The harpoon could be a big heavy thing. Perhaps we crash an astroid into Mars that is trailing a cable. That doesn't really jive with the planetary protection principals but it might be the kind of thing we need to consider to get the job done.

> As an incidental benefit of solving this we would be able to perform sustainable agriculture anywhere on Earth.

True. Although sustainable on Earth usually also means economical. I would like to see a sustained habitat on the moon.