Super-Venuses

There are a lot of headlines about exo-planets. It would seem the general public has a modicum of interest in whether there are other planets on distant solar systems, and the continual addition to the collection of known solar systems or at least some selection of planets of distant solar systems keeps the interest up. One of the more prevalent news stories is about how some telescope or some astronomer has reported some super-Earth hundreds of light years from us.

There is a selection effect, in that small planets are harder to detect than a larger planet would be in the same orbit. Larger planets induce more wobble in their parent star and cause a bigger reduction in light when they fly in front of the star. So far, not many planets of the same size as the Earth have been found, but there are multiple super-Earths, which are planets only a small multiple of the mass or diameter of the Earth. This will certainly change, as budgets permit even higher resolution telescopes to be constructed and turned to the search for yet more exo-planets.

There is one Earth-sized planet that seems to be largely neglected, Venus. Venus is almost the same size as the Earth, both in mass and diameter. It is located about 72% of the distance to the sun as Earth, meaning solar radiation is about twice as much. From this alone, Venus should be hotter than the Earth, and it is, but the temperature difference is greatly exaggerated by the fact that Venus has a carbon dioxide atmosphere about a hundred times the mass of Earth’s. This produces a great greenhouse effect, which helps to explain why the equator of Venus has something like 465ºC, rather uniformly because the thick atmosphere spreads the heat.

Venus and Earth would produce identical signals to some alien world watching our star, using the same kinds of instruments as we currently use to find exo-planets. The same mass means that the wobble induced by Venus would be the same as that induced by Earth in the same orbit, and the same diameter means that Venus would block as much of the sun as Earth in the same orbit. So the conclusion from this is that when an astronomer issues a press release indicating they have discovered another super-Earth, or even something similar to Earth, they could just as easily produced a press release saying they discovered a super-Venus, or something similar to Venus. Atmospheres are quite thin compared to the diameter of a rocky planet, so it will be a while before reputable measurements of the atmospheric mass are available, which is what would be needed to directly discriminate between a Venus and an Earth in some distant solar system.

There are two explanations for why the atmospheres of Venus and Earth are so radically different. One might hark back to the formation of the Earth, during the first period of bombardment by asteroids, when a large planetoid is believed to have impacted the Earth, producing the moon. This impact might have blown off much of the atmosphere during the impact, and even more might have been ripped off if the moon started out its life in an orbit very close to the surface of Earth, from which it was torn. Venus has no moon, and it might be quite unlikely that such an impact, with just the right masses, velocities, and miss distance center-to-center, would happen to other planets in other solar systems. If this hypothesis is correct, we should be detecting super-Venuses instead of super-Earths, and soon, Venuses instead of Earths.

Another possible mechanism is that life ate up all the carbon dioxide in the atmosphere of a primitive Earth, producing oxygen in its place, creating the lightweight atmosphere Earth now has. This hypothesis has some difficulties. If there was an Earth with a huge carbon dioxide atmosphere at the present Earth orbital radius, it too would have a greenhouse effect that would raise its temperature above that where life could form. The older sun was somewhat less bright, but not that much less bright so as to allow this form of atmospheric modification to occur.

Just consider for a moment the situation in the galaxy if rocky planets forming in solar systems like the one we inhabit almost invariably have heavy carbon dioxide atmospheres, and there is rarely a situation with the right type of planetesimal collision to strip it down. Using our G2 sun as an example, if there was a Venus-like planet at Earth’s radius, there would not be life because of the high temperature, which means no aliens and no star travel. If there was a Venus-like planet further out toward Mars or even beyond, there would be a range of orbital radii where life could survive. A Venus-like planet in that range, if it somehow originated life, might have the same phenomena happen as happened on Earth: carbon dioxide disappears and oxygen appears. Exactly how so much carbon dioxide goes away might be a further question, but just suppose life is potent enough to have this happen almost completely. But when the carbon dioxide goes away, into rocks or sediments or living creatures, the greenhouse effect diminishes, and the planet gets colder and colder.

We had an ice age on Earth, nicknamed snowball Earth, which did not kill off all life. Quite possibly there was an equatorial area where there was no ice cover. But if Earth had been out at a Mars radius, the equatorial safe zone would likely not exist. The whole Earth would be a snowball, ending the chances of life surviving and evolving. And it would likely stay that way. Thus, one option for the non-existence of aliens traveling to Earth and giving us their business cards is that all the planets out in these distant solar systems are Venuses, too hot for life, or snowballs, too cold for life. Earth, with its fortunate collision four billion years ago, somehow was transformed into a planet where life could both originate and evolve. Perhaps there are other planets like ours, with a moon lingering as evidence of the collision, but the numbers would be drastically less than the count of super-Earths (really super-Venuses) would indicate.

Colonizing Mercury

An alien civilization might want to attempt to preserve its own chance of survival, as a species, in the event of some monstrous calamity, such as an asteroid of large size smashing into the planet. One form of insurance is to have a colony somewhere which would not be affected by the catastrophe. It would be nice if there was another planet similar to the home planet in the same solar system, maybe a bit hotter or colder, but close in attributes to the home planet. If there is no such planet, what do they do? What types of planets might serve as runners-up?

In our solar system, we have no such planet. Mars is the closest thing, and we talk about colonizing it someday. But in a solar system with no Mars, what else could they do? How about some planet like Mercury, close to the sun, with no atmosphere, maybe phase-locked with some resonance relation between its orbital time and its rotation time?

If there was some diffusion of heavier elements toward the sun inside the pre-planetary disk, it might be that there is more of the iron-and-heavier elements there. This is not necessarily a good thing from an insurance planet point of view, but it is if you want to mine these resources for some other purpose and make the insurance goal a subsidiary to the mineral exploitation one. To simplify, let’s just consider the insurance goal. Mercury has a reputation for being hot, due to its location near the sun. This is true, in general, but not completely. Mercury rotates in a 3:2 resonance between rotation and orbit, which means that it has a solar day twice as long as its year, 176 earth days. The temperature along the equator rises to about 425ºC at the subsolar point, but drops to -180ºC on the dark side.

To colonize a planet, and have the colony be self-sustaining, it must have two things, energy and resources. If a Mercury-like colony cannot be self-sustaining, then it is of no use as an insurance planet for an alien species. Mercury certainly has energy to spare. A water tank at any latitude other than one near the poles will cycle between steam temperatures and ice temperatures. It doesn’t take much imagination to design something to produce power from this temperature cycling. Suppose there is a surface tank able to withstand high pressure steam, and underground, where temperatures stay closer to the average, another tank of the same size with a turbine between them, as well as a pump. During morning, the water in the surface tank turns to steam, spins the turbine making electricity, which is used at the time both to power the habitation requirements, and to create storable energy, for example, hydrogen and oxygen from water. During night, a set of fuel cells would convert the two gases back to water while producing energy. Water is pumped from the lower tank to the upper tank shortly after dawn and the cycle begins again.

Living under the surface means a more constant temperature in the habitable area. The average equatorial temperature is about 120ºC, meaning that would be the underground temperature, too hot for humans. The polar temperature is always about -180ºC and unless there was significant heating from the core, that would be the underground temperature there also. At some latitude between zero and ninety degrees, the underground temperature should be 70ºC, quite comfortable. Aliens might prefer a different temperature, but the basic idea of having poles very cold from virtually no sunlight and an equator heated up by intense solar heating implies there is a latitude where the average temperature is the one desired, and that would be the one underground.

The next question is: are there mineable resources there that cover the entire range of elements that the aliens might need? The follow-up question is, can they be mined at a sufficiently low cost so that everything needed for life can be produced, with energy to spare?

Mercury is very dense, more so than all other planets except Venus, and should have a wide variety of minerals. The proper questions would be asked about the light elements, those needed for life, such as hydrogen, carbon, nitrogen and oxygen. Oxygen is present in many earth minerals, as oxides, sulfates, borates, phosphates and carbonates. The last of these is a source of carbon, as are pure carbon minerals such as graphite and silicon-carbon combinations, specifically moissanite. Hydrogen is perhaps the most versatile element and is found in thousands of minerals. Nitrogen is the likely show-stopper, as it is much more rare than the others, yet critical for life of any sort we know of.

Thus the question turns into one related to the presence of elements necessary for life, especially nitrogen, at latitudes on Mercury near the comfort zone, and not too deep. Energy would need to be used for excavating, transporting, crushing, extracting, processing, forming, and manufacturing activities. Two key variables will control the answer to this question. What is the level of reliability that the alien society can achieve, as this relates to the redundancy count needed to ensure the colony never runs out of power or foodstuffs or any other critical material? What is the degree of recycling they can achieve, as recycling typically uses less energy than all the separate activities that are involved with obtaining a supply of fresh resources from underground?

In other posts, it was postulated that successful alien civilizations will stress both of these key variables. After some time, an alien society that wants to prolong its existence on its home planet will come to recognize that one limit on its longevity as a civilization is how much mineral wealth of the planet is wasted as opposed to how much is conserved. Reliability and recycling are the main activities that reduce resource usage, and there is no reason to think that these procedures, techniques and habits would not be easy to transfer from the home planet to an insurance colony on a forbidding planet like Mercury. Building in reliability and manufacturing easy-to-recycle products is not something that immediately will spring out in the civilization, but assuming it is sufficiently intelligent, it will happen, sometime around the time they go through their genetics transformation.

By the way, establishing a sustainable colony on a barren and hostile planet like Mercury opens up a wide range of options for an alien civilization which plans to travel to other solar systems. Mercury has some similarities to planets around dwarf stars, and what is learned from their experience with a Mercury-like planet may serve them well, as they can go to a much nearer star to establish a colony. Dwarf stars are the predominant type of star in the galaxy, meaning they are everywhere and close as well.