Peridot (olivine) rich rock called peridotite is known to be the upper mantle source rock for most basaltic magmas on earth. The freezing point of olivine is very high so it is generally present as suspended crystals in primary basaltic magmas. Olivine crystals generally settle or filter out first. After olivine crystals are removed from the magma it may move and cool then pyroxene crystals begin to form and settle out.
Many alkali basalts are thought to form by fractional crystallization of the pyroxene minerals orthopyroxene and clinopyroxene. Clinopyroxene crystals prefer iron to zinc. Thus the high zinc to iron ratios seen in the chemistry results could be explained by the formation of pyroxenite rocks in the upper mantle of Mars. The residual magma may have erupted at the surface as alkalai basalt to form the rock we now know as 'Jake'. Alkali basalts have enhanced levels of mantle-incompatible elements found in earth's continental crust. If 'Jake' formed this way, Mars is just a tiny bit more earth-like than we thought. Norwegian Eclogite
Additional scientists who spoke at the news conference did not resolve the dilemma raised by the zinc data. However, they did give strong clues that the zinc/iron enrichment is real. Dr. Ed Stolper of Cal Tech/JPL spoke of the possible presence of eclogite in the source rock for the magma that produced the basalt. Eclogite in the source rock would enrich zinc in the melt relative to iron. The fact that he mentioned eclogite suggests that the rock has a high ratio of zinc to iron. This is a big clue that this rock could lead to the major discovery of earth-like plate tectonics on Mars. Dr Stolper is a world class expert on petrology and the geology of igneous rock so his words should be considered carefully.
All this compositional stuff is nifty, but we're not yet telling a story. The problem is, it's hard to tell a full story when all you've got is one rock. To learn more about what the composition of this rock means for the story of its origin, you really have to see how it fits in to the larger context of rocks in the area. It's just one rock, and it's not even in place; there's little context to help us understand how it formed, what the geologic environment was like. Stolper mentioned that the rock is a relatively uncommon type on Earth, generally found in ocean island volcanic settings (think Hawaii) or associated with eclogites (which are pretty unusual themselves). But just because that's where it found on Earth, doesn't mean that there was the same kind of environment on Mars. (And how exactly Earth generates melts with this composition is still a subject of debate.)
The journalists on today's teleconference pushed Stolper hard for statements about what this rock says about Mars' history, about the environment that prevailed when Jake formed. His answers likely dissatisfied them; they were equivocal and noncommittal. But that was appropriate. The news today is that in investigating a common rock type (basalt) to cross-calibrate two instruments, they found an uncommon composition, something unique, something that requires a dynamic, evolving geologic environment of an as-yet-unknown nature, something that opens up a new space of possibilities for types of igneous geology that can be imagined for Mars' past.
So, NASA scientists will be carefully running more tests on additional equipment on the Curiosity rover to find out what the surprising initial results mean. They will carefully study additional samples in detail to broaden their knowledge beyond what they can learn from one rock. We don't yet know if it formed like some Hawaiian alkalai basalts by settling and filtering of crystals from a normal basaltic magma or if it formed in association with plate tectonic subduction of basalt like some of the alkalai basalts found on Hainan Island did. Either way, Mars is more earthlike than we knew.
The high-magnesian olivine phenocrysts, high mantle potential temperature, and the presence of recycled oceanic crust in the source region provide independent support for the Hainan plume model that has previously been proposed largely based on geophysical observations. The Hainan plume thus provides a rare example of a young mantle plume associated with deep slab subduction.
If the Mars rock 'Jake Matijevic' formed under similar conditions as the Hainan alkalai basalts - a mantle plume associated with deep slab subduction - this will be a stunning discovery with profound implications for the history of Mars.