Last week we talked about life in some very strange places like in sinkholes at the bottom of lakes and in your own gut. But this week we’re going to talk about some even more exotic places. We’re going to move out into the solar system and examine the most likely places among our neighbors that may have life.
Back in January, I told you about the discovery of methane on Mars. Although it didn’t make a big splash in the news media (someone was inaugurated about the same time) it could conceivably be one of the biggest stories of the 21st century because free methane is vanishingly rare. Because it only has one carbon atom, methane combines with virtually everything and as far as we know, it can be produced by only two processes.
It can be produced by volcanic activity. Although we don’t see all the ash, dust and gas produced in volcanic eruptions, it is possible that there are underground reservoirs of methane on Mars that were produced long ago. Methane from these ‘tanks’ may be slowly seeping to the surface and producing the small amounts of methane in Mars’ atmosphere.
But the more intriguing possibility is that the methane on Mars was produced by life. As I said in January’s article, you produce methane every day of your life. It’s a common by-product of the bacterial metabolic processes that digest your food for you. Cows produce methane, your dog produces methane, it’s produced in abundance by teen-aged boys who find it a great source of humor. Everything that eats produces methane as a result of that eating. Methane production, in short, is a sign of LIFE.
So is it volcanism or life that’s producing the methane on Mars? Whatever the answer is, I’ll never be able to tell my Astronomy students again that Mars is a “dead world”. We still have working rovers on the surface of Mars and the satellites that orbit Mars see much more of the surface of that planet than the satellites of Earth see of home. So with all those robots to send us information, maybe next week or next year we’ll get our answer to the methane mystery on Mars.
Besides carbon, the other requirement for life-as-we-know-it is liquid water. Those underground bacteria could easily have access to water on Mars in the form of ice, which their internal heat could melt or there could be liquid water below the ice, just as there is in Antarctica.
Liquid water is a comparative rarity in the solar system. There’s literally tons of water, but most of it is ice. Until very recently the only place we knew of with lots of liquid water is Earth. But those roaming robots have shown us that there are at least three other places out there that probably have more liquid water than Earth; much more.
One of them is Europa, the ice moon that circles Jupiter. When we sent robots to orbit Jupiter, we discovered that Europa wobbles as it orbits Jupiter. You can tell if an egg is hard-boiled by spinning it. A hard-boiled egg is a solid body inside and it spins just like a top. A raw egg has liquid inside it and that liquid has two different densities. A raw egg won’t spin for very long; the sloshing liquid inside brings it to a quick halt.
The fact that Europa wobbles implies that there’s liquid inside. Since the surface of Europa is frozen water ice, the implication is that the inside of Europa is filled with liquid water with a rock core at the center. Europa probably has a liquid water ocean that’s around 20 miles deep. Europa is about the size of our moon and may contain more liquid water than the seas of Earth.
But the big boy of the ‘water’ scenario is Jupiter’s moon Ganymede. Ganymede is the largest moon in the solar system and it’s bigger than Mercury. Ganymede has its own magnetic field, which implies that it has an iron core like Earth and it also wobbles as it orbits Jupiter. If there is a liquid water ocean inside Ganymede, it could be as much as 300 MILES deep. Ganymede is probably the biggest reservoir of liquid water in the solar system.
But notice that we say ‘maybe’ and ‘probably’ and ‘could be’. We speculate about liquid water in the moons of Jupiter (and that also includes the last Galilean satellite, Callisto) but there is a moon in the solar system about which all speculation about a liquid ocean has ceased. And that’s Saturn’s moon, Enceladus.
We have a robot called Cassini in orbit around the planet Saturn and it’s discovered something very interesting about Enceladus, the brightest little moon in the solar system. We’ve known for a long time that its surface had to be made of something very reflective. The surface of Enceladus is solidly frozen water ice and it’s as reflective as new-fallen snow. And Enceladus also wobbles as it goes around Saturn.
Then Cassini found something really interesting. There were what the team called ‘tiger stripes’ at Cassini’s south pole. A closer look during one of Cassini’s scheduled fly-bys of Enceladus revealed these ‘tiger stripes’ were huge cracks in the ice.
Then they discovered something REALLY interesting. Liquid water is erupting from these cracks. We don’t have to infer from the wobble that Enceladus contains liquid water, we KNOW it does. And not only is it erupting liquid water, there are also organic chemicals in the water.
The ‘tiger stripes’ on Saturn’s ice moon Enceladus. (Credit: NASA/JPL/Space Science Institute)
Could microbial life exist inside Enceladus, where no sunlight reaches, photosynthesis is impossible and no oxygen is available? To answer that question, we need look no farther than our own planet to find examples of the types of exotic ecosystems that could make life possible on Saturn’s geyser moon. The answer appears to be, yes, it could be possible.
In recent years, we’ve found life on Earth that thrives in places where the sun doesn’t shine and there’s no oxygen because no photosynthesis takes place there. Microbes have been discovered that use the energy from chemical reactions between different kinds of minerals, and others that live off the energy from naturally-occurring radioactive decay.
There are at least three ecosystems found on Earth that harbor the kind of life that could survive in the sunless seas of Enceladus. Two are based on methanogens (hmmm, this is fancy Latin for ‘methane generator!), very primitive bacteria relatives that thrive in harsh environments without oxygen. Deep volcanic rocks along the Columbia River and in Idaho Falls host two of these ecosystems, which get their energy from chemical interactions. The third ecosystem is powered by the energy produced by natural radioactive decay, and was found deep below the surface in a mine in South Africa.
So the evidence points to the feasibility of life in Enceladus. But how would it get its start? A major problem in answering that question is that we don’t know how life originated on Earth, nor have we been able to reproduce Earth’s first spark of life in the laboratory. But here’s the good news: there are a lot of theories for how life originated on Earth. Now the question is — do they apply to Enceladus?
Two theories for the origin of life on Earth do seem to apply to Enceladus–the "primordial soup" theory and the “deep-sea vent” theory.
Primordial Soup Theory
Charles Darwin first proposed that life originated in a soup of organic material created from non-biological sources. The possibility that this is exactly what happened was demonstrated in a famous expe
riment in 1953 when the chemists Stanley L. Miller and Harold C. Urey cooked up a primordial soup of chemicals thought to have been present on early Earth before life began. A spark, simulating lightning, was passed through this highly reduced mixture of methane, ammonia, water vapor and hydrogen. Within two weeks, a few amino acids, some of the building blocks of life, had formed in the soup. While Miller and Urey did not actually create life, they demonstrated that very complex molecules like amino-acids could spontaneously assemble from simpler chemicals.
(The actual ‘gunk’ created in the Miller-Urey experiments has been recently examined using new techniques. This also included an experiment Dr. Miller performed that introduced steam into the experiment. In his 1953 paper, Dr. Miller had reported that he had detected five amino acids produced by the original apparatus. The recent examination found small amounts of nine additional amino acids in those samples. In the residues from the apparatus with the steam injector, the scientists detected 22 amino acids, including 10 that had never before been identified from the Miller-Urey experiment.)
So where did this organic soup come from? Here on Earth, it’s possible that all the ingredients were already in place. Another theory is that the right soup mix ingredients arrived as incoming comet material and interplanetary dust.
Deep Sea Vent Theory
The deep-sea vent theory for the origin of life on Earth might apply to Enceladus as well. In this scenario, life on Earth began at the interface where chemically rich fluids, heated by tidal or other mechanisms, erupted from the sea floor into the existing ocean. Chemical energy is derived when reduced gases, like hydrogen-sulfide and hydrogen emerge from the vent and contact an oxidant, like carbon dioxide. Hot spots like this could also occur on the sea floor of Enceladus.
We don’t know how long it takes for life to start when the ingredients are there and the environment is suitable, but it appears to have happened quickly on Earth. It could have happened just as quickly on Enceladus.
For life to persist once it has been established requires an environment of liquid water, the essential elements and nutrients, and an energy source. Enceladus has liquid water and both simple and complex organic chemicals. Some kind of energy source is producing the erupting geysers. Hmmmm, liquid water, organic chemicals and an energy source.
As a scientist said who studies the moon Europa: “Do you really think you can have a liquid water ocean for five billion years and NOT have life in it?”
Are we alone? More and more evidence is giving us the answer “Probably not!”
About two miles below the ground in a South African gold mine, scientist Duane Moser stands next to the fracture zone (white area) where he and Li-Hung Lin found bacteria that live in an ecosystem driven by radioactive decay with no oxygen, no light and no organic input. (Credit: Photo by Li-Hung Lin. Image courtesy of NASA)
Cruise on over to the Deep Website at www.thedeepradioshow.com to learn more about life in other places and many other topics. Enjoy!