The outer solar system’s frigid moons, each with a liquid-water ocean under its frozen surface, are the hottest spots for extraterrestrial life.
Titan, Saturn’s moon, contains a thick layer of brackish water under its frozen surface, studded with liquid hydrocarbon lakes. Enceladus, Titan’s Saturnian sister moon, has ejected geyser-like plumes from cracks near its south pole, revealing its underlying sea.
Plumes also originate from Jupiter’s Europa, which has a liquid depth that exceeds all of Earth’s seas combined.
A “second genesis” may have happened in any of these aquatic extraterrestrial sites, just as life sprang billions of years ago on Earth.
Astrobiologists are currently studying the ocean-bearing moons’ habitability, or the complex geochemical circumstances necessary for life to develop and thrive.
By 2030, NASA’s Europa Clipper mission may begin orbital explorations of Jupiter’s mysterious moon.
Another mission, the Dragonfly, a nuclear-fueled flying drone, is set to land on Titan as early as 2036. These missions, as amazing as they are, are only preludes to future initiatives that might more directly seek extraterrestrial life.
But how can astrobiologists know when they find life in those weird sunless locations so unlike our world?
The search for life on Earth is often focused on subtle chemical traces rather than a fossilised form or a little green humanoid waving hello.
Jezero Crater, a dry lakebed that may preserve traces of former life, contains organic compounds and salts discovered by NASA’s Perseverance Mars rover.
Scientists using telescopes to study Venus may have detected phosphine gas, a possible by-product of microbes floating in the planet’s temperate zones.
Both living organisms can make many basic biosignatures, abiotic geochemical processes, which is the problem.
On Earth, microorganisms produce a lot of phosphine, but Venus’ phosphine, if it exists at all, might be related to erupting volcanoes rather than an alien life in its clouds.
Such uncertainties can lead to false positives when scientists believe they have discovered life where none exists.
However, if an organism’s biochemistry and physiology differs greatly from that of terrestrial species, scientists may encounter false negatives, where they miss life despite proof of its presence.
Researchers must navigate between these two interrelated astrobiological dangers, especially when considering the possibility of life on distant worlds like the ocean moons of our solar system.
However, research published lately in the Bulletin of Mathematical Biology proposes a new method.
Astronomers should look beyond specific chemical tracers like phosphine to discover how living creatures rearrange materials throughout vast ecosystems, say the scientists.
That is true even if the biochemistry employed by the organism was essentially alien.
ANALYZING A SEA CHANGE IN LIFE
Stoichiometry, which examines the elemental ratios that exist in the chemistry of organisms, ecosystems, is used in this study.
The researchers began by noticing that chemical ratios inside groupings of cells fluctuate with remarkable regularity.
The Redfield ratio—a 16:1 average proportion of nitrogen to phosphorus indicated by phytoplankton blooms throughout the world—is a notable example.
Other sorts of cells, such as bacteria, have their own set of, particularly constant ratios.
When it comes to recognising life in an alien environment, stoichiometry may be the key.
They also vary with cell size, giving a second check on any oddly consistent but perhaps abiotic chemical ratios on another world
Protein molecule concentrations in bacteria, for example, drop as cells get bigger, but nucleic acid concentrations rise.
Biological particles, unlike nonliving particles, have “ratios that consistently change with cell size,” according to lead scientist Chris Kempes of the Santa Fe Institute.
It’s difficult to explain how various cell sizes affect element abundances, as Kempes, Levin, and others did.
They focused on the fact that when cell sizes expand in a fluid, their abundance decreases quantitatively, as a power-law with a negative exponent.
The size distribution of cells (or cell-like particles) in a fluid allows astrobiologists to predict elemental abundances inside materials.
In essence, this may be a strong approach for detecting living creatures in a collection of unknown particles, like a sample of European seawater.
In a system where particles exhibit regular relationships between elemental ratios and size, but the surrounding fluid does not, Kempes says, “we have a strong indicator that the ecosystem may include life.”
TESTING THE WATERS
“Ecological biosignatures” are the latest in a decades-long quest to link life not only to fundamental physics and chemistry but also to its distinctive environments.
For example, it would be absurd to assume that organisms on a warm, stony planet had the same chemical biosignatures as those on an aquatic moon.
This is highly significant, adds NASA Chief Scientist Jim Green.
“We are now reaching the age when we can apply universal principles to what we know about life evolution.”
How can we use this broader view of biosignatures to better understand Europa, Titan, and Enceladus?
At the present, more than the European Space Agency’s Europa Clipper orbiter will be required, according to Green; possibly a follow-up mission to the surface would suffice.
“We aim to collect far more comprehensive measurements with Clipper, fly through the plume, investigate Europa’s development over time, and acquire high-resolution images,” he says.
This will lead us to the following stage, which is to descend to the earth.
Earth will need the next generation of instruments and ideas.
Finding the ecological biosignatures described by Kempes et al. will involve measuring the size distribution and chemical composition of cells inside their original fluid.
Scientists use Flow cytometry as a method to categorise cells by size. Bringing equipment to an alien moon’s deep water would be tough enough.
Due to the lack of available energy, scientists anticipate that any life will be single-celled, small, and rare in these dark abysses.
To begin with, a fine-tuned flow cytometer capable of detecting particles in this size range would be necessary.
Our current flow cytometers aren’t cutting it, says Sarah Maurer, a biochemist and astrobiologist at Central Connecticut University.
“Some cell types require substantial pretreatment or would not pass through a cytometer,” she says.
space flight will require Improved Earth-based cell sorting and filtering instruments.
Heather Graham of NASA’s Laboratory for Agnostic Biosignatures and Goddard Space Flight Center says both fronts are making headway.
The next stage, she says, will be to implement new technology in remote places across the world that have some of the world’s most harsh and impoverished ecosystems.
Astrobiologists can improve gadget specifications by identifying chemical ratios linked with life in our planet’s calm waters.
And, just maybe, finally reveal a second genesis, written within the mathematics of a subsurface ocean’s chemistry.
The author is Natalie Elliot