Covered Interest Rate Parity

Two currencies. Financial ones...notot electric ones. They buzz along, such that a Martian dollar buys two American dollars. There exists a whole network of derivatives of those currencies: futures, forwards, various interest rate machinations. And the concept basically states a kind of immutable law in finance that well-defines the relationship of how these two currencies should trade, relative to each other. Or said another way, the law runs: the interest rate differential between two currencies in cash markets or money markets or equivalent short-term paper...should equal the differential between the forward and spot rates of the currencies.

That is, you'd take whatever price the Martian paper's forward contracts are trading at, then subtract them from that equivalent forward rate, and you should get the "correct" or market clearing interest rate for the currencies.

Of course, none of this will matter after the mother ship has blown Earth to kingdom come, but...it gives us something to keep ourselves occupied with in the meantime.