The short answer to this question is that we don’t know - although there is a whole field of study dedicated to trying to figure it out. What I’m about to describe are not truly theories - they’re suggestions. If you could transport life around a solar system, how could you do it? We don’t have any evidence to show that these are the ways in which it happens, or even if nature needs the help.
The hypothesis that suggests that either bacterial life or the building blocks for it could be transported via comets or meteorites is called panspermia. It proposes that once life were to arise somewhere in the universe, a large impact upon that planet could create such an explosion that pieces of rock carrying bacteria were flung into space, where they cruise about until crash-landing on another life-friendly planet.
We know that meteorites blasted off other planets can make it to our own planet, since we’ve collected a few meteorites that originated on Mars (as discussed in a previous post), so the real question is whether or not bacterial life is hardy enough to survive this whole process, including the blasting off the original planet and the landing on the new planet. If it can, then this might be one way to get bacterial life from Planet A to Planet B within a solar system, however unlikely it might be for those pieces of rock to be flung in exactly the right directions. We have been testing how resilient some bacteria can be to exposure to outer space, which is a pretty unforgiving environment. Some microbes from the English town of Beer managed to partially survive a 533 day stint on the outside of the ISS, which is pretty impressive, but there’s no guarantee that a rock blasted off the surface of another planet will be covered with a particularly space-hardy breed of microbe, nor is there a guarantee that the meteoroids will arrive at another planet in a short period of time.
In order to transport entire bacteria, a lot has to go exactly right. But it might be a little easier to transport not the live bacteria, but the the organic molecules that are required to build them. Amino acids are one such set of complex organic molecules, and are often called the building blocks of life. If a comet or meteorite can bring amino acids to a planet, it might help to jump-start the development of life on that planet by skipping the steps required to build those molecules from scratch. These molecules don’t need to have formed on the surface of another life-bearing planet - surprisingly complex molecules can form in the gas between stars in our galaxy. Because these simpler building blocks may be more widely available than planets with life that are also being bombarded with impacts, it may be easier to distribute these molecules than it is to distribute entire bacteria.
Until we have more concrete proof of either of these processes happening out there, they will remain suggestions, but we’re working towards testing each of the steps individually to get a sense of how plausible all of the steps together might be.
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Unlike the shooting stars we see in meteor showers, which burn up entirely in our atmosphere, meteorites are the fragments we find that have made it all the way down to the ground. Like anything that enters our atmosphere, meteorites have also been burned by the friction of our air, which means that the pieces that we find are only a fraction of the original object; the rest of it was burnt up as a colourful vapor in our sky, like the pieces of grit that make up a meteor shower.
Meteorites are usually made of a combination of one of two substances: rock, or iron. With the infinite inventiveness of astronomers, we duly called them stony, iron, or stony-iron, if they’re a relatively even blend of the two. Stony meteorites can (and often do) contain some small fraction of iron, but only those meteorites which are almost entirely metal are classified as iron meteorites. The vast majority - over 90% - of meteorites that fall to earth are stony, leaving less than 10% of all meteorites as primarily iron. However, the iron ones are a little more well known, and that’s because for a long time, the iron ones were much easier to identify as meteorites.
Stony meteorites look like stones (see the picture just below), so unless you know exactly what you’re looking for, they can be hard to spot. In certain parts of the world, they can be a little easier to pick out - the deserts of Africa or the high deserts of Chile will let unusual-looking rocks stand out a little more. The best place to look for stony meteorites, however, is the glaciers of Antarctica. The ice there is so old and so slow-moving that meteorites that fall there will stay put for a long time. Meteorites are also quite easy to spot there, since anything dark will stand out for miles against the white ice.
That said, even though they look like one of many other stones on our planet, they are still fantastically interesting to scientists. Some of these stony meteorites are some of the oldest untouched rocks in the solar system, and they give us a sense of what the early solar system was made of and how it built itself up.
We also have some stony meteorites which have been blasted off of other worlds. In particular, we have a little collection of about 30 different meteorites which were originally part of Mars, one of which is shown just below. These are chunks of the surface of Mars which were flung so far away from the surface after another impact that they never fell back to the surface, and wandered the solar system until they encountered the Earth. These martian meteorites tend to be much younger than the rest, and are composed of a different set of elements and minerals, so we can pick them out without too much trouble.
Iron meteorites are much easier to spot than the stony ones. This is partially because it’s relatively unusual to find weathered chunks of iron sitting on the ground, so they are more easily recognized as out of place. As with the stony meteorites, they are most easily found in deserts and in the glaciers of Antarctica, where they are likely to stand out more starkly from their background, but they can also be relatively easily picked out in other places. Iron meteorites don’t wear down as much by erosion as the stony ones do, so they’re also more likely to stay around for a longer period of time. The iron meteorites are not actually completely made of iron, but contain a significant fraction of nickel, and this blend of metals is part of what makes them unique.
This one has been sliced in half and polished to show off the metal crystals inside it - but without this extra treatment, they look much less dramatic.
Opportunity, one of our Mars rovers, discovered a basketball-sized iron meteorite on Mars in 2005, which people got very excited about. The discovery of that meteorite was the first time we’d found a meteorite on a planet other than our own. This is much more what iron meteorites tend to look like before being sliced in half.
Iron meteorites and stony meteorites also tend to behave a little differently as they come through the atmosphere. Stony ones tend to be a little more fragile against the force of the atmosphere, and as a result, they can shatter more easily before they hit the ground. Iron meteorites are a little more resilient; heating up the outer layers of a metal lump doesn’t create enough stress to fracture them the way the stony ones would.
The Chelyabinsk meteor that exploded over Russia a little over a year ago also left behind some pieces for us to examine; the largest of the pieces were extracted out of a lake. One of the confirmed pieces is shown at the top. The meteorite pieces confirmed that the object that came through our atmosphere was a stony type meteor. It had already been suspected of being stony since it had exploded so brilliantly in the air.
Both iron and stony meteorites give us a fascinating look at the early solar system, and help us to understand how the planets formed and when, so finding more and more meteorites helps us to understand the variety that was present when the planets were first being formed.
Thanks for your patience while my thesis was being completed, everyone!