<P> Planetary geologists have measured the relative quantities of oxygen isotopes in samples from the Earth, the Moon, Mars, and meteorites, but were long unable to obtain reference values for the isotope ratios in the Sun, believed to be the same as those of the primordial solar nebula . Analysis of a silicon wafer exposed to the solar wind in space and returned by the crashed Genesis spacecraft has shown that the Sun has a higher proportion of oxygen - 16 than does the Earth . The measurement implies that an unknown process depleted oxygen - 16 from the Sun's disk of protoplanetary material prior to the coalescence of dust grains that formed the Earth . </P> <P> Oxygen presents two spectrophotometric absorption bands peaking at the wavelengths 687 and 760 nm . Some remote sensing scientists have proposed using the measurement of the radiance coming from vegetation canopies in those bands to characterize plant health status from a satellite platform . This approach exploits the fact that in those bands it is possible to discriminate the vegetation's reflectance from its fluorescence, which is much weaker . The measurement is technically difficult owing to the low signal - to - noise ratio and the physical structure of vegetation; but it has been proposed as a possible method of monitoring the carbon cycle from satellites on a global scale . </P> <P> In nature, free oxygen is produced by the light - driven splitting of water during oxygenic photosynthesis . According to some estimates, green algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on Earth, and the rest is produced by terrestrial plants . Other estimates of the oceanic contribution to atmospheric oxygen are higher, while some estimates are lower, suggesting oceans produce ~ 45% of Earth's atmospheric oxygen each year . </P> <P> A simplified overall formula for photosynthesis is: </P>

Oxygen (o2) is formed in photosynthesis as a direct result of