To those who know rocks, this chondrite meteorite could not be mistaken for an Earthly stone. Photos by Andrew Alden

The other week I mentioned, in talking about concretions, that people can be fixated on the idea that they have found a dinosaur egg or meteorite. This last week meteorites featured in two news stories, one excitingly true and the other almost certainly bogus.

The exciting story was about a set of meteorites recovered in the desert of Morocco, a few months after their fall from space had been recorded. That doesn't happen very often—once meteors arrive in the atmosphere, their unguided trajectory means that a rather large area must be searched to find them. What was extraordinary was that these rocks were from Mars, certified as such this week by an expert scientific panel.

Meteorite hunting has become a cottage industry in the Sahara Desert, where conditions are ideal for space rocks to be preserved and for practiced observers to spot them. The locals who found the new Martian rocks sold them to dealers, who in turn marked up the price to almost a thousand dollars per gram even before the meteorites were formally certified as Martian.

Meteorites are most easily found in two places on Earth, the Sahara and Antarctica. In the case of Antarctica, they fall on the ice cap, where no other rocks exist at all. Movements of the ice can concentrate these meteorites, including the rarest stones from Mars and the Moon, in certain areas that are surveyed regularly and exclusively by scientists. That's good for science. For its part, the Sahara is good for the rest of us who can acquire these rarities for our own pleasure. And scientists can still study Saharan stones because meteorite hunters must donate pieces of their finds to a museum to qualify for authentication, without which the stones have no value. It's a tidy system with little impact on the environment.

The California deserts are also promising places for meteorite hunters. At least one Martian stone has already come from the Mojave. Meteorite hunting is simple in principle, yes, but far from a casual hobby. First you acquire a very intimate knowledge of the rocks that belong there, and then you examine approximately a million rocks to find one that doesn't belong there. And with that, you start to learn about meteorites. I love rocks inordinately, but I think I would still go mad. Dr. Randy Korotev is a genuine meteorite expert at Washington University who gets torrents of email from would-be meteorite finders. On his excellent "What to Do" page, he says that of over 2000 serious inquiries over the years, only eight people had real meteorites.

Easier to dream of fabulous wealth falling into your back yard. That dream, fed by an extremely rare handful of true stories, can blind people to the obvious. And that leads me to the Castro Valley man who got a reporter to feature his story in the San Jose Mercury News last week. His story did not even point to a meteorite, let alone prove it. He said he responded to his dog's barking and found a fresh pit in his back yard with a smoking, red-hot stone in it. That scenario is a old urban legend about meteorites that is never true. He said he talked to experts from Lawrence Livermore National Lab, who had him hold a magnet against the stone and try to cut off a piece of it. He claimed that after finding it both non-magnetic and hard enough to break a hacksaw blade, those experts told him that was positive evidence. None of that is what an expert would say. And the object he showed a photographer had a silvery color and finish (which could not be iron because it was non-magnetic), and a multiply-layered structure that is very common in Earth rocks. In short, it looked nothing like a fresh meteorite and everything like an ordinary metamorphic rock. But he was fervent enough in his belief to fool a reporter, and at least one editor, into running the story anyway.

This is the back side of the chondrite shown at the top. Note the dark fusion crust and the hollows, called regmaglypts, carved by erosion in passing through the Earth's atmosphere. And a magnet sticks to it because it has small grains of iron metal throughout it.

Easier to save up some of your birthday money and buy a nice little meteorite at a rock show from a large, well-run dealership. My advice is to wait until the afternoon of the last day for the best price; dealers hate to pack up their inventory. That's how I got my 1/3-pound chunk of meteoritic nickel-iron from the Sikhote-Alin fall. There's nothing like the feeling of this ancient space metal in your hand.

The Sikhote-Alin fall occurred in eastern Siberia in 1947. Specimens like this are readily available.

I've written more about Martian and lunar meteorites on my About.com site. I also have a photo gallery there. Dr. Tony Irving has a deep and erudite page on Martian meteorites.

Asteroid 2005 YU55 - Radar image taken in 2010 - Credit NASA/Cornell/Arecibo

At about 1,300 feet across, this roughly spherical, charcoal-black space rock would give us quite a wallop if it were to hit the Earth. A bit larger than a typical football stadium (including a bit of the parking lot), if this asteroid were to strike Earth's ocean a powerful tsunami result, and if it struck land, a city-sized hole in the ground. Not to mention a lot of fireworks.

Fortunately, the asteroid's trajectory is well known, and poses no threat to us (at this time).

Asteroids and comets that can come close to the Earth—Near Earth Objects, or NEOs—have been a concern to life on Earth since it began. From the end of the "era of heavy bombardment," when the young Earth endured frequent impacts by asteroids and comets, large and small, debris leftover from the formation of our Solar System still meets up on occasion with our planet. Craters from past large impacts can be found today, camouflaged by millennia or eons of erosion, sedimentation, and tectonic activity—Earth's scar-healing processes.

The crater left by a 10-mile-sized asteroid (that would be the stadium, parking lot, and the surrounding major metropolitan area) believed to have contributed to the demise of the dinosaurs lies hidden and buried at the tip of the Yucatan Peninsula: the Chicxulub crater. (No, Chicxulub is not an all-women car service station….) Other craters masquerade as round lakes and other landscape sculptures.

And some are quite candidly impact craters, like "Meteor Crater" near Winslow Arizona. When I was in the Peace Corps in Cameroon, my house was 2 kilometers from a round lake that is apparently a meteorite crater. (That's Lake Ejagham; check it out at 5.750000 degrees north latitude and 8.987778 degrees east longitude.) Lake Ejagham is about a kilometer in diameter and 60 meters deep (not counting sediment infill). The meteorite that created it wasn't nearly that big—probably the size of a very small house….

Now imagine an object the size of asteroid 2005 YU55 striking Earth, land or sea. It wouldn't cause our demise—except for unfortunate bystanders—but it would create havoc around ground zero.

And even though 2005 YU55 will not hit the Earth on November 8, all NEOs that pass that close (within the Moon's orbit) are considered near misses, and are scrutinized by the "eyes of the Earth": radio dishes and optical telescopes across the planet.

Chabot's own NEO observing team will aim the eye of our 36-inch telescope, Nellie, on the asteroid and measure its trajectory, contributing to our knowledge of this particular NEO's orbit and improving our ability to predict its future passes.

This time, it's a mere field goal.

Scanning electron microscope image of the Orgueil meteorite.
Credit: Dr. Richard Hoover/Journal of Cosmology

The recent publication of the investigation of a rare class of meteorite (the CI1 Carbonaceous Chondrite) by Dr. Richard Hoover of NASA's Marshall Space Flight Center has caused another stir among scientists and the news media regarding possible origins of life on Earth, and life in the Universe in general. Exciting stuff—though the report stimulated the "usual" spectrum of responses, ranging from the starry-eyed wow! to the cool-headed let's wait and see to a tepid-at-best we've heard this hype before….

In a nutshell, Dr. Hoover's study suggests that encased within the minerals of the studied meteorites are chemical signatures of life and fossils of microbes, some of which are very similar to known Earthly cyanobacteria, and some that are not very like Earth life forms at all. The implication is that life (at least these would-be meteoritic microbes) originated outside of Earth, on some other parent body—possibly a comet—and that the formation of life may be common and ubiquitous in the Universe. It's a very big implication—and as some respondents have cited, big implications require big proof (okay, I'm paraphrasing Carl Sagan's, "extraordinary claims require extraordinary proof").

One of the challenges in this type of investigation is in distinguishing between "indigenous" mineral and microbe-like forms in the meteorites (that is, those that may have come along with the meteorite during its fall to Earth) and contamination by Earthly microbes—because, come on, if the Earth's surface is nothing else, it is absolutely teeming with life in every nook and cranny!

Other implications have been mentioned beside life from "out there" seeding the early Earth and giving rise to us (in fact, there is no definitive proof that life originated on Earth—and according to one theory, life could not have started here).

One idea goes the other way: life started on Earth early on, prior to the end of the period of heavy bombardment of our world by asteroids and comets, and the blasts of some of those collisions could have kicked the Earthly specimens back into space…maybe only to return to us eons later as "evidence of extraterrestrial life"—which would be ironic.

As part of his analysis, Dr. Hoover included a range of known Earth life as control references–samples from mastodons, Egyptian mummies, insects preserved in amber, fossilized cyanobacteria from ancient rocks, and several others—to compare to the samples from his subject meteorites. According to his publication, the comparison of the samples has shown certain organic chemicals found in terrestrial life lacking in the meteorites—a number of amino acids, and nitrogen—which suggest that the meteorite samples may not bear contamination by Earthly life.

The quest for life beyond Earth has gone on for a long time, and a lot of tantalizing clues and possible evidence have been making the rounds. Microbe fossils in meteorites from Mars? Life not-as-we-know-it living in the waters of Mono Lake? Methane plumes from beneath the Martian surface? Organic molecules in the tails of comets and on Saturn's moon Titan? And now, possible organics and fossils in the rarest of carbonaceous chondrite meteorites?

For the moment, I'll wait and see how scientists review Dr. Hoover's study—because, yes, I have heard before what turned out to be hype—but in the back of my mind…wow!

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Close-up of a nickel-iron meteorite discovered on Mars by
the rover Opportunity. Image Courtesy of NASA.

Trundling along the wind-swept plains of Meridiani Planum, NASA's Mars rover Opportunity stumbles upon yet another rock that looks like it could be an iron meteorite. Does the image of a Gold Rush era prospector leading his burro across desert sands and pecking at a rock with a hammer come to mind? Ah, but more precious than gold is common sand and stone when it's on Mars!

Sounds a bit lonely, like the lives of real gold prospectors; right now, Opportunity is the only active explorer on the surface of the entire planet, surrounded by the now eternally sleeping derelicts of the Vikings, Pathfinder, Spirit, Phoenix, and a few lost souls like Beagle and some Soviet landers….

But, it really is an amazing time to be alive. Each new report from our exploration of space—Mars in particular—reminds me of the state of our knowledge of the solar system when I was a starry-eyed child, back in the 1960s. I recall having to imagine what the surface of that planet might look like; I remember poring over pictures in books and posters of artists' concepts of what the Martian desert might be like, or the even more mysterious shrouded surface of Venus, or how a scene of Saturn and it rings from the surface of one of its moons—Titan, Enceladus, Iapetus–might look.

(Speaking of Venus, I remember a page from the old Time-Life book series–I think it was "The Planets" volume–that showed different possibilities of what might lie under the perpetual shroud of cloud: a rocky desert, a vast ocean, a steamy swamps. Isn't imagination great for filling in the gaps in our knowledge? Remember, prior to 1964, we had sent no spacecraft to any planets, so all we had to go on was what we saw through ground-based telescopes.)

But back to Opportunity, lone prospector of Mars now traversing its 15th mile as it marches steadily on to Endurance Crater. NASA sent the rover to the meteorite suspect it spotted from a distance to take a closer look. Why? We sent the rovers to Mars to examine Martian geology, not interplanetary debris that happened to fall from the sky. Sounds a bit like going to Paris to sample French cuisine and running into the McDonalds.

Actually, even a meteorite can tell us something about Mars. Depending on when the meteorite fell, an examination might reveal clues as to the thickness of the atmosphere at the time. And a reading of the content of certain radioactive isotopes (though Opportunity does not possess this capability) might reveal how long ago the meteorite stopped being exposed to interplanetary radiation—see my earlier blog, Mars Rock Talks, Opportunity Listens, for more on that.

Though the highly imaginative ideas about Mars and other places in our solar system are being summarily swept away by the extremely revealing close-up images taken by our robotic explorers, the reality of those places is at least as enthralling. No, no steamy, dinosaur-filled swamps on Venus; no Martian-constructed water-bearing canals on the Red Planet; no strange black obelisks on a Moon of Saturn (that we know of). But, being privileged to stand in the wheels of Opportunity and gaze across real Martian sands at a lump of extra-Martian iron through the rover's eyes is, just, awesome….

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An 1833 depiction of the Leonid Meteor Shower

Each year at this time, Earth plows through a belt of dust left behind by the periodic passage of comet Swift-Tuttle, causing a significant bump in the number of meteors that might be seen. We usually think of shooting stars—meteors—as bits of dust or small pebbles that fly into Earth's atmosphere at high speed and are vaporized in a blaze of glory by their flight of friction.

In reality, it's the Earth running into the dust specks, not the other way around. It's a bit like a car on a freeway speeding through a cloud of insects, if you think of the windshield as the Earth's atmosphere and the smears and streaks of "bug guts" as the flashy demise of meteors. Earth's orbital speed is about 18 miles per second, which explains the fast and fiery trajectory of shooting stars.

But what happens to the material in the meteor after its luminous tail fades? Well, what do you think? It's a trail of dust high in the atmosphere: eventually, it is pulled to Earth, like any other dust particles, and settles down around us, invisibly, to become part of the landscape, and part of the air we breathe.

How much meteor dust, as well as larger lumps of meteorite that strike the Earth's surface before completely burning up, is there?

It's not known absolutely how much meteoritic dust bathes our world every day, and estimates range widely depending on how the calculation is done, but one (maybe conservative) figure I found is 40,000 metric tonnes per year—about 110 metric tonnes per day. This estimate is for the total amount of material falling on Earth from space, be it in the form of the largest meteorites that occasionally collide with the ground or the continual "rain" of the tiniest micrometeorites that aren't even large enough to register a tail visible to our eyes. All of it. (And, as I said, only one estimate.)

Now let the slicing and dicing of that number begin!

40,000 metric tonnes is about the weight of one hundred fully loaded Boeing 747-400 jumbo jets, or about an eighth the weight of the Empire State Building, or about 48,000 1967-model Volkswagen Beetles.

So, if that amount of material were distributed evenly around the world, how much of it would fall on, say, one typical city block? (I'll assume a city block is a tenth of a mile square—or 1/100 of a square mile.) Given Earth's total surface area of about 197 million square miles, and 100 city blocks in each square mile, that gives 19.7 billion city blocks. Dividing 40,000 metric tonnes by 19.7 billion city blocks yields an astounding…2 grams of material per city block…per year…. Conveniently, that's the weight of half a sugar cube, so is easy to visualize…but not so impressive a figure after all. Guess I won't be panning for meteor dust anytime soon.

But what about over time? Assuming the same rate of meteor-dust fall, since the dawn of civilization my city block will have received 14 kilograms of the stuff—enough to fill a bucket or two….

Of course, ultimately, ALL of the material that the Earth is composed of (all 5.97 billion trillion metric tonnes of it) originally "fell to Earth" from the protosolar nebula, which the entire Solar System formed from. (I had to throw that in to end this blog with something a little more impressive than a couple bucketfuls of dust.)

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Scene from the 1951 film The Thing From Another World

On Monday, November 30th, 2009, NASA/Johnson Space Center announced that a recent study strengthens the argument that chemical and structural features in a Martian meteorite—ALH84001—may be evidence of fossilized microbial life on Mars from the distant past. While not absolutely conclusive that the meteorite bears the remains of ancient Martian life, the results of the study show that alternate, non-biological explanations for some of the meteorite's properties are not consistent with new findings.

Meteorite ALH84001, discovered in Antarctica in 1984 and chemically identified as having originated on Mars, hit the news in 1996 when researchers hypothesized that microscopic features and chemical constituents in the rock could possibly be the fossilized remains of ancient Martian microbial life. The hypothesis was controversial among scientists, and alternate, non-biological processes were offered by opponents as possible explanations for the meteorite's features.

The recent reexamination of the meteorite was focused on one of the leading non-biological explanations for the existence of magnetite crystals in the sample. Magnetite is an iron-bearing, magnetic mineral that can be produced both biologically and through inorganic processes. Some forms of life on Earth—including microbes–produce magnetite crystals in their cells that help them orient to Earth's magnetic field.

The NASA/JSC team that performed the new analysis of ALH84001 concluded that new data on the magnetite crystals, obtained with more powerful analytic instrumentation than used in the 1996 study, are not consistent with the leading non-biological explanations. This, they argue, strengthens the biological explanation for the origin of the magnetite.

The new analysis also obtained scanning electron microscope data that yielded more detail on shapes within the alleged microbe fossils. The new shapes that emerged from the data closely resemble shapes within Earthly microbe fossils—further strengthening the hypothesis that the meteorite contains fossils of life, and thus that life at one time existed on Mars.

The evidence for the possibility of life on Mars, past or present, has been growing over the past decade, or longer—evidence that Mars was once much warmer and wetter than it is now, and that it had rivers, lakes, and possibly oceans of water, making it an environment possibly conducive to the formation of life. We have also detected methane rising out of the soil of Mars, which some suggest could be a byproduct of current biological activity, underground.

NASA will continue to examine the Martian meteorite, focusing their study on further detailing the structures of alleged microbe fossils and possible chemical signatures of life that remain in the rock.

So, we're still waiting for the day—but with all the tantalizing clues emerging from our exploration of Mars and the Martian meteorite, it feels very much like that day is somewhere on the horizon. But, wouldn't it be ironic if we were to make the first definitive detection of extraterrestrial life right here on Earth, with evidence that's been just laying around since before the beginning of human civilization?

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Block Island—a half-ton meteorite found on Mars by NASA's
Opportunity rover.Image credit, NASA/MER Opportunity

Okay, dramatic desert car scene ended. That would be the Hollywood movie version of what NASA's Mars Exploration Rover Opportunity did recently, on the lonely desert highway that it's scouting on Mars.

On its determined long trek from Victoria Crater to the larger Endeavour Crater (a 12-mile span that Opportunity has completed about one fifth of over the past year), the rover passed by an X-box-sized block of iron that presented the appearance of a meteorite. It snapped a picture in passing, which was eventually transmitted to Earth and examined. By this time, Opportunity had already traveled about 180 meters beyond the block (dubbed "Block Island"). This is when the rover was commanded to backtrack all the way to the find (though it's doubtful it worked up a rooster tail).

Upon returning to Block Island—quite obviously an iron-nickel meteorite by appearance alone, but whose composition was confirmed by the rover's alpha particle X-ray spectrometer instrument—Opportunity took more pictures, including extreme close-ups with its microscope camera, which revealed surface patterns similar to those found on Earth iron-nickel meteorites that have been exposed to long-term weathering by wind and sand.

As interesting as stumbling upon a half-ton meteorite on the dusty plains of Mars' Meridiani Planum is, what this particular chunk of weathered iron is telling scientists sparks the imagination. In a nutshell, given the thinness of Mars' current atmosphere, scientists wouldn't expect a meteorite of this size to survive impact intact, at the speed it would be going. One of the possible explanations for Block Island's rock-houndable state is that when it fell to Mars, Mars' atmosphere was substantially thicker than it is now.

Further examination of the meteorite may reveal clues as to how long ago it fell through Martian skies. Evidence that Mars' atmosphere was warmer and thicker in the distant past, as well as the possibility that there was liquid water on the surface, has been mounting over the years. The age of this meteorite-fall could shed more light on the history of Mars' environment. If it fell billions of years ago, Block Island would weigh in as more evidence to support our current suspicions. If, however, we find that it fell more recently, then this could indicate that the atmosphere was more substantial later in Mars' history than we thought.

Imagine, if you will, a Mars that looks even more Earthlike than it does now: seas of water with waves rolling into shorelines, great clouds sending downpours of rain and snow onto mountains and plains, streams and rivers snaking through the landscape. Maybe, maybe, even some form of life?

All that from a rock? Yes, rocks talk, if we listen.

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A few weeks ago, this asteroid came really close to hitting Earth.

Called “2009 DD45”, the asteroid was estimated to be around the same size as the one that exploded in the atmosphere near the Podkamennaya Tunguska River in remote Siberia on June 30th, 1908, flattening 80 million trees across eight hundred square miles of remote forest. Of course, if an asteroid of this size were to hit a city or in an ocean offshore from a populated area, tens of thousands of people would likely die.

Then, just as the last of the night sky observers were completing their collective sighs of relief, on March 17th, 2009 another Tunguska-class asteroid, 2009 FH, passed by about 53,000 miles from Earth. Thankfully, neither of these asteroids actually hit us. But astronomers didn’t even observe 2009 DD45 until 4 days before its closest approach. It's orbit was calculated and it was determined that it would miss the Earth. But it's likely that asteroids of this size are fairly frequently buzzing by the Earth. And until recently, most of them have been undetected.

In 1998, NASA started the Spaceguard Survey which set out to discover 90% of those Near Earth Asteroids (NEAs) 1 km in diameter and larger. An impact by an asteroid this size would likely cause global destruction and an end to much of life as we know it so it’s definitely reassuring that 10 years after its inception, the Spaceguard Survey had found about 80% (CK) of them. But unfortunately, once we’ve found them, there’s still no international concensus or infrastructure in place in how to deflect or destroy them. But the Survey is limited by its mandate to find those mass extinction-sized asteroids as well as by the size and sophistication of the telescopes that are dedicated to searching the skies.

As former Apollo 9 astronaut, Rusty Schweickart said in a recent phone conversation, "in the process of finding the big ones, you also find a bunch of small ones, and the smaller ones are obviously far more numerous than the large ones." But it will take many more resources and new telescopes to continue searching for and tracking the smaller ones. And unfortunately, once we’ve found them, there's still no international consensus or infrastructure in place in how to deflect or destroy them. Raising awareness and building alliances amongst governments and space agencies is Schweikart's current "mission". He founded the B612 Foundation and Association of Space Explorers to tackle these goals on different fronts.

The message that I hope is conveyed with the Asteroid Hunters TV segment is that we are not immune from asteroid impacts here on Earth. Rusty Schweikart puts it best in a portion of his interview that didn’t make it into the final program:

"Well, asteroids and comets are good news and bad news, you know? But for them we wouldn’t be here, and on the other hand, if we don't actually take some action now, at some point we won’t be here anymore, because there's no question that we will be hit by asteroids, and we’ll probably be hit by, we would be hit by comets as well. Unless, we use the technology that we have and the brains that we have in order to protect the Earth from asteroid impacts, and we can do that. We can basically now, with current technology, assure that no asteroid ever hits the Earth again. That can do any serious damage."
-Rusty Schweikart

Here's a little exercise from Rusty that you can do to get a sense of what we know today about exactly what's out there:

• Go to: neo.jpl.nasa.gov/risk
• See two tables, the first table says "Recently Observed Objects" and the table below says "Objects not recently observed." You’ll notice in the bottom table that Apophis is the 4th one listed.
• Click on "Apophis". At the top you see a bunch of boxes, like the diameter at .27 km, or 270 meters.
• Down below that you see 3 lines, those are the 3 potential impacts. The first one is April 13, 2036. Go over to the right on that line you'll see the column "Impact Probability" is 2.3 x 10-5 – click on that. So there is the probability, 1 in 43,000 of that particular impact.
• Now if you go back to the main table you can do the same thing with every single one of those lines.
• Now go to the very top of the page and hit "Discovery Statistics." Scroll down to a blue and red graph "Known Near-Earth Asteroids". This shows the discovery rate beginning back in 1980 going up to almost current time. Notice the knee in that curve in 1998 – that’s when the Spaceguard Survey began.
• Scroll down to table just below the graph and look across that table to the far right side, to see that a a total of 6166 NEOs (of ALL sizes) have been discovered.

Rusty concludes that, "…what we really care about is not only the things that large, we care about things that can hurt us. Things that can hurt us go down to 40 to 45 meters or so. Instead of there being 940 of them, there are more like 600,000 of them. So the new charge for NASA, which they have so far ignored, is to find 90% of the objects 140 meters and larger by 2020. You can't reasonably set a goal to find everything down to 40 meters because it's just beyond the capability of telescopes and the money available. So NASA, working with Congress, set the goal at 140 meters. Now nevertheless, when you are looking for 140 meter objects, it’s going to take bigger telescopes than the ones to find a kilometer. Therefore we are going to find many many smaller objects as well. So 10 to 15 years from now, instead of that number on the far right hand column being 6000, it will be 1 million."

Watch the Asteroid Hunters television story online.

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The 1992 'Peekskill' meteorite and its point of
impact in Peekskill, New York. Credit: "Pierre Thomas

The first thing I tell people is that I'm not a meteorite expert, but that I have a contact who is. This rarely discourages them from wanting to bring their rocks in for a look.

The first sample was brought in by a family who said they collected the chunk of iron from Lake Tahoe. This one actually looked promising to my mostly untrained eye: a fist-sized chuck of magnetic metal, with pits and holes and an overall melted look. I took some pictures to send off to our regional expert and told the family I'd call them to let them know what he said. The response to the pictures was pretty certain: it wasn't a meteorite, but a chunk of metallic slag. I was told that this is a common mistake; that often bits of slag from old foundries or other sources are taken for meteorites.

The second sample brought to me didn't really strike me as a meteorite, by appearance. It was metallic, but not magnetic; it was pretty heavy for its size; it didn't have any obvious signs of melting, and no real pits or holes-other than one, deep, tunnel-like hole the width of a finger. It didn't appear jagged or shrapnel-like, as fragments from an exploding metallic meteorite often do. Finally, it had wide, flat facets that looked much more like the result of natural rock cleaving as pieces of Earth's crust break apart.

I went ahead and performed a density measurement on the sample. It was pretty heavy, so our sensitive balance scales wouldn't handle the load. Instead, I resorted to our "learn your weight on other planets" scale-the one that tells you how much you would weigh on the Moon, Mars, and other planets, in addition to your Earth weight. (I found this scale useful when I had a package to mail and needed to know the weight; by selecting the Moon weight of the package, I would pay only one-sixth the normal Earth rate!)

The double-fist-sized sample was 11.3 pounds, which converted to 5126 grams. Then, I selected a graduated beaker from our lab, filled it with water and submerged the sample. Reading the difference in water level with and without the sample, I measured a volume displacement of 750 cc. So, the density-mass divided by volume-turned out to be about 6.83 grams/cc. That's twice the typical density of silicate-type rocks (stone), and fairly close to that of pure iron.

I sent the owner off with my appraisal that the rock didn't present the appearance of a meteorite, and though the density was in neighborhood of that of iron, the appearance (black, inside and out) and non-magnetic nature suggested some other metal or metal-stone mixture. As always, I encouraged him to seek an expert appraisal.

Let's face it, all rocks found on Earth are ultimately of extraterrestrial origin-though what we regard as Earth rock has been on Earth for many billions of years, and shaped, reshaped, and metamorphosed by eons of weathering and geological activity. Meteorites, then, are only the newcomers….

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