Possible #meteorite fall in Norway – Finding space rocks with seismic

On November 8, 2013 by Mike
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The energy from the November 6, 2013 fireball over southern Norway arrives more than 7 minutes after the fireball is observed visually. This delayed arrival can help us calculate the location of the terminal burst.

Last night (November 6th) at approximately 20:19 a bright fireball was observed by many people across southern Norway. As well as visual effects, many of those who saw the event were also treated to an audio show when the sonic booms from the fragmentation and terminal burst reached them several minutes later. Sadly, I didn’t see (or hear) the event from Stavanger, but news of it excited me all the same. Why is that? Well, from the reports that had come in, it sounded like the blast may just have been strong enough to be detected by some of the earthquake seismometers scattered around Norway. And I like seismic data. It’s simple and, like astronomical data, can tell an incredible story if one looks at it in the right way.

The area of Valdres was mentioned in the NRK article describing the event so I’ve examined the nearest seismic stations to this area. The signal was weak at most stations, however, some of the Norsar NOA detectors showed a decent signal. Decent enough to attempt to triangulate the burst location. In addition to seismic observations, infrasound detectors can provide useful information on distance and direction to a burst. Because of the nature of infrasound, it is not unusual to detect large fireballs more than a thousand km distant. How cool is that?

What follows is a very quick demonstration of how one can use seismic data from earthquake sensors to help localize a possible meteorite fall area. The solutions shown are very simplistic. Full solutions take into account atmospheric temperatures, wind speeds and directions at different altitudes, and are not terribly different from the way that geophysicists deal differing rock types and velocity anisotropies in the subsurface. If you’d like more detailed info on how an airblast makes it’s way to a seismic detector, take a look at this article by Wayne Edwards and colleagues.

BurstAreaThis Google Earth image shows 320m/s (green) and 350m/s (orange) circles for 4 of the detectors from the NOA station. Also shown are two sets of red and white circles. These correspond to very rough, back-of-the-envelope solutions from the apparent infrasound signal at the Lycksele and Kiruna stations located at distances of about 600km and 900km respectively from the burst location.  What you’ll immediately notice is that the seismic solutions (orange and green) tend to converge in an area about 165km NW of Oslo. More precisely, about 10km west of the town of Røn. Neat. So that’s where the meteorites are? Not likely…

When an asteroid like this enters the Earth’s atmosphere it is travelling at something like 20km/s. With all of that pent up energy, the thin upper atmosphere must, at first, seem like a bit of a joke. But, as the atmosphere becomes more dense, pressures build up and, at some point (many 10’s of km altitude), exceed the strength of the asteroid. When this happens, the rock disintegrates in a spectacular fashion causing the bursts seen from the ground. With each fragmentation (yes, there can be many along the path), the asteroid slows until it reaches a sub-luminous velocity (maybe 4 or 5 km/s). It then continues onwards in ‘dark flight.’ So, even though a fireball may no longer be visible, it can still be moving along its incoming trajectory at quite a speed. If the incoming trajectory is vertical, then any meteorites will be scattered on the ground below the burst point. Non-vertical (most) trajectories, however, will result in a meteorite strewn field that is elliptical and displaced downrange from the atmospheric burst point. So, to find the meteorites, we need to look further along the path of the meteor.

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From the 4 stations used here, the most-likely burst location (assuming a burst altitude of 35km) is about 10km west of the town of Røn. Keep in mind that any meteorites will most-likely continue to travel towards the south before striking the ground.

So, where do you look for this one? Well, without good information on the direction that this one was headed (we do know it was south-ish) your best bet would be to go to the area and speak to people who saw the event. Get them to show you where they first saw the meteor and where they saw the burst. Measure the compass directions of these points and plot them on a map along with as many other observations you can find and, fairly quickly, you’ll find that a estimate of the direction of travel emerges. With that in hand, move down range of the burst point in, say, 5km, increments and search the ground with every stop. There’s no guarantee that you’ll find a piece of this space rock, but if you do, it will be an experience you will never forget. And remember, you most certainly won’t find anything if you don’t go out and look.

Now that you’ve decided to become a meteorite hunter, here are a few basic things to keep in mind.

1) It may or may not be legal to search for and collect meteorites in your area. In Norway, it’s legal. In other countries it’s not. That’s all of the legal advice that I’ll give you.

2) A metal detector tends to slow you down. Especially in an environment like the mountains of Norway. Besides, not all meteorites will make your metal detector beep. Your eyes are your best tools. Use them.

3) You’re looking for rocks that (if freshly fallen) will most likely be very black (sometimes shiny, sometimes not) on the outside. And I mean, really black. If the surface is broken, you will notice that the crust is very thin (less than a mm) and the inside will typically be a light grey colour.

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A naturally broken piece of the Moss meteorite. Notice the black fusion crust and the light grey interior. And the aluminum foil. Notice the aluminum foil…

4) Meteorites will typically feel heavy for their size. This is because most have some amount of iron in them. Of course if you have an iron-nickel meteorite, it will be immediately obvious.

5) Meteorites will not hurt you. They’re not radioactive (not in the glowing green sense) so you don’t need to worry about frying yourself.

6) That being said, fresh fallen meteorites do not like to be touched too much. The oils from your fingers can damage them and make it difficult to do certain types of research. If you find something, it’s best to either pick it up with latex gloves or with a piece of aluminum foil (and then wrap it in the foil). If you can’t do that, you can always use a plastic shopping bag as a makeshift glove.

7) Speaking of research, it’s a good idea to think about loaning your new found rocks to scientists for research. A great deal of non-destructive work can be done and you’ll get the satisfaction of having contributed to science. And, keep in mind that some work should be done as soon after recovery as possible. If in doubt, speak with your local meteoriticist. 😉

8) If you decide to sell your meteorites, make sure that you know what they’re worth. The value varies considerably with classification but, for a fresh fall may range from a few dollars a gram (most common types) to thousands of dollars a gram (for the rarest specimens). Don’t get ‘screwed’ but do realize that a dealer still needs to put a lot of work into preparing your specimen for further sale.

9) Most of the rocks that you find will not be meteorites. Finding meteorites can be tricky but, if you stick to it, you may get lucky.

p.s. If you do go out, feel free to send me a note. I’d love to hear you story. And, if you find something, you’re welcome to let me know.

 

 

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