Shadow measurements on Viking Orbiter photography have yielded depths for 172 fresh martian craters spanning a diameter range of 0.7 to 80 km. Most craters studied are shallower than their lunar and mercurian counterparts. While the martian data exhibit a break in the depth/diameter distribution similar to those found for the Moon and Mercury, the ''inflection'' occurs at a smaller diameter on Mars, and the slopes below and above the break are respectively less than and greater than those of the other two planets. In addition to possible substrate-related transient cavity modification mechanisms, flash vaporization of proposed subsurface H2O would alter the original impact-induced velocity field by enhancing horizontal to subhorizontal target flow an excavation, thus yielding shallower craters. On the basis of the observed distribution, it is suggested that this process is more active at diameters near the break in slope (~4 km), declining in efficiency with increasing diameter, and can account for many dissimilarities between the martian distribution and those of the Moon and Mercury, which cannot be reconciled with gravity , impact velocity, or projectile differences alone.