Water drives NASA’s solar-system exploration program. Wherever there’s a chance water could exist, either as liquid or ice — like on Mars, Jupiter’s moon Europa, and Saturn’s moon Enceladus — future exploration will be a focus for scientists.
This works for planetary-exploration missions and for extrasolar-planet telescopes like Kepler. For many years, planetary scientists and biologists have held fast to one maxim: Water is essential for life. Great interest focuses on the possibility of extraterrestrial life, even if it’s merely microbial. So, follow the water to find where the big planetary-exploration dollars will be spent.
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The seed of finding extrasolar life where there’s water goes back to Harvard biologist Lawrence Henderson, who wrote in his 1913 work The Fitness of the Environment, “Life necessarily must be based on carbon and water, and have its higher forms metabolizing free oxygen.” For decades, scientists took this statement as gospel, but it was questioned in the 1970s. Carl Sagan wrote he doubted it “because Lawrence Henderson was made of carbon and water and metabolized free oxygen. Henderson had a vested interest.”
Life in the universe, astronomers and biologists now admit, could be based on completely different chemical systems than ours. The first thing to do is to define what we mean by “life.”
By definition, living things have several properties that separate them from rocks and dirt. Living things on Earth are arranged into cells. They are also highly organized, on different levels and with different tasks. They take in energy from the environment and excrete waste products. They exhibit homeostasis, stable internal conditions that are required to stay alive. They grow and change, showing differentiation and mutation. They also reproduce, passing genetic material to their descendants. Anything in the universe that exhibits these characteristics would be considered a living being.
The movement of molecules plays a key part in making a place hospitable for life. A flowing solvent can trigger molecule movement and, thus, the energy reactions life requires to take in energy and excrete waste. On Earth, that solvent is water. But water may not be the molecule-moving solvent everywhere in the universe. Clues to alternative environments have come from strange places as well as from our own familiar planet. On Earth, microbes can remain alive in the absence of water nearly indefinitely. They go dormant, take in no energy, and do not reproduce or grow. Yet microbes can revive and become active again at a much later date.
If liquids other than water might help create and sustain life, which ones might they be? Water does have an amazing number of properties that help support life. But could another fluid — ammonia, methane, formamide, or sulfuric acid — create a place where exotic life forms could flourish?
At Saturn’s large moon Titan, the Huygens probe discovered signs of abundant liquid methane that acts as Titan’s “water.” It also found abundant ethane. There is no evidence of life on Titan, and Huygens was not designed to detect it; however, some scientists speculate self-replicating molecules might exist in Titan’s methane-rich environment or may have even produced some of it.
Is water needed for life, or could other solvents do? That’s the key question, but it ties in to another, even larger one: Does extraterrestrial life exist in the universe? With at least 200 billion stars in our galaxy and 125 billion galaxies in the universe, life has plenty of places to gain a foothold. There are also plenty of opportunities for life to flourish based on solvents other than water.