Siberia in the crosshairs

kw: book reviews, nonfiction, science, meteorites, craters

Take a look at Manicougan Crater in Quebec, 100 km (60 miles) in diameter and 214 million years old. It is one of hundreds of impact craters known; see the Impact Database.

I was hoping to find a nice image of a crater in Siberia, but none are this photogenic. The Popigai crater in north-central Siberia is the same size as Manicougan, but is only 35 million years old. Popigai is one of the places visited by Dr. Roy A. Gallant, and reported about in his book Meteorite Hunter: The Search for Siberian Meteorite Craters. Northern Siberia is a lot harder to reach than central Quebec.

Meteorite Hunter chronicles seven of Dr. Gallant's expeditions ("trips" is too tame a word), facilitated by his friend and colleague Katya, Ekaterina Rossovskaya, from 1992 to 2000. For the Popigai expedition, the number of researchers, family members, guides and staff numbered more than 25. The impact sites visited were Tunguska (of course!), Sikhote-Alin, Chinge, Pallas, Tsarev, Popigai and Teleutskoye. Four of these were meteorite falls in the Twentieth Century. Contrary to some reports, the Tunguska impact of 1908, really an aerial explosion, killed at least two people.

Any visit to Siberia is an ordeal. Crossing the continent via the Trans-Siberian Railroad is difficult enough, but getting to the geologically interesting locales requires vehicles that can manage deep bogs, cross rivers and climb boulder-strewn hills. A driver told the author that the best vehicle to use on Siberian roads is a helicopter. Indeed, three of the seven locations did require helicopter transport. Then I read that in 1992 the author was 68. Clearly, he has the endurance of a man half his age.

Dr. Gallant's interest is not simply geological. As he explains in his last chapter, Earth is still being pummeled by a rain of cosmic dust, pebbles and larger objects that amounts to between 40 and 60 tonnes daily, or 14,000-22,000 T/y. According to NASA's Near Earth Object program, an object 10 meters across or larger hits Earth about every ten years. Calculated another way, such an object passes inside the Moon's orbit every day. The Barringer Crater in Arizona, 1.2 km across, was excavated by a 50m metallic object. The most reliable estimate by Russian researchers is that the Tunguska object was about 156 meters in diameter, but it must have been stony or even a stone/ice combination.

The Google Maps Meteor Crater Viewer shows these known craters for North America:

A similar view for northern Asia and part of Europe:

The Mercator projection used exaggerates the size of Siberia, because it is so far north. Its area is about 15 million square km, while that of North America is 25 million. But the seeming sparseness of craters in Siberia just indicates how little explored it is, compared to North America and Europe. There are hundreds of craters yet to be found.

Among known craters, this list from Wikipedia is instructive:

This shows the eleven known craters, 20 km or greater in size, that are 65.17 million years of age or younger. It may be that the two oldest, Chicxulub and Boltysh, were simultaneous, though the Ukranian crater's size indicates it had less than 1% of the energy of the other. Then there is a cluster of three craters aged 35 million years, including Popigai. There was an extinction event at that time, though it wasn't quite as bad as the one 65 million years ago, the younger impacts having perhaps one-fourth the energy of the older ones. The youngest crater on the list, Karakul, is but 5 million years old. It is likely that continued study will turn up dozens to hundreds more craters in coming years, to help us better estimate our chances of getting wiped out. Doing something about it is another thing entirely.

Reading this book reminded me of reading the "dinosaur hunter" books by Roy Chapman Andrews, many years ago. The thrill of adventure is the same, as is the frank assessment of the difficulties encountered. Of course, getting around Siberia still requires the good will of Russian bureaucrats, and some of that was hard for Katya to earn. Fortunately, the people who aren't bureaucrats were very welcoming. But getting from somewhere like Moscow to a remote Siberian village mainly required braving abominable travel conditions, and biting insects that are unrivaled (I've encountered mosquitoes the size of horse flies in southern Kansas, but the Siberian ones must be even worse). And, you'd better love beet soup. Borscht is the common fare in most places. The author reports it is all well worth it. Lovely scenery, welcoming people, and amazing science to be done. There is much more yet to do.

My meteorwrong

kw: information, space science, meteorites

Proving myself wrong about a rock

I've carried this rock around for so many years I've forgotten where I found it. I suspect, though, it was on a trip to San Benito County, California, with a group of fellow Geology students looking at the ultramafic rocks there.

"Ultramafic" refers to rocks with high Magnesium and Iron content; the County is host to a sequence of rocks derived from the upper part of the Mantle during a continental collision.

The connection of this rock with that trip is only in retrospect. Since finding the rock in a box several years ago, I've considered it might be a meteorite, mainly because it is rather heavy and sort of "looks right". Recently I decided to find out.

I decided to first measure its density. I don't have lab equipment available, so I used what I found in the kitchen: a spice scale (capacity 0.5kg) and a 0.5 liter measuring cup. I weighed the stone (210g), then put the stone in the cup and added water until it was covered, turned it all ways with a spoon to knock off all bubbles, then topped it to 500cc, the top line on the cup. I fished out the stone and read "a little below 450", which I estimated as just above 440cc. So the difference was less than 60cc, and I called it 58-to-60cc. Divide these limits into 210, and the result is 3.5-3.6 g/cc. That's in the right range for a stony-iron meteorite.

Checking relative accuracy, I decided I wasn't quite satisfied. The scale is marked each 10g, and can be read to 5g with care, so 210±5 meant a range of ±2.3%. The measuring cup, however, is only marked each 50cc, and it is hard to read a fifth of a marking, particularly because their spacing changes on the tapered cup. Generously assuming I can read to 10cc accurately, I still was faced with a range of ±10cc out of about sixty, or about ±17%. And there's no telling how properly printed the markings are...

So I used weight instead of volume. I tied a thread harness to the rock, so it would hang flat. I put the rock into the cup with enough water to cover it when I lifted it clear of the bottom. The whole rig exceeded the capacity of the scale, but not when I lifted the rock above the bottom of the cup, yet still under the water. I could just make it: 490 grams. Then I lifted the rock all the way out, and the reading dropped to, as nearly as I could tell, between 435cc and 440cc. I split the difference at 52.5±2.5cc, an error figure of ±4.8%. Now we're getting somewhere.

The result was 4.0g/cc, ±5.3%, or a range from 3.8-4.2. The former measurement, 3.55 with a ±17% range, covers a possible span of 2.9-4.2! Anyway, it doesn't contradict the new measurement. Now...what has a density of 4?

Stony meteorites have a density in the range 3.0-3.3. Iron ones are near 8, and even stony-iron ones are typically less than 3.8. My stone is either too heavy or too light to be a meteorite, and is not magnetic, so stony-iron is out. A closer look provides a further clue.

This broken surface, which is in the shadow to the left of the first photo, shows holes inside the rock.

A quick look at the Meteorite Realities page confirmed my suspicion that no meteorite formed with internal holes; there is no gas in space to open the holes, and these are clearly gas bubble holes. Though they appear to constitute only a few percent of the rock, they indicate that the bulk density without porosity ought to be several percent greater, at least 4.2-4.4 g/c. Now I'm in trouble...

The rounded lump at the arrow is reddish compared to the solidly gray background. Is it a garnet? Perhaps the garnet (or whatever) is one of the denser types, but it has to constitute most of the rock to have much effect. The usual sort of garnet peridotite seldom exceeds a density of 3.5, and this rock isn't green enough to be a peridotite (think the black sands of Hawaii, which are really blackish green, formed of peridot sand).

This image, around the end of the stone from the other two views, shows a squarish, reddish crystal (see the arrow). Maybe a garnet, or possibly a spinel. The reddish stain below gave me another clue, and there are several around the stone (see the first image). Now I remembered that trip to San Benito. I still don't know if that is where I got the rock, because we also visited a number of mines on that trip and later ones during my Senior year as a Geology student.

What is clear is that this is a combination of garnet or spinel in a siderite-hematite mix. These occur together in metamorphic assemblages, and both San Benito County and other places I frequented in those years were metamorphic terranes with plenty of sources for iron ore, which is just what this is.