Research project P4S/251/ISOCOM (Research action P4S)
Comets are small icy bodies formed in the outer Solar System about 4.6 billion years ago and stored in distant reservoirs such as the Kuiper Belt and Oort Cloud. Occasionally, gravitational perturbations send some into the inner Solar System, where they release gas and dust, forming a coma and the characteristic tails visible from Earth. Because they have remained largely unchanged, they preserve information about the conditions under which the Solar System formed.
The composition of comets reveals which molecules are present and in what amounts, while isotopic ratios (such as D/H or ¹⁸O/¹⁶O) provide additional constraints on the conditions under which this material formed.
Although comets can be observed from Earth, remote measurements probe the coma along the line of sight and measure an integrated signal that cannot be directly traced back to the nucleus, whereas in-situ measurements, such as those obtained during the Rosetta mission to comet 67P/Churyumov-Gerasimenko, provide a more direct link to the source material.
The ROSINA Double Focusing Mass Spectrometer (DFMS) dataset obtained during the Rosetta mission consists of more than 1.5 million in-situ mass spectra and has enabled several key measurements of comet composition and isotopic ratios.
Most studies to date have focused on spectra acquired during periods of high comet activity, when signal-to-noise ratios are highest and the data are easier to calibrate and interpret.
The ISOCOM project aims to investigate signals close to the detection limit in the DFMS dataset, including those associated with periods of lower comet activity, as well as low-abundance species and isotopes. For such weak signals, instrumental and calibration effects that are negligible at high signal-to-noise become more increasingly important.
To enable this analysis, ISOCOM first establishes a consistent and accurate calibration of the data. This forms the basis for constructing sum spectra through which the signal-to-noise ratio improves. Weak signals can thus be analysed more robustly than in individual spectra and signals that are not observable in single measurements become accessible.
ISOCOM systematically applies this approach to the full DFMS dataset, generating combined spectra across a range of observational conditions and enabling a more comprehensive exploration of the data.
The resulting spectra provide a basis for identifying and prioritising signals of interest. While known isotope systems, such as sulphur-bearing species and the water D/H ratio, form a natural starting point, the same approach can be applied to other signals present in the data.
This approach enables a systematic exploration of the dataset, providing access to signals that are not clearly observable in individual spectra and allowing a more complete assessment of comet 67P’s composition.
