Adsorption of CO and NH3 on iron salts in space-like conditions: implications for icy moonssurface chemistry and observations

Astrophysical ices accrete on substrates whose chemical composition vary significantly according to their environment.
In the interstellar medium, ices mantles grow onto dust grains composed of amorphous silicates and carbonaceous materials
[1], while more diverse mineralogical substrates are present in circumstellar regions [2]. On the surfaces of icy moons in
the Solar System, several saline species have been identified [3]; these species are thought to originate from subsurface
oceans, making their characterization essential for inferring the chemistry of these internal water bodies. In this study,
we investigated the interactions between astrochemically relevant molecules and saline substrates through laboratory
experiments. Specifically, we evaluated the adsorption of carbon monoxide and ammonia onto iron salts; the oxidation
states of Fe could indicate the redox conditions of the oceans under the surface. Infrared spectra show the emergence of
spectral profiles reflecting the interactions between ices and salt substrates. Under ultra-high vacuum conditions, while
pure CO typically desorbs at 25–30 K, the new spectral features of CO persisted on the salt substrate up to approximately
180 K, a clear effect of strong chemical interactions. Furthermore, signatures of NH3 remained visible at temperatures
as high as 300 K. These results suggest that saline surfaces significantly enhance the thermal stability of volatile species,
with profound implications for the chemical evolution of icy planetary surfaces and their observations.
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Society Open Science 3.12 (2016).
[2] Keller, L. P., et al. ”Identification of iron sulphide grains in protoplanetary disks.” Nature 417.6885 (2002): 148-150..
[3] Carlson, R. W., et al. ”Europa’s surface composition.” Europa 283 (2009).
[4] Napoleoni, Maryse, et al. ”Probing the oxidation state of ocean worlds with SUDA: Fe (II) and Fe (III) in ice grains.” The
Planetary Science Journal 5.4 (2024): 95.