Distant Encounters
The search for extraterrestrial life begins with understanding life on Earth.
Evidence supporting the possibility of finding life in our solar system is growing, due, in part, to discoveries made on Earth. Spacecraft and landers are gathering information about the material around and on planets, moons and asteroids, revealing that the building blocks of life are scattered throughout the solar system. So, it’s not surprising that our home planet is contributing important data.
NASA astrobiologists rely on all that data to do their job. Mary Voytek, the senior scientist and head of the Astrobiology Program in the Science Mission Directorate at NASA Headquarters in Washington, DC, says the search for life is at the core of NASA’s work.
“The law that was enacted to form NASA was revised in 2017 to include as one of the goals to understand the origin, evolution and distribution of life in the universe,” she explains. “So, we focus on understanding how life emerged here on Earth and what steps led to that in order to understand if it could have arisen somewhere else.”
Life can and does exist in the absence of sunlight.
This understanding helps scientists identify the conditions that can support life and where they might exist now. The first challenge is that there is no single scientific definition for life. Voytek jokes that, if asked, 100 scientists will come up with 110 different definitions of life. So, NASA focuses on what can be measured: biosignatures – single or combined features are indicative of life.
“What we mean by ‘life’ is a system where you can have changes that are passed down through generations resulting in an adaptation to something in the environment,” says Voytek.
By focusing on biosignatures, it’s possible to develop the necessary scientific instruments to look for and collect relevant data. Living beings from bacteria to humans metabolize nutrients, reproduce, and change over time, which requires energy, specific elements, and liquid water. Those basic elements are CHNOPS – carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. It was thought that the Sun was the sole source of energy for ecosystems to function effectively until the discovery of new life forms diversified the list of requirements.
"...at hydrothermal systems here on Earth, there’s enough energy to drive bacterial metabolism and drive the reactions that could have led to the emergence of life."
In 2000, a new type of deep-sea hydrothermal vent in the North Atlantic revealed life can and does exist in the absence of sunlight. Hydrothermal vents, cracks along the ocean floor that vent water heated by Earth’s geothermal energy, have unique marine ecosystems. The water can reach temperatures of 750 degrees Fahrenheit or more, which results in chemical reactions that remove oxygen and chemicals from the water. This makes the water more acidic, which then causes the surrounding rocks to leach metals.
The water/rock interaction and chemical energy provide everything some species of marine life need to thrive. Called extremophiles, these organisms exist in environmental conditions considered extreme compared to those of human beings.
Hydrothermal vents are found in highly volcanic regions on the sea floor. Some deep-sea vents generate enough chemical energy to drive bacterial metabolism, letting them support ecosystems that don’t rely on the Sun. Evidence of this kind of water/rock interaction found on the icy moon of Enceladus indicates its subsurface ocean might have an environment that could support life. (Image Credit: P. Rona/OAR/National Undersea Research Program (NURP); NOAA)
“We know at hydrothermal systems here on Earth, there’s enough energy to drive bacterial metabolism and drive the reactions that could have led to the emergence of life,” explains Voytek. Knowing that organisms also live and thrive many degrees below freezing tells researchers that places where ocean temperatures are close to freezing could support life, she says.
Data gathered by the Cassini spacecraft provided evidence that life might exist in water beneath the icy surface of Saturn’s moon Enceladus. Plumes of frozen water were observed being ejected into space from the southern surface of the moon. Cassini flew directly into the plumes and tested the samples it collected. The craft’s mass spectrometer – a machine that identifies the molecules and their isotopes in a sample by their mass and electrical charge – identified carbon and nitrogen in the plumes. However, this particular mass spectrometer wasn’t designed to run the complicated chemical analysis required to get more detailed information. In late 2019, a study of the data revealed that the plumes contain the sort of nitrogen – and oxygen – bearing organic compounds that help form amino acids, which are building blocks of life on Earth.
This evidence, combined with what scientists know about chemistry, is enough to identify Enceladus as a viable location to visit with more sophisticated life-detecting experiments.
“We are pretty certain there’s water below the ice shell, and it’s pretty clear from the surface that there’s exchange between the surface and the water below,” says Vo"ytek. “We see silicates, which are evidence of a hydrothermal process similar to that on Earth.”
Another ice-covered moon that appears to eject plumes of subsurface water into space is Europa, a moon of Jupiter. While it seems likely that a subsurface ocean does exist and could support life (See Exploring Icy Moons), scientists require direct evidence to know for sure. The technology needed to autonomously drill through miles of ice to obtain samples from such an ocean is in development. Until then, one way to get comprehensive information about the content of the plumes is to design a mission with existing technology.
"Extraordinary claims require extraordinary evidence."
NASA is exploring the possibility of using a swarm of small satellites known as CubeSats to fly through Europa’s plumes and analyze the vapor multiple times. To carry the little satellites to the Jovian moon, NASA’s Space Technology Mission Directorate is funding the study of a mission concept called SPEAR, the Swarm-Probe Enabling ATEG (advanced thermoelectric generator) Reactor. Nuclear electric propulsion would allow a lighter, smaller and less expensive spacecraft to transport the CubeSats. While more sophisticated instruments are being designed specifically to enable the Europa Clipper spacecraft – scheduled to launch in 2024 – examine the contents of Europa’s plumes, the CubeSat swarm could add to the current pool of data.
But more than one of Saturn’s moons might help answer the question of whether Earth is the only place in the solar system where life exists. Scientists believe that organic material found on Titan, the planet’s largest moon, could be from microbes that evolved to use the liquid methane found on the surface instead of water to enable metabolic function (See Dragon Flying).
“The bottom line in all this is a quote from Carl Sagan that we go back to all the time,” says Voytek. “‘Extraordinary claims require extraordinary evidence.’ There are a lot of things that hint at life, but confirming that it’s actually life is going to take extraordinary evidence.”
Life as we know it requires three ingredients: energy, organic molecules, and liquid water. Astrobiology, our search for life beyond Earth, is a search for planets, dwarf planets, and moons that harbor substantial liquid water. We call these places “ocean worlds.” We’re learning that ocean worlds could be ubiquitous in the galaxy. (Video Credit: NASA)
To ensure that such evidence is valid, NASA relies on planetary protection guidelines. These are designed to ensure that contamination from terrestrial life doesn’t interfere with the search for extraterrestrial life. While avoiding contamination of experiments and samples returning to Earth is critical, protecting other planetary bodies and their potential inhabitants from human activity still needs to be added to the standards.
A 2019 review of the protection requirements drafted in the 1950s also recommended incorporating scientific standards developed over the course of 60 years that now serve as best practices. This would allow NASA to provide the leadership necessary to ensure government and private sector activities are carried out professionally and ethically.
“We want to know if we’re alone or not. There are intellectual, spiritual, and scientific reasons to want to know if life exists elsewhere,” says Voytek. “Finding life elsewhere will ultimately lead to a better understanding of ourselves and the biology here on Earth.”
Originally published in Technology Innovations (2021) by NASA