Chapter 30
What is a biomarker? Give some possible examples of biomarkers we might look for beyond the solar system.
A biomarker is a feature—a chemical substance, a structure, or a signal—that could only have been formed by life. Beyond our solar system, we can only detect planet-scale biosignatures—biological impacts so great that they affect the way a planet appears in reflected or emitted electromagnetic radiation. An example of such an exoplanet biomarker would be unusual atmospheric composition, such as the mutual presence of methane and oxygen. While this would be a strong indication of life, it would not be unequivocal because methane and oxygen can be produced in the absence of life under special circumstances. Another possible example might be very short, very energetic pulses of visible light or infrared radiation or radio waves that are not just natural static, but are coded with information given off by huge structures in space built near or around stars. Both would be biomarkers of technologically advanced civilizations.
What is a habitable zone?
A habitable zone is the range of distances from a star where, if water existed on the surface of a planet, that water would likely be liquid.
Give a short history of the atoms that are now in your little finger, going back to the beginning of the universe.
All the hydrogen atoms in your little finger have been around since the universe first cooled enough for protons and electrons to get together into atoms. Elements heavier than hydrogen in your finger were fused in stars through a process called nucleosynthesis. Fusion of lighter elements inside the "furnace" of stars creates new heavier elements, which are then dispersed when stars explode or lose material more peacefully. The newly made elements eventually join the clouds of gas and dust between the stars. Out of these, new stars form and further heavier elements are created inside their furnace. Elements up to iron can be formed during normal fusion inside stars, and elements heavier than that are formed during the violence of supernova explosions. The atoms in your little finger were made available in this way to the cloud from which the solar system formed. They then became part of the planetesimals that collided with each other to form the proto-Earth. Then through twists and turns of planetary evolution and life's evolution on our own planet, the atoms found their way to your little finger. But they likely won't be there for long because a human lifetime is but a blink of the eye compared to the age of the universe.
What is the "cosmic haystack problem"? List as many of its components as you can think of.
Because so many factors go into detecting a signal from extraterrestrial intelligence, some astronomers have compared the effort to searching for a needle in a haystack. Some of the problem's components include the origin and direction of the signal containing the message from among all the possible directions one could "listen," the frequency chosen for that signal from among the vast range of potential frequencies in the electromagnetic spectrum, the frequency width of that signal, the strength of that signal compared with background noise, the continuity of that signal (whether it's on all the time, or only sweeps over us periodically), the frequency drift of that signal (caused by the relative motion of the sources to Earth), the system used for encoding any message in that signal, and whether we would recognize the nature of the message, when it was coded by alien minds.
Where in the solar system (and beyond) have scientists found evidence of organic molecules?
Beyond our solar system, organic molecules have been found in giant clouds of dust and gas between stars (the "interstellar medium") and in star-forming regions. In our solar system, besides Earth, organic molecules have been discovered on comets, in meteorites, on Saturn's moon Titan, in the plumes of water expelled from Saturn's moon Enceladus, and on Neptune's moon Triton.
Why are Mars and Europa the top targets for the study of astrobiology?
Five decades of observation of our neighbor world, Mars, strongly suggest that in the distant past it had an environment (thicker atmosphere, running surface water, perhaps even lakes) that could have sustained life on its surface. Even if such life no longer survives on Mars, its "fossils" might still be found on the red planet. Life could also exist on modern Mars just below the surface, where liquid water is thought to exist. Europa is a top target because of the high likelihood of an extensive salty ocean under the thick ice shell that covers this moon of Jupiter. This ocean, substantially deeper than Earth's ocean, is probably in contact with a rocky seabed and may be warmed by internal heat; thus the interaction of water and rocks could provide a chemical energy source for life.
Can you name five environmental conditions that, in their extremes, microbial life been challenged by and has learned to survive on Earth?
Five environmental conditions that microbial life has overcome are extreme temperature, pressure, salinity, acidity, and radiation.
Why is traveling between the stars (by creatures like us) difficult?
Interstellar travel is difficult for many reasons. The first is certainly the vast distances between the stars. Even at speeds very close to the speed of light, the maximum theoretical speed achievable, it would require four years or more to travel between stars. At more realistic speeds, trips would take far longer than a human lifetime. And the faster you go, the more expensive (in fuel costs) the trip would be. Since we can't depend on fuel being available at our destination, such travel would require carrying all the fuel necessary for both the trip there and the return trip and require accelerating all that fuel to tremendous speeds—a truly gargantuan effort, and an extraordinarily expensive one. To be sure, those issues only come up if creatures like us are along on the trip. Travel by machines (such as robots, computers, or smartphones) could proceed much more slowly and less expensively. As this book went to press, a billionaire in Silicon Valley gave $100 million to a project to find technology that could get a very tiny probe to the nearest star using laser propulsion. See: Project Breakthrough Star-shot: https://breakthroughinitiatives.org/News/4
What are two characteristic properties of life that distinguish it from nonliving things?
Life extracts energy from its environment and has a means of encoding and replicating information in order to make faithful copies of itself.
Why is the simultaneous detection of methane and oxygen in an atmosphere a good indication of the existence of a biosphere on that planet?
Oxygen and methane chemically react with each other, so we would not see them together unless there are active sources for both. At least on Earth, biology is responsible for essentially all the oxygen and the majority of the methane in our atmosphere.
What are the advantages to using radio waves for communication between civilizations that live around different stars? List as many as you can.
Radio waves travel at the speed of light, are cheap to produce (they are the lowest-energy electromagnetic waves), are not significantly absorbed by interstellar clouds, go right through planetary atmospheres, and, most importantly, can be modulated in a way that carries information.
What is the Copernican principle? Make a list of scientific discoveries that confirm it.
The Copernican principle is the idea that Earth and the Sun are in no way specially favored bodies in the universe. Several discoveries confirm this, including (in order of discovery) the following: Earth orbits the Sun and is not the center of our solar system, our Sun is one among billions of other stars in the Milky Way Galaxy and is not in any central position within the Galaxy, our Galaxy is one among billions of other galaxies in the universe, and planets are commonly found orbiting other stars. You could also discuss that the elements that make up most of Earth and the Sun are commonly found in other stars and other planets.
What are the three requirements that scientists believe an environment needs to supply life with in order to be considered habitable?
The requirements are a solvent (water may be the best example), the biogenic elements (CHNOPS) in biologically accessible form, and energy.
