Other research
— Charge transfer inefficiency —
WIP
— Gravitational lensing —
WIP
— Neutron lifetime —
Free neutrons decay in about 880 seconds, but the two types of experiments (“bottle” and “beam”) have a long-standing disagreement on the exact value by a lot compared with their measurement precision. This lifetime is an important parameter for our understanding of the formation of helium in the early universe and key tests of particle physics.
We demonstrated the feasibility of an entirely different way to measure the lifetime from space, using data from the MESSENGER spacecraft as it flew by Venus and Mercury. Neutrons are produced when cosmic rays strike atoms in the surface or atmosphere of a planet. MESSENGER scooped up neutrons at different altitudes above each planet, so by studying how many neutrons survive the long flight to high altitudes we can estimate the lifetime. This was made more complicated by the quirks of MESSENGER’s neutron detectors – which were not intended for this experiment – and the varied composition of Mercury’s surface, among other things.
This figure shows the comparison of three models with different neutron lifetimes with the data during the Venus fly-by, for the two detectors that were facing in (a) and opposite (b) the direction of MESSENGER’s flight.
Our measurement is consistent with the lab-based results but, being based on limited data from a mission designed for something entirely different, has a large uncertainty. We are now investigating the best way to measure the lifetime precisely with a new mission.
— Lunar exosphere —
The Moon has an extremely thin atmosphere (a “surface bounded exosphere”) which is so tenuous that the particles can fly around without bouncing off each other. The most abundant component is argon.
Radioactive decay of potassium in the Moon’s outer layers produces argon that leaks out through the lunar surface, joining a handful of other gases in the exosphere.
Argon particles hop around for a short while before either being lost due to interactions with radiation from the sun or getting stuck in the extreme “cold traps” in craters around the poles. Studying the lunar argon exosphere can teach us about the solar wind, the lunar interior and outgassing, the efficiency of volatile sticking in polar cold traps and the kinetics of adsorption and desorption in low pressure environments.
By performing Monte Carlo simulations of the transport of argon molecules through the exosphere, we showed that only a localised source of argon can explain the persistent excess observed over the western maria. This figure shows our model as solid lines and the data from the LADEE spacecraft as points. The dashed lines show the alternative hypothesis of locally varying surface interactions.
We also produced the first simulations to show that the long-term fluctuations in the exosphere’s global argon density could be explained by seasonal variations in the polar cold traps.