I Love Impacts!

Today I am traveling to the Lunar and Planetary Science Conference in Houston, TX. The LPSC, as it is called, is a great conference, where scientists from all over the world gather to discuss everything about planetary science.  I will be microblogging from the conference on Google+ so follow me there for short updates on the exciting research being presented.

But today is a travel day. Knowing that I would spend much of the day sitting on buses, planes, and in airports, I went out for a pre-emptive run this morning. My usual entertainment on my runs includes science fiction short stories brought to me by Escape Pod, the science fiction podcast magazine. Today's run featured Episode 386: Finished. The story itself was interesting, but the best part came at the end, in the "Feedback" section, where the editor mentioned a comment I had made on the Escape Pod forums in February. The comment was in response to Episode 381: Elias, Smith and Jones. There the forum discussion veered into an analysis of whether a pea-sized object would cause any damage to a space ship. I piped in with some details and cool pictures about hypervelocity impacts. And this was one of the comments mentioned in the Feedback.

I am pretty proud of this comment (check it out here). Writing it was one of the things that helped me decide I wanted to start a planetary geology blog. So, that comment post can really be considered to be my first blog post.  And I wanted to take some time to expand upon it here, because I really do love impacts.

So, this is what happens when an object moving at very fast speeds hits another object.  Anything faster than about 2 km/s is called a hypervelocity impact and at these speeds, collisions behave very differently than what we are used to in everyday life experiences. For one, the resulting crater is much bigger than the object that hit the target.

This is what happens when a 12mm aluminum ball hits an 18 cm thick aluminum plate at 7 km/s.  The ball we see here is not the original ball that formed this crater. That ball would have been vapourized by the impact. This is a different, same-sized ball, shown here for comparison. Notice how the back of the plate has separated from the rest of the plate, even though the area between the crater and delaminated part looks untouched. This is telling us that a lot more damage happened here than what can be seen.
Image credit: European Space Agency

When this happens on planetary scales, the result is a fairly symmetrical bowl-shaped depression.   That is, if the impactor, the thing that did the hitting, is fairly small.

This impact crater is approximately 1 kilometer in diameter. It is located in the basalt flows of Mare Imbrium on the near side of the Moon.
Explore this crater and its surroundings in more detail by viewing the Lunar Reconnaissance Orbiter Camera data at the ACT-REACT Quick Map Website.
Image credit: NASA/GSFC/Arizona State University

When the impactor is larger, interesting things start to happen in the target rock.  The resulting crater becomes too large to support itself, so the sides collapse inward, forming slump terraces along the outer rim and filling in the crater floor. Also, the floor rebounds upwards, creating a central peak structure. Often, material melted by the impact will fill the floor of the crater, contributing to its flatness.

This is the complex crater Copernicus on the near side of the Moon. It is approximately 93 kilometers in diameter and is visible from the Earth with the naked eye. 
Explore this crater and its surroundings in more detail by viewing the Lunar Reconnaissance Orbiter Camera data at the ACT-REACT Quick Map Website.
Image credit: NASA/GSFC/Arizona State University

With still larger impacts, the collapse of the sides is so extensive that multiple rings can form, creating a bulls-eye pattern.  Extensive amounts of impact melt can cover the floors of basins, hiding some of the characteristic features.

This Orientale impact basin, on the western limb of the Moon. The diameter of Oriental, as defined by the outer ring, is approximately 930 km wide.
Explore this crater and its surroundings in more detail by viewing the Lunar Reconnaissance Orbiter Camera data at the ACT-REACT Quick Map Website.
Learn more about the Orientale Basin at the LROC Featured Image Website.
Image credit: NASA/GSFC/Arizona State University

Of course, there are lots of different deviations from this simple progression that can occur. We see many of them throughout the solar system. But those will have to wait for future blog posts...