Following on from my last post highlighting the scientific objectives of NASA’s Genesis mission, today I’m going to focus on oxygen. Analysing the isotopic composition of oxygen in the solar wind was the highest priority science objective of the Genesis mission.
Oxygen has 3 stable isotopes: 16O, 17O and 18O. Different parts of the solar system have distinct oxygen compositions. Planetary materials vary in their relative abundance of 16O – oxygen isotope ratios are one criteria used in distinguishing different classes of meteorites. The cause of these variations is unknown, and prior to the Genesis mission it was not possible to define the oxygen composition of the primordial solar system. But understanding the origins of these variations is vital to understanding the origins of the solar system.
A group of researchers from the Department of Earth and Space Science at the University of California Los Angeles (UCLA), USA developed a new instrument specifically for analysing Genesis samples: the MegaSIMS. This new instrument was necessary for a number of reasons:
- The concentration of solar wind oxygen implanted into the collector targets is low.
- The solar wind is implanted only just below the surface of the collector targets, and needs to be distinguished from surface contamination from atmospheric oxygen (oxygen make up about 20% of our atmosphere).
- Only a limited amount of sample material is available.
As the name might suggest the MegaSIMS is HUGE! It fills a whole room, just look at the photos on their website. But rumour has it that you have to have tiny hands to be able to reach inside to where the samples sit!
Their data show that the sun appears to be highly enriched in 16O relative to the Earth, Moon, Mars and bulk meteorites. But remember that the sun accounts for about 99.86% of the mass of the Solar System, and preserved the average isotopic composition of the solar system. Therefore we should turn this statement around to say that rocky materials in the inner solar system were enriched in 17O and 18O relative to 16O. This probably occurred due to non-mass-dependent chemistry before the first planetesimals formed.
Further details of this work can be found in The Oxygen Isotopic Composition of the Sun Inferred from Captured Solar Wind, by K. D. McKeegan et al., published in Science in 2011 (Vol 332, pg 1528-1532).
Next time we’ll look at oxygen’s neighbour in the periodic table – nitrogen.
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