When we ask the question of whether life arose on Mars, we address possibly the most important philosophical question of human existence. We ask if we are alone. We ask if life is as common in the universe as it is here. When we begin the question, and wish to ask it seriously, then we must understand what it means to search for life. This will lead us to study biology and chemistry, as well as climate science, geology, comparative planetology, stellar atmospheres and interplanetary media. However, we must start with what we know, and on what constitutes life, we find that we have a conundrum. Shall we be satisfied in finding a virus or a prion or even simple proteins? Shall we call "it" alive, if it does not have characteristics like our own, say devouring rocks at a rate that is nearly imperceptible or living at such a high speed that it appears to be an explosion? In short, we must define what we mean by our search, and isolate the processes that constitute life.
In light of the potentially nebulous nature of defining life, perhaps if we start at home, where we know life exists. We could start with people and porpoises, ants and aardvarks, but since we don't have any reason to think these forms of life exist elsewhere, we shall look at others. All of the forms of life that we consider "complex" have one thing in common: they are composed of a huge number of specialized cells that do different functions for the organism as a whole. But, if we look at the simplest forms, where the entire package of life is packed into one cell, which interacts with its environment, reproduces, eats and creates waste, then we can begin to understand early life. The simplest forms of life, the bacteria, eukarya, and archaea, each appear to have a common root set of ancestors, or more specifically, the organisms that bridge the differences between these three phylogenic trees, are all thermophiles or hyper-thermophiles. These organisms all do not use photosynthesis and live in hot springs or at the bottom of deep sea vents. They live in water with no oxygen, no sunlight and where the temperatures can greatly exceed boiling. These simple DNA-based organisms may not be the simplest. The fact that the family trees of these tiny, hot bugs all merge together in this way suggests that the earliest forms of life on Earth didn't need sunlight. What they needed was a lot of water, a good heat source, and a chemical potential in the rocks to feed upon.
As an example, if we look at the sides of a deep-sea vent, we find a whole host of life that lives under a mile of water, near hydrothermal, magmatically-driven vents. Take a trip to the American Museum of Natural History, and look at the astounding exhibit on the Black Smokers in the Hall of Planet Earth. Here, a whole food chain is held up by bacteria that thrive in 570 degree Fahrenheit water mixed with minerals ripped out of the rocks below. The extensive sulfide minerals that make up these vents are a veritable smorgasbord for these thermophilic bacteria. They thrive and support the life functions of a special tube-worm that lives only on these vents. If you visit the Museum, you'll see these dead tube worms stuck to the rock.
In addition to this form of life, we also know that the vast majority of the history of life on Earth was in the form of single-celled life. We know from stromatolite fossils and banded-iron formations, that between 2.5 billion years ago and about 500 million years ago, the only life on Earth was bacteria. The banded-iron rocks have alternating strata of oxygen-rich and poor layers. These sedimentary rocks are thought to be formed as huge ocean-floating bacterial colonies exhaled oxygen into the Earth's atmosphere and water. The iron bound with the oxygen when it was there, and sank to the ocean floor. These rocks preserve the record of ancient life on Earth, which was all simple bacteria.
Thus, if we look for life on Mars, we will look for places where bacterial life or its cousins the archaea and eukarya could have taken hold. We would look for places filled with warm to hot water, devoid of oxygen, which has a good mineral content in abundance, specifically sulfides, since that matches the deep-sea vents and the toxic water-filled strip mines on Earth. We would look for places where the water is not highly acidic, since that breaks down proteins in living cells. We would look for what is essentially a fossilized dinner table set for simple bacterial or even simpler RNA-based life.
Amazingly enough, this has been found on Mars.
On December 7, 2011, NASA's Opportunity rover found an outcropping of pure hydrated calcium sulfate: gypsum, or if you like, drywall. Opportunity, with this find conclusively proved that water existed in copious quantities on the surface of Mars. The "Homestake" deposit would have formed perhaps 3 billions of years ago from water dissolving calcium out of volcanic rocks, which then combined with the sulfur in those rocks or in volcanic gasses. This mix would have filled rock fractures and been trapped until the mixture solidified and cooled. Eventually a meteorite strike would later expose it to the surface, for an intrepid rover to find a few billion years later. Having now proved that water copiously filled volcanic rocks enough to change their chemistry and create hydrated minerals, the stage was set for Curiosity to land on Mars to search for an actual habitable environment.
When Curiosity arrived, its goal was not to find life, or even find fossilized life. Fossils occur on Earth due in part to tectonic forces, abundant water, and diverse ecosystems. There has to be an enormous amount of life itself in order to make fossils, but no one expected the La Brea Tar Pits to be anywhere near Curiosity. So, an March 12, 2013, we learned that Gale Crater on Mars was indeed habitable by simple organisms. The percussive drill from the rover drilled into a rock to expose it to air for the first time in 3 billion years. The result was a grey powder from inside the rock. The color of the powder itself was significant, because all other drill samples and internal samples by Opportunity and Spirit showed rock that had been affected by highly acidic water. The gray powder contained a lot of hydrated, clay-bearing minerals. In fact, the type of hydrated minerals found at Yellowknife Bay implies that the Ph of the ancient water that once flowed through there was drinkable. The minerals there would have made it taste like a refreshing Perrier. The clay minerals found there appear to have been intermixed with the basalt rock which is so common on Mars. The habitability rests in the fact that the clay minerals that were found can act like a battery. There is chemical potential there in the rocks for simple organisms to feed upon. This is exactly the same process that is found on the thermophilic bacteria on the black smokers on Earth, except it wasn't so hot. There was so much water at Curiosity's location, that they've even discovered rocks that have been clearly smoothed by tumbling for years in a river.
This is amazing. It means that if we took some chemosynthetic, rock-eating bacteria on Earth, put them on the rock drilled up by Curiosity, flooded the rock with a lot of water, provided a copious heat source, such as a steady volcano nearby, the bacteria would grow and eat the rock.
This means that Curiosity found a place for bacteria not unlike Sardi's in the theater district.
We still don't know whether life actually arose on Mars, but we are now certain that Mars was a bacteria's favorite restaurant. The table was set, the waiters were waiting, the doors were open, and the music was playing. Did anyone walk in and nibble on those tasty non-oxidized rocks and sulfates and sulfides? We don't know. But at least we now know it could have happened.
Curiosity still has a lot of work to do, and it's only 9 months into a two-year mission. It still needs to cross the dunes and scale Mount Sharp, seeking how the clay-bearing rocks change in depth and deposition. Perhaps among the hummocky hills, we'll see the telltale signs of fossilized stromatolites or their distant cousins. If we look for life on Earth, we find it everywhere we look and everywhere we're not expecting. Life has a way of adapting to its environment. Once the self-replication with RNA was developed out of the primordial organic sludge 4 billion years ago, the process never stopped. If Mars had a foothold, it is incredibly tempting to say that Mars once did have life. But we don't know that. Carl Sagan stated that extraordinary claims require extraordinary evidence. It will have to be an amazingly careful and ingenious mission to determine if life arose on Mars three or four billion years ago. Perhaps it will take human prospectors and scientists to actually go there and look in person. Of all the reasons for humanity to go to Mars, this is the only one worth dying for: to answer once and for all, are we alone?
- Homestake vein seen by Opportunity.
- Curiosity's discovery of habitability on Mars.
- Curiosity's analysis of clays at Yellowknife Bay.
- The First Scoop
- Dry Streambed on Earth like Mars Gale Crater
- Mount Sharp: The Promised Land
- The Base of Mount Sharp where the real work will begin
- The Grand Canyon is similar to Mount Sharp
- On Earth we have some places that look a lot like Yellowknife Bay, with the same rocks and the same dry environment.
What a fossilized ancient streambed looks like on Mars
- Compare the water-rolled rocks on Earth to Mars
Published in Eyepiece