If it moves, it’s probably alive: Searching for life on other planets

The search for extraterrestrial life has always been a key motivator of space exploration. But if we were to search Mars, Titan, or the subsurface oceans of Europa or Enceladus, it seems like all we can reasonably hope to find is extremophile microbes. And microbes, just a few microns long and wide, will be difficult to identify if we’re relying on robots working with limited human supervision and without all the fancy life-detecting gear we have here on Earth.

To solve that problem, a team of German researchers at the Technical University in Berlin figured that, instead of having a robot looking for microbes, it would be easier and cheaper to make the microbes come to the robot. The only ingredient they were lacking was the right bait.

Looking for movement

Most ideas we have for life detection on space mission rely on looking for chemical traces of life, such as various metabolites. Most recent missions, the Perseverance rover included, weren’t equipped with any specialized life-detecting instruments. “On Mars, the focus was on looking for signs of possible ancient life—fossils or other traces of microbes,” says Max Riekeles, an astrobiologist at the Technical University Berlin. “The last real in-situ life detection missions were performed by Viking landers, which is quite a while back already,”

We didn’t fit more advanced instruments that could reliably look at chemical biosignatures of microbes living on Mars on the most recent mission because such instruments would add too much mass, boost energy consumption, and require additional computing power. So, Riekeles and his colleagues suggested a much simpler and lighter life detection system based on the most obvious biosignature of them all: motility. When you see something move on its own, you can tell it’s alive, right?

But how do you get an alien microbe moving? From previous research, Riekeles knew most microbes, even those living in extreme environments, are attracted to L-serine, an amino acid used by organisms on Earth to build proteins. The microbes sense the presence of L-serine in their surroundings and move toward it, a behavior known as chemotaxis. “Also, there seems to be evidence L-serine was found outside of Earth, and it was present in the Martian environment,” Riekeles said.