Seeking Signs of Life from Suitably Sustained Microbes in Subsurface Environments

In September 2023, The Kavli Foundation and the Laukien Science Foundation announced a new fellowship program to support scholars in the Harvard Origins of Life Initiative. Through their research, Kavli-Laukien Postdoctoral Fellows are exploring how initial conditions on planets may enable life to emerge and subsequently evolve.

Kavli-Laukien Fellow Peter Higgins is studying the potential for subsurface environments on rocky worlds or on icy Solar System moons to stay habitable long enough for life to emerge, propagate and evolve, or even sustain sufficient biological activity to generate detectable signatures.

Even from space, Earth’s habitability is plain to see. Vast green expanses on the landmasses reveal the presence of photosynthetic organisms, while even broader stretches of blue ocean indicate a key ingredient for life, water, is amply available.

The other worlds of our Solar System offer less promising visages. None boast surface water, nor display any signs of macroscopic plant or animal life. The best bet, then, for alien life right here in our cosmic backyard is some combination of small and secret. For instance, perhaps microscopic creatures are eking it out in Martian groundwaters or Titan’s hydrocarbon lakes, or lay concealed under the icy shells of Titan, Europa, and Enceladus in these moons’ theorized internal oceans.

With surface exploration continuing on Mars, plus an exciting fresh slate of robotic explorers including the Jupiter Icy Moons Explorer (JUICE), the Europa Clipper, and the Titan-bound Dragonfly slated to reach their astrobiologically promising targets in the early 2030s, the race is on to better understand life’s possibilities in these extraterrestrial climes.

Peter Higgins is dedicated to this cause. He arrived at Harvard University as a Kavli-Laukien fellow in September 2024 to pursue a project assessing where habitable conditions might occur in the subsurfaces of rocky and icy worlds, how long such conditions might persist, and how we might infer the presence of microbial life.

“The key question that underpins what I'm working on is simply, ‘Where can life take hold?’” says Higgins.

To this end, Higgins will be using a model called NutMEG—standing for “nutrients, maintenance, energy and growth”—that he began developing as a PhD student at the University of Edinburgh. With the NutMEG model, Higgins is working off of baseline data from studies of Earthly microbial life and habitable, subsurface environments to venture into the alien territory of rocky worlds like Mars and icy moons. By tweaking a range of biological, chemical, and geological parameters, the model can compare and predict overall suitability for life in a wide variety of places.

“The model provides us with a mechanism to test hypotheses about environments elsewhere that may be slightly different from the natural settings we see on Earth, but whose chemistry may be compatible with Earth-like biology,” says Higgins.

In this way, NutMEG can churn out estimates for biomass levels, meaning the anticipated amount of life that a given environment could realistically support over time if conditions remain stable. “With NutMEG, we can push the boundaries,” says Higgins. “We can slightly perturb the environment and slightly perturb the biology and see how things change. And that can reveal new places to think about and hopefully explore someday.”

Helpfully, NutMEG also makes predictions about the production of detectable “biosignatures”—chemical signals strongly suggestive of biological origins and not deriving solely from nonliving, geological sources. Such telltale chemical cues vary substantially in context. But to take Earth as an example, the atmospheric concentrations of oxygen and methane indicate ongoing replenishment of each, at levels which cannot plausibly happen via purely geological processes. For the oxygen, we have the tremendous global biomass of photosynthesizing plants to thank, while the methane is mostly courtesy of microbes in wetlands and cattle digestive tracts.

For other rocky and icy environments in our Solar System, and potentially beyond, any such atmospheric biosignature might be far more subtle. (Not least because small worlds, such as Enceladus, have very little atmosphere to speak of in the first place.) A big reason for weak observable biosignatures is that the bulk of life’s activities may be happening underground, for example in isolated watery habitats or subsurface seas.

“In all of the other settings in the Solar System that are of astrobiological interest, almost all of the water and energy is below the surface,” says Higgins. He will be exploring what kinds of surface markers might still hopefully emerge of life doing its business out of sight. “For example, on icy moons, we need to look at how biosignatures can be transported from habitats at the seafloor through an ocean and ice sheet to possible plumes at the surface,” adds Higgins.

Other factors to consider in the biosignature arena are that the total biomass present might be quite little, and the metabolism of individual organisms could be very slow—akin to some forms of underground Earthly life. As a result, any biosignature could be dwarfed by abiotic processes—the opposite scenario of our planet, where life all but shouts its presence, thus further complicating detection. “There may be a higher risk on places like Enceladus and Europa that abiotic processes are overprinting biotic processes,” says Higgins. “So we’re trying to work out whether weak biosignatures are something that we can tease out with future models and observations from spacecraft.”

Higgins expects to work on these project particulars for the next few years, looking ahead to when new up-close observations of astrobiologically tantalizing Solar System destinations start streaming in from space probes next decade.

“There are so many new and exciting frontiers in astrobiology,” says Higgins. “The search for life is shifting up a gear.”