Earth has been hit numerous times in recent months by some large chunks of space rock. One of the larger meteoroids to enter the atmosphere was the November fireball over Saskatchewan, Canada. In this case, an estimated 10 tonne meteorite slammed into the atmosphere, creating a bright bolide (fireball), exploding into fragments. Fortunately, eyewitnesses were able to pinpoint the location of possible debris. Sure enough, after an extensive search in the rural area of Canada, meteorite fragments were found.
However, these fragments did not impact the ground at the hyper-velocities that the original fireball was travelling at, the Earth’s thick atmosphere created an efficient barrier, through air resistance, breaking up the bolide. In this case, an energetic explosion was observed for miles around. Fragments from the fireball then fell at a maximum speed of terminal velocity, bouncing off the ground. Some fragments sat proudly on top of frozen ponds – the debris final kinetic energy was so low that little damage would have been done even if the small rocks scattered over a populated area (unless, of course, someone got hit on the head – they would have had a very bad day).
OK, so we’re well protected from most bits of junk space can throw at us. Most meteoroids, from the size of a grain of sand to the size of a small bus, will burn-up, break-up or explode high in the atmosphere, scattering bits on the ground. But what about Mars? What if Mars gets hit by a sufficiently-sized meteoroid?
Even if the meteoroid does break apart, unfortunately the atmosphere is too thin to slow the debris sufficiently. A lack of air resistance makes for more impressive impact craters. Watch your heads future Mars colonists, you could be faced with a shotgun blast from space…
A future Mars colony will have to withstand very harsh conditions without having to worry about meteorite impacts. Fortunately, although more meteoroids will reach the ground on Mars (as the Martian atmosphere is 100× thinner than Earth’s, thereby reducing the efficiency of air resistance on a falling rock), it remains unlikely that a settlement will get hit (although the probability would be greater on Mars than on Earth).
However, what if there is a bolide event high in the Mars atmosphere? The fragments of the fireball could be spread over a wide area, possibly causing trouble for any man-made structures below. It would be like choosing between being shot by a bullet or buckshot; both are bad news, but getting shot by a shotgun sprays a wider area with damage. There’s a greater chance of getting hit by a spray of smaller meteorite fragments than one large one.
I am sure a single meteorite would cause all kinds of hassle for a Mars colony if it impacted nearby, but small, high velocity debris could wreak havoc with pressurized space habitats nonetheless.
Looking at this recent High Resolution Imaging Experiment (HiRISE) image from NASA’s Mars Reconnaissance Orbiter, it becomes clear that impacts by bolide fragments are a common occurrence on the Red Planet. This cluster of impacts occurred sometime between 2003 and 2007, and are spread over an area a few hundred metres wide. The original meteoroid was probably only a metre in diameter. If an object of this size hit the Earth’s atmosphere, depending on the meteoroid’s composition, the fragments would have burned up very quickly, not reaching the ground. On Mars, the fragments create many small impact craters. As can be seen by the ejected material from the craters, these particular meteorites hit at very high energies.
Interestingly, the two largest impact craters have a dark stripe running between them. NASA has an idea as to what formed this feature:
Although small Martian crater clusters are common, this example is unusual because there is a dark line between the two largest craters. We hypothesize that atmospheric breakup coincidentally made two nearly equal-size objects that impacted close together in space and time so the air blasts interacted with each other to disturb the dust along this line. — NASA/HiRISE