As we’ve discussed in this series of posts, impact cratering events are highly energetic. They do much more than explode a huge hole in the ground – such events change the nature of the rocks themselves. Target rocks can be broken to pieces, shattered into dust, shocked with high pressures, completely melted, or even turned into vapor. We can’t see the details of all of these sorts of rocks from orbit, but high resolution imaging and the data returned from robot landers can give us a good idea of the large-scale changes impacts make to their target rocks.

Rays and Dust

The youngest craters on the Moon often have bright rays. These rays might be found far away from the crater that produced them. The material that forms them was emplaced with very high energy. Rays remain an enigma. But they do seem to vanish relatively quickly – space weathering processes constantly mature the rays until they disappear. Are they mostly dust, or a string of boulders and other larger particles? We are still learning …

The lunar near side. The bright crater Tycho is easily seen in the lower center of the Moon. Its rays can be traced across the entire near hemisphere. From http://lroc.sese.asu.edu/images Image credit: NASA/GSFC/Arizona State University.

Pieces and Boulders

Impact events break apart the coherent target bedrock into which they form. These impacts create and then eject pieces of all sizes, from pebbles to boulders. Some of these end up inside the crater, while others are thrown outwards into the ejecta. Boulders can tell us a lot about the nature of the rock in which the crater formed.

A small, unnamed crater (740m in diameter) with ample boulders strewn about the interior walls (and into the exterior ejecta). Image width is 847 m, LROC NAC M127328861L
from http://lroc.sese.asu.edu/images?author_id=31&page=6
Image Credit: NASA/GSFC/Arizona State University.

Melt Rocks

In addition to breaking and pulverizing rock, impacts are so energetic they can completely melt rock. This melt can be strewn about the impact site in tiny blebs, or can pool into ponds of melt that eventually re-solidify into hard rock. Some melt can even flow across the surface like lava from a volcano, before it finally cools and hardens.

Impact melt flow on the surface of the Moon. Image is 500 m across. From http://lroc.sese.asu.edu/images?author_id=31&page=6
Image Credit: NASA/GSFC/Arizona State University.

Mixed Rocks

Of course in an impact event all of this is happening at once. Rock is broken and shocked and melted and mixed all at the same time. So it is not surprising to see images where streamers of impact melted rock are mixed in along with other impact ejecta, boulders, and rays.

“High-reflectance ejecta and low-reflectance impact melt streamers surround this fresh impact crater. NAC M189994606LR, image width 2.25 km” From http://lroc.sese.asu.edu/images Image Credit: NASA/GSFC/Arizona State University.

Rocks near impact sites can be complicated – a mix of shocked rocks, broken and pulverized rocks, melted rocks, and more. From remote sensing we can see the larger picture, but what about up close? What do impact rocks look like when we have samples in our hands? That’s for our next post in the series!