Q: When you do a Field Promotion in Martian Chess, where does the new piece come from?

A: For both types of promotion, the pip count of the new piece is the same as the pip count of the old pieces. Since scoring is by pip count, you can take the new piece from either player and give them the old pieces. Their score remains the same.

(Thanks to Jeff Wolfe for fielding this question a bunch of times, and writing it up for the FAQ.)