Argon-40 is produced by the decay of potassium-40, the radioactive isotope of this common rock-forming element. Europa is mostly rock by mass, so should have its fair share of potassium, producing ⁴⁰Ar over geologic time (but especially early on). As a noble gas, argon-40 will make its way out of Europa's deeper interior, either carried by water released by dehydrating minerals heated as Europa thermally evolves or even by magmas produced that reach Europa's ocean floor, which can then be hydrothermally altered and leached. Europa's ocean thus acts as the major storage reservoir for all the ⁴⁰Ar ever produced and able to escape the rock below. An additional amount is produced by potassium ions dissolved in the ocean itself, just like on Earth. Eruptions of ocean water to the icy surface will create vapor plumes that, if Europa Clipper can catch them in the act, will allow measurement by the MASPEX mass spectrometer of any ⁴⁰Ar present (just one of many spectacular Clipper observations possible!). Of course, plumes and other 'cryovolcanic' activity mean likely loss to space of ⁴⁰Ar as well. Measurement of Europa's argon thus provides a critical constraint on Europa's total geological vigor through time.