Monday, October 25, 2010

The importance of tides and tidal heating for habitability.

The importance of the moon to the origin of life on Earth is a debated topic. It is also very relevent to the search for extraterrestrial life because theories that suggest that the moon was crucial for life’s origination on Earth, may imply that researchers should look to exoplanets with their own moons in their search for extraterrestrial life. The importance of the moon largely revolves around the importance of tides for the development of life. However, much more generally, not just the tidal effects of moons, but the mechanism of tidal heating in various types of exoplanets should be scrutinized in order to better understand the effect this has on the habitability of planets. Below I will discuss one theory about the importance of tides for the emergence of life on Earth, and will also discuss some important things to think about in terms of the effect of tidal heating on habitability.

The effect of our Moon’s tides

Richard Lathe, a scientist at Pieta Research in the UK, has recently discussed his theory about the cruciality of tides to the origin of life on Earth. He claims that without tides, life could not have evolved.

The story begins approximately 4.5 billion years ago when the moon is believed to have form. At about 4 billion years ago, when life is believed to have evolved, the Moon orbited much closer to us than it does now; in addition, the Earth itself rotated much faster. The combined result was that tidal cycles occurred every two to six hours, with tides extending several hundred kilometres inland. Due to the much greater tidal strength, coastal areas saw dramatic cyclical changes in salinity. Lathe believes that these frequent, cyclical changes in salinity enabled the formation and evolution of self-replicating molecules.

One theory for the origin of life suggests that DNA or RNA formed when small precursor molecules in the primordial ‘soup’ polymerized into long strands which then served as templates, creating double-stranded polymers similar to DNA. In order for this to happen, an external force was needed to dissociate the two strands. Lathe uses PCR (polymerase chain reactions)—a commonly used experiemtnal method of amplifying DNA—to explain this need for a constantly changing environment. In PCR, DNA is cycled between two temperatures in the presence of appropriate enzymes--at the lower temperature of about 50 °C, single DNA strands act as templates for synthesizing complementary strands and at the higher temperature of about 100 °C, the double strands break apart, doubling the number of molecules. When the temperature is lowered, the synthesis begins again.

In the case of tides, when they rolled in, the salt concentration was very low and this would cause DNA to dissociate because of the repulsion of the charged phosphate groups. However, when the tides rolled out, the salt concentration would dramatically increase, which would encourage the strands to associate. The tides could thus lead to the repeated association and dissociation of double-stranded molecules similar to DNA. The tidal force is absolutely important, because it provides the energy for association and dissociation of polymers.

There are many problems to this theory; one major one being that it assumes a very specific origin of life. This origin would have had to occur in the ocean which is a problem for those who believe that life may have more efficiently evolved on surfaces, around hydrothermal vents, or in cell membranes (which can not stabilize in the high salinity of oceans). It also presumes that DNA and RNA were the first replicating molecules, however, some believe that much simpler "genetic" material formed first, from the crystallisation of clay minerals.

Despite these limitations, the importance of a constantly changing environment needs to be highlighted. This is probably a crucial factor for life to evolve. This is also something that needs to be taken into consideration as we think about the necessary compontents of a habitable planet and where to most efficiently search for life.

Tidal Heating of Terrestrial Extra-Solar Planets and the Implications for Habitability

Tidal heating may affect a planet’s habitability in various ways including:
(a) It may be great enough to drive plate tectonics for a length of time that depends sensitively on the host star and planet’s masses and the planet's initial orbit,
(b) It may be so great as to make the planet uninhabitably volcanic (Io),
(c) It may drive outgassing from the planet’s interior, continually replenishing the planet’s atmosphere against loss, and
(d) It may also be sufficient to produce a habitable subsurface ocean on an icy planet, or to mitigate life- challenging “snow-ball” conditions on a terrestrial planet (Europa).

Tidal heating may be critical for the creation and maintenance of the atmospheres of terrestrial planets. As heat induces internal convection within the mantles of these planets, volatiles trapped in the mantle may be outgassed, feeding the atmosphere.
In the habitable zone of M stars, atmospheres may be depleted by vigorous stellar activity or impact erosion and in this case, tides may be crucial to drive adequate outgassing to replenish the atmosphere, enabling the planet to remain habitable.
In other cases, planets may be habitable even without an atmosphere. Tidal heating of an icy planet may generate a habitable subsurface ocean, analogous to Jupiter’s moon Europa. For these planets, an atmosphere may not be necessary for habitability, if, as proposed for Europa, life could exist below the icy surface. As a source of caution, tidal heating may be an obstacle to life when volcanic activity becomes too intense, such as the case of Io. Indeed, tidal heating of the innermost planet in the recently discovered system HD 40307 may exceed Io’s, suggesting that, if the planet is rocky, it may be volcanically active. This would clearly not be a habitable place.

For the Earth, tidal heating is negligible, and adequate heating is provided by decay of radionuclides. Plate tectonics help stabilize a planet’s atmosphere and surface temperature many millions of years. Because a stable surface temperature is probably a prerequisite for life, plate tectonics may be required for a planet to be habitable. However, if in situ formation of terrestrial planets is common, many terrestrial planets we find may have too little radiogenic heating to drive long-lived plate tectonics. The implication of the existence of tidal heating is that even without radiogenic heating, tides may provide adequate internal heating and hence may be critical in determining planetary habitability.

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