Within the enormous clouds where planets and stars form, temperatures are incredibly cold, allowing gas to condense onto dust grains to form ice. These icy grains are vital for chemistry as they enable the formation of a rich variety of molecules seen around us – ranging from simple two-atom molecules to large organic compounds. They also 'stick' together to form the early seeds of comets and planets.
These complex multi-component interstellar ices are mainly made of Amorphous Solid Water (ASW). This is where the water molecules are disordered, unlike the crystalline ice you would find in your freezer.
ASW, when it's created under certain conditions, like those in space, is known to have high porosity and large surface areas. This has consequences for the scope and range of chemical and physical processes that can occur on/in the ice.
Most experimental studies in Astrochemistry rely on indirect measures of porosity, such as measuring gas uptake and observing specific surface structures in infrared spectra. Unfortunately, these have been shown to be either unreliable or limited methods. This means that the detail of the porosity in ASW remains unresolved, with much debate on the overall structure.
To shed light on this issue, a team of researchers from The Open University and ISIS used neutron scattering as a direct and non-interacting method of measuring ASW porosity and structure. They used a complex setup to grow space ice in situ on both Nimrod and SANS2D.
Their study, published in PCCP, aimed to understand how the ASW structure and porosity is impacted by the temperature of its formation, or deposition, as this depends on the region of space that the ice grows in.
“Nimrod's unique hybrid nature gave us an unprecedented look into the ASW structure. We could simultaneously track the larger-scale features like porosity in the SANS region and the overall crystallinity in the diffraction region, all throughout the deposition," explains Dr Zac Amato, lead author, whose PhD was jointly funded by ISIS and The Open University as part of a Facility Development Studentship.
At low deposition temperatures, the ASW structure was confirmed to be highly porous with a large surface area. But this changed significantly when they increased the temperature.
By using the available SANS region of Nimrod and combining the results with experiments on SANS2D they were able to, for the first time, provide direct experimental evidence for the presence of two main types of pores.
Their results have led to a new picture of the overall ASW structure, which involves microporous islands with voids between them. This differs significantly from prior models visualising ASW to date.
“Overall, these neutron scattering studies have drastically changed our picture of the structure of interstellar ice and the role this plays in processes that lead to planet- and star-formation," adds Zac.
These complex experiments relied upon the support of the Pressure and Furnace, Cryogenics, Electronics and Soft Matter groups at the ISIS, who helped the group set up the dedicated deposition setup and were on hand to help solve any problems. In particular, Chris Goodway from the Pressure and Furnace team helped the team with the design and construction of the setup.
The data reduction and analysis was done through ISIS's server IDAaaS, using Mantid and Gudrun, which are supported by the computing and disordered materials groups at ISIS.
Read the full paper at DOI: 10.1039/D5CP00270B
