I've wanted to write about self-replicating solar robots for awhile.
I've been interested in the idea of sending self-replicating solar robots to the Moon for awhile, but just recently, I read an article on this topic by professor Klaus Lackner. He calls his self-replicating robots, auxons.
While the idea has been around for awhile, it looks like Klaus Lackner and his co-authors (Darryl Butt and Christopher Wendt) have done the best job of thinking through all of the chemical reaction that must occur to derive the materials needed produce self-replicating robots.
A self-replicating robot has to collect enough electricity from sunlight in order to be able to build a duplicate of itself. I've defined the work return on investment as the ratio of the total net work (electrical work in this case) generated over the lifetime of the machine divided by the total exergy (electricity also in this case) to build the machine. A self-replicating solar robot requires a 'work' return on investment greater than one, and for the robot colony to grow the return on investment must be much greater than one. Also important is the rate of return on investment, which is related to the net electricity generated per unit time divided by the upfront electricity consumed in building the solar auxon. (The "net" in the numerator means the gross electricity produced minus reoccurring electricity expenses associated perhaps with labor, maintenance, fuel costs. For the solar auxons, the reoccurring work is the electricity required to move, repair, and clean.) The rate of return is typical given in units of %/yr. You can invert the rate of return to approximately calculate the pay-back time and then double that value to find the time required to double the population of the solar robots.
Lackner and his co-authors found that the solar colony could double in size every few months. Though, their numbers here seem to rather optimistic because the payback time for solar PV panels is on the order of magnitude of 10 years, and that's using PV solar cells chemistry that probably consumes less electricity than the cell chemistry that would have to be used by the self-replicating robots. On the other hand, the doubling rate for blue-green algae is on the order of 20 hours. I have yet to check their numbers, so I am just speculating right now. (I'd like to do a full analysis, perhaps as a class project in a future course I teach.)
I'm particularly interested in the self-replicating solar robots because they seem to be the best way of populating the Moon. (Unlike Mars, it seems unlikely that water-carbon-based lifeforms, such as algae, could survive on the Moon. We could probably populate Mars with the introduction of the strong greenhouse gases [to melt the ice caps] and some algae from the Earth.) If the Moon could be covered with solar robots, we could possibly use the Moon as a staging ground for further exploration of the solar system.
The chemical composition of the Moon (depending on location) is roughly 45% silica (SiO2), 20% alumina (Al2O3) and 10% iron oxide (FeO). The metals in these three materials would make up the main components of the self-replicating robots (Si, Al, & Fe). As Lackner found, one would have to develop innovative chemical processing techniques in order to make the Si, Al and Fe from the oxides.
While it's possible to produce silicon from some electro-chemical reactions, the robots could also use electricity to run a high temperature plasma arc that heats the silicon dioxide to 4000 K, at which point the oxygen is released in the moon's atmosphere. Unfortunately, the oxygen won't stay up there that long because the moon can't hold on to its gases as long as the Earth can. (I calculated that the probability of an oxygen molecules escaping from the Moon's gravity is 424 million times larger than the probability of an oxygen molecules escaping from the Earth's gravity.) Although, it still may be possible to build up a sizable pressure of O2 in the atmosphere once the self-replicating robots start to cover the entire Moon.
Once the solar robots populate the Moon, the electricity they generate could be used to produce hydrogen from water so that we could fuel hydrogen rockets for further travels into the solar system.
My question is: would we consider self-replicating solar robot colonies to be living organisms? I think that self-replicating solar robots fit the definition of life, but I admit that it'd be a primitive form of life unless they could modify their computer program, i.e. their "DNA".
What would be interesting would be to compare the size of the computer program required to be encoded into each robot vs. the size of DNA for blue-green algae. My guess is the sizes of each would be comparable.
In the future, I plan to calculate the return on investment and the rate of return of the self-replicating solar robots in order to estimate how quickly they could cover the entire surface of the Moon.
A back of the envelope calculation of return on investment can be made from: 1) the exergy requirement for producing Si, Al and Fe; 2) the collection efficiency of each robot; 3) the life time that each robot lives; and 4) the amount of Si, Al, Fe required for each robot.