WHEN NASA MET PILBARA — MARTIN VAN KRANENDONK & MITCH SCHULTE
Why WA’S rocks are the best chance of finding life on Mars
In 2020, two rovers will be launched from Earth in the treasure hunt for signs that life was once on Mars. Astrobiologist MARTIN VAN KRANENDONK took NASA Mars 2020 Program Scientist MITCH SCHULTE and rover instrument scientists to the vast and precious Pilbara, in WA, to show them how to travel through geological time to spot biology hidden in plain sight.
Here are their accounts of what happened next…
You can feel like you’re walking a shoreline 3.5 billion years ago, seeing life get a start
‘‘ WE’VE BEEN INVOLVED in the study of ancient life but also in the search for life on Mars and on early Earth for many years now.
This area in northwest Australia – the Pilbara – contains the oldest, best-preserved evidence of life on the planet. Some natural systems get bashed around while others not so much. So the Pilbara has really just been a fluke of preservation. It’s because of the high degree of melting that took place beneath the Pilbara during its long period of formation, which left behind a depleted, buoyant, and cold residue that has kept the interior part of the craton protected from damage through the later aeons. The other factor is that Earth was subject to strong impacts by large meteorites until about 3.8 billion years ago in the Late Heavy Bombardment. So the Pilbara rocks are really some of the first that were formed and didn’t get pummelled; they were able to survive because Solar System evolution was dying down at that time.
There’s an aspect to life that most people are probably not aware of. Early microbial life precipitates rock – and these microbes deposit different minerals through their metabolism to make food energy. So when you see the different colours in the rocks, sometimes that’s a function of the geology, but often it’s the microbes themselves that have precipitated different minerals. You might be familiar with fool’s gold – pyrite, one of the most common shiny metals. It can actually form from two different processes: it can happen through magmatism, where heat and fluids come up out of the mantle, or it can be precipitated by the activity of microbes. Some microbes take a molecule of sulphate – SO4 – and reduce it to make H2S, which is highly reactive with iron. On early Earth there was no free oxygen in the atmosphere or the oceans, so the seas would have been full of iron. As soon as microbes made H2S, it would have combined with iron to make pyrite.
The oldest stromatolites in the Pilbara are all made from pyrite, precipitated by microbes. It’s what gives the rocks at the surface a rusty red colour; it’s rusted pyrite. Other microbial communities precipitated calcium carbonate – a light brown mineral. The activity of life is actually changing the geology and we can see those textures and minerals in the rocks as exclusive and unique to life. So that provides also another way of exploring for life – not just the textures, but the mineral assemblages themselves.
We spend our careers looking at the features and minerals associated with the earliest microbial life on Earth and thinking in great depth about ways to positively discriminate between biology (life) and geology. We humans are biology, but we are also a chemical system, although a chemical system alone doesn’t necessarily mean it’s biology.
So right at the forefront of our scientific field is the challenge to discriminate between geology and merely chemical processes, and those that are uniquely made by biology. We spend a lot of time
finding techniques and new methodologies to be able to discriminate between those two.
Because if you go to Mars you don’t want to say, “Oh look we just found some interesting chemistry, or some interesting-looking rock textures”. You want to be able to prove that the remains were made exclusively by life. And when you’re dealing with ancient rocks, you’re actually dealing with the chemical traces and the fingerprints of microscopic micro-organisms. That’s when you have to find ways to be confident – to be able to say “yes, this was made by life – biology – and not just by some kind of chemistry or physical process”.
The rocks in the Pilbara are 3.5 billion years old, which is about the age of most of the crust on Mars. We know now that Mars had a warm and wet atmosphere early in its history – and that this all happened around the same time as the rocks in the Pilbara were being formed, including their stromatolites. And it’s tremendously exciting to think that perhaps life flourished on Mars just as it did on Earth.
Basically, the record of life in the Pilbara is the ancestry of all life on our planet. When you touch it and walk around it – on these ancient rocks I can still see the ripples on the beach and which way the water flowed – you can actually transport yourself back in time and feel like you’re walking along the edge of a shoreline 3.5 billion years ago, seeing life getting a start on this very young planet.
In 2019 we hosted the instrument specialists for the Mars Rovers that are being launched by NASA and the European Space Agency, as well as the heads for Mars Exploration for NASA. These scientists came to look at these rocks to think: How can I use my instrument? What am I trying to look for? Where are we going to sample for life on Mars?
For many of the scientists it was their first time seeing traces of early life. They found it a real eyeopener: the complexity of the science that goes into investigating life on Earth and the importance of the context of the rocks. Rocks are rocks, but when you look in more holistic ways at the environment, where they were deposited and the things around them, that context is really important to understanding life itself – not just where it lived, but how to identify it, and determine how it made its living.
There were two things I’ll take from the trip.
One was the understanding that when people see these features in the outcrops, when they’re actually surrounded by the geology, it’s very different to seeing a picture of a close-up rock texture at a conference, or a rock in the lab. We had a chance to discuss that and to show how the context from the outcrops was an important part in the search for life on Mars.
But I also gained an appreciation of the enormity of the task that these scientists face. It’s one thing for me and others to walk around free – we can go and look where we want. I have the luxury of being able to take as many samples as I want and bring them back to my lab. But the Rover scientists might only be able to collect one sample from a layer. And their kind of search is very dependent on the instruments that they have – they’re highly sophisticated, but they’re still extremely limited in a number of ways.
It’s a tremendously complex business. It really comes down to the detail: how physically can I drill on this outcrop if we’ve got a slant of more than 10 degrees? And even if I can identify a layer of interest, is it better to sample five centimetres to the left, or five centimetres to the right? What would give me the best chance of success? All from a million miles away. ->
‘‘
I’M A GEOLOGIST by training – my PHD was in geochemistry – and when you’re going through school, the rocks in WA are one of those bucket-list places that you need to see, because they contain some of the oldest evidence for life on Earth – they’re a really big deal. This is the kind of thing that we’re hoping to find on Mars.
The surface there now is cold and dry and dusty and there’s no liquid water, so we don’t think we’re going to find anything alive in the rocks on the surface. But we think there were conditions conducive to life on the surface of Mars or near the surface of Mars when there was liquid water at the surface, with the evidence of river channels and alteration of minerals by liquid water, for example.
I really wanted to get the mission teams to the Pilbara to get an appreciation of a) what we’re looking for and b) how difficult it is here on Earth to do this and see how special these rocks are here. A lot of the people who work on building instruments for these space missions are not geologists and they don’t always know what they’re looking at when they’re looking at the rocks. That makes sense, because when you’re trying to figure out the optics of the spectrometers, for example, you really need more of a physics background.
I don’t know if you’ve seen the Pilbara rocks – and if you haven’t you should go – but the outcrops of bedrock are not really that large. You have to use the clues to tell you what kind of environments were there three-and-a-half billion years ago to figure out where to look for the evidence for life in the rocks. So it’s the exact kind of thing that these Rovers are trying to do on Mars from hundreds of millions of miles away, with just a robot rather than a person who can walk around and return to the area a number of times.
The NASA Rover has a payload of seven different instruments that are designed to look at the chemistry, mineralogy and the geology of the materials on the surface of Mars (the rocks and
the sediments). We’ll also be looking for organic material. They all work together and there’s overlap in the kind of things they can do. Some work a little bit more rapidly, some do a little bit more detailed analysis, but they all tell us what we want to know.
The Europeans are also sending the Exomars Rover to Mars next year. NASA has an instrument flying along with the Exomars and our Rover has a couple of European instruments. There’s a lot of collaboration going on.
We’re going to be very careful not to touch the rocks on Mars that we sample with the instruments because we want them to be as pristine as possible when they come back.
We’ll use the instruments to understand the geology and the context of where we are and then once we’re convinced that we have found the one that we actually want to sample we’ll make sure that we go to a spot where the rocks haven’t been touched with those instruments. We’ll collect those cores into tubes and the tubes will be sealed up and left on the surface of Mars for us to come get later.
We have 43 sample tubes, which will have to include a number of blanks – “witness” tubes that can be used to monitor Earth-based contamination. We’re planning to collect 15–20 containers during the prime mission. We’re working very hard at NASA to get congressional and presidential administration approval to begin a mission to go get those and bring them back to Earth to study. The very earliest we could send that mission would be 2026, and the samples wouldn’t come back until the end of the decade.
One of the biggest lessons the scientists learned in the Pilbara is that they have a hard job ahead of them. Which is fair, and that was part of what I wanted to impress upon them. If the Rover landed in the valley where you know that there’s material that contains evidence of life but you don’t know that it’s there, would you do all the right things to lead you to the evidenceof-life goldmine?
So it’s important to get a sense of scale in terms of appreciating how precious and rare these samples are.
There were a great number of discussions among the Mars2 020 team and between the
Mars 2020 and Exomars teams about how to approach operations on the surface after seeing these rocks for themselves. It was very useful for non-geologists especially to get an appreciation for “okay, this is what your instrument is trying to do and this is the kind of stuff you’re actually going to be looking at on Mars”.
Martin’s spent so much time out there and he knows the area really, really well, so he was able to help all the instrument specialists see that the context matters. When you start reading, the rocks really do tell you a story.
It’s the exact thing we’re trying to do on Mars, from millions of miles away with just a robot