Context: An international team of scientists, including from India, has discovered strong evidence indicating the presence of ozone on Jupiter’s moon, Callisto.
Importance of Ozone:
- Ozone molecule is composed of three oxygen atoms bonded together.
- The ozone layer is found in the lower part of the earth’s stratosphere, around 15–35 km above ground, and serves as a shield as it completely absorbs ultraviolet-B and ultraviolet-C radiation.
- Ultraviolet-B (wavelengths 290–320 nanometres) and ultraviolet-C (100-280 nanometres) can damage DNA, trigger mutations, inhibit plant growth and increase the risk of skin cancer and cataracts in humans.
- Without the ozone layer, UV-radiation levels would be much higher on Earth’s surface, rendering it uninhabitable for many species and disrupting entire ecosystems.
- Rationale of the study: 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, their possibility of hosting life.
Callisto and its unique environment:
- Callisto is one of Jupiter’s largest moons and the third-largest moon in the Solar System after Ganymede (Jupiter’s moon) and Titan (Saturn’s moon).
- 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 lacks the extensive seismic activity, which is present in some of Jupiter’s other moons like Europa.
- It has relatively few geological features which suggest that Callisto’s surface is geologically inactive or relatively stable for a long time. This stability could be vital to preserve any subsurface ocean or potential habitats beneath the icy crust.
The Experiment:
- The Scientists modelled the surface of Callisto icy surface conditions in a lab, and analysed the chemical reactions occurring on the surface of Callisto when sunlight hits its surface (caused by ultraviolet radiation).
- 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 do this, the researchers used vacuum ultraviolet photons (of wavelength 137.7 nanometres), which mimic the solar radiation that reaches the moon’s surface.
- 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.
- Finally, they irradiated the ice with vacuum-ultraviolet photons and recorded its ultraviolet absorption spectrum during and after the irradiation using a photomultiplier tube detector.
- The ultraviolet absorption spectrum revealed the formation of ozone after the sulphur dioxide ice samples were irradiated.
- 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.
Significance:
- 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 such as amino acids.
- The investigation into the chemical evolution of ‘SO2 astrochemical ice’ (ice primarily composed of sulphur dioxide (SO2) in the presence of ultraviolet irradiation) would shed light on the chemical processes and composition of the surface of Callisto.
- Hence, the findings would help in understanding Callisto’s environment and the potential habitability of icy moons in the Solar System.