Gale Crater – a pile of possibilities
I couldn’t let the MSL landing go without at least a quick note – congrats to NASA! Though there’s still room for disaster with any space mission, the incredibly tricky EDL (entry, descent and landing) is over and Curiosity Rover is sitting happily on the surface of Mars, sending back images.
I’ve been writing about what’s cool about each planet, so what’s so great about this landing site, Gale Crater?
An Ancient Crater Lake
As I said in this post, we know Mars was once pretty wet, and that around 3.5Ga, it cooled and hydrous clay minerals (phyllosilicates) in rocks gave way to sulphur-bearing evaporites which form when water dries up. Gale Crater has a great stack of sedimentary rocks in its centre (Mount Sharp) and remote sensing tells us those at those at the bottom contain phyllosilicates and that these slowly transition into sulphur-bearing rocks as you go up the stack1. It’s suggested the crater contained a lake which deposited sediments throughout this vital period. By simply driving up the stack, we can see how Mars’ environment evolved over time.
Apart from the geological story, this is just as important for astrobiology: when the clays were being formed, conditions may have been right for life on Mars. Then, when things started cooling, that life would have been increasingly concentrated in the last few warmer places. It’s been suggested that Gale crater is one such place: the impact that formed it may have heated the ground around it and set up a hydrothermal system which would have kept conditions warm for hundreds of thousands of years ((Schwenzer et al. (2012) Planet. Space. Sci.: 70, 84-95)). So this is one of the places where life may have survived the longest, even if it’s extinct now.
Recent Water Flows
The crater’s also important for the later part of the water story. That mountain of sedimentary rocks has clearly been eroded away around its edges or it would fill the entire crater. Mount Sharp is actually higher than the north end of the crater, which has eroded landforms and channels in it. So it looks like a big outflow of water washed in from that direction, from the Elysium region. The timing? Late Amazonian2 , the time when the Cerberus plains (my own favourite topic) were formed by outflow over just that region.
So what we have here is a deep, 150km wide crater which may have contained standing bodies of water on and off over a span of over 2 billion years3. That whole whole story is stacked up for us right there in Mount Sharp and across the floor around it. It can give us the full history of hydrous activity on Mars and might harbour signs of ancient or not-so-ancient life. A good choice for a landing site for the biggest, best rover we’ve ever sent out? I think so.