Gratteri crater is a young Martian crater which still retains secondary craters, and some of them have unique ejecta deposits similar to mudflows splattered by impacts, suggesting that they might have been formed on wet sand-like material. Interestingly, the morphologies of Gratteri secondary craters vary among regions. We conducted crater formation experiments onto wet sand targets with different water contents and observed that the crater size and morphology changed with target water content. We established a crater size scaling law for wet sand which can account for water content change. Our scaling law suggests that the region 103 km away from the Gratteri crater might have had a higher water content than the region 32 km away (≥4.6 wt.% vs. <2.0 wt.%, respectively), judging from the sizes and morphologies of its secondary craters in each region.