(Continued)

This is a "martian meteorite," with the stirring name EETA79001. Samples of this meteorite were used to show for the first time that, unknown to us, samples of Mars actually had been in our possession for years! The surface facing the camera is so flat because the original rock was sawed into two pieces as it was prepared for distribution to scientists for analysis. Observations like that one lead to a simple and obvious question: can material be ejected from the target planet or satellite completely, never to return again? Well, the answer appears to be a loud "yes!" We have, here at the Johnson Space Center (JSC) in Houston, meteorites that are indistinguishable in many ways from lunar samples that the Apollo astronauts returned. In fact, the only differences are chemical ones, and you have to be pretty well-versed in lunar geochemistry to recognize that the "lunar meteorites" probably came from parts of the Moon that the Apollo guys didn't visit. We also have meteorites that, most planetary scientists believe, came from Mars! While we can't apply the same sort of argument used to prove that the lunar meteorites came from the Moon (We don't have samples of Mars that astronauts or automated spacecraft picked up and returned.), we have an indirect way of showing that a martian origin is very likely for these special meteorites: the Viking measurements of the martian atmosphere. Photograph of the martian surface taken by Viking Lander 1. The large boulder ("Big Joe") is about 2 meters across and a meter high. How in the world can the martian atmosphere be related to the origin of these meteorites? Well, when the Viking landers were passing through the atmosphere of Mars on their way to their amazingly successful landings in 1976, they were measuring the composition of its gases. Now, if pieces of a martian meteorite are ground up and heated, the gases that are baked off by the heat are almost indistinguishable from those measured in the atmosphere of Mars by the Viking landers! This is a view of some of the older terrain on Mars. Impact craters are everywhere, and the small channels and valleys visible possibly indicate the presence of water at some time in the past. Note the elongate crater just to the left of the center of this picture; it was probably formed by an oblique impact -- that is, one in which the projectile's trajectory made an angle of only a few degrees (maybe 15 or 20?) with the martian surface. Samples from this part of Mars almost certainly would have been "reprocessed" by the many craters and maybe by flowing water. The large, fresh crater in the upper right is about 35 km in diameter. "Okay, but how can a rock contain the gases of its home planet's atmosphere?" you might ask. Well, they were forced into the minerals composing the rock by the same process that lifted the rocks off the martian surface: IMPACT. In fact, we've taken rocks very similar to the martian meteorites, put them in a target chamber full of a simulated martian atmosphere, and used a small cannon to fire projectiles at the rocks at speeds approaching 2 kilometers per second (km s-1 or km/s). We then recovered what was left of the rocks (not much), heated them up (in a very small chamber), and bingo -- we got simulated martian atmosphere out of them! A very simple, but very telling, experiment, demonstrating that the shock waves that create the crater can also force any gases that are present into the structures of the minerals making up the rocks. The next question that the geochemists and petrologists had was logical, but extremely difficult to answer: what crater or craters mark the site where the martian meteorites originated? There are a lot of reasons why this is such a tough question to tackle, but one of the principal ones is pretty basic. We know that material is ejected from growing craters, but we just don't know how fast these fragments travel. If we knew that, we could find all of the craters on Mars that appear to be the right age, measure their sizes, and calculate how much of the ejected material from each crater could have traveled at speeds greater than the escape velocity of Mars. We could then go to some other equations that have been derived to relate the actual sizes of ejected fragments to crater size, and then determine what fraction of the escaping ejecta would be large enough to satisfy the cosmic-ray exposure data for these potential martian meteorites. Given the ages of the martian meteorites, the crater or craters that would have ejected them would be relatively young and, therefore, would look nice and crisp. So, this would narrow down the possible "source" craters somewhat, since there aren't a lot of very fresh craters on Mars, relatively speaking. Even if we couldn't narrow down the search to a single crater, it would make the This is Olympus Mons, the largest known volcano in the solar system. It's about 600 km in diameter, over 20 km high, and would easily cover the state of Colorado. Because it's composed of solidified lava that originated deep within Mars, geochemists and petrologists would examine samples from Olympus Mons with a different approach than they'd use on "reworked" samples from the heavily cratered terrain pictured in the photograph above. jobs of the geochemists and petrologists much easier, since they'd be able to interpret their data more solidly on the basis of the few possible sites of origin of the meteorites. For instance, their analytical approach would be different than if the meteorites had come from the the southern highlands of the planet than if they were originated near one of the large volcanoes on Mars. In fact, their data could reduce the list of potential source craters even further. This is just one example of the need for hard numbers to be placed on the ejection velocities, which are important for other, equally perplexing problems. There have been attempts to measure ejection velocities in the laboratory, with some success. It's a difficult thing to do, though, and requires a lot of tedious experimental setup and a lot of time spent in analyzing the data once they're collected. We wrote a proposal to NASA a couple of years ago, requesting funding to allow us to try a new way of measuring ejection velocities from craters that we make in the Experimental Impact Laboratory here at JSC. If it worked, we figured, it would be a simple but effective way of photographing individual particles ejected from craters made in a wide variety of materials, from solid rock to plain old quartz sand. We submitted the proposal NASA's "Origins of Solar Systems Program," which was formed to fund research into the various processes involved in the formation and early history of our solar system and, by extension, any other solar systems that are out there. The proposal was reviewed by the Origins of Solar Systems Review Panel and, wonder of wonders, it was approved!! If there were any dead people in our building when I opened the letter of approval, they would have gotten up and gone someplace where there weren't screaming banshees. It was right after Christmas at the time, though, so there weren't many people around. Not very professional, I admit, but tough darts. I was pretty stunned, happy, and excited, because now we could try to develop this new system for measuring ejection velocities in our lab, and (hopefully) derive good measurements that the theoreticians could use and that the folks who write computer models could incorporate into their programs that simulate impact events. This, we hope, will be the story of our efforts to develop this method of studying impact craters in the laboratory. We'll start with the first steps in the process, trying to explain why we're doing things the way we are, what works and why it does, and what doesn't work and why it doesn't. This will not be a nicely packaged, slick description of things the way you might see them in a documentary on TV. For starters, we're not sure how long it'll take us to get things working the right way. Neither are we guaranteed success. Experimental science seldom follows a nicely planned schedule, and there are a lot of frustrations that occur before, during, and after the successes. If you stick it out, you'll see both sides of the coin and, in the process, we hope that you'll get an idea of what this type of research is like. When it's distilled to the basics, you'll probably find that scientific research is pretty much like life in general: you never know what kind of hand you'll be dealt on any given day. You do your best, sweat out the details, and hope that in the long run you've thought of all the important stuff. Who knows? Maybe, after all is said and done, it'll even look pretty neat to you, and you might consider going into science yourself. So, here goes. Wish us luck. To be continued . . . Related Links of Interet: Impact Cratering Experiments at JSC