Jerry,

If the masses of Mars, Titan, etc. are as radically different as you seem to suggest, then how do the probes make it there at all? A few failures doesn't justify a rewrite of physics especially when there have been so many successes using our current understanding. Is G an unchanging value, that real science suggests, for most of the trip except for a bit at the end where it goes goofy now? Instead of grasping at straws try a few books on physics instead.