Insights into Resolving the Sub-surface Structure of Cometary Analogues using THz Spectroscopy

The composition and structure of comets provide key insights into planetary formation processes and the conditions present in the early Solar System. Especially dynamically new comets (DNC) with a long orbiting period have remained mostly untouched and are assumed to have preserved their structure for billions of years. The findings from previous missions like Stardust or Rosetta/Philae have indicated that comets are highly porous, and the water ice is mostly covered with dust (Groussin et. al. 2019). However, these findings were mostly conducted on larger scales, and little is known about the structure at smaller scales. Resolving the sub-surface structure on a sub-centimeter level could be valuable to challenge planetary formation theories.

Further in-situ exploration is required to discover the relationship between non-volatile and volatile materials. Since water ice is mainly transparent to THz frequencies (Ioppolo et. al. 2014), and the spatial resolution achievable at these frequencies is on the millimeter scale, we explore the application of in-situ THz time-domain spectroscopy to analyze the sub-surface structure of comets. THz spectroscopy offers higher spatial resolution than ground-penetrating radar, while still being capable to penetrate the upper surface layers. While THz spectroscopy has a lower spatial resolution compared to infrared (IR) spectroscopy, IR cannot penetrate the surface, limiting its applicability for subsurface studies. Moreover, the fingerprint absorption spectrum of water vapor enables THz spectroscopy to analyze sublimation above the surface. Investigating the activity of comets further provides valuable insights into the subsurface structure.

Our novel laboratory setup COCoNuT (Characteristic Observation of Cometary Nuclei using THz-spectroscopy) provides the capabilities to simulate the conditions we expect to encounter on a comet and perform proof-of-concept measurements with a commercial spectrometer (Stöckli et. al. in revision).

We present our first measurements with cometary analogue samples composed of dust and ice.

 

Groussin, O., Attree, N., Brouet, Y. et al. The Thermal, Mechanical, Structural, and Dielectric Properties of Cometary Nuclei After Rosetta. Space Sci Rev 215, 29 (2019).

Ioppolo, S., McGuire, B. A., Allodi, M. A., and Blake, G. A. (2014). THz and mid-IR spectroscopy of interstellar ice analogs: methyl and carboxylic acid groups. Faraday Discuss., 168:461–484.

Stöckli, L. L., Brändli, M., Piazza, D., Ottersberg, R., Pommerol, A., Murk, A., Thomas, N. (2025). Design and Commissioning of a THz Time Domain Spectro-Goniometer in a Cryogenic Comet Simulation Chamber. Rev. of Sci. Instruments, in revision.