Many of us have wondered whether or not there could be other living things out there in the universe. Do you think we could find livings things on another planet, on a moon, or maybe even somewhere else? And, maybe a more important question, how might we find that life if it exists?
The first and maybe simplest answer would be to just look with a good camera or even our eyes and if we see a big alien with legs walking about or one with wings flying around, then we know we’ve found it. But what if the life on another planet is all microbial and not easy to find? Or, what if life on another world has now gone extinct? We can still use the tools of science to try to figure out if there are or were living things there.
Often in our daily lives we can find signs of life and know that a plant or an animal was present without really seeing it. A bird’s feather, some dog poop, dandelion seeds flying through the air, and a person’s footprints leftover in some mud are all signs of living things. Being able to find such signs is so important for astrobiology that we even have a word to describe them: we call these kinds of things “biosignatures”. A biosignature is any characteristic, element, molecule, substance, or feature that can be used as evidence for past or present life. It also needs to be something that can’t be made without the presence of life. It can be something like a leaf or a feather, but could also be fossils stored away in the rocks, organic molecules made by life, and even differences in the chemistry of an atmosphere or a body of water.
A morphological biosignature is one that we can tell was made from life based on its shape and size. The word “morphology” comes from the Ancient Greek words “morphé” (for “form”) and “lógos” (for “study” or “research”). So, morphology is really about studying the form of living things, and a morphological biosignature is one that’s a formation or structure leftover from living things. For instance, microorganisms living together in goopy microbial mats in shallow waters can create layered structures of minerals called stromatolites. There are microbial mats that are doing this today and there are fossilized versions of microbial mats from billions of years ago. If we were to go exploring on another world like Mars and found layered minerals from stromatolites in a rock, then that might be a biosignature. There are a bunch of morphological biosignatures that we might consider, including various types of fossils, etchings or layering in rocks, and even direct observations of cells or active living things.
Chemical biosignatures include a huge range of possible ways that life can leave its mark within the chemistry of rocks, bodies of water, and even atmospheres. For instance, biological macromolecules such as lipids, carbohydrates, nucleic acids, and proteins might all be used as biosignatures. Looking on alien worlds for DNA and RNA might be a little too Earth-centric, since we’re really not sure if alien life would use the same information storage molecules as us, but if we find those or other nucleic acids on an alien worlds then they could be important for us to consider as possible signs of life. We could also look for the kinds of lipids (fats) that are used to make up the membranes of living cells. On Earth, leftover lipids from life that existed long ago has allowed some scientists to piece together the kinds of organisms that were alive in certain places long before we humans were around.
It turns out that many molecules have two or more versions, based on how the chemical bonds form inside of them. We call these versions “chiral”. Chiral comes from the Greek word for “hand”. This is because chiral molecules are mirror images of each other, just like your hands. Your left hand and right hand are mirror images of each other; they have the same shape and structure, but if you lay your left hand on top of your right hand you can see that they are obviously different (the thumbs stick out in opposite directions). Our studies of non-living materials both on Earth and from asteroids and comets shows us that non-living things tend to have an equal mix of left-handed and right-handed chiral molecules. But all living things on our planet prefer one or the other when making biological molecules. Inside of your cells, all of the amino acids in your proteins are of the left-handed form (we call them “L” form, from the Latin word “Laevo” (on the left)), while all of your simple sugars inside of your body are of the right-handed form (we call them “D” form, from the Latin word “Dexter” (on the right)). Although life here on Earth is our only example of life so far, it’s possible that alien life would also pick either the L or the D form for its organic molecules, and this is something we can search for as a chemical biosignature.
Another chemical biosignature is the ratio of isotopes of chemical elements. An isotope of an element is the same element, but with a different number of neutrons in the nucleus. For instance, carbon-12 and carbon-14 are both carbon atoms, but carbon-12 has 6 neutrons and carbon-14 has 8 neutrons. It turns out that life as we know it really prefers using lighter isotopes of chemical elements (those with less neutrons). When an organism is catalyzing a chemical reaction, more energy can be used for metabolism and growth if the organism uses molecules in the reaction that have lighter isotopes. We’ve discovered that we can use measurements of the ratios of the lighter isotopes to the heavier isotopes within samples from nature as biosignatures.
What if we can’t get to the surface of a planet or moon to study the rocks there? For instance, what kinds of biosignatures might we look for on exoplanets? Using our telescopes on the Earth and in orbit of our planet, we can now look at the atmospheres of exoplanets to see what kinds of gas molecules are abundant there. One thing we look for, are gases that might be signs of life. For instance, early in our planet’s history our atmosphere didn’t have any of the oxygen that we breathe as O2\. Just how plants breathe in CO2 and breathe out O2 today, living things on Earth breathing out O2 are responsible for all of that oxygen we have in our atmosphere. So, it’s possible that oxygen or another gas used by life in an atmosphere might be a biosignature.
What if there are other forms of life out there in the universe that have developed civilizations and technology? The same way that we’ve been beaming our television and radio waves into space for over a century, maybe some intelligent life out there has been transmitting messages into space as well. If we were to receive these messages with our radio telescopes we might be able to use them as a type of biosignature called a technosignature (one that shows us that technological life made it). Looking for such technosignatures is part of something called SETI, or the Search for Extraterrestrial Intelligence. Not only does work in SETI involve listening for radio waves, but also considers many other ways that an intelligent alien civilization may be detectable. That includes looking for gas molecules in an exoplanet atmosphere that indicate industrial activity, looking for emissions of light from traveling spacecraft, and looking for signs that a civilization is advancing to a point where they are consuming energy from stars. That might sound like science fiction, but some scientists are considering how we might observe giant arrays of solar panels or other, perhaps way more advance, technologies that are collecting energy from a star.
Our continued efforts to study life here on Earth and learn about the kinds of biosignatures that life leaves behind will help us as we keep asking ourselves if we’re alone in the universe and whether or not we might find signs of alien life. Finding stromatolites in ancient rocks on Mars, measuring isotope ratios that signify life in a lake on the moon Titan, detecting what could be biological activity in gas compositions on exoplanets, or receiving a message from an extraterrestrial civilization would be monumental discoveries for all of us. However, if we do find potential biosignatures from alien life, we’ll certainly want to do the best job we can in making sure it’s a real biosignature and not something created abiotically.
Disciplinary Core Ideas
ESS1.A: The Universe and Its Stars: The star called the Sun is changing and will burn out over a lifespan of approximately 10 billion years. (HS-ESS1-1) ▪ The study of stars’ light spectra and brightness is used to identify compositional elements of stars, their movements, and their distances from Earth. (HS-ESS1-2, HS-ESS1-3)
ESS2.E: Biogeology: The many dynamic and delicate feedbacks between the biosphere and other Earth systems cause a continual coevolution of Earth’s surface and the life that exists on it. (HS-ESS2-7)
LS4.C: Adaptation: Changes in the physical environment, whether naturally occurring or human induced, have thus contributed to the expansion of some species, the emergence of new distinct species as populations diverge under different conditions, and the decline – and sometimes the extinction – of some species. (HS-LS4-6)
PS1.A: Structure and Properties of Matter: Each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons. (HS-PS1-1) ▪ The periodic table orders elements horizontally by the number of protons in the atom’s nucleus and places those with similar chemical properties in columns. The repeating patterns of this table reflect patterns of outer electron states. (HS-PS1-1) ▪ The structure and interactions of matter at the bulk scale are determined by electrical forces within and between atoms. (HS-PS1-3)
PS1.B: Chemical Reactions: Chemical processes, their rates, and whether or not energy is stored or released can be understood in terms of the collisions of molecules and the rearrangements of atoms into new molecules, with consequent changes in the sum of all bond energies in the set of molecules that are matched by changes in kinetic energy. (HS-PS1-4, HS-PS1-5) In many situations, a dynamic and condition-dependent balance between a reaction and the reverse reaction determines the numbers of all types of molecules present. (HS-PS1-6) The fact that atoms are conserved, together with knowledge of the chemical properties of the elements involved, can be used to describe and predict chemical reactions. (HS-PS1-2, HS-PS1-7)
PS3.A: Definitions of Energy: Energy is a quantitative property of a system that depends on the motion and interactions of matter and radiation within that system. That there is a single quantity called energy is due to the fact that a system’s total energy is conserved, even as, within the system, energy is continually transferred from one object to another and between its various possible forms. (HS-PS3-1, HS-PS3-2) ▪ At the macroscopic scale, energy manifests itself in multiple ways, such as in motion, sound, light, and thermal energy. (HS-PS3-2, HS-PS3-3) These relationships are better understood at the microscopic scale, at which all of the different manifestations of energy can be modeled as a combination of energy associated with the motion of particles and energy associated with the configuration (relative position of the particles). In some cases, the relative position energy can be thought of as stored in fields (which mediate interactions between particles). This last concept includes radiation, a phenomenon in which energy stored in fields moves across space. (HS-PS3-2)
LS1.A: Structure and Function: Systems of specialized cells within organisms help them perform the essential functions of life. (HS-LS1-1) ▪ All cells contain genetic information in the form of DNA molecules. Genes are regions in the DNA that contain the instructions that code for the formation of proteins, which carry out most of the work of cells. (HS-LS1-1) ▪ Multicellular organisms have a hierarchical structural organization, in which any one system is made up of numerous parts and is itself a component of the next level. (HS-LS1-2)
LS1.C: Organization for Matter and Energy Flow in Organisms: The process of photosynthesis converts light energy to stored chemical energy by converting carbon dioxide plus water into sugars plus released oxygen. (HS-LS1-5) ▪ The sugar molecules thus formed contain carbon, hydrogen, and oxygen: their hydrocarbon backbones are used to make amino acids and other carbon-based molecules that can be assembled into larger molecules (such as proteins or DNA), used for example to form new cells. (HS-LS1-6)
LS3.A: Inheritance of Traits: Each chromosome consists of a single very long DNA molecule, and each gene on the chromosome is a particular segment of that DNA. The instructions for forming species’ characteristics are carried in DNA. (HS-LS3-1)
LS4.A: Evidence of Common Ancestry and Diversity: Genetic information provides evidence of evolution. DNA sequences vary among species, but there are many overlaps; in fact, the ongoing branching that produces multiple lines of descent can be inferred by comparing the DNA sequences of different organisms. Such information is also derivable from the similarities and differences in amino acid sequences and from anatomical and embryological evidence. (HS-LS4-1)
ESS1.C: The History of Planet Earth: Continental rocks, which can be older than 4 billion years, are generally much older than the rocks of the ocean floor, which are less than 200 million years old. (HS-ESS1-5) Although active geologic processes, such as plate tectonics and erosion, have destroyed or altered most of the very early rock record on Earth, other objects in the solar system, such as lunar rocks, asteroids, and meteorites, have changed little over billions of years. Studying these objects can provide information about Earth’s formation and early history. (HS-ESS1-6)
Patterns: Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena. (HS-PS1-2, HS-PS1-5)
5-12 Astrobiology Graphic Histories. Issue 1: The Origin of Science. These astrobiology related graphic books are ingenious and artfully created to tell the story of astrobiology in a whole new way. The complete series illustrates the backbone of astrobiology from extremophiles, to exploration within and beyond the solar system. This issue traces the roots of Astrobiology in the search for life beyond Earth. NASA. https://astrobiology.nasa.gov/resources/graphic-histories/
6-12 Astrobiology Math. This collection of math problems provides an authentic glimpse of modern astrobiology science and engineering issues, often involving actual research data. Students explore concepts in astrobiology through calculations. Relevant topics include Organic Molecules Detected on a Distant Planet (page 93) and Estimating the Temperatures of Exoplanets (page 101). NASA. https://www.nasa.gov/pdf/637832main_Astrobiology_Math.pdf
9-12 SpaceMath Problem 349: Exoplanet Orbits and the Properties of Ellipses. Given the formula for the orbits of newly-discovered planets, students determine the basic properties of the elliptical orbits for the planets. [Topics: properties of ellipses] https://spacemath.gsfc.nasa.gov/astrob/7Page13.pdf
9-12 SpaceMath Problem 338: Asteroids and Ice. Students calculate how much ice may be present on the asteroid 24-Themis based on recent discoveries by NASA [Topics: mass=density x volume; volume of a spherical shell] https://spacemath.gsfc.nasa.gov/astrob/6Page154.pdf
9-12 Second Genesis: The Quest for Life Beyond Earth. Second Genesis is a 20 minute digital short that follows planetary scientist Carolyn Porco as she explores what it takes to look for life beyond Earth, and what conditions are required for life to exist. This short offers explanations and examples of chemical and physical signatures of life (or biosignatures). PBS. http://www.pbs.org/the-farthest/second-genesis/
9-12 Mission: Find Life. Extreme Biosignatures with Niki Parenteau. These videos clips (8) are from The Mission: Find Life! exhibit at the Pacific Science Center in Seattle, WA. They show how astrobiologists search for life elsewhere in the Universe, studying extreme environments to understand the potential habitability of extraterrestrial environments, and examining how life might arise on planets orbiting stars different from our Sun. The exhibit features research at the Virtual Planetary Laboratory and ran March 18-September 4, 2017. VPL. https://www.youtube.com/watch?v=fsSnLthHqPY&list=PLaKWGoQCqpVDiJl9NBwJ4E7Nwf3tn-yzB&index=3