How to Tell if the Rock You Found Might Be a Meteorite
Two Parts:Looking for Visual Identifiers and Testing the Rock’s Physical Properties
If you’ve come across a rock that looks positively out-of-this-world, there’s a possibility it may be a meteorite. Although meteorites are relatively rare on Earth, they’re not impossible to find in the wild. However, you’ll want to make sure your find is indeed a stony or iron rock of cosmic origin and not a piece of ordinary terrestrial material. By checking for common visual and physical markers of a meteorite, you can determine whether the rock you’ve found is actually extraterrestrial in origin.
Part 1
Looking for Visual Identifiers
Two Parts:Looking for Visual Identifiers and Testing the Rock’s Physical Properties
If you’ve come across a rock that looks positively out-of-this-world, there’s a possibility it may be a meteorite. Although meteorites are relatively rare on Earth, they’re not impossible to find in the wild. However, you’ll want to make sure your find is indeed a stony or iron rock of cosmic origin and not a piece of ordinary terrestrial material. By checking for common visual and physical markers of a meteorite, you can determine whether the rock you’ve found is actually extraterrestrial in origin.
Part 1
Looking for Visual Identifiers
Discern if the rock is black or rusty brown. If the rock you’ve found is a freshly fallen meteorite, it will be black and shiny as a result of having burned through the atmosphere. After a long time spent on Earth, however, the iron metal in the meteorite will turn to rust, leaving the meteorite a rusty brown.[1]- This rusting starts out as small red and orange spots on the surface of the meteorite that slowly expand to cover more and more of the rock. You may still be able to see the black crust even if part of it has begun to rust.[2]
- The meteorite may be black in color but with slight variations (e.g., steely bluish black). However, if the rock you’ve found isn’t at all close to black or brown in color, then it is not a meteorite.
2. Confirm that the rock has an irregular shape. Contrary to what you might expect, most meteorites are not round. Instead, they are typically quite irregular, with sides of varying size and shape. Although some meteorites may develop a conical shape, most will not appear aerodynamic once they land.[3]
- Although irregular in shape, most meteorites will have edges that are rounded rather than sharp.[4]
- If the rock you’ve found is relatively normal in shape, or is round like a ball, it may still be a meteorite. However, the vast majority of meteorites are irregular in shape.
3.
Determine whether the rock has a fusion crust. As rocks pass through the Earth’s atmosphere, their surfaces begin to melt and air pressure forces the molten material back, leaving a featureless, melt-like surface called a fusion crust. If your rock’s surface looks like it has melted and shifted, it may be a meteorite.[5]
Determine whether the rock has a fusion crust. As rocks pass through the Earth’s atmosphere, their surfaces begin to melt and air pressure forces the molten material back, leaving a featureless, melt-like surface called a fusion crust. If your rock’s surface looks like it has melted and shifted, it may be a meteorite.[5]
- A fusion crust will most likely be smooth and featureless, though it may also include ripple marks and “droplets” where molten stone had moved and resolidified.
- If your rock does not have a fusion crust, it is most likely not a meteorite.
- The fusion crust may look like a black eggshell coating the rock.[6]
- Rocks in the desert will sometimes develop a shiny black exterior that looks similar to fusion crust. If you found your rock in a desert environment, consider whether its black surface might be desert varnish.
4
Check for flow lines where the surface may have melted. Flow lines are small streaks on the fusion crust from when the crust was molten and was forced backwards. If your rock has a crust-like surface with small streak lines across it, there’s a good chance it’s a meteorite.[7]
Check for flow lines where the surface may have melted. Flow lines are small streaks on the fusion crust from when the crust was molten and was forced backwards. If your rock has a crust-like surface with small streak lines across it, there’s a good chance it’s a meteorite.[7]
- Flow lines may be small or not immediately apparent to the naked eye, as the lines can be broken or not completely straight. Use a magnifying glass and a discerning eye when looking for flow lines on the surface of a rock.[8]
5
Identify any pits and depressions on the rock’s surface. Although the surface of a meteorite is generally featureless, it may also include shallow pits and deep cavities that resemble thumbprints. Look for these on your rock to determine both if it’s a meteorite and what type of meteorite it is.[9]
Identify any pits and depressions on the rock’s surface. Although the surface of a meteorite is generally featureless, it may also include shallow pits and deep cavities that resemble thumbprints. Look for these on your rock to determine both if it’s a meteorite and what type of meteorite it is.[9]
- Iron meteorites are particularly susceptible to irregular melting and will have deeper, more defined cavities, whereas stony meteorites may have craters that are smooth like the rock’s surface.[10]
- These indentations are technically known as “regmaglypts,” though most people who work with meteorites will suffice to call them “thumbprints.”
6
Make sure the rock isn’t porous or full of holes. Although craters and cavities on the surface may indicate that your rock is a meteorite, no meteorite has holes in its interior. Meteorites are dense pieces of solid rock; if the rock you’ve found is porous or bubbly in appearance, it’s unfortunately not a meteorite.[11]
Part two
Testing the Rock’s Physical Properties
Make sure the rock isn’t porous or full of holes. Although craters and cavities on the surface may indicate that your rock is a meteorite, no meteorite has holes in its interior. Meteorites are dense pieces of solid rock; if the rock you’ve found is porous or bubbly in appearance, it’s unfortunately not a meteorite.[11]
- If the rock you’ve found has holes in the surface, or appears “bubbly” as if it was once molten, it is definitely not a meteorite.[12]
- Slag from industrial processes is often confused for meteorites, although slag has a porous surface. Other commonly mistaken types of rock include lava rocks and black limestone rocks.
- If you’re having trouble discerning between holes and regmaglypts, it may be useful to view side-by-side comparisons of these features online to learn how to spot the difference.[13]
Part two
Testing the Rock’s Physical Properties
1
Calculate the rock’s density if it feels heavier than normal. Meteorites are solid pieces of rock that are usually densely packed with metal. If the rock you’ve found looks like a meteorite, compare it to other rocks to ensure it’s relatively heavy, then calculate its density to determine if it’s a meteorite.[14]
Calculate the rock’s density if it feels heavier than normal. Meteorites are solid pieces of rock that are usually densely packed with metal. If the rock you’ve found looks like a meteorite, compare it to other rocks to ensure it’s relatively heavy, then calculate its density to determine if it’s a meteorite.[14]
- You can calculate the density of the potential meteorite by dividing its weight by its volume. If a rock has a calculated density higher than 3 units, it is much more likely to be a meteorite.[15]
2
Use a magnet to see whether the rock is magnetic. Nearly all meteorites are at least somewhat magnetic, even if only weakly. This is due to the high concentration in most meteorites of iron and nickel, which are magnetic. If a magnet is not attracted to your rock, it’s almost certainly not a meteorite.[16]
Use a magnet to see whether the rock is magnetic. Nearly all meteorites are at least somewhat magnetic, even if only weakly. This is due to the high concentration in most meteorites of iron and nickel, which are magnetic. If a magnet is not attracted to your rock, it’s almost certainly not a meteorite.[16]
- Because many terrestrial rocks are also magnetic, the magnet test will not definitively prove your rock is a meteorite. However, failing to pass the magnet test is a very strong indication that your rock is probably not a meteorite.
- An iron meteorite will be much more magnetic than a stone meteorite and many will be strong enough to interfere with a compass held close to it.[17]
3
Scratch the rock against unglazed ceramic to see if it leaves a streak. A streak test is a good way to test your rock to rule out terrestrial materials. Scrape your rock against the unglazed side of a ceramic tile; if it leaves any streak other than a weak grayish one, it is not a meteorite.[18]
- For an unglazed ceramic tile, you can use the unfinished bottom of a bathroom or kitchen tile, the unglazed bottom of a ceramic coffee mug, or the inside of a toilet tank cover.[19]
- Hematite and magnetite rocks are commonly mistaken for meteorites. Hematite rocks leave a red streak, while magnetite rocks leave a dark gray streak, indicating that they are not meteorites.[20]
- Keep in mind that many terrestrial rocks also do not leave streaks; thus, while the streak test can rule out hematite and magnetite, it will not definitively prove your rock is a meteorite on its own.
4
File the surface of the rock and look for shiny metal flakes. Most meteorites contain metal that is visibly shiny under the surface of the fusion crust. Use a diamond file to file a corner of the rock and check the interior for telltale metals on the inside.[21]
File the surface of the rock and look for shiny metal flakes. Most meteorites contain metal that is visibly shiny under the surface of the fusion crust. Use a diamond file to file a corner of the rock and check the interior for telltale metals on the inside.[21]
- You’ll need a diamond file to ground down the surface of a meteorite. The filing process will also take some time and a good bit of effort. If you’re unable to do this on your own, you can take it into a laboratory for specialist testing.[22]
- If the interior of the rock is plain, it is most likely not a meteorite.
5
Inspect the inside of the rock for small balls of stony material. Most meteorites that fall to Earth are of the type to have small round masses on the inside known as chondrules. These may look like smaller rocks and will vary in size, shape, and color.[23]
Inspect the inside of the rock for small balls of stony material. Most meteorites that fall to Earth are of the type to have small round masses on the inside known as chondrules. These may look like smaller rocks and will vary in size, shape, and color.[23]
- Although chondrules are generally located in the interiors of meteorites, weather erosion may cause them to be visible on the surface of meteorites that have been exposed to the elements for a sufficient amount of time.
- In most cases, you will need to break open the meteorite to check for chondrules.[24]
How Do We Know That It's a Rock From the Moon?
Many people have approached us over the years wanting to know if a rock that they possess is a Moon rock. The most common story we hear is that the rock was given to a relative in the 1970's by an astronaut, a military person, or a NASA security guard. We have chemically tested five such rocks and none has been a moon rock. Other people suspect that they have found a lunar meteorite. None of the many samples that we have been sent has been a lunar meteorite, except those from meteorite dealers, persons who bought lunar meteorites from a dealer, or experienced meteorite prospectors who found them in the deserts of northern Africa or Oman.
Lunar meteorite QUE 94281 - An unattractive rock that could pass for a cinder or piece of slag. It weighed 23 grams, just less than an ounce. (From NASA photo S95-14590)
No lunar meteorite has yet been found in North America, South America, or Europe. They undoubtedly exist, but the probability of finding a lunar meteorite in a temperate environment is incredibly low. Many experienced meteorite collectors have been looking and none have yet succeeded. Realistically, the probability that an amateur will find a lunar meteorite is so low that I cannot raise much enthusiasm to examine the many rocks that I have been asked to examine. If I wanted to find a lunar meteorite myself, I would not scour the Mojave Desert. I'd look through rock collections at colleges and universities. It's not unreasonable that a lunar meteorite exists in an old drawer somewhere because a sharp-eyed geology student or professor found a funny-looking rock years ago in a place it didn't belong. It would not surprise me to learn that some "expert" proclaimed that the rock was not a meteorite because it didn't look like an ordinary chondrite, it didn't attract a magnet, or it didn't contain a high concentration of nickel. Both visually and compositionally, lunar meteorites "look" more like terrestrial (Earth) rocks than do "normal" meteorites (ordinary chondrites). It would be easy to overlook a lunar meteorite. A weathered lunar meteorite would look remarkably unremarkable.
Here I discuss some aspects of lunar geology, mineralogy, and chemistry that guide us in our attempts to identify lunar material.
Lunar MineralogyOnly four minerals - plagioclase feldspar, pyroxene, olivine, and ilmenite - account for 98-99% of the crystalline material of the lunar crust. [Material at the lunar surface contains a high proportion of non-crystalline material, but most of this material is glass that formed from melting of rocks containing the four major minerals.] The remaining 1-2% is largely potassium feldspar, oxide minerals such as chromite, pleonaste, and rutile, calcium phosphates, zircon, troilite, and iron metal. Many other minerals have been identified, but most are rare and occur only as very small grains interstitial to the four major minerals.
Some of the most common minerals at the surface of the Earth are rare or have never been found in lunar samples. These include quartz, calcite, magnetite, hematite, micas, amphiboles, and most sulfide minerals. Many terrestrial minerals contain water as part of their crystal structure. Micas and amphiboles are common examples. Hydrous (water containing) minerals have not been found on the Moon. The simplicity of lunar mineralogy often makes it very easy for me to say with great confidence "This is not a moon rock." A rock that contains quartz, calcite, or mica as a primary mineral is not from the Moon. Some lunar meteorites do, in fact, contain calcite. However, the calcite was formed on Earth from exposure of the meteorite to air and water after it landed. The calcite occurs as a secondary mineral, one that fills cracks and voids (see Dhofar 025). Secondary minerals are easy to recognize when the meteorite is studied with a microscope.
pyroxene - A group of magnesium-iron-calcium silicates, common on the Earth and Moon.
clinopyroxene - A form of pyroxene; typically contains some calcium; most common in mare basalts [Ca(Mg,Fe)Si2O6].
orthopyroxene - A form of pyroxene; contains little calcium; most common in highlands rocks [(Mg,Fe)SiO3].
olivine - A magnesium-iron(II) silicate; common on the Earth and Moon [(Mg,Fe)2SiO4].
ilmenite - An iron(II)-titanium oxide; more common in lunar basalts than in terrestrial basalts [FeTiO3].
feldspar - A group of alumino-silicate minerals; common in the crusts of the Earth and Moon.
plagioclase - A form of feldspar; a calcium-sodium alumino-silicate [(CaAl,NaSi)AlSi2O8].
anorthite - A mineral; the calcium-rich extreme of the plagioclase feldspar; the most common mineral of the lunar crust, but not so common on Earth.
anorthosite - A rock consisting mainly of anorthite.
Lunar RocksMost of the lunar crust, that part called the Feldspathic Highlands Terrane or simply the feldspathic highlands, consists of rocks that are rich in a particular variety of plagioclase feldspar known as anorthite. As a consequence, rocks of the lunar crust are said to be anorthositic because they are plagioclase-rich rocks with names like anorthosite, noritic anorthosite, or anorthositic troctolite (see table below). The ratio of iron-bearing minerals to plagioclase probably increases with depth in the feldspathic highlands at most places. For example, rocks exposed in the giant South Pole - Aitken impact basin on the far side are richer in pyroxene than typical feldspathic highlands.
rock name
rock name mineralogy
anorthosite >90% plagioclase
noritic anorthosite and anorthositic norite 60-90% plagioclase, the rest mostly orthopyroxene
gabbroic anorthosite and anorthositic gabbro 60-90% plagioclase, the rest mostly clinopyroxene
troctolitic anorthosite and anorthositic troctolite 60-90% plagioclase, the rest mostly olivine
norite 10-60% plagioclase, the rest mostly orthopyroxene
gabbro 10-60% plagioclase, the rest mostly clinopyroxene
troctolite 10-60% plagioclase, the rest mostly olivine
In much of the northwest quadrant of the nearside of the Moon, in the region known as the Procellarum KREEP Terrane, the crust contains less plagioclase and more pyroxene. The original rocks of this anomalous crust were probably mostly norites and gabbros.
Photomicrograph (crossed polarizers) of a thin section of an impact-melt breccia, Apollo 16 sample 65015. The light-colored clasts are mainly grains of anorthosite or plagioclase. Poikiloblastic pyroxene grains are also evident. Field of view: 3.3 mm. Click on image for enlargement. (Photo by Brad Jolliff)
The feldspathic crust of the Moon began to form about 4.5 billion years ago. While it was forming and for some time afterwards, it experienced intense bombardment from meteoroids and asteroids. The rocks of the lunar crust have been repeatedly broken apart by some impacts and glued back together by other impacts. As a consequence, most rocks from the lunar highlands are breccias (brech'-chee-uz), a word for a rock composed of fragments of older rocks. Breccias occur on Earth, but they are much less common than on the Moon. Also, most terrestrial breccias were not formed by meteoroid impacts but by faulting. Lunar breccias are subdivided into a variety of categories such as impact-melt, granulitic, glassy, fragmental, and regolith breccias. In impact-melt and glassy breccias, rock fragments called clasts are suspended in a solidified (crystalline or glassy) melt matrix formed by meteorite impact.
In fragmental and regolith breccias, there is little or no molten portion, just fragmental debris that was lithified (formed into a rock) by the shock pressure of an impact. Because breccia refers to texture and anorthositic or feldspathic refers to mineralogy, rocks from the lunar highlands are variously called anorthositic breccias, feldspathic breccias, or highlands breccias. Because the lunar crust has been battered so intensely, there were very few hand-sized rocks collected on the Apollo missions that are unbrecciated remnants of the early igneous crust of the Moon. Thus it is no surprise that all of the lunar meteorites from the Feldspathic Highlands Terrane and the Procellarum KREEP Terrane are breccias.
In fragmental and regolith breccias, there is little or no molten portion, just fragmental debris that was lithified (formed into a rock) by the shock pressure of an impact. Because breccia refers to texture and anorthositic or feldspathic refers to mineralogy, rocks from the lunar highlands are variously called anorthositic breccias, feldspathic breccias, or highlands breccias. Because the lunar crust has been battered so intensely, there were very few hand-sized rocks collected on the Apollo missions that are unbrecciated remnants of the early igneous crust of the Moon. Thus it is no surprise that all of the lunar meteorites from the Feldspathic Highlands Terrane and the Procellarum KREEP Terrane are breccias.
On Earth, volcanoes are often cone-shaped mountains because they are a pile of ash and lava ejected from a vent. The lavas are viscous and solidify before they flow very far. Because of their iron-rich composition and lack of water, lunar lavas were much less viscous, more like motor oil. When lunar lavas erupted onto the surface they didn't form big volcanoes, they simply flowed and filled low spots. Also, because the Moon has no atmosphere and little gravity, ejected ash dissipated widely instead of piling up near the vent. As a result, lunar lava deposits are flat, thin, and cover wide areas.
Top: The Pu`u `O`o eruption of the Kilauea volcano in Hawaii. (Image courtesy of USGS/Hawaiian Volcano Observatory ) Bottom: Northwest Mare Imbrium (Sea of Rains, bottom) and Mare Frigoris (Sea of Cold, top). The embayment on the left is Sinus Iridum (Bay of Rainbows) and the basalt filled crater on the right is Plato (109 km diameter). Mare Imbrium fills the Imbrium impact basin. Mare Frigoris is one of the few mare in which lava filled a low spot that is not an impact basin. (Plate A13 from the Consolidated Lunar Atlas) |
Starting about the time of the period of intense bombardment, the lunar mantle partially melted. The resulting magmas rose through the crust to the surface, ponding in low spots. These low spots were mainly the huge craters, called basins, that were left by impacts of the largest meteorites. Lunar volcanism continued for about 2 billion years.
The Latin word mare is pronounced mar'-ay in English. The plural of mare is maria, which is pronounced mar'-ee-ah. Basalt is usually pronounced bah-salt'.
On Earth, volcanic rocks solidify from molten lava (magma). The most common type of volcanic rock is basalt. The ancient astronomers called the round, dark areas on the surface of the Moon seas because they were smooth dark areas surrounded by areas of higher elevation. The features were given Latin names like Mare Serenitatis for Sea of Serenity. We now know that the lunar maria are basalt flows, so we call the rocks of the maria mare basalts. Mare basalts are composed mainly of clinopyroxene, but all also contain plagioclase and ilmenite, and some contain olivine. The maria are darker than the highlands because (1) mare basalts are rich in iron-bearing minerals, (2) iron-bearing minerals are dark colored, and (3) plagioclase is light colored. In contrast to the highlands, most of the rocks collected on maria by the Apollo astronauts are actual basalts, not breccias composed of fragments of basalt. This is one of several reasons why we know that the basalts mostly formed after the time of intense bombardment. Mare basalts cover about 17% of the surface of the Moon, but it is estimated that they account for only about 1% of the volume of the crust.