A new derivation of the 12C/13C and 15N/14N isotopic ratios in the stratosphere of Jupiter revealed by ALMA

The composition and isotopic ratios in different bodies of the solar system are important tracers of their formation and of the primordial composition and physical/chemical conditions of the protoplanetary disk. Numerous measures have been performed these past 50 years in meteorites (Meibom et al. 2007), comets (Rousselot et al. 2014), and in giant planets atmospheres (Owen and Encrenaz 2003 and many others). They show a variety of values compared to the solar ones (Marty et al. 2011). Measuring the abundances and the isotopic ratios in the giant planets is of prime interest as they should reflect the initial composition of the solar nebula and the properties of the disk from which they were formed (Marboeuf et al. 2018).

One way to assess this is to look at the ¹²C/¹³C and ¹⁵N/¹⁴N isotopic ratios. The relative abondance of ¹⁵N in N-bearing species is a strong indicator of the formation processes of the giant planets, because it gives hints on the abundances of N₂ and NH₃, the two major N-bearing species in the solar system. The ¹⁵N/¹⁴N ratio from NH₃ in Jupiter’s and Saturn’s atmospheres was found to be close to solar (Fouchet et al. 2000, Fletcher et al. 2014), implying that the main contributor of primordial N in the two planets atmospheres was N₂ which is expected to present a low ¹⁵N enrichment in the solar nebula (Fletcher et al. 2014). The ¹²C/¹³C ratio in hydrocarbons is also close to solar in the giant planets’ atmospheres (Niemann et al. 1998, Orton et al. 1992, Combes et al. 1979 and others).

However, the atmospheres of the giant planets are subject to drastic changes as they constantly interact with their surroundings. A major example was in July 1994 with the comet Shoemaker-Levy 9 (SL9) impacts in Jupiter’s atmosphere, which led to altered atmospheric composition and anomalous isotopic ratios (Matthews et al. 2002). New molecules previously undetected were brought/produced in Jupiter’s stratosphere, presumably by shock-induced chemistry (Zahnle et al. 1995), such as HCN (Marten et al. 1995). Matthews et al. 2002 found that the post-impact ¹²C/¹³C and ¹⁴N/¹⁵N isotopic ratios in HCN were super-terrestrial, respectively with a mean increase factor of 3 and 10, probably indicating an unusual cometary composition depleted in ¹³C and ¹⁵N.

In this paper, we present new derivations of the ¹²C/¹³C and ¹⁵N/¹⁴N isotopic ratios in Jupiter’s atmosphere from HCN observations. We used the Atacama Large Millimeter/submillimeter Array (ALMA) observations of Jupiter at two lines of two HCN isotopes, H¹³CN at 345.34 GHz and HC¹⁵N at 344.20 GHz. This dataset is part of project 2016.1.01235.S that was taken in March 22, 2017, i.e., almost 23 years after SL9 impacts. With a radiative transfer code, we derive the vertical abundance profiles of the two isotopes and we give new estimates of the ¹²C/¹³C and ¹⁵N/¹⁴N isotopic ratios in the stratosphere of Jupiter using the vertical profile of HCN derived in Cavalié et al. (2023). We compare our new derivations 1) with the intriguing post-SL9 values of Matthews et al. (2002) to confirm their findings or to trace any evolution of the isotopic ratios 23 years after SL9 impacts; and 2) more generally with the last 50 years observations of the atmosphere of Jupiter and of other solar system objects to better understand isotopic ratios in the giant planets.

 

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