A Traveler's Guide to Mars

"Again, it’s a little bit dated. We’ve gone through a decade of amazing Mars exploration. This book partly tells the history of the exploration of Mars. In 1976 the Viking missions landed—the first and last life detection experiments ever carried on a spacecraft—and then did not discover life (Carl Sagan was very much involved at the time). There’s a perception that, at that point, NASA or Congress lost interest in going to Mars, or the public did. So it wasn’t until twenty years later that they finally sent a tiny rover to Mars. After that a continuous campaign of missions mapped the planet in all of its beautiful geological history. This book came out when we were getting the very first high-resolution space images of Mars, fantastic photography which reveals Mars as far more complex than we thought we had left it back in 1976. So now we realise it has a whole history of climate change and recent glaciation, and that there are a lot of ice bodies close to the equator. We didn’t anticipate this, and it has lots of implications for human exploration on Mars. Essentially, this recent exploration made it much more Earth-like. “As you read through this book you’ll come across this conundrum: where do you go to search for ancient life on Mars?” It’s a good book, both in terms of providing the reader with a fantastic history of the exploration of Mars (even though it doesn’t have the latest missions in it) and making the geology of Mars much more exciting. And how can you go wrong? The images that you’re looking at in this book are superlative. In terms of a book that’s readily available to the lay person who’s never had any knowledge of Mars before, it’s a good way of getting an excellent introduction. Mars appears to have had liquid water on its surface in the past. You see these beautiful deltas that look very much like the Mississippi River delta. You have water on the surface, but, as it says in the book, how could Mars have had an ocean in its northern hemisphere three and half to four billion years ago when the Sun was actually cooler then? We have no means of explaining how Mars could have been warmer than it is today. The idea that Mars had a very thick atmosphere and greenhouse gases—sufficient to raise the temperature up to the melting point of ice in the presence of a sun which had only 70% of its current luminosity—has failed totally. The geologists that work on Mars today are stuck in this Martian conundrum. Episodically, it may have been warm enough under a combination of situations: impact debris that somehow generated a temporary warmth that melted water which flowed across the surface. Then everything froze back up again. If you’re left with that scenario, it’s challenging you as to where to look for life. That’s really the implication of this book. In the next mission that goes to Mars from NASA, in 2020, we’re going to send a rover to collect samples for a return to Earth. Those samples are supposed to contain, hopefully, biomarkers of ancient Martian life. So where do you go? Do you go to these fluvial deposits, even if they only formed within a thousand years? Or do you go look for something which is from the subsurface and may have been in existence for a billion years? As you read through this book you’ll come across this conundrum: where do you go to search for ancient life on Mars? I don’t think the conundrum is ever going to be solved. I think that these deposits that look like they were formed in the presence of liquid water on Mars were actually formed in very short episodes in time. That has implications. On Mars, an ocean will not have existed for very long and it would have existed under a very thick layer of ice. What does that mean about the potential for life originating on the surface? And if it did originate on the surface it would have inhabited the subsurface. There’s no doubt about that, it will penetrate down. The fluids will go underground and life will go with it. If you’re life, you will survive longer beneath the surface of Mars than you would on the surface because that icy lake that you’re in is going to evaporate and dry up and you’re going to get exposed to radiation because there’s nothing to shield you from the radiation in the atmosphere. But if you’re down there, you’ll be doing all this stuff that we know subsurface life has the ability to do. We know from the evolutionary record that, on Earth, the most primitive organisms are those that exist in the subsurface, whereas organisms that grow by photosynthesis appeared later on. It takes time to develop photosynthetic enzymes, it could have taken a billion years to develop that on Earth. On Mars you may not have had that amount of time to do that before the radiation came and destroyed you. That’s the battle that we’re now engaged in. It’s a hypothetical battle. Do we expect photosynthesis to have evolved on Mars? If it had, then, yes, we’d expect to see beautiful mats existing on the surface in the lakes. But if it had not, then it has to be life that exists like chemolithoautotrophes, feeding off the rocks, and where do you expect them to live for a long period of time? In the subsurface."