second law of Thermodynamics?

Is the co2 dispersed into space or is it contained in the capsule?

The second law states that energy can't be created or lost, only converted into a different form. If the capsule was a sealed environment and nothing - not even sunlight or electricity derived from sunlight intruded - there would be no change in the mass of the capsule.

What would have changed was that the energy present had been degraded ie entropy had increased. This would be in the form of heat produced as the animals respired. The second form would be found in the conversion of the calories in the feed into a low-value waste-product. Without the addition of more electrical energy or calorific inputs the system would degrade to the point that it would not longer support life.

Its mass would not change.

That's actually the first law of thermodynamics. The second law of thermodynamics is the one that states that enthropy in a closed system can never decrease.

I'm assuming some small amount of dust was accumulated on the capsule and therefore the mass increased by a miniscule amount, according to the law of "even space needs some sweeping up."

Probably not going to get you full credit, though.

Correct, correct. I was a little bit pre-occupied with the concept of how entropy would be expressed via rabbits (I have issues.... you can check on the appropriate threads).

Entropy will increase, but it can't be 'reset' without the addition of more energy in the form of sunshine or food.