ALMA Observations Reveal Isotopic Ratios in Jupiter's Stratosphere 23 Years After Comet SL9 Impact
Using ALMA radio telescope data from 2017, researchers measured carbon and nitrogen isotopic ratios in hydrogen cyanide (HCN) in Jupiter's stratosphere, finding values consistent with enrichment in heavier isotopes relative to Jupiter's bulk composition. The HCN traces back to the 1994 collision of Comet Shoemaker-Levy 9 (SL9) with Jupiter, which deposited new molecules in the stratosphere that persist to this day. The findings help resolve a longstanding puzzle from 1998 measurements that had reported anomalous depletions in heavier isotopes never seen elsewhere in the Solar System.
A new study published on arXiv uses Atacama Large Millimeter/submillimeter Array (ALMA) observations from 2017 to derive isotopic ratios of carbon (12C/13C = 73±5) and nitrogen (14N/15N = 245±29) in HCN within Jupiter's stratosphere. These molecules were originally deposited by the dramatic impact of Comet Shoemaker-Levy 9 in July 1994, which triggered shock-induced chemistry that created HCN, CO, CS, and other compounds at pressures below 0.1 mbar. Earlier measurements taken in 1998, just four years after the impact, had reported unusually strong depletions in heavier isotopes that were unprecedented in Solar System science, suggesting either an exotic cometary composition or an unknown fractionation mechanism. The new 2017-era values instead show enrichment in heavier isotopes relative to Jupiter's bulk solar-Jovian values, a reversal that the authors interpret as either a direct signature of cometary material mixed into the Jovian stratosphere or the result of 23 years of chemical evolution — or both. The study employed radiative-transfer calculations to probe atmospheric pressures ranging from 0.03 to 1.8 mbar, providing a more detailed vertical profile than previous work. The results contribute to the broader effort to use isotopic ratios as tracers of material reservoirs across the Solar System and to understand the long-term chemical legacy of large cometary impacts on giant planet atmospheres.
What's missing
The study acknowledges that it cannot definitively distinguish between two competing explanations for the observed isotopic enrichment: direct cometary contribution versus chemical fractionation over 23 years. Additionally, the isotopic composition of Comet SL9 itself was never directly measured, limiting the ability to confirm the cometary origin hypothesis.
What different sources said
- arXiv astro-phCenter
New 12C/13C and 14N/15N isotopic ratio measurements in Jupiter's stratosphere revealed by ALMA
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