With 29 km behind her, and Cape York now less – considerably less, in fact – than 4km away, it’s tempting to assume that Opportunity’s landfall on that rocky ‘island’ is a done deal, to tell ourselves that we’re practically there. Of course, nothing could be further from the truth: every sunrise Oppy sees may be her last, dawn breaking on the fateful, horrible day she finally succumbs to some mechanical failure, software glitch or the harsh martian environment itself. Nevertheless, Cape York now feels tantalisingly close, and although we can’t see it – it’s beyond and actually beneath Oppy’s local horizon – we can almost feel it, can’t we?

But what is all the fuss about Cape York, anyway? Why, when Oppy was concluding her studies of Victoria Crater (wow, doesn’t that seem a lifetime ago now?!) did the MER team decide to leave all Victoria’s beautiful and relatively easy-to-reach layered rocks and bays and capes behind and strike out for that little ‘island’ on the ludicrously far away rim of the enormous Endeavour crater? What was calling to them from that huge impact scar, what was so fascinating about its eroded, ancient rim that it was worth sending Oppy off towards it, on the longest, most challenging leg of her epic martian trek so far?

Well, scanning the many MER websites, science papers and articles, one word is used over and over, again and again, to justify the long, long trek to the rim of Endeavour – “phyllosilicates“. Do a Google search for “phyllosilicates+Mars” and you will find many papers and articles written about the subject, so clearly they’re a big deal. But if you’re not very scientifically literate, or if you don’t know a lot of the background story behind the MERs, and martian exploration in general, the wow! factor of phyllosilicates can be a bit hard to explain and understand. They’re something to do with… clays? Water?

I decided the best way to get to the bottom of the whole phyllosilicate story was to just ask one of the scientists involved in the MER mission. Can’t hurt, right? So, Google searching for “phyllosilicates+Mars” brought up a long list of academic papers and science magazine articles… with one name dominating: James Wray, of Cornell University, Ithaca, New York. Basically, if a paper or article deals with martian phyllosilicates, the search for them, and the consequences and implications of finding them, it will probably have his name on it.

Target acquired! 🙂

So, cheeky as ever, I sent James some questions, asking for a ground-up beginners guide to just why these minerals are such a big deal, for Oppy and the MER mission, and for Mars exploration as a whole. And even though he is horrendously busy right now, preparing for the final big Mars Science Laboratory landing site selection meeting, again, as every other NASA scientist I have ever emailed has been, he was more than happy to answer my questions, clear things up for me, and help me – and through this blog, hopefully help many of you out there, too – appreciate why Oppy was sent tearing off across the Meridiani desert, towards an impossibly far away crater.

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Opportunity is now less than 4km away from Endeavour Crater, and closing-in on “Cape York”, the small rocky island/ledge/outcrop on the crater’s NW rim. We know that oce she reaches Cape York, after crossing Botany Bay, she’ll go up onto Cape York in search of “phyllosilicates” which have been detected spectroscopically from orbit and heralded as the “Holy Grail” of both the MER mission and the MARS SCIENCE LABORATORY mission, but can you give us a very basic beginners guide to what they are and why they are so important?

Phyllosilicates are a class of minerals. In particular, those we see at Endeavour crater belong to a subset of phyllosilicates known as clays. You probably didn’t have any clays in your childhood mineral collection (at least I didn’t); they are very fine-grained, so they don’t form big beautiful crystals. But you probably have them in your garden. Clays are present in soils all around the Earth–essentially everywhere that water percolates through silicate rocks on a regular basis, clays will form.

In fact, clays *require* water to form, much like the sulfates that Opportunity has been studying for over 7 years now. Those sulfates were a historic discovery. But, as you know, they are interpreted to record only shallow ponds of salty water that may not have persisted very long. Clays probably indicate something different. On Earth, clay formation typically requires liquid water to be present much longer than is required for the formation of sulfates (or other salts). But not always. So the implications of detecting clays on Mars are not yet certain, but there is the tantalizing possibility that they mark places where water persisted for longer than anywhere we have yet studied with a rover. Therefore, we would like to study them ASAP!

Another difference between clays and sulfates is the pH at which they form. One of the sulfates found by Opportunity in all the bedrock studied to date is jarosite, which forms only in very acidic fluids. Most life on Earth wouldn’t like such acidity, although some microbes are specialized for it. What did ancient Martian life prefer? I would say we can’t possibly know until we find it. But since clays form at higher (close to neutral) pH, they indicate a type of water chemistry that we may not yet have explored, and may have had a different habitability potential than the environments that formed sulfates. We’ll learn much more from getting a close-up view.

Just how significant for Mars research would it be if Opportunity reaches and gets to study phyllosilicates?

Opportunity may provide our first chance to get “ground truth” on Martian clays. Even on Earth, geologists sometimes develop hypotheses based on aerial (or orbital) reconnaissance of interesting landforms. Then when they physically explore these landforms on the ground, they learn the “truth” … which might be quite different from what they previously guessed! We have yet to experience this process of enlightenment for clays on Mars.

Opportunity was built, launched, and landed before any clays had been detected from Mars orbit. So Opportunity was not designed for the specific objective of studying clays. (Nor, of course, was either rover expected to operate as late as 2011!) But she does carry instruments that may be capable of confirming their presence. One of these would have been MiniTES, but that instrument now seems unlikely to return useful data again. Another is the Mössbauer spectrometer. Mössbauer senses only minerals with iron in them; luckily, the orbital spectra show that the Endeavour clays contain iron! But as the Mössbauer spectrometer ages, it takes much longer for it to make a single good measurement. So while the Mössbauer might definitively confirm clays by staring at one spot for awhile (weeks? months??), it won’t make enough of these measurements to map the clays. For that, I think our best hope will be the APXS. Beyond finding them, both Mössbauer and APXS can potentially answer questions about the chemistry of these clays that are impossible to resolve from orbit.

Orbital spectroscopy shows that Mars formed several different types of clays, at different times and places across the globe. Opportunity won’t teach us about all of those clays, but the type of clays at Endeavour (“Fe/Mg-smectites”) may be the most common type on Mars. So studying them close-up could yield new insights into widespread water processes that occurred early in Mars history.

Once Opportunity gets up onto Cape York, what will these phyllosilicate-rich areas look like to her cameras? And through her Microscopic Imager?

We don’t know! Pancam and MI images of these rocks will be some of the most interesting data that Opportunity can provide. By analogy, the sulfates in the rocks at Eagle crater told us that water had been there, but did that water ever reach the surface? The answer (yes!) came from images showing ripple patterns preserved in parts of the layered bedrock. Other rock outcrops lacked these ripples, and instead had layers that must have been deposited in ancient dunes, not in ponds. These clues in the layering occur at scales of centimeters or smaller, too small to see from orbit even with the powerful HiRISE camera. HiRISE shows us that the Endeavour rim rocks appear to be layered and fractured at scales of a meter or more, but what we’ll see at smaller scales with Pancam and MI is anyone’s guess. I can’t wait to find out.

And finally, many papers and articles about phyllosilicates use the word “life” . Can you clear up for us what the connection is? Does the presence of phyllosilicate minerals in an area simply suggest that that area was once underwater, or do they show that that area might once have been a place where life could have existed? Some over-optimistic people claim Opportunity’s MI might even image micro-fossils in phyllosilicates. Possible?

Aside from indicating a watery environment, there is another way in which clay minerals might relate to life. Clays have a tendency to bind other molecules to their surfaces. These molecules can include organics, and it has been suggested that clays may have facilitated the origin of life here on Earth by providing places where organic molecules could collect and undergo chemical reactions. Whether this is true or not, the ability of clays to concentrate organic matter is one reason why they’re such a high priority for exploration by MSL, which will be able to identify organics.

Opportunity cannot identify organics, so possible evidence for life could only come from images. Fossils of individual microbes would be too small to see with the MI, but larger structures created by life could be resolved in principle. For example, “stromatolites” (fossilized microbial mats) provide evidence for some of the oldest life on Earth. But we have absolutely no reason to expect any such fossils at Endeavour crater, where the rim rocks are likely older than any known Earth fossils. Nevertheless, exploring these rocks with Opportunity may yield valuable new information about the clay-forming environment on ancient Mars, therefore helping to guide future searches for evidence of life.

James, thanks very much for answering some questions for Road to Endeavour.

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So, there you go… that’s why phyllosilicates are such a big deal, and that’s why Oppy is screaming across Meridiani, desperate to reach Cape York and find some of them.