Tracing the Origins of Life
DMS, DMDS and the Chemical Precursors of Life in the Universe
In a recent research article
A recent study utilizing the James Webb Space Telescope's MIRI instrument has detected mid-infrared spectral features in the atmosphere of exoplanet K2-18 b that are consistent with the presence of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS), both considered potential biosignature gases. The observed abundances of these molecules are significantly higher than what would be expected from abiotic sources, suggesting a possible biological origin. However, further observations are necessary to confirm these findings and to rule out non-biological explanations.
Src: https://arxiv.org/abs/2504.12267
How Scientists Detect Potential Biosignatures on Distant Planets?
The search for life beyond Earth has been one of the most profound scientific pursuits of the modern era. While telescopes have peered deep into the universe for signs of intelligent civilizations, a more fundamental and promising line of investigation focuses on chemical precursors of life—molecules that are essential to or strongly associated with biological processes. Among these, Dimethyl Sulfide (DMS) and Dimethyl Disulfide (DMDS) have emerged as notable candidates.
These and other complex organic molecules could serve as potential biosignatures, indicating the presence or past presence of life. But what exactly are DMS and DMDS, how do they relate to life, and how can we detect such molecules from light-years away?
DMS and DMDS: Biological Molecules with Astrobiological Significance
Dimethyl Sulfide (DMS):
DMS is a volatile sulfur compound most commonly produced on Earth by marine phytoplankton, tiny oceanic organisms. It plays a critical role in the Earth’s sulfur cycle and contributes to cloud formation, influencing the planet’s climate. Because it is predominantly biological in origin, DMS is considered a biosignature gas—a molecule that may only exist in significant quantities if produced by life.
Dimethyl Disulfide (DMDS):
DMDS is chemically related to DMS and is also associated with biological activity. It is found in decaying plant matter and is released by certain bacteria. Though less volatile than DMS, it is still relevant in biogeochemical cycles and is of interest in astrobiology due to its association with microbial metabolism.
Other Known Precursors of Life
While DMS and DMDS are sulfur-based compounds, several other molecules are widely regarded as precursors or indicators of life. These include:
Amino Acids: Building blocks of proteins. Over 70 amino acids have been identified in meteorites, suggesting they can form in space.
Nucleobases: Components of DNA and RNA. Some, like adenine and guanine, have been detected in carbon-rich meteorites.
Formaldehyde (H₂CO): A simple organic compound that can polymerize to form sugars, essential in prebiotic chemistry.
Methane (CH₄): While not definitive proof of life, its abundance and variability in an atmosphere can hint at biological processes.
Phosphine (PH₃): A highly debated molecule recently proposed as a possible biosignature in Venus's atmosphere.
Acetonitrile (CH₃CN): Detected in interstellar clouds, this molecule plays a role in prebiotic chemistry.
Isoprene and other terpenes: Produced by plants and microbes, these are volatile organic compounds that could be used to infer life in oxygen-rich environments.
How Are These Molecules Detected From Earth?
Detecting biosignature gases like DMS or DMDS in the atmosphere of a distant planet is a challenging yet fascinating task. Scientists rely on spectroscopy, the study of how matter interacts with electromagnetic radiation.
1. Transit Spectroscopy
When a planet passes in front of its host star (a transit), starlight filters through its atmosphere. Molecules in the atmosphere absorb specific wavelengths of light, creating a unique pattern—an absorption spectrum. By comparing the light before, during, and after the transit, astronomers can identify the chemical fingerprints of atmospheric gases.
2. Emission and Reflection Spectroscopy
In some cases, scientists analyze the light emitted or reflected by the planet itself. Instruments onboard telescopes like the James Webb Space Telescope (JWST) are sensitive enough to detect faint infrared signatures of various molecules, including potential biosignatures.
3. Radio Astronomy
Some complex organic molecules in space emit radio waves as they rotate and vibrate. Using radio telescopes like ALMA (Atacama Large Millimeter/submillimeter Array), astronomers have detected numerous life-related molecules in interstellar clouds and protoplanetary disks.
4. Direct Imaging (Future Possibility)
Though still in its infancy, direct imaging of exoplanets aims to capture light directly from the planet, separate from the glare of the host star. Future missions like LUVOIR and HabEx aim to apply this technique to observe Earth-like planets and search for biosignatures.
Challenges in Detecting DMS and DMDS
While detecting simple molecules like methane or water vapor is now routine, detecting complex biosignature gases like DMS or DMDS remains difficult due to:
Low Abundance: These molecules may exist in trace amounts, requiring extremely sensitive instruments.
Spectral Overlap: Their spectral lines may overlap with those of other gases, making identification ambiguous.
False Positives: Some molecules can be produced abiotically (without life), so context is crucial to interpretation.
Signal-to-Noise Ratio: Atmospheric interference, instrumental limitations, and cosmic noise can distort data.
To understand and communicate the complex process of searching for life beyond Earth, we prepare a flow chart that outlines each key step in the journey of biosignature detection. The goal is to visually represent the logical sequence of scientific tasks—from selecting candidate planets to analyzing chemical data—that guide our search for extraterrestrial life.
The quest to uncover life elsewhere in the universe has shifted from the philosophical to the practical. DMS, DMDS, and a suite of other life-related molecules are offering real, tangible clues. Their detection in extraterrestrial environments could mark a revolutionary moment in science, reshaping our understanding of biology, chemistry, and our place in the cosmos.