Like Asking an Alien for a Signature: A New Way to Search for Life Beyond Earth
Long before a wrinkled creature from a distant civilization looks at us with sad, intelligent eyes and asks to be taken home, life beyond Earth will likely be revealed to us in a handful of molecules sampled from an ancient rock on Mars, a speck of ice from one of Jupiter’s or Saturn’s moons, or an underground ocean flowing beneath the frozen surface of another celestial body. In a study published in the scientific journal Nature Astronomy, an Israeli-American research team led by scientists from the Weizmann Institute of Science proposes a new kind of fingerprint for the existence of such life.
The new method is expected to offer a relatively simple way to address one of the oldest questions, are we alone? For many decades, scientists have searched for biosignatures that indicate the existence of life beyond Earth, meaning chemical or physical traces that serve as a kind of fingerprint of life out there and make it possible to distinguish biological material from nonbiological chemical material. Some methods focus on the ratios between “right-handed” and “left-handed” molecules, a chemical property called chirality, while others examine the ratios between different isotopes. 4 View gallery The Europa Clipper probe above Europa, with Jupiter in the background on the left (illustration: NASA/JPL-Caltech)
However, deciphering these signs usually requires knowing how the sample formed and how it changed over time, information that is often unavailable. In addition, spacecraft cannot carry every instrument scientists would like to send into space, and samples originating outside Earth are almost never clean or intact. It is also important to remember that radiation changes molecules, geological processes can resemble biological ones, organic matter can break down, mix or become contaminated over time, and perhaps the biggest challenge of all, identifying organic material does not necessarily indicate the existence of life, since amino acids and other compounds can also form through chemical processes that have nothing to do with life.
“The main advantage of our approach is that it provides a simple way to identify organic material of biological origin, as opposed to nonbiological organic material formed in the early Solar System,” said Prof. Itai Halevy, who headed the research team together with Prof. Yoav Kaspi, both from the Department of Earth and Planetary Sciences at the Weizmann Institute of Science. The study was led by Dr. Gideon Yoffe, a postdoctoral researcher in Prof. Kaspi’s lab, who combined tools from statistics, ecology and planetary science. The team also included Dr. Fabian Klenner of the University of California, Riverside, and Dr. Barak Sober of the Hebrew University of Jerusalem.
“Many methods currently used in the search for life beyond Earth are limited because they require complex processing of organic material or unique analytical methods that cannot currently be carried out in space,” says Dr. Yoffe. The new approach bypasses many of these limitations because it relies less on complex chemistry and more on statistical patterns. The method originated in ecological research, where it was developed to characterize animal diversity in different habitats. Dr. Yoffe, whose training is in statistics and data science, adapted it to astrobiology, the field of research focused on the search for life beyond Earth. 4 View gallery From right, Dr. Gideon Yoffe, Prof. Itai Halevy and Prof. Yoav Kaspi (photo: Weizmann Institute of Science)
The researchers’ central idea was to examine molecular diversity, based on the understanding that life reorganizes chemistry according to its needs, sometimes increasing diversity and sometimes reducing it. Instead of focusing on individual molecules, the researchers examined statistical patterns in groups of molecules, their degree of dispersion, the relative amount of each one, and how dominant or rare they are. To test the method, the team analyzed more than one hundred organic and inorganic samples, including 3-billion-year-old rocks from Earth, dinosaur eggshells, preserved dinosaur feathers in amber, and samples collected in space from the asteroids Ryugu and Bennu.
The study began with amino acids, the building blocks of proteins. These acids can also form outside a biological context, as a result of collisions between simpler molecules. But in space, such encounters are especially rare, so the likelihood of spontaneous formation of complex amino acids is low. Therefore, in nonbiological chemical systems, simple amino acids tend to be more common, while larger and more complex amino acids are rarer. Life, by contrast, behaves differently. Living systems survive thanks to their ability to produce the molecules they need for function and survival, even when their energetic production cost is high. As a result, instead of a random molecular collection, biological systems leave behind patterns that are not necessarily controlled by the simplest building blocks. Consequently, samples originating from living organisms are repeatedly found to be more diverse in molecular composition than samples with no biological origin. This difference is not limited to amino acids alone, the researchers also identified it in fatty acids, indicating that this is a fundamental biological signature.
“Life produces the building blocks it needs to function,” summarizes Prof. Halevy. 4 View gallery Searching for life in space excites many people. The surface of Europa (photo: NASA / JPL-Caltech)
The method developed by the researchers was born as part of an Israeli proposal for a future space mission called Eureka. Prof. Kaspi, Prof. Halevy, Dr. Yoffe and their partners are developing the mission proposal together with Israel Aerospace Industries, with the aim of launching a small spacecraft to one or two of the Solar System’s icy moons, most likely Europa, and perhaps Enceladus as well, beneath whose ice shells vast oceans are hidden. The IAI Space Division, part of Israel Aerospace Industries, is participating in the mission planning and is leading the spacecraft design.
“These subsurface oceans are especially interesting, because conditions there may allow life to form,” explains Prof. Kaspi. Future missions may sample material originating from those oceans, including molecules formed near underwater hydrothermal vents similar to those known on Earth. “Our approach does not require especially sophisticated equipment and is easy to implement. All we need is a way to measure the relative amounts of the different molecules, for example using a mass spectrometer.”
Even so, for some of us it may sound like science fiction: to obtain the data on molecular diversity, the scientists propose firing a laser beam from the spacecraft toward the ice of another world and waiting for the molecules in it to reflect light. In those reflections, the scientists will look for a fingerprint of life, with emphasis on complex amino acids and other compounds. One of the major advantages of the method is that it can also work with samples that have been through a lot along the way, material altered by heat or radiation, or damaged by time or cold.
“Space is a tough neighborhood, especially in the Jupiter system, where its powerful magnetic field causes energetic particles to constantly bombard the surfaces of its moons,” explains Dr. Yoffe. “Beyond the significant scientific contribution and the chance to discover life beyond Earth, for us the space mission to the icy moons of Jupiter and Saturn is also an educational inspiration for the next generation of scientists and engineers in Israel,” say officials at Israel Aerospace Industries. “We are certain that every boy and girl who watches the spacecraft and follows its journey will also want to discover the universe and become those who lead Israel to the next breakthroughs.”
4 View gallery Europa (photo: Kevin M. Gill/NASA/JPL-Caltech/SwRI/AP)
The new method is not limited to icy moons. It could also be applied to meteorites, to material originating from asteroids, or to samples from ancient rocks on Mars. In a sense, it connects many branches of space science: it could be used to analyze data from telescopes scanning the atmospheres of distant worlds, samples brought back by spacecraft from asteroids and comets, meteorites that reached Earth, or drill cores taken by robotic space vehicles from ancient rocks.
Discovering life beyond Earth could change the way we imagine the “first contact” with extraterrestrials. Perhaps no voice will come from the sky and no one will greet us or ask to be taken to our leader, at least not at first. The first encounter is expected to be silent and to take place inside a data set, in the patterns hidden within a collection of molecules. Even so, it will be no less dramatic.
“Ever since I was a child, I have been fascinated by everything related to the search for life beyond Earth,” says Dr. Yoffe. “In my view, such a discovery would be one of the most exciting scientific discoveries ever.”
The article was first published in Masa HaKesem HaMada'i, the Weizmann Institute of Science.
