  The number of splashed fingers generated by a solid projectile's impact onto
a viscous liquid layer is experimentally studied. A steel sphere is dropped
onto a viscous liquid pool. Then, a fingering instability occurs around the
crater's rim, depending on the experimental conditions such as projectile's
inertia and the viscosity of the target liquid. When the impact inertia is not
sufficient, any fingering structure cannot be observed. Contrastively, if the
impact inertia is too much, the random splashing is induced and the counting of
fingers becomes difficult. The clear fingering instability is observable in
between these two regimes. The number of fingers $N$ is counted by using
high-speed video data. The scaling of $N$ is discussed on the basis of
dimensionless numbers. By assuming Rayleigh-Taylor instability, scaling laws
for $N$ can be derived using Reynolds number $Re$, Weber number $We$, and
Froude number $Fr$. Particularly, the scaling $N=(\rho_r
Fr)^{1/4}We^{1/2}/3^{3/4}$ is obtained for the gravity-dominant cratering
regime, where $\rho_r$ is the density ratio between a projectile and a target.
Although the experimental data considerably scatters, the scaling law is
consistent with the global trend of the data behavior. Using one of the scaling
laws, planetary nano crater's rim structure is also evaluated.
