Mercury is different from the Moon and Mars. Mariner showed that the planet was covered by smooth plains and cratered planes but their origin was unknown.
So what were they formed by? Scientists studied the surface to see if the planes were formed by volcanic flows or material ejected from Mercury’s impact basins.
New images from NASA’s Messenger spacecraft showed that at least the younger planes resulted from volcanic activity. But it was still not known how far into the past volcanic activity occurred or how much of the planet’s surface may have been resurfaced by volcanic activity.
New insights brought forward today calculate that the first 400 to 500 million years of the planets evolution are not recorded on its surface. This study did show that the oldest terrains that are visible have an age of 4.0 billion to 4.1 billion years.
This result took quite a bit of work measuring the sizes and numbers of craters on the most heavily cratered terrains using the images from the Messenger spacecraft. From this they concluded that the craters were created on or at about the same time pointing to volcanic creation.
These results also showed that the oldest terrains coincided with the late heavy bombardment period (a heavy impact time of asteroids and comets).
When you put all the findings together they show that not only was the resurfacing due to volcanism but also due to the impact of rocks etc while the volcanism was in process.