The Moon with the Mare Humorum location, inset.
d. This photo shows Mare Humorum as it appears today.
a. This illustration shows the Mare Humorum region as it probably looked about 4 billion years ago, when it would have been completely covered in craters.
c. A few hundred million years later, heat from radioactive decay built up enough to melt the Moon's upper mantle. Molten lava welled up throught the lithospheric cracks, flooding the impact crater.
b. Around that time, a huge impact excavated the crater that would later become Mare Humorum. The impact fractured the Moon's lithosphere and erased the many craters that had existed earlier.