Processing of analogues of plume fallout in cold regions of Enceladus by energetic electrons

Abstract

Context. Enceladus, a small icy moon of Saturn, is one of the most remarkable bodies in the solar system. This moon is a geologically active object, and despite the lower temperatures on most of its surface, the geothermally heated south polar region presents geysers that spouts a plume made of water (∼90%), carbon dioxide, methane, ammonia, and methanol, among other molecules. Most of the upward-moving particles do not have the velocity to escape from the gravitational influence of the moon and fall back to the surface. The molecules in the ice are continuously exposed to ionizing radiation, such as UV and X-rays photons, cosmic rays, and electrons. Over time, the ionizing radiation promotes molecular bond rupture, destroying and also forming molecules, radicals, and fragments. Aims. We analyse the processing of an ice mixture analogue to the Enceladus fallout ice in cold resurfaced areas (north pole) by 1 keV electrons. The main goal is to search for complex species that have not yet been detected in this moon, and to determine relevant physico-chemical parameters, such as destruction and formation cross-sections and the half-life of the studied molecules in the ice. Methods. The experiment consisted of the electron irradiation of an Enceladus-like ice mixture (H 2 O:CO 2 :CH 4 :NH 3 :CH 3 OH) in an ultra-high vacuum chamber at 20 K. The analysis was made by infrared spectrometry in the mid-infrared region (4000−800 cm −1 or 2.5−12.5 μm). Results. The absolute dissociation cross-sections of the parent molecules, the formation cross-section of daughter species, and the half-life of the parental species in a simulated Enceladus irradiation scenario were determined. Among the produced species, CO (carbon monoxide), OCN − (cyanate anion), HCONH 2 (formamide), and H 2 CO (formaldehyde) were tentatively detected.

Photochemistry and desorption induced by X-rays in water rich astrophysical ice analogs: implications for the moon Enceladus and other frozen space environments

“…The production of OCN À is very enhanced at 80 K. Pilling and Bergantini 10 also reported this dependence of the OCN À production with temperature in another experiments also employing so X-rays in astrophysical related ices. Bergantini et al 43 have presented several sets of electroninduced chemistry of similar ices containing initially H 2 O, CO 2 , NH 3 and CH 4 . The reaction routes are able to describe the formation of daughter species such as OCN À , NH, CH 3 OH, HCONH 2 , and others.…”

“…Bergantini et al, 43 also employing infrared spectroscopy have studied similar ice mixture at 20 K, in the presence of ionizing 1 keV electron beam. From their data (Fig.…”

“…A comparison between the results of radiochemistry (ion and electron bombardment) and X-ray induced photochemistry (Xray irradiation) of astrophysical ice analogs presents lots of similarities indicating that both penetrating and ionizing agents can change considerably the chemistry of frozen species in space. 22,31,43,50 Adapted from Pilling and Bergantini 10 and Gueymard 59 and considering r À2 law Triton orbit ($30 AU) 9 Â 10 6 7.0 Â 10 3 1.1 Â 10 4…”

Processing of astrophysical ices by soft X-rays and swift ions

“…Such fast primary electrons lose energy by creating electron-hole pairs at a distance that is generally short compared with the penetration depth L of the incoming photon. As indicated by Bergantini et al (2014b), the range of 1 keV electrons is around 60 nm inside such astrophysical analog samples and for low-energy electrons (∼ 5 eV), the mean-free path inside such ices is around 1-2 nm; ii) Excitation of secondary electrons by fast primary electrons (photoelectrons). Once a secondary electron is created, it will undergo a short random walk in the material while losing energy due to the creation of phonons.…”

Synthesis of formamide and isocyanic acid after ion irradiation of frozen gas mixtures

“…On the other hand, previous studies have shown that processing of various ice mixtures by UV photons (Bernstein et al 1995;Henderson & Gudipati 2015), energetic electrons (Jones et al 2011;Bergantini et al 2014;Henderson & Gudipati 2015), and ions (Gerakines et al 2004;Demyk et al 1998) produce both formamide and isocyanic acid. The experimental results obtained so far are discussed in some detail in Sect.…”