Astronomers Find “Hope Of Life” On Saturn (Planetary Science / Chemistry)

The hydrogen cyanide (HCN) molecule in the planetary atmosphere is key to the formation of building blocks of life. The HCN present in photochemical reactions, create lipids, amino acids and nucleosides, the 3 basic building blocks of life. Earlier in 2016, we detected hydrogen cyanide (HCN) in the atmosphere of Saturn moon “Titan”. The discovery of an HCN emission line in the atmosphere of Saturn may be crucial in the formation of certain complex bio substances, as well as prebiotic molecules. Without the existence of HCN, CO, and H2O, the development and evolution of prebiotic molecules with carbon-based metabolism is difficult in the atmosphere of Saturn.

Now, Manna and Pal presented the spectroscopic detection of the rotational molecular line of nitrile species hydrogen cyanide (HCN) in the atmosphere of Saturn using the archival data of the Atacama Large Millimeter/Submillimeter Array (ALMA) in band 7 observation. Their observations given us a “hope of life” on saturn.

They detected strong rotational emission line of HCN at frequency ν = 354.505 GHz (>4σ statistical significance). They also detected the rotational emission line of carbon monoxide (CO) at frequency ν = 345.795 GHz. The statistical column density of hydrogen cyanide and carbon monoxide emission line is N(HCN)∼2.42×1016 cm¯2 and N(CO)∼5.82×1017 cm¯2. The abundance of HCN and CO in the atmosphere of Saturn relative to the H2 is estimated to be f(HCN)∼1.02×10¯9 and f(CO)∼2.42×10¯8.

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Figure 1: Molecular rotational emission spectrum of HCN in the atmosphere of Saturn at the frequency of ν = 354.505 GHz with transition J=4–3 using ALMA band 7 observation. The spectrum was made by integrating the reduced ALMA data cubes from the center of Saturn with 1.1″ circular region. The spectral coverage in the figure is 200 MHz with a total integration time of 476.402 second. In the spectrum, the solid red line corresponds to the best fit model of column density 2.42×1016 cm¯2 © Manna and Pal
Figure 2: Molecular rotational emission spectrum of CO in the atmosphere of Saturn at the frequency of ν = 345.795 GHz with transition J=3–2 using ALMA band 7 observation. The spectrum was made by integrating the reduced ALMA data cubes from the center of Saturn with 1.1″ circular region. The spectral coverage in the figure is 250 MHz with a total integration time of 476.402 second. In the spectrum, the solid red line corresponds to the best fit model of column density 5.82×1017 cm¯2 © Manna and Pal

They also discussed possible chemical pathways to the formation of the detected nitrile gas HCN in the atmosphere of Saturn.

“In the atmosphere of Saturn, HCN is produced due to the photochemical reaction of CO2, and CH4.”

They explained that atmosphere of Saturn primarily consists of H2, CO2, CO, and CH4 which are very essential for the production of HCN. The formation of HCN in the nitrogen-based atmospheres requires the N≡N bond and it needs to find a carbon atom. First, the nitrogen bond is split from NH3, and second, carbon molecules need to win over oxygen for the competition of getting free nitrogen. The first condition is very easy because NH3 molecules are present in the atmosphere of Saturn, which has a large absorption cross-section over N2. The hot temperature is necessary to split the hydrogen atom from NH3.

Figure 3: Suggested chemical pathways to the formation of HCN in the atmosphere of Saturn. The rate constant for this chemical reaction to formation of HCN is found in the STAND2019 chemical network which is a modification version of the STAND2016 network © Manna and Pal

After the split of the nitrogen bond, it reacts with nearby molecules as well as other stable molecules. The single nitrogen atom finds another single nitrogen atom to create N2. We already know a big amount of H2 exists in the atmosphere of Saturn. So the nitrogen is rapidly reacted with hydrogen to create NH2 and then will continue to react with abundant hydrogen to the formation of NH4. Similarly, the nitrogen atom reacts with the oxygen atom to produce NO2. According to authors, the trace species HCN are producing in the atmosphere of Saturn if the nitrogen bonds are reacted with carbon and hydrogen. In the atmosphere of Saturn, the carbon to oxygen balance is a key to understand nitrile chemistry. Fig. 3 above provides the possible reaction of the chemical network to the formation of HCN from CO2, CO, CH4 in the atmosphere of Saturn via photochemical pathways.

In addition, they mentioned that the formation mechanism of HCN in the atmosphere of Saturn is varied with temperature, molecular composition, impact rate, surface process, and rate of lighting. In the atmosphere of Saturn, the HCN can be produced by the photochemical pathways and atmospheric lightning.

“In our predicted chemical reaction, some reactions are photochemical which denoted as hν and it requires photon for the production of HCN. We used the STAND2019 chemical network for chemical modeling which is a modification of the STAND2016 network which includes H/C/N/O chemistry with 3 carbons, 2 nitrogens, 3 oxygens, and 8 hydrogens and valid up to 30000 K temperature and also include some gas-phase reactions involving Na, K, Mg, Fe, Ti, Si, and Cl. The STAND2019 chemical network included 5000 reactions involving over 350 amount of chemical species. In the planetary atmosphere, CO2 and CH4 are very important species for the formation of other molecular gas due to photochemical reaction.”

Finally, they concluded that, in the atmosphere of Saturn, further spectroscopic observations of the complex molecular gas will aid in confirming the origin and formation mechanism of the observed trace gases.


Reference: Arijit Manna, Sabyasachi Pal, “ALMA detection of hydrogen cyanide and carbon monoxide in the atmosphere of Saturn”, Arxiv, pp. 1-6, 2021. https://arxiv.org/abs/2104.10474


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