The Chemical Processing of the Surfaces of Pluto & Triton

Pluto (former planet) and Triton (Neptune’s largest moon) are thought to have formed from the pristine solar nebula that gave birth to our solar system. Their surfaces are chemically similar in many respects including being dominated by molecular nitrogen (N₂), and the presence of methane (CH₄) and carbon monoxide (CO). Over the age of the solar system, these ices have been subjected to radiation in the form of cosmic rays, magnetospheric particles (on Triton), and solar effects (the solar wind and a photon flux) that chemically modify the surfaces of these bodies. In order to better understand the unaltered material from which these bodies and the solar system formed and to achieve a glimpse of the surfaces today, it is necessary to investigate the elementary chemical reactions that have affected the surfaces of Pluto and Triton since their formation. We have carried out radiolysis experiments on a number of simple ices and their mixtures that are relevant to the surfaces of Pluto and Triton including N₂, N₂:CH₄, N₂:CO, N₂:CO₂, CO, and CO₂. Experiments were carried out at 10 K using energetic electrons as a source of radiation and the progress of chemical reactions were monitored by infrared spectroscopy and mass spectrometry. Several new molecules and interesting chemistries were observed as a result of this low-temperature non-equilibrium chemistry because the formation of terrestrially unstable molecules has been possible. We have developed an extensive picture of the types of reactions that are active in these outer solar system ices so that a broader understanding of the elementary chemical processes that occur in these environments can be obtained. The surface composition of Pluto and Neptune’s moon Triton is dominated by solid nitrogen. Minor amounts of less than 1 % of carbon monoxide and methane have been also detected. Carbon dioxide is present only on Triton (<0.1 %). To understand the chemical processing of these surfaces at, it is important to simulate the interaction of cosmic ray particles with nitrogen-rich ices at appropriate temperatures (10-40 K). We started with pure nitrogen ices (α-phase, β-phase, amorphous) and subjected them to high energy electrons to mimic the δ-electrons generated in the trajectory of the galactic cosmic ray implant.