Question

the weight of 80 kg of mass on mercury is 296 N and almost identical to the weight of the same mass on Mars but mercury has much less mass than Mars explain how this can be.(what else could affect gravitational force?)

Answer 1

Where Gravity rely's on only mass and distance and nothing else, so the weight on the planets will vary like you have stated. However Mars is smaller than Mercury, so the weight on Mars will be less, and the weight on Mercury will be more. Think this way.

More Mass = More Gravity = More Weight

Less Mass = Less Gravity = Less Weight