Friday, January 19, 2018

Identifying Meteorites

All About Meteorites

Photo: Meteor, 8.5.2016 by Migebuff. CC-BY-SA 4.0
There is just something about meteorites. Something deep within us stirs when we hold a rock from “outer space.” Or when we realize that the lump of stone in our hand is billions of years old. It is no wonder that so many people are searching to find their own meteorite. How do you know if you find one? Could that black rock in the garden be a meteorite? It looks interesting. How could you even tell? We at the Delta College Planetarium are here to help give you the knowledge to do a first pass and filter out the regular old earth rocks from the specimens that deserve further scrutiny.

What are meteorites? Are they different from meteors and where do they come from? Astronomers use the word “meteor” to mean one very specific phenomenon; the bright quick streaks of light we occasionally see in the night sky, sometimes called “shooting stars”. Meteors occur when tiny bits of dust and rock slam into the Earth’s atmosphere. The dust or rock itself is called a “meteoroid”. When the meteoroid collides with the atmosphere it quickly rams into the air in front of it. As the air is compressed, it heats up so much that it glows white-hot. This glowing event is a “meteor”.

Most meteoroids end their stories here. The air radiates enough energy to melt and break up the meteoroid. However, some meteoroids are large enough, and strong enough that they survive the shock of entering the Earth’s atmosphere. If a meteoroid survives to the surface, that object graduates to being a “meteorite.”

Basalt is common earth rock that is frequently
misidentified as a meteorite.
Photo: Basalt by James St John. CC-BY 2.0

Meteorites are special because they do not look anything like Earth rocks, chemically speaking. The chemical and physical properties that formed them do not exist on the Earth. Most meteors started their lives in the asteroid belt between the orbits of Mars and Jupiter. In rare cases, a meteorite might have originated from a planetary body, like the Moon or Mars.

A stony meteorite specimen that fell near Tindouf, Algeria.
Photo: NWA869 Meteorite by H Rabb. CC-BY-SA 3.0

Meteorites come in three basic flavors. The first is stony meteorites. These are, true to their name, mainly made of stony materials. These are by far the most common kind of meteorite, but they can be difficult to detect because they frequently look like Earth rocks.

Example of a typical iron meteorite.
Photo: Octahedrite (Canyon Diablo Meteorite) (4.55 Ga) 2
by James St John. CC-BY 2.0

The second kind are iron meteorites. Unsurprisingly, these are made of iron. Far rarer than their stony cousins, iron meteorites are much easier to identify because of just how much iron there is in their chemical makeup.

Stony-iron meteorites feature nodules of minerals
surrounded by iron.
Photo: Pallasite-Esquel-RoyalOntarioMuseum-Jan18-09
by Captmondo. CC-BY-SA 3.0

The third kind of meteorite is sort of a mixture of the two previous types. They get the creative name stony-iron meteorites.

What about the black rock in the garden? Could it be a meteorite? Maybe. Many meteorites share common features that you can check for at home.

One common feature is meteorites tend to be heavy for their size, much heavier than you would expect from an Earth rock. This is because most meteorites have pure iron metal inside of them, even the stony ones. The presence of the iron also means most meteorites will react strongly to a magnet. If a magnet falls off or is only weakly holding on to the rock, then it is probably not a meteorite. In some cases, a stony meteorite might have almost no iron in it, and wouldn't react strongly to a magnet. In those instances, look for other strong indicators of extraterrestrial origin, such as a fusion crust, decribed below.

The iron present in even a stony meteor will have formed into small irregular blobs distributed throughout the specimen. You can scratch the suspected meteorite with a file looking for these blobs. They will appear as bright, shiny metal similar in appearance to chrome. Just a faint metallic sheen will not do, it should shine brightly.
Note how the bright iron blebs are spread randomly through the meteorite.
Iron rich rocks on the earth lack this irregular distribution.
Photo: Fisher by Jon Taylor. CC-BY-SA 2.0

If your suspected meteorite fell from space, it will likely have a layer of fused rock on the outside, called the “fusion crust”. Some meteors break up after the point where a fusion crust would form, exposing some of the inside of the meteorite, but most meteorites will have some of this fusion crust over part or the entire meteorite. The crust forms from the molten rock that develops on the outside as the meteoroid moves through the atmosphere. It will appear dark grey to charcoal black in color. The crust is usually quite thin, only a few millimeters deep. Lines might be present from the flow of hot gasses over the meteorite.
The fine lines in the fusion crust mark where channels of hot gases flowed over
the meteorite while it was entering the atmosphere.
Photo: Allende meteorite, crusted individual, 5g by Jon Taylor. CC-BY-SA 2.0

Smooth divots can appear in the surface of a meteorite, called thumbprints. The surface of a meteorite should still appear relatively smooth and rounded. Remember, this object plunged through the atmosphere. Atmospheric entry will have blasted away any hard angles or sharp edges. There also should be no bubbling or air pockets; these are telltale signs that a rock formed on the Earth.
These divits are called thumbprints because they resemble the mark left behind
when sticking one's thumb into clay.
Photo: Sikhote Alin Meteorite by H. Rabb. CC-BY-SA 3.0

Even though the air sculpts meteorites as they move through the atmosphere, they rarely form aerodynamic shapes. Meteorites are usually irregularly shaped with rounded points, so they are almost never round. Round metallic objects are frequently from human manufactured origins.

Having never been exposed to air, the iron in meteorites is completely unoxidized. Once it becomes exposed to the Earth’s atmosphere, the iron in the meteorite will quickly begin to rust. Suspected meteorites should show signs of this rusting, frequently turning the common rusty red color.

You can also perform a streak test. Take an inexpensive, unglazed, white ceramic tile from a home improvement store and drag the suspected meteorite across the tile. What color streak did the rock leave behind? If the streak is brown, it might be a meteorite. Meteorites produce a brown colored powder when finely ground. If the color is red or black, it is unlikely to be a meteorite and is probably some form of Earth rock.

Does it mean that a rock that passes all these tests is a meteorite? Meteorites are quite rare and Earth rocks can masquerade as them well enough to pass these tests. Ultimately, the only way to know for sure is to test the sample’s chemical composition in a laboratory.

Still have questions about meteorites?

The Delta College Planetarium and the Sunset Astronomical Society is excited to be hosting a special free program all about Michigan Meteorites. Craig Whitford from Michigan State University will give a presentation in the Delta College Planetarium at 7pm on Friday, March 9, 2018. Craig is the Meteorites Collection Coordinator at the university’s Abrams Planetarium in East Lansing and is leading the effort to put up a major exhibition there in 2019 thanks to a $100,000 grant from the National Institute of Museum and Library Services. The program will focus on meteorite finds from across the state and discuss the characteristics to look for in determining if a particular rock is a meteorite or not. For more information about the event, and to see the Delta College Planetarium show schedule visit our website.

Tuesday, February 14, 2017

Get Ready for the August Solar Eclipse!

On August 21, 2017, in some parts of the United States around mid-day, the Sun is going to disappear. Then, about 2 minutes later, it is going to come back. How is the Sun going to accomplish this “vanishing trick?” On that day, the Moon will move into just the right position to completely block the disk of the Sun seen from parts of the United States. We call this a total solar eclipse.

What is a Solar Eclipse?

Solar eclipses happen when the Moon passes between the Sun and the Earth. The Moon doesn’t sit perfectly on a line drawn between the Sun and the Earth. Instead, it orbits at an angle of about 5 degrees with respect to that line.

Image Credit: SMU Physics
Usually when the Moon passes between the Sun and the Earth it will pass above or below the Sun as seen from the Earth because of that angle. However, when the Moon is at just the right spot in its orbit at just the right time, it will actually block the Sun as seen from the Earth. The Earth passes into the Moon’s shadow. The Moon is a lot smaller than the Earth, so the shadow the Moon casts is also a lot smaller. In fact, the Moon is so far away, and so small, that the darkest part of its shadow, called the umbra, just barely reaches the Earth. If an observer is standing on the Earth in the path of the Moon’s umbra, they will see the Moon totally cover the disk of the Sun. Astronomers call the line traced by the Moon’s umbra the “path of totality.”

Image Credit: The New Student's Reference Work, 1914

But how can that be? Isn’t the Sun really, really big? And the Moon, it’s even tinier than the Earth! How could it cover up something as big as the whole Sun? It is all about being at the right place at the right time. The Sun is about 400 times bigger than the Moon, but the Sun is also about 400 times further away from Earth than the Moon is from the Earth. From the surface of the Earth, the Sun and Moon look nearly the same size in our sky, even though if we could place them side by side it wouldn’t even be close.

Why is this eclipse special?

Total solar eclipses are not incredibly rare. They occur every few years. However, the path of a total solar eclipse is tiny compared the surface of the Earth, so an eclipse being visible from any particular point on the Earth is quite rare. The last total solar eclipse to happen in the United States was in 1991 (and then only in Hawaii), and the next one won’t occur until 2024.

What makes this solar eclipse special is that the path of totality stretches across all of North America, from Oregon to South Carolina. This means that nearly every person in the United States will be within a day’s drive from seeing a total solar eclipse.

Anyone standing in the thick yellow line in the middle of the map will see a total solar eclipse. Those outside the yellow line will not be able to see Moon completely cover the Sun, and will only see a partial solar eclipse.

Where can I see the eclipse?

The closest place to Bay City to see the total eclipse will be in southern Illinois, about 600 miles away. That is pretty far away, but if you do make the journey you will be treated to a special sight that not many people get to see.

Maybe southern Illinois is too far away, is it even worth trying to see the eclipse from Bay City? Yes! While you won’t be able to see the Moon completely cover the Sun from Michigan, you will be able to see the Moon cover nearly the entire disk of the sun. This is a partial solar eclipse. The Moon will cover over 80% of the Sun at the height of the eclipse from Bay City. For those that want to see the total event but can’t get to the path of totality, or those that find themselves looking up into clouds, the eclipse will also be streamed live on the internet.

How do I view the eclipse safely?

The first rule of viewing an eclipse is never look directly at the Sun. Looking directly at the Sun can permanently damage your eyes.

The second rule is NEVER EVER look at the Sun through any kind of magnifying device (such as binoculars or a telescope) that does not have a proper solar filter securely installed on it. Looking at the Sun through an unfiltered telescope WILL damage your eyes, quickly and permanently. If you are not sure if something is a proper solar filter, do not use it.

Eye protection when viewing the Sun is very important. Although invisible, sunlight contains dangerous infrared and ultraviolet rays. These rays of light are damaging to the human eye, so observers need to take special precautions. Solar eclipse glasses are an inexpensive option. These glasses are made with a thin plastic filter that blocks out nearly all the of the Sun’s light, including all of the dangerous rays, and allows a person to look directly at the Sun safely.

Do not use sunglasses! They will not block enough of the harmful rays of the Sun to protect your eyes. No, two pairs isn’t good enough either. There is no number of sunglasses you could wear to protect your eyes from the harmful rays of the Sun.

Image Credit: flickr user Mr.TinDC, CC-BY-ND 2.0
Special solar filters for telescopes and binoculars can be used to view magnified images of the Sun. Only use filters that cover the entire aperture of the telescope. Never use a solar filter that attaches to an eyepiece, as they are prone to failure and can quickly damage your eyes. Not sure if your filter is a solar filter? Ask an astronomer. Always inspect your filter for cracks and pinholes before using. Never look through a telescope with a filter you are not 100% sure will protect your eyes.

But what if you don’t have solar eclipse glasses or a proper solar filter for your telescope or binoculars? There many indirect viewing techniques you can use. You can build a pinhole viewer out of a box and some tin foil. The viewer will project an image of the Sun on to another surface that is safe for viewing. You can also use a telescope to project an image of the Sun onto another surface, but there are a couple of things to consider before using this method. All that energy from the Sun’s light is passing through the lenses or reflecting off the mirrors of the telescope. This could cause them to heat up and crack, damaging the telescope. There will also be a dangerous beam of light from the telescope’s eyepiece to the projection surface where all of the harmful rays of the Sun will be concentrated. Do not stick your eye or skin in that beam, as it could injure you easily. With those considerations in mind, telescopic projection can be a safe technique for viewing a magnified image of an eclipse.

What will I see during the eclipse?

Image Credit: Tom Ruen, CC BY-SA 4.0
The Moon will slowly creep in front of the Sun. It will take the Moon nearly 90 minutes to cover the Sun. For most of the process, there will be no noticeable darkening of the surrounding area.

Image Credit: Arief R. Sandan (Ezagren), CC BY 1.0
About 15 minutes before totality, it will begin to get noticeably darker outside. As the Moon moves to cover the entire Sun, the remaining portion of the Sun will look as though it were a string of beads. These are called “Baily’s beads,” and are caused by portions of the Sun still being visible between mountains on the Moon. When only one Baily’s bead remains, the eclipse may resemble a diamond ring.

Image Credit: Luc Viatour /, CC BY-SA 3.0
When the moon finally covers the sun, it will get very dark out. The stars will be visible, and the surrounding horizon will look like sunset. Around the edge of the Moon, you will be able to see the Sun’s corona, the outer layers of the Sun’s atmosphere. Usually the Sun is so bright that it washes out any view of the corona, but because of the eclipse you will be able to see this feature with your own eyes. During totality, the eclipse is safe to look at without eye protection. But still take care, because as soon as any part of the Sun reappears from behind the Moon, so do those dangerous infrared and ultraviolet rays. The exact length of the totality will depend on your location, but for most places in the path of totality it will be longer than 2 minutes.

Then, as the Moon moves along its orbit, the Sun will peak out, spilling its light all around. Baily’s beads will be present again, and it will take about 90 minutes for the Moon to exit the disk of the Sun.

Where can I learn more about the eclipse?

Stop by the Delta College Planetarium. We’re always ready to answer your questions. Our new show Eclipse: The Sun Revealed, opening Saturday, May 13, is all about the science and history behind solar eclipses.

Another fantastic resource is There you will tons of great maps and information leading up to the August 21, 2017 solar eclipse.