In the past two decades over a thousand planets have been found orbiting stars other than our own, many of which appear to be small rocky planets in the habitable zones of their stars (i.e. the zone in which such a planet might host liquid water, thought to be an essential precursor for life). The Transiting Exoplanet Survey Satellite, due for launch in 2017, is intended to look for Earth-like planets around nearby stars. To do this it will look for biomarkers, chemical signs thought to be indicative of life, in the atmospheres of any planets that it finds. One of the most obvious such biomarkers to look for would be molecular oxygen (O2) which makes up 20% of the atmosphere on Earth, and which is thought to be entirely produced by the actions of photosynthetic Bacteria, Algae and Plants.
However in order to be sure that such biomarkers are genuine signs of life, scientists need to be confident that they could not be formed in other, abiotic ways. Oxygen, for example, can also be formed by photodissociation of water molecules (H2O) by ultraviolet light, and therefore could potentially make up a high proportion of the atmosphere of planets that receive high amounts of such radiation. Large amounts of ultraviolet light are also though to be extremely harmful to life, so that such planets could probably be ruled out of any search for life quite easily, but the potential remains that free oxygen could be produced in other, less easily detectable ways.
An artist's impression of the planned Transiting Exoplanet Survey Satellite. NASA.
In a paper published in the journal Nature Scientific Reports on 10 September 2015, Norio Narita of the Astrobiology Center at the National Institutes ofNatural Sciences in Tokyo, the National Astronomical Observatory of Japan and the Graduate University for Advanced Studies, Takafumi Enomoto and Shigeyuki Masaoka of the Graduate University for Advanced Studies and the Institute for Molecular Science, and Nobuhiko Kusakabe, also of the National Astronomical Observatory of Japan describe a process by which rocky planets could potentially develop atmospheres containing substantial levels of oxygen without photosynthetic organisms or exposure to high levels of ultraviolet radiation.
Narita et al. observe that the mineral titania (a form of titanium dioxide, TiO2), can act as a catalyst for the photodissociation of water molecules by light at near-ultraviolet wavelengths (280-400nm). They further note that the mineral appears to be common in the universe, having been detected in dust outflows around giant stars and supernovas as well as on the Moon and other bodies in the Solar System. Substantial amounts of exposed titania on the surface of a planet with water in its atmosphere could potentially lead to a build-up of molecular oxygen in the atmosphere of the planet.
SEM image of the surface of a grain of titania. Tong et al. (2015).
Narita et al. calculate that were the Earth's surface to be covered by titania, and that levels of near-ultraviolet radiation measured at the Hateruma Observatory between 2000 and 2014 are typical for Earth over a longer period then the level of oxygen seen in our atmosphere could be produced in about 20 000 years, and the all the water in the Earth's oceans could potentially be photodissociated within 20 000 000 years, though they calculate that the amount of exposed titania at the Earth's surface is actually less than 250 km2.
From these calculations Narita et al. conclude that it would be possible for an Earth-like planet orbiting a Sun-like star to develop levels of molecular oxygen in its atmosphere similar to those seen on Earth through the catalytic action of titania without the presence of any form of life.
Next Narita et al. examined the possibility of the titania process creating oxygen-rich atmospheres around Earth-like planets orbiting non-Sun-like stars. They modelled Earth-like planets around a series of hypothetical stars; an M6 Red Dwarf star with an effective surface temperature of 3000K (compared to 5778K for the Sun), a radius 15% of that of the Sun, a luminosity 0.09% of the Sun's and an Earth-like planet orbiting at a distance of 0.03 AU (3% of the distance at which the Earth orbits the Sun); an M0 Red Dwarf with an effective surface temperature of 3800K, a radius 50% of that of the Sun, a luminosity 7.2% of that of the Sun and an Earth-like planet orbiting at a distance of 0.27 AU; a K2 Orange Dwarf Star with an effective surface temperature of 5000, a radius 73% of the Sun's, a luminosity 33% of the Sun's and an Earth-like planet orbiting at 0.58 AU; and an F6 Yellow-white Dwarf star with an effective surface temperature of 6300K, a radius 150% of the Sun's, a luminosity three times that of the Sun and an Earth-like planet orbiting at a distance of 1.73 AU. In each case they found that an oxygen-rich atmosphere could potentially be created by the titania catalyzed photolysis of water.
Narita et al. caution that the formation of oxygen by the photolysis of water will not automatically lead to the development of an oxygen-rich atmosphere. On Earth photosynthetic organisms are thought to have produced oxygen for millions of years before it began to build up in the atmosphere, as free oxygen reacted with a variety of substances present on the Earth's surface, most notably iron, and only once these oxidisable substances had been used up beginning to accumulate in the atmosphere, and as similar reactive substances are likely to be present on any alien world, then titania would need be present at the surface of the planet and suitable near-ultraviolet radiation reach that surface, for long enough for all such reactions to occur before oxygen began to accumulate in the atmosphere.
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