Of course, a Martian prior appropriations water rights regime would require a change in international law, which currently forbids any nation from claiming extraterrestrial resources. While this law has value—in avoiding over-exploitation or inequitable exclusivity in extraterrestrial resource rights—it does little to encourage investment in developing those resources. Prior appropriation would create incentives to invest in the exploration of Mars.

Thinking about water rights on Mars is valuable for three reasons. First, missions to develop Martian natural resources may be lucrative business in the future. Mars has potentially valuable mineral resources, including copper, zinc, gold, and the kinds of rare earth minerals used in lasers, microelectronics, and batteries. Indeed, these rare earth minerals are so rare, we may end needing to leave Earth to find a long-term supply. Second, someday interplanetary colonization may be essential for the survival of the mankind. Third, and of more immediate concern, contemplating the water rights regime on Mars may help us better understand what we did right, and what we did wrong, in establishing the prior appropriation regime in the western United States and how to think about the changes to water law that may be necessary to adjust to a changing climate.