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Astron. Astrophys. 341, 304-311 (1999)

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2. The H2O - O3 criterion

It is important to specify the conditions when the simultaneous detections of H2O and O3 becomes a proper test for the presence of a biological activity on an exoplanet. The main concern is, of course, to avoid false positives i.e. detection of abioticO3.

2.1. Possible sources of abiotic O2

In his original paper, Owen (1980) already considered this question and since that time, it has been revisited several times. Before Noll et al.'s paper, the main process considered for producing large amounts of abiotic O2, and consequently O3, was the photodissociation of gases as CO2 and H2O. The former can produce only very limited quantities of O2 (Rosenquist & Chassefière, 1995) but the latter requires a more detailed study.

The corresponding chain of reactions can be summarised as:

[EQUATION]

If hydrogen escapes from the atmosphere of the planet, the result is the presence of abiotic O2 (Chassefière, 1996) that in turn can produce O3 through additional photochemical processes.

2.2. Telluric planets with temperate climates

When a planet has a surface temperature similar to that of the Earth, a cold trap develops in its atmosphere that precipitates most of the evaporated H2O as rain or snow and prevents it from becoming a major constituent of the upper layers of the atmosphere where liberated hydrogen could escape. The hydrogen produced by H2O photolysis has then to go through a long diffusive path before escape becomes possible. During this journey there is a high probability of recombining with oxygen and rebuilding H2O. The overall destruction rate of H2O and production of abiotic O2 is then very low. On Earth, this production is only [FORMULA] times that by biological photosynthesis (Walker 1977) i.e. 1 ppm is abiotic.

Is a cold trap expected on a telluric exoplanet? For planets with plate tectonics which are located inside the Continually Habitable Zone (CHZ) of their star (Kasting et al. 1993a), the CO2 cycle provides an efficient regulation of the surface temperature and provides the conditions for the development of this cold trap. An increase of the temperature produces an increased of H2O evaporation, more rain, which in turn dissolves more atmospheric CO2. The acidic water attacks silicate rocks and fixes the CO2 in the planet's mantle as carbonate sediments. The greenhouse heat trapping due to CO2 decreases and the surface temperature cools back (Kasting 1997; Duplessy & Morel 1990). Conversely, a surface cooling decreases the water evaporation and decreases the fixing rate. As plate tectonics and volcanism provide a steady source of CO2, the concentration of this gas increases in the atmosphere and the resulting greenhouse effect combats the initial temperature decrease.

This regulation has to be very efficient because it has never failed during the 4.6 Gyrs of the Earth's life, although plate tectonics has produced major changes in the distribution of continents and oceans (e.g. opening and closing the Straits of Gibraltar) that changed Earth's albedo and might have caused a climatic runaway.

2.3. Greenhouse runaway

The CO2 regulation is efficient as long as the planet temperature is not high enough to prevent the formation and preservation of carbonates. Conservatively, Kasting et al. (1993a) estimated that planets at normalised distances from their star larger than 0.95 AU do have a cold trap. The normalisation is a multiplication of the actual distance by [FORMULA] where L and [FORMULA] are the star and Sun luminosities. Normalization results in black sphere planet temperatures that are independent of the star luminosity.

If the planet is too close to its star, the regulation by CO2 can fail and the planet suffers a runaway greenhouse effect. This probably happened to Venus. When the trap is not cold enough, large quantities of H2O vapor enter the upper atmosphere, the water vapor greenhouse effect increases, as does the surface temperature. More water evaporates, increasing the heating and so on. Water becomes a major constituent of the atmosphere and its UV photolysis takes place in the highest layers of the atmosphere. Hydrogen can escape after a short diffusion path during which the probability of rebuilding a water molecule is small. Massive hydrogen escape and abiotic O2 production result.

However, there are several criteria to recognise such a situation:
(1) the temperature runaway is expected only for planets at renormalised distance from their star significantly smaller than 0.95 AU. In the missions proposed to search for life by using the presence of O3 as a criterion, both the star luminosity and planet separation from it will be directly observable, and it will be possible to determine the black sphere temperature independent of the star's distance. This will enable us to estimate whether a planet can be in a hot and moist runaway phase.
(2) a runaway process is a fast event. The time required for photodissociating the whole content of free H2O at the surface of the planet is relatively short (0.1- 0.3 Gyr). The duration of the following phase, when O2 is being depleted from the atmosphere will depend upon the rate at which O2 sinks can capture oxygen. It can last longer than the first phase because the O2 sinks can be temporary saturated (finite volcanic outputs...). Consequently, if the criterion is: "the simultaneouspresence of O2 and H2O ", the probability of finding an exoplanet in such a phase is small. If a sample of planetary systems can be studied, only a small fraction of them could be in such a situation (Owen, 1980), in addition to the characteristic feature of their location out of the CHZ of their star.

In conclusion, this production of abiotic O2 will not be misinterpreted as evidence of photosynthesis.

2.4. Icy planet without volcanism

There is another situation that can lead to an atmosphere rich in abiotic O2: that where oxygen sinks are inactive which allows the low production of O2 by UV photolysis to accumulate (Kasting, 1997).

When a planet is small enough, its volcanism disappears some time after the planet formation and so do the associated oxygen sinks. Quantitative calculation should be done but there is probably room between the sizes of Earth and of Mars for a planet large enough to retain a thick atmosphere (0.1-10 bar) but small enough for volcanism to vanish after 2 Gyr or thereabouts.

The sink due to water weathering can also be lacking if the planet is cold enough to have very low water evaporation, scarce snow falls but no liquid water rainfall.

Consequently, a planet with a size in between those of Earth and Mars, located outside the CHZ, could have an atmosphere rich in abiotic O2.

There are features that would allow the identification of this situation too:
(i) the planet is located outside CHZ;
(ii) it has a small size;
(iii) the temperature of its surface is lower than the H2O freezing point;
(iv) the strength of the water bands in the planet spectrum are weaker because of the low water content of its atmosphere.

Modelling is needed to quantify points (ii) and (iv).

This production of abiotic O2 will not be misinterpreted as evidence of photosynthesis either.

2.5. Formulation of the H2O - O3 criterion

The preceding considerations lead to the formulation:
"a simultaneous detection of significant amounts of H2O and O3 in the atmosphere of an exoplanet located in the Continuously Habitable Zone of its star, i.e. at renormalised distances from the star between 0.95 and 1.15 AU, is a good signature of biological activity".

We have shown that this criterion does not suffer from a confusion with abiotic production by UV photolysis in the planetary atmosphere. Now, we consider the objection by Noll et al. (1997) where abiotic presence of O2 is due to infall of comets whose icy grains would be rich in O2.

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© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998
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