An international team of scientists, including from India, has discovered strong evidence indicating the presence of ozone on Jupiter’s moon Callisto, shedding light on the complex chemical processes taking place on icy celestial bodies in the Solar System.
The study was published in the March 2024 issue of the journal Icarus. It outlines the researchers’ investigation into the chemical evolution of “SO2 astrochemical ice”, which is ice primarily composed of sulphur dioxide (SO2) in the presence of ultraviolet irradiation.
This shed light on the chemical processes and composition on the surface of Callisto. By analysing the data of the UV absorption spectra of the irradiated ice samples, the team was able to identify a distinct signature indicating the formation of ozone.
They corroborated their findings by comparing them with data from the Hubble Space Telescope to understand Callisto’s environment and the potential habitability of icy moons in the Solar System.
The importance of ozone
The earth has life not just because it found a way to originate here: it also had the resources to thrive, evolve, and diversify. These resources include sunlight containing the ‘right’ frequencies of radiation, water, a stable atmosphere providing a stable supply of the requisite gases and at the right temperature, and various compounds required for the life-forms’ biochemical processes.
This said, not all emissions from the Sun are good for organisms on the earth. Ultraviolet radiation in particular is harmful to many species (but also useful to some others). Two of its components, called ultraviolet-B and ultraviolet-C, of wavelengths 290-320 nanometres and 100-280 nanometres respectively, can damage DNA, trigger mutations, and increase the risk of skin cancer and cataracts in humans.
Ultraviolet light has also been known to inhibit plant growth and have detrimental effects on various organisms. This is why the ozone layer is a crucial part of the earth’s atmosphere: it completely absorbs ultraviolet-B and ultraviolet-C radiation.
The ozone molecule is composed of three oxygen atoms bonded together. The ozone layer, found in the lower part of the earth’s stratosphere, around 15-35 km above ground, serves as a shield. Without the ozone layer, ultraviolet radiation levels would be much higher on the planet’s surface, rendering it uninhabitable for many species and disrupting entire ecosystems.
Scientists are currently studying various celestial bodies in the Solar System that show signs of ozone, suggesting the existence of stable atmospheric conditions and, by extension, the possibility of their being able to host life.
Callisto and its unique environment
After Saturn, Jupiter has the most moons in the Solar System. Callisto is one of Jupiter’s largest moons and the third-largest moon in the Solar System after Ganymede and Titan.
But more than its impressive size, Callisto is distinguished by its composition. Despite being as big as the planet Mercury, it has less than half as much mass. Callisto is primarily composed of water ice, rocky materials, sulphur dioxide, and some organic compounds. These substances make the moon a potential candidate for supporting life in the Solar System beyond the earth.
Callisto’s surface is heavily cratered, indicating a long history of being struck by asteroids and comets. (It may have the oldest surface in the Solar System, in fact.) It also lacks the extensive seismic activity seen on some of Jupiter’s other moons, such as Io and Europa.
The presence of relatively few geological features suggests Callisto’s surface is geologically inactive. In other words, its surface has likely been relatively stable for a long time. This stability could be vital to preserve any subsurface ocean or potential habitats beneath the icy crust.
The detection of sulphur dioxide on Callisto’s surface has encouraged this team of scientists to conduct spectroscopic observations to gain a better understanding of the moon’s surface composition and formation.
Recreating conditions on the earth
Scientists led by R. Ramachandran, of the Atomic, Molecular, and Optical Physics Division, Physical Research Laboratory, Ahmedabad, set out to investigate the chemical evolution of sulphur dioxide ice under irradiation, leading to the formation of ozone.
Scientists have previously demonstrated this process in laboratory experiments. The current team’s aim was to recreate the conditions required for this process on the surface of Callisto when sunlight hits its surface. To do this, the researchers used vacuum ultraviolet photons, which mimic the solar radiation that reaches the moon’s surface.
The experiments were conducted at the National Synchrotron Radiation Research Centre (NSRRC) in Taiwan, which provided access to high-energy radiation sources required to recreate the radiation coming from the Sun.
To model the surface of Callisto, the researchers placed a substrate of lithium fluoride in a chamber with very low pressure. This environment recreated conditions similar to those found in outer space. The sulphur dioxide ice samples were deposited onto the substrate, setting the stage for the final step: observing the absorption spectrum.
The absorption spectrum is the unique fingerprint of a substance. It shows the wavelengths of light it absorbs, providing insights into its composition and properties.
The team carefully controlled the temperature of the sulphur dioxide ice samples throughout the experiment.
The samples were initially kept at a low temperature of around 9 K (-264.15 degrees C), in line with the conditions on Callisto’s surface. Then they slowly warmed it up to 120 K to resemble different environmental scenarios.
In this process, they irradiated the ice with vacuum-ultraviolet photons (of wavelength 137.7 nanometres) and recorded its ultraviolet absorption spectrum during and after the irradiation using a photomultiplier tube detector. This device measures low levels of electromagnetic radiation by converting photons into electrical signals.
Ozone and potential habitability
The ultraviolet absorption spectrum revealed the formation of ozone after the sulphur dioxide ice samples were irradiated. This was evidenced by a distinct signature in the absorption spectrum.
The researchers also compared their experimental data with data collected by the Hubble Space Telescope, which had also suggested the presence of sulphur dioxide and ozone on the surface of Callisto in 1997.
The discovery of ozone on Callisto suggests the presence of oxygen, which in turn is a fundamental ingredient required for the formation of complex molecules required for life (as we know it), such as amino acids, raising questions about the moon’s habitability. This extends to other icy moons in our Solar System, potentially informing our understanding of habitable conditions beyond Earth.
In addition to the ozone, the researchers observed an unidentified band in the absorption spectrum – similar to that observed on Ganymede in 1996 – hinting at a common molecular source in their surface compositions or chemical processes.
This finding could provide valuable insights into geological and atmospheric processes on these moons. In particular, it could help us to understand the precise mechanisms that led to the formation of Jupiter and its moons, which remain topics of active research.
Tejasri Gururaj is a freelance science writer and journalist.
