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. 2015 Sep 10:5:13977.
doi: 10.1038/srep13977.

Titania may produce abiotic oxygen atmospheres on habitable exoplanets

Norio Narita  1   2   3 , Takafumi Enomoto  3   4 , Shigeyuki Masaoka  3   4 , Nobuhiko Kusakabe  2
Affiliations

Titania may produce abiotic oxygen atmospheres on habitable exoplanets

Norio Narita et al. Sci Rep. .

Abstract

The search for habitable exoplanets in the Universe is actively ongoing in the field of astronomy. The biggest future milestone is to determine whether life exists on such habitable exoplanets. In that context, oxygen in the atmosphere has been considered strong evidence for the presence of photosynthetic organisms. In this paper, we show that a previously unconsidered photochemical mechanism by titanium (IV) oxide (titania) can produce abiotic oxygen from liquid water under near ultraviolet (NUV) lights on the surface of exoplanets. Titania works as a photocatalyst to dissociate liquid water in this process. This mechanism offers a different source of a possibility of abiotic oxygen in atmospheres of exoplanets from previously considered photodissociation of water vapor in upper atmospheres by extreme ultraviolet (XUV) light. Our order-of-magnitude estimation shows that possible amounts of oxygen produced by this abiotic mechanism can be comparable with or even more than that in the atmosphere of the current Earth, depending on the amount of active surface area for this mechanism. We conclude that titania may act as a potential source of false signs of life on habitable exoplanets.

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Figures

Figure 1
Figure 1. Schematic illustration of the photocatalytic oxygen formation on titania photocatalyst in the presence of water and an electron acceptor.
Photon absorption of titania affords an electron (e−) – hole (h+) pair on the photocatalyst. The electron reduces the acceptor and the hole oxidizes water to oxygen on the surface of the titania photocatalyst.
Figure 2
Figure 2. NUV and total solar flux density data taken at the Hateruma Observatory.
The upper panel (a) shows data taken on a clear day (23 June 2013) and the lower panel (b) does the same for a cloudy day (20 June 2013). The horizontal axis indicates time in Japanese Standard Time (Universal Time + 9 hr). The violet line (left axis) plots NUV flux density and the yellow line (right axis) does total solar flux density. The data were obtained in one-minute intervals.
See this image and copyright information in PMC

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