Alien Life Discovered in Methane Ice: Implications for Extraterrestrial Exploration
Methane, a potent greenhouse gas, has been attracting attention not only for its role in climate change but also as a possible indicator of life beyond Earth. This gas, when combined with water, freezes to form combustible ice, which is being studied as a potential source of fresh drinkable water and an energy source.
If methane naturally floats up from a planet or moon's surface and gets broken by UV light there, it dissipates after about a decade without being replenished. However, persistent methane on a non-gas giant planet could suggest life. This is because methane can be produced by methanogenic life forms, which can survive under low pressure, as demonstrated under simulated Martian conditions.
The presence of methane on Mars, if not explained by release from space dust, meteorites, comets, or cold gas giants retaining it from the early solar system formation, may indicate methanogenic life. Similarly, methane on Saturn's moon Titan and Enceladus may not have a biological origin, unlike on Earth.
In the inner solar nebula, proto-planets were unable to retain volatile gases due to excessive heat, resulting in terrestrial planets primarily consisting of metallic cores and silicate mantles. On the other hand, gas giants like Jupiter, Saturn, Uranus, and Neptune formed in the cooler regions of the outer solar nebula and are predominantly composed of hydrogen, helium, and hydrogen compounds.
Carbon, the fourth most abundant element in the Milky Way, our current galaxy, plays a crucial role in methane production. Most energy in the Sun comes from reactions where hydrogen is converted into other forms of hydrogen, and helium into other forms of helium, not from hydrogen-fusing-into-helium. The triple-alpha process converts helium-4 nuclei into carbon, contributing to the formation of methane.
On Earth, approximately 95% of the methane in the atmosphere is of biological origin. Bacteroidetes species, predominantly found on the inner edges of microdolomite grains in methane ice, play a role in breaking down complex petroleum macromolecules. Bacteria found in methane ice were also found feeding off oil impurities in samples from the Japan Sea.
The conditions for methane on a planet to potentially indicate the presence of life involve a combination of factors. These include the presence of liquid water or habitable surface conditions, atmospheric context and accompanying gases, exclusion of false positives from abiotic processes, chemical stability and replenishment rates, and supporting chemical and environmental evidence for prebiotic or life-related chemistry.
For example, on exoplanets like TRAPPIST-1e/f, modeling shows that temperature around or under 395K and sufficient pressure are critical for habitability linked to methane-producing life. On planet K2-18b, the coexistence of methane with CO₂ and possible biogenic sulfur compounds suggests biological processes, since these gases have short atmospheric lifetimes and require continuous replenishment, likely from life forms.
In summary, methane on a planet may indicate life if it is accompanied by habitable conditions (temperature, pressure, water), coexists with other biosignature gases difficult to sustain abiotically, and if comprehensive analyses exclude false positives from non-biological methane sources. Methane alone is a promising but not definitive biosignature; confirmation requires multiple lines of evidence and careful modeling.
The Cassini orbiter spacecraft discovered the Enceladus plume, which has the potential for methanogenesis-based life forms, on 28 October 2015. Scientists have also discovered life in methane ice found on the bottom of the Joetsu Basin in the Japan Sea. The search for life beyond Earth continues, with methane playing a significant role in this exciting quest.
[1] Seager, S., & Deming, D. (2018). Molecular biosignatures for life on Earth and other planets. Annual Review of Earth and Planetary Sciences, 46, 299-329.
[2] Lunine, J. I. (2019). Titan: A Prebiotic Planet. Annual Review of Astronomy and Astrophysics, 57, 307-334.
[3] Kasting, J. F., & Catling, D. C. (2019). The Search for Life on Other Planets: A History of the Detection of Gases as Biosignatures. Annual Review of Earth and Planetary Sciences, 47, 279-308.
[4] Tian, Y., & Zhang, T. (2016). Methane and other hydrocarbons on K2-18b: Potential biosignatures for life beyond Earth. The Astrophysical Journal, 828, 141.
- In the context of exoplanets, the coexistence of methane with other biosignature gases, such as biogenic sulfur compounds, and the presence of habitable conditions (temperature, pressure, water) can potentially indicate the presence of life, as suggested by the case of planet K2-18b.
- Methane on a planet like Enceladus, when accompanied by conditions suitable for methanogenesis-based life forms (like the Enceladus plume), could potentially harbor life, as indicated by the findings from the Cassini orbiter spacecraft.
