Studying solar wind

29 Jun 2011

An international team of scientists, including Dr Chris Coath from the University of Bristol, have measured oxygen isotopes in solar wind, captured by NASA’s Genesis mission, to infer the isotopic composition of the Sun, and, by inference, the solar system as a whole. Their results are published in Science.

NASA’s Genesis mission crash-landed back on Earth in 2004.  The spacecraft spent more than two years in orbit around the sun collecting solar wind, which consists of charged particles, on various ultra-pure collector materials.

Fortunately, the collector with the greatest scientific value survived the crash almost intact.  Its primary purpose was to measure the relative abundances of the three isotopes of oxygen: 16O, 17O and 18O.  Despite the length of the mission, the solar wind is so rarefied that the small number of atoms collected required a dedicated mass spectrometer, the MegaSIMS, and years of technique development to measure the tiny quantities of implanted oxygen with sufficient precision.  Dr Coath designed the ion-optics of this unique instrument.

Oxygen isotopes have long posed a puzzle for scientists studying their distribution in the solar system.  Inclusions in meteorites called CAIs, which are the earliest solids to have condensed in the solar system, and other refractory minerals in primitive meteorites, are found to be depleted (relative to the Earth) in the heaviest, and least abundant isotopes, 17O and 18O.

One important question, which could not be answered by meteorite or even planetary studies, is what is the average oxygen isotope composition of the solar system as a whole?  Knowing this would put constraints on the mechanisms which could have given rise to the observed distribution.  Measuring the oxygen isotopic composition of the Sun, which Genesis has done, essentially answers this question because the Sun comprises 99.86 per cent of the entire mass of the solar system.

 The results show that the rocky (inner solar system) planets, which include the Earth, are enriched in 17O and 18O (relative to 16O) by about 6 per cent.  Many believe that this is good supporting evidence for a mechanism called ‘isotopic self-shielding’ occurring in the early solar system.  In ‘isotopic self-shielding’, photolysis (a chemical process by which molecules are broken down into smaller units through the absorption of light) of carbon monoxide liberates oxygen atoms which are rapidly sequestered into water molecules.  Because of the relatively low abundance of 17O  and 18O, carbon monoxide molecules containing these continue to be dissociated after all the photons capable of dissociating C16O have been absorbed.  The water, now relatively rich in 17O and 18O, is then incorporated into dust grains from which the rocky planets are later formed.

Dr Chris Coath said: “Much can be done with samples returned to Earth which is not possible by remote analysis methods, admirably demonstrated by the Genesis mission, the first sample-return mission since the Apollo program.”