Meteorite or Meteor-wrong? – Students Attempt to Find Out!

Students at Perimeter College are now getting a hands-on meteorite experience in their astronomy lab!  This innovative lab puts a wide variety of meteorites in front of students, inviting them to touch the history of the solar system.  The students are encouraged to examine the meteorites, to learn how they form, and to explore what signs to look for to indicate that these are indeed rocks from space.  The students are then given a box of unknown samples and the opportunity to apply their newfound knowledge and a few simple tests.  The goal is for students to determine for themselves whether there is evidence to determine whether each of the unknown samples is a meteorite or not.  The students have been very excited by this opportunity for experiential learning and finding answers themselves; in our end-of-semester evaluations this spring, almost all said it was their favorite activity in lab.  This suggests that we created a memorable learning experience that will stick with the students well beyond the semester in which they took our course.  Therefore, we want to encourage others teaching college astronomy labs to consider the creation of a similar lab to take advantage of the opportunity to inspire positive feelings about science in these students pursuing other fields of study.

We are long-time astronomy professors at Perimeter College, a two-year liberal arts access college within Georgia State University.  We teach Introduction to Solar System Astronomy courses to one hundred or more college freshmen and sophomores each semester.  A very few of these students may choose to pursue astronomy as a field of study, but the vast majority take our courses in order to fulfill their core educational science requirements.  Therefore, this course, which includes a lecture and a lab, is our chance to help students understand how scientific thinking works, to apply critical thinking in a scientific setting, and to grow their appreciation for science as a whole.

The story of the “Meteorite or Meteor-wrong?” lab began as we were discussing ways to expand the experiential learning opportunities for students.  The two of us started thinking about constructing a lab with meteorites independently; one of us was originally interested in giving a hands-on show-and-tell experience while the other of us was inspired by a conversation with a geology colleague after a member of the local community brought in a piece of slag, wondering what it was.  The resulting lab is a two-part experience where students first visually study and learn about meteorites and then test samples, allowing students to argue whether each sample is or is not a meteorite.

The “Properties of Meteorites” day of lab offers several different stations that students rotate through, observing various characteristics of meteorites.  The students learn to identify fusion crusts and regmaglypts and observe flowlines on Sikhote-Aline meteorites with loupes.  They examine polished, acid-etched slices of iron meteorites to see Widmanstätten patterns and find out that the large crystals formed when iron cools very slowly in the core of a protoplanetary body.  They inspect stony meteorites with various levels of metamorphism, observing chondrules and determining that these meteorites are attracted to magnets.  Students also observe stony iron meteorites and hypothesize about what could cause the crystals in pallasites and the mixing in mesosiderites. Finally, students observe carbonaceous chondrites and marvel at these stones that are essentially unchanged from pieces formed in the solar system’s original nebular disk.

Our students also examine lunar meteorites from both the highlands and the maria as well as Martian shergottites and eucrites from Vesta.  Testing of these meteorites shows that though most meteorites are attracted to magnets even if they don’t look like they contain iron, there are some meteorites that do not contain significant amounts of ferromagnetic material.  We help them to realize that this tells us that some meteorites do come from differentiated bodies in which crustal material does not contain enough iron to be attracted to a magnet

During the “Do You Think It’s a Meteorite?” day of lab, small groups of 3 or 4 students employ four main tests to study several unknown samples: visual inspection with a loupe, magnetic attraction, determination of density, and a nickel test.  Developing this section of the lab was an adventure for us – while we have ample experience constructing lab materials, meteorites are not an area of expertise for us.  We therefore consulted with several colleagues who are geologists and chemists as well as sources such as The Field Guide to Meteors and Meteorites by O. Richard Norton and Lawrence A. Chitwood and determined that these four tests, while not sufficient to positively determine that a sample was a meteorite, would allow our students adequate information to put forward reasoned thoughts about each sample.

1. Visual inspection: Students examined the surfaces of the samples using simple plastic double loupes which provided 10x magnification.

1. Magnet test: Students estimated the level of ferromagnetic materials in samples using a dipole cow magnet suspended from an electroscope stand.

1. Density test: Students calculated the density of each sample based on data collected using a specific gravity device and digital scale with 0.01-g precision.  Students measured the weight of a sample while it was submerged in water and when it was not.  The difference in these weights, compared with the actual weight of the sample when it was not submerged in water, gives the specific gravity of the sample.  The specific gravity of a sample is simply the density of the sample compared to the density of water at 4∞ C; because the density of water at this temperature is 1.0 g/cm³, it gives us the density in those units.

1. Nickel test: Finally, students tested their samples for the presence of nickel using a 1% solution of dimethylglyoxime dissolved in methanol.  They rubbed a Q-tip moistened with distilled white vinegar (5% acetic acid) on the sample to dislodge and collect nickel ions, then touched the vinegar Q-tip to a second Q-tip that contained the dimethylglyoxime solution and household ammonia.

Students are encouraged to think critically about what they are doing at each step of the lab.  They need to learn how to perform the experiments correctly, but they also need to know how their observations relate to the question at hand: Is this a meteorite or not?  Each experiment provides evidence, but none are conclusive by themselves. In fact, our true goal is for the students, by the end of this meteorite experience, to be able to assess their level of confidence in their results.  Because these results are reached by a process of critical thinking and drawing conclusions from multiple lines of evidence, students build proficiency developing logical arguments.

We prepare the students for the lab by giving them guidance on how to interpret the results and meteorite facts to provide context.

Visual Observation:  Some of the samples were small uncut meteorites, so students could see the fusion crust.  The loupes also allowed students to see crystals in some Earth rock samples that were unlike anything they had observed about meteorites in the first part of the lab.  One of the samples was an uncut meteorite that had chondrules partially visible.

Magnet test:  The suspended magnet setup was quite sensitive to ferromagnetic materials in the samples; the magnet would deflect gently even when the small stony and carbonaceous chondrite samples were held nearby.  Iron meteorites and samples such as hematite and magnetite strongly attracted the magnet.  Because almost all meteorites contain iron, students can use the magnet to quickly find the “meteor-wrongs”; they are the ones that the magnet does not react to. Pyrite is a shiny rock that grabs your attention, but the magnet does not react: not a metallic meteorite!  Using the magnet test as a limiter does eliminate rare achondrites, but at the end of the lab we circle back around and ask students to consider what types of meteorites they might have missed due to the criteria they used.

Density test: Students compare the densities they find for their samples to the known density ranges of various kinds of meteorites and Earth rocks. The general rule-of-thumb is that most terrestrial rocks have a bulk density of 3 g/cm³ or less, but there is significant overlap, and our samples include a CM chondrite that has a bulk density around 2 g/cm³. Students are provided with a chart of bulk densities of terrestrial and meteoritic minerals and have to think carefully to make sense of the results.

Nickel test:  The nickel test is the most complex to make sense of.  Students must think logically about their results – whether or not their Q-tip turns pink indicating the presence of nickel – and situations where a straightforward interpretation of “the presence of nickel means it’s a meteorite and the absence of nickel means it’s not a meteorite” is not correct.  They must consider both what would cause a “false positive”, a positive test for nickel in a sample that is not a meteorite, and what would cause a “false negative”, a negative test for nickel in a sample that actually is a meteorite.

For metallic samples, a positive result on the nickel test provides strong evidence because there are no naturally occurring terrestrial iron-nickel alloys, and slag is typically low in nickel content. A metallic sample that passes both the magnet test and the nickel test is likely a meteorite!

For a rocky sample, a positive result for the nickel test argues in favor of the meteorite hypothesis.  Most meteorites contain significant amounts of iron and nickel whether or not they look metallic or have metallic flecks in them.  Because of the primordial nature of meteorites, their nickel-to-iron ratio is similar to the early solar nebula, significantly higher than nickel-to-iron ratios found in terrestrial rocks.  Because the vast majority of the Earth’s nickel is in its core rather than its crust, almost all terrestrial rocks lack significant nickel.  Nickel ores are the notable exception to the rule.  Interestingly, however, these ores are typically not magnetic and therefore would not pass the magnet test.  Observing photographs of nickel ores can also help here because they often look quite different than stony meteorites.  Together, these characteristics would allow students to realize that a nickel ore would be a “false positive”, not a meteorite though it tested positive for nickel.

“False negatives” must also be considered. Testing with dimethylglyoxime is sensitive to nickel, but we have found some negative results for known meteorites.  This is because our solvent is not getting enough nickel ions in solution to produce a noticeable color change to react with the dimethylglyoxime and ammonia.  Fusion crusts, even those of iron meteorites, do not contain large amounts of nickel.  In addition, unbrecciated achondrites meteorites truly lack metal content; this test misses these rare meteorites, giving a “false negative” result.

We plan to add additional meteorite samples in the form of slices, which should increase the number of samples that test positive.  In addition, we are also going to experiment with using a solvent of 1% hydrochloric acid to liberate more nickel atoms.

Putting it all together: In the final step of the lab, students compile the results for all the tests on their samples and draw a conclusion for whether each one is or isn’t a meteorite, including rating themselves on how confident they are in their results.

The students then get to see an identification sheet that tells what each of the samples actually are, which invites lively discussion about how the lab went for them and what their conclusions were. This final reflection on their thought process is great for building critical thinking skills.  Some samples are easy to rule out as common types of meteorites based on the magnet test alone; some samples pass the magnet test and may look like meteorites, but they fail the nickel test due to negative results.  This could be because the sample is in fact a terrestrial rock, or it could be a meteorite that fails the nickel test due to its fusion crust.  Likewise, the bulk density test provides compelling evidence for samples far from the density of terrestrial rocks, but there are outliers that can complicate the results: slag has a density that masquerades as an iron meteorite, and CM chondrites have densities comparable with many terrestrial minerals.

We hope that the primary take-aways from this “Meteorite or Meteor-wrong?” lab are good memories of the opportunity to conduct scientific testing and the understanding that reasoned conclusions are drawn by weighing all the testing evidence together!

We want to express appreciation for our Department Chair and Associate Chair, Dr. Solomon Fesseha and Dr. Jay Dunn, for their support of this project, both financially and ideologically.  We want to thank Dr. Dion Stewart for his help in designing the experiments.  We also want to thank Melissa Silva at Aerolite Meteorites, who helped us build our collection and shared information and ideas with us.  It has been a lot of fun making this idea become a reality.  In the process, we have learned a lot and, hopefully, shared our excitement with our colleagues.  Our hope is that astronomy faculty and lab coordinators at other colleges might consider implementing a lab such as this.  The costs were surprisingly reasonable, and can be incurred over time if the lab is implemented part by part.  We would be glad to share our newfound expertise to anyone interested.  You can reach us at bskelton [at] gsu.edu and cstringer4 [at] gsu.edu.