It seems that when the discussion of Martian and Lunar meteorites comes up two questions are always asked. First how do they get to Earth? And secondly, how do you know they are from the Moon or Mars?
Most meteorite are pieces of asteroids. Chunks of rock that orbit the Sun and are much smaller usually than any of the planets. When asteroids smash into each other bits are sent in every direction to wander in space on their own. If they get close enough to the Earth our planets gravity can capture them and pull them in. This is the story that most meteorites have as their history. On very rare occasions however, a tremendous impact on a planet or moon of a planet like our Moon for instance may also create bit of rock that may someday reach Earth. Not everything that is needed to answer every question about meteorites from the Moon and Mars is yet known. But, it would appear from what we do know that some sequence of events like what follows is reasonable.
The Moon and Mars are large bodies with substantial gravity. So launching rocks by asteroid impacts from large bodies into space requires a great amount of energy to be imparted to the chunks. Otherwise they would simply fall back to the surface. Some scientist think that the way it happens is in very shallow angle impacts the Martian and Lunar rocks are plowed in front of the impacting asteroid and are accelerated to escape velocity. On both the Moon and Mars are found a few very elliptical shaped craters that may be candidates for having made rocks that have made their way to Earth. Though the gravitation force is greater than asteroids in space it is less for mars and the Moon than for Earth. Getting rocks off the Earth to become potential meteorites is ever a harder proposition. But, it seems to scientists that plowing and throwing the rocks off the surface is one possible method.
Once the rocks are out in space they wander around in wild orbits with some that come near to the Earth’s region of the solar system. How do we know when they are found as meteorites that Mars or the Moon was their original home? We have had some spectacular successes in the space program. Several missions to Mars and the Moon each have helped us to know enough about those world to recognize rocks from there when tested. First of the characteristics that are observed in Martian and Lunar meteorites is their lack of chondrules. They have structures that make it clear they formed a larger bodies where gravity prevented the small spherical chondrules from forming. Lunar and Martian rocks have crystalline minerals similar to those found on Earth. Often volcanic minerals called basalts. The rocks maybe made up of small angular bits of these minerals. Such rocks are called breccias.
The atmosphere of Mars was accurately sampled and analyzed by probes, so we know the detailed gas signature. When meteorites are found that are made of material different than from asteroids test are done to see if trapped gas has the Martian signature. Many have now been found that do. The Moon on the other hand has no atmosphere but the rocks still preserve a signature of the isotope ratios which we can compare with samples returned by both manned and unmanned missions to the Moon. We have hundreds of pounds of lunar rocks, yet many of the meteorites we now know are also from there are very different. We landed in only a handful of locations. The meteorites provide samples form other locations.
Moon Meteorites and Mars Meteorites are collected for many reasons I’m sure. But among those reasons is because they are rare and what comes back from manned and unmanned missions will never be available to collectors. Meteorites found in deserts and elsewhere sample material from many places in our solar system including our Moon and at least Mars in addition to the asteroids. The hunt is on for meteorites from Mercury, Venus and also pieces of the Earth that may have been blasted into space.