Researchers find high carbon monoxide and methane in Comet 13P/Olbers

Researchers measured the native ice composition of Halley-type Comet 13P/Olbers after perihelion on Aug. 16, 2024, using the iSHELL spectrograph at the NASA InfraRed Telescope Facility, according to Astrobiology Web.

Astrobiology Web reported that the observation took place when the comet stood 1.37 astronomical units from the Sun and 1.99 astronomical units from Earth. An astronomical unit is the average distance from Earth to the Sun. Those distances describe the comet’s position relative to the Sun and Earth during the August 16, 2024, iSHELL measurement of its coma gases.

The study appeared in The Planetary Science Journal under the title Surprisingly Enriched CO and CH4 in Halley-type Comet 13P/Olbers: Clues to Its Interstellar Heritage. Perihelion is the point in a comet’s orbit closest to the Sun. The journal title frames the findings as clues to the comet’s interstellar heritage.

Astrobiology Web reported that iSHELL is a 1─5 micrometer cross-dispersed, high-resolution facility spectrograph located at the NASA InfraRed Telescope Facility. It delivered spectral resolving power of about 4.5 × 104. The instrument used two settings to target rovibrational emissions, light given off as molecules rotate and vibrate. Infrared wavelengths are light beyond the red end of visible light.

Seven parent molecules had sublimated, or turned directly from solid ice into gas, from nucleus ices into the coma, Astrobiology Web reported. Native ice composition refers to the ices that were originally part of the comet. One setting simultaneously measured water, carbon monoxide, and carbonyl sulfide together, while the other simultaneously measured methane, ethane, methanol, and formaldehyde together. The researchers obtained meaningful abundances for all seven species targeted in the coma of 13P, the cloud of gas and dust surrounding a comet’s nucleus.

Carbon monoxide showed an enrichment of about six times the mean abundance ratio for measured Oort cloud comets, a distant reservoir of icy bodies, Astrobiology Web reported. Methane reached an enrichment of about three times the mean for Oort cloud comets. Carbon monoxide and methane were the two most volatile species systematically targeted at infrared wavelengths in comets.

Astrobiology Web noted that the comparison of carbon monoxide used mean abundance ratios derived from comets in the Oort cloud reservoir and was reported in the same sentence as the smaller methane enrichment. This enrichment factor appeared alongside the roughly sixfold carbon monoxide enrichment and near-average methanol and ethane abundances. Methanol and ethane abundances were near average among measured comets.

Astrobiology Web noted that these near-average levels appeared alongside the substantial enrichments of carbon monoxide and methane in 13P/Olbers. The chemistry suggests large endowments of carbon monoxide and methane on interstellar grains. Interstellar grains are tiny particles of dust and ice found between stars. Inhibited surface chemistry on pre-cometary grains contained in the nucleus is another possible explanation.

The researchers present large interstellar-grain endowments and inhibited surface chemistry as alternative explanations for the observed volatile chemistry, Astrobiology Web reported. Pre-cometary grains are grains that existed before the comet formed. Assuming the composition is primordial, the natal environment of 13P may have been about 10 kelvin or colder. That low-temperature estimate applies only if the measured abundances reflect the comet’s primordial composition rather than material altered after formation.