Thursday, October 1, 2015

Self-heated Planets

A recent discovery by those scientists who capture neutrinos led to a surprising announcement. The amount of heat generated by radioactive decay in the Earth was much larger than had been expected and it was a significant contributor to the heat content and therefore the temperature of the planet. This is just an initial uncorroborated result, but it raises a few questions, some of which are difficult and some of which are amusing.

If the Earth has so much radioactivity, how can the outer planets in our solar system, and the satellites around them, be so different? They are remarkably cold, as would be expected from the lack of solar thermal flux they receive. If the rocky materials that make up the Earth are the same as that inside these planets, they should be warmer. If they are not the same, how exactly was there a differentiation in the primordial dust disk that surrounded our sun and eventually congealed into the planets? One suggestion might be that the early disk was much more compact, but as the sun turned on and started blasting out solar wind and photons, mostly the lighter elements were thrust further outward, and the heavier ones, not so much. Then when the aggregation started, heavier stuff was around closer into the sun, and lighter stuff, perhaps silicate materials and lighter, was out farther. Something to calculate, perhaps.

Different classes of supernova
do different things to the materials from their outer layers and any materials in space near them. Higher fluxes of protons and neutrons would lead to a higher percentage of heavier elements around larger supernovas. Heavier elements form as part of the probabilistic distribution of elements in the different layers of any star, but the sudden explosion leads to a shower of nucleons which can transmute lower elements into higher ones, although the opposite may happen as well. Suffice it to say, it is not obvious that different regions of star forming clouds have identical distributions of heavier elements, and therefore one can ask the question of whether there are planets around some star which have more radioactive elements than the Earth does. One can even ask if there are some planets which have sufficient radioactive elements so that they can be self-heating, and remain warm, even in a far orbit.

The extreme case is of a rogue planet, one which has been cast free of its connection with the originating solar system or even one formed on its own. This process can take place via the interaction of planets in an early solar system, when one quickly or slowly drives another out so far that random gravitational forces can drag it loose from its solar system. Alternatively, a stellar encounter can cast planets into the wild beyond like billiard balls flying from a good break shot. Some astronomers have estimated that there are as many rogue planets as there are orbiting planets. But, can there be warm rogue planets?

What exactly would it mean to have this alternate source of thermal energy for the origination of life? Consider as an extreme example, a rogue planet which stayed as warm as Earth, not because it was near a star, but because it was being heated from within. The source of heat means that the magma inside the planet is relatively warmer than the magma inside the Earth. This has implications for geological activity. There should be more. Over the history of the Earth, there has been many instances where major eruptions have occurred, leaving behind on the surface of this planet something referred to as basalt floods. This is a large area where magma has escaped to the surface. On an internally heated planet, this might occur more frequently and over larger areas. There might also be less thickness in the crust, with the molten lava buried less deeply. This would lead to more eruptions of individual volcanoes, and more magma bubbles near the surface, leading to more vents, both on land and under the sea.

It should be obvious that this type of planet would be more likely to originate the simplest form of life, if the hypothesized conditions for this origination were both met, and one was more energy-rich mineral flows coming into the oceans. The other condition was a soup of amino acids, and these might be formed by volcanic action, possibly. So, it is at least a novel speculation that life could emerge on a warm rogue planet. However, with no solar photons, chlorophyll would play no role, and chemotrophs would be the limit of life’s evolutionary options. How far can chemotrophs evolve?

Chemoautotrophs can propagate in numbers in accordance with the availability of chemical nutrients, their energy source. In a self-warmed planet, there should be much larger supplies of this chemical energy than on a planet with more ordinary constituents, i.e. less radioactivity. It would be expected that chemoheterotrophs would arise in greater numbers to dine on the chemoautotrophs. And it would be expected that evolution would do what evolution does, and try out every variety of mutation that it could devise. In a rich field of chemical nutrients, and a plethora of other lifeforms to feed on, chemoheterotrophs might develop in ways that parallel sea life on Earth. The whole point is that more efficient ways of collecting food organisms will win out, and then other niches will be found and filled, until a whole ecology exists.

One difference exists, and that is without any photons at all, all creatures will be blind. All sensing will be by the detection of vibration, touch, chemical sensing, or thermal effects. Sound might be adopted as a communication means, but with limitations due to the difference between sound generation we see in land animals and in oceanic animals. Perhaps long tentacles will become the standard appendage. Perhaps means of fastening oneself onto surfaces will become very diverse. And even perhaps some creatures will develop light sources, and ways to exploit self-generated light. Obviously, the ocean on a self-heated planet which has a hundred million years to experiment with lifeforms will be very different from what we see here on Earth.

It is not clear that evolution could devise any lifeform that would emerge onto any continents that exist on such a self-heated planet. Unless there was a chemical energy source that could be absorbed by a land creature, anything emerging from the shallows would expire from lack of nutrients. In another blog, it was noted that in some extreme situation, intelligence just might develop underwater. One requirement was a lack of a moon and a lack of tides. A rogue planet certainly meets that criteria. So, at the fringe of speculation, it is possible that a self-heated rogue planet could have some lifeform that mastered the basics of intelligence, which is tool-using and communication, while remaining underwater. Exactly how far this could go is anybody’s guess.

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