Our Solar System is thought to have formed from a cloud of dust and gas called the Solar Nebula that collapsed under its own gravity about 4 and a half billion years ago. In order to understand exactly how the Solar System formed, the processes that have occurred during its evolutions, and the similarities and differences we observe between different planets and other bodies, we need to know the composition of that Solar Nebula, i.e. the composition of the starting material of the Solar System.
The sun make up about 99.86 % of the total mass of the Solar System, therefore its composition can effectively be considered to be the average for the Solar System. One of the outer layers of the sun, the photosphere, is believed to be relatively unchanged from the average composition of the solar nebula. So by measuring the composition of the photosphere we are, in effect, measuring the average composition of the starting material of our Solar System. The Sun is too hot to sample directly, but we can access it via the solar wind, the hot plasma ejected from the Sun.
NASA’s Genesis mission collected samples of the solar wind and returned them to Earth for chemical analysis. The aim of the mission was to measure the elemental and isotopic composition of most elements in the wind, from lithium to uranium. Genesis launched in August 2001, and travelled to a point in space known as Lagrangian Point L1, where the gravity of the Earth and Sun cancel each other out. The spacecraft then spent 2 and a half years orbiting around L1, facing the sun. During this time solar wind ions were implanted into the collector arrays.
The sample capsule returned to Earth in September 2004. The plan was that parachutes would open to slow the capsule’s descent, and then it would be caught mid-air by something akin to a long fishing rod hanging out of a helicopter. Unfortunately this part of the mission did not go quite to plan. The parachutes failed to open and the capsule hit the ground at almost 200 miles per hour.
The sample capsule was taken to Johnson Space Center in Houston to assess the damage. The impact of the landing had smashed the collector arrays into thousands of pieces. Fortunately the amazing people working in the curation team at NASA have been able to identify over 10000 pieces of the collectors, including determining which material the pieces are made of and which array they originated from. They have also developed cleaning techniques to remove dust from the landing site without damaging the samples. The solar wind is implanted only a fraction of a millimetre below the surface, so even the smallest scratch on the surface would remove the region where the solar wind is implanted.
So in spite of the “non-nominal landing”, it is still possible to fulfil the scientific objectives of the mission – they have just been delayed a little because of the need to identify and clean the samples.
I nominated NASA’s Genesis mission for Mission Cup 2021 because I have done a lot of work on Genesis samples, reporting the first measurements of the isotopic composition of xenon in the present day solar wind. Xenon is an important element for tracing the chemical evolution of the Solar System. The isotopic composition of xenon in Earth’s atmosphere is odd! Prior to the Genesis mission, the only data we had about the xenon isotopic composition of the solar wind was from lunar samples, which also contain contributions from other sources. But atmospheric xenon cannot be derived from solar wind xenon. So it was important to have a direct measurement of the isotopic composition of the solar wind to see if that is the same as or different to the composition determined from the lunar samples. The short answer is that it is the same, and we still can’t derive atmospheric xenon from solar wind xenon!
Our work analysing xenon in the Genesis samples is published in 2 papers:
Crowther S.A. & Gilmour J.D. (2012) Measuring the elemental abundance and isotopic signature of solar wind xenon collected by the Genesis mission, J. Anal. At. Spectrom., 27, 256-269
Crowther S.A. & Gilmour J.D. (2013) The Genesis solar xenon composition and its relationship to planetary xenon signatures, Geochim. Cosmochim. Acta, 123, 17-34