I guess the idea is stability. Seconds are defined as a constant pattern for some physical activity of an element. If you are on a colony on Mars, you can measu

I guess the idea is stability. Seconds are defined as a constant pattern for some physical activity of an element. If you are on a colony on Mars, you can measure that with a complex scientific gizmo and the specification sheet and you are golden. With mass.. you have to get a copy of a reference mass or a tuned measuring instrument up with a shaky spacecraft, then down again and hope it didn't break too much. Having something that would allow to create a reference object on site with only the specification sheet and scientific gizmos is a better way to go about it. edit: spelling