Mars Up Close, Part 2: John Grant

John Grant: Thank you all for being here tonight. And listening to us tell you a little bit about Curiosity. So, starting out with why Gale Crater. Well, here is a map of Mars. We've landed in six locations before Curiosity. Why do you need to go for a seventh? Haven't we-- haven't we been all over the planet? It looks kinda like a shotgun blast to me here in terms of the yellow marks. Well, Mars is a smaller planet than the Earth. But without oceans it's got about the same land area as the surface of the Earth. And I travel a fair amount, more than I'd like to but if I had only been to six places on the surface of the Earth, I wouldn't know much about it. Let me give you one example. There is a place where I used to live in Western New York, called Letchworth State Park. It's called the 'Grand Canyon of the East'. A fabulous place. But if I had never been out west and didn't know about the Grand Canyon, it would be kind of a different story in terms of where I might choose to go. But what if the rocks in the Grand Canyon weren't of interest and the ones in Letchworth State Park, even though it's smaller, were.

So what you really need when you are looking at landing sites is not only do you have to know something about the planet and where the good places are but you've got to bring together the expertise to be able to look at those places and find out which ones are the most relevant for the kinds of things that you want to accomplish with your mission. So, we started with about 65 landing site candidates from our Science Laboratory, Curiosity. We whittled it down to the final four that you see here in white. Holden Crater, Eberswalde Crater, Mawrth Vallis and of course Gale Crater where we ended up going.

And just to give you a sense of what we found attractive about those four places and I'll point out that the yellow ellipsis that you see here are all 25 kilometers long by 20 kilometers high. We've got Mawrth Vallis on the upper left. There is this stratigraphy, these layers of rock that we see there that we see a sense of different kinds of clays from top to bottom and it's an ancient, ancient set of rocks that tell us something about the earliest history of the planet. A question about Mawrth was how did those rocks get to be there and we never really could get a handle on that. Although work has been moving forward. Down here in the lower left we see a place called Holden Crater. It's about the same size as Gale Crater, about 100 miles across and here we see along the side of the crater sediments that have been washed down into a big fan. It's the best alluvial system preserved on Mars.

Over here on the lower right we see Eberswalde Delta. A place where we might go and sample sediments that have been deposited by water running down through a river in the ancient past. The problem or question about these two on the bottom were they occur relatively late in Mars' history compared to when we think it was the wettest. And if we got there we're seeing a fairly narrow period in the history of Mars. So we want to know relative to habitability when those things occurred, we might want to sample a place with a section that's preserved is much older.

And that brings us to Gale up here in the upper right. You'll notice that in this 100 mile across crater, 150 kilometers across, there is this mountain of material. Five kilometers or three miles high. And the layers that are in that mountain are very much like pages or chapters in a book. And when we look at the chemistry or the chemical composition of those layers, we see a transition from the kinds of things that were deposited in water in wet conditions near the bottom to drier things as we move up through the section. And so Gale became a chance to sort of read the chapters related to the changing environment on Mars, from when it was wetter to when it's relatively dry today. And that's why we picked Gale Crater. Gale Crater lies across this transition on Mars.

What you see here, this topography where the yellows and the reds are relatively high and the blues and purples are low. And Gale right there in the middle straddles this boundary called the 'planetary dichotomy'. And what we see here already visible is this mound; this mountain of material, Mount Sharp. If we zoom in on the crater proper, we are looking across here towards the south and on the north side between the small crater that you see in the middle of the mountain is about where we were planning to land. So we wanted to land and be able to drive directly over to the flank of the mountain transitioning up between these layers of material, these chapters in the book, telling us about the environmental history of Mars.

But there was another target there. A small channel that runs down the wall of the crater. This is a closer view now. The top of the crater is now to the north, so I've turned things around on you. This is an image from the Mars Reconnaissance Orbiter. There is this incredible synergy between the cameras that we have in orbit and the pictures that we can take of the surface, and the things that we can do on the surface to ground truth that data. This data, we can resolve things that are about 20 meters across. The camera has a resolution of six meters per pixel. And we can see that channel now coming in from the upper left and draining out into a fan of sediment. We call this an alluvial fan. And there are many examples of this on Earth that we see in places for example like Death Valley. So this became a very important secondary target, to the layers in Mount Sharp, for our exploration.

Well, here is the view from the surface after Curiosity landed. This is one of my favorite images from the mission because to me it's almost like you are standing in Death Valley looking up the west at the Panamint Mountains. And seeing this fan of material, you can see the bright outline of Peace Vallis, this channel coming down and the sloping sort of deposit of sediment that's being laid down by that river some time in the past. We are way down at the very far end of that where we landed with Curiosity. In fact we weren't even sure if we were going to be on those sediments.

Now here is an overhead view, this time from the HiRise camera, which can image things that are about three feet across. It has a resolution of about 25 centimeters or about a foot. You can see where Curiosity landed, the two blue spots over here on the left. And those are the tracks of the rover. When we landed, because we had mapped the surface of our landing ellipse before we got down to the surface of the planet we knew that there were some different terrains that we were likely to sample, depending on where we landed. And because we landed at that spot there, we happen to be very close to three different kinds of material that were sort of like a Rosetta Stone we thought for understanding the geology of our traverse as we went across to Mount Sharp. The most of the material for the left two-thirds of the image was one type and the lower right corner was another, and then the brighter whitish-blue stuff up there is a third. Seeing this juncture of these three different materials knowing that they held to clues to what was going on in Gale and that they might be related to that alluvial fan, drove us literally in the opposite direction than we intended once we got down on the surface. Mount Sharp is down to the left-- to the lower left, your left, my right. We went east, we went to your right. And it paid off.

Very soon after we got started, we started to find these outcrops. The pebbles that you can see weathering out, that are rounded. Becky Williams who is here tonight was the lead author on a science paper that first described these as 'deposits related to that river flowing down that side of the canyon'. Now, in Death Valley it doesn't, it doesn't rain very often. But when it does, that water comes out fast. And I can almost imagine when I see these sorts of things, you know water up to my thigh kinda thing draining across the surface depositing these gravels. What an amazing and different place this must have been three, three and a half billion years ago.

We got into this place called Yellowknife Bay and here's our dried up lake sediments. This is a mudstone. We drilled down into this rock. The rocks weren't red, they are gray. They tell us something about the conditions in the past. When we look at the chemistry it tells us that these clays that are in here were deposited in conditions that were relatively neutral, that is not too acidic, not too basic. But we've already sampled sediments washed down in a river and ancient lake deposits. And we haven't even gotten to Mount Sharp yet.

So, clearly the first bit of information that we got back from Curiosity, really sort of set us on our ear, in terms of understanding Mars as a planet and how it evolved. Since leaving Yellowknife Bay and these ancient lake deposits we've been traversing across a landscape that's littered with sedimentary rocks. You can see some of those here in the foreground. And we've been sampling these as we've gone, trying to understand more about the relationship between sediments washed down in this river, Peace Vallis and out onto this fan of sediments versus the sediments that have been shed off the side of Mount Sharp. There's still kind of an uncertainty in terms of where some of these different sediments have come from and what their origin is in relationship to the timing of the formation of this big, huge mountain in the middle of the-- of the crater.

But as you can see this image, we haven't lost sight of Mount Sharp. It's always sort of been dead in our sights. And we are actually more than half way there from when we left the Yellowknife Bay area. And in fact we see images like this almost daily with Curiosity and it reminds us exactly why we came here and why this is the goal. Because you can see the layers here that I described earlier as sort of chapters in a book. And you can see here how they transition from sort of darker and grayer layers near the bottom to lighter sediments at the top. And knowing what we already know about Gale Crater from our exploration of these lake and sediments washed down in the fan, we are very eager to get here and understand more about the evolution of the Martian climate as these sediments were deposited.