Make your own impact craters

posted: 3978 days ago, on Thursday, 2007 Jan 04 at 05:35
tags: astronomy, Moon, outreach.

A choice of chocolate catastrophies


Of the many ways life on Earth could be wiped out, the idea of a killer asteroid grips the imagination. Hollywood has given us Meteor!, Deep Impact, Armageddon, and everybody "knows" that a meteor strike killed the dinosaurs.

Space rocks have certainly smashed into our planet: there are some 174 confirmed impact structures on Earth. These range in size from as small as 15 metres (Haviland, Kansas, U.S.A.) to our very own 300km uber-crater at Vredefort.

On the Moon, some 30,000 craters larger than 1km are known, with the largest about 360km across. Binoculars will show dozens of craters, and a high-power telescopic view is truly awesome. (OK technically, the smallest known lunar craters are microscopic, having been found in Moon rocks returned to Earth from the Apollo missions.)

Seeing craters on Earth is all-together more difficult. The total area covered by craters is 220,674 km² which sounds a lot, but this is just one-tenth of one percent of the Earth's land surface. The chances that you're living near one, is tiny.

Exploring with Google Earth

However, using Google Earth, you can explore these sites and enjoy their somewhat alien but incredible beauty. For those of you who don't know about Google Earth, it is a free program that you download and install, and once launched, you can visually navigate anywhere on Earth, zooming in or out to heart's content.

Satellite images, and sometimes low-flying aerial surveys, brings the globe to life. Check out the pyramids at Giza, SALT's glittering dome in Sutherland, or a bird's eyeview of your house. The only caveat is that you need a good internet connection – if you're on dial-up you could probably walk to your destination faster.

A thorough list of craters can be found in the Earth Impact Database, maintained by the Planetary & Space Science Centre at the University of New Brunswick in Canada. I went through the list, and looked up each exposed crater (some are underground, or under the sea) – not all of them are identifiable from the satellite images.

The smallest one I could find was the 80-metre Veevers crater in Western Australia. The table below lists those that show up really well on the Google Earth maps.

Table 1: Confirmed terrestrial craters that look pretty on Google Earth

Crater name and locationLatitude, longitudeDiameter (km)Age (Ma)
 
 
Vredefort, South AfricaS 27° 0', E 27° 30'3002023
Sudbury, Ontario, CanadaN 46° 36', W 81° 11'2501850
Manicouagan, Quebec, CanadaN 51° 23', W 68° 42'100214
Acraman, South AustraliaS 32° 1', E 135° 27'90~ 590
Siljan, SwedenN 61° 2', E 14° 52'52376.8
Clearwater West and East, Quebec, CanadaN 56° 13', W 74° 30'36 / 26290 ± 20
Shoemaker, Western AustraliaS 25° 52', E 120° 53'301630 ± 5
Mistastin, Newfoundland/Labrador, CanadaN 55° 53', W 63° 18'2836.4 ± 4
Presqu'ile, Quebec, CanadaN 49° 43', W 74° 48'24< 500
Lappajärvi, FinlandN 63° 12', E 23° 42'2373.3
Haughton, Nunavut, CanadaN 75° 22', W 89° 41'2339
Gosses Bluff, Northern Territory, AustraliaS 23° 49', E 132° 19'22142.5
Oasis, LibyaN 24° 35', E 24° 24'18< 120
Elgygytgyn, RussiaN 67° 30', E 172° 5'183.5 ± 0.5
Gweni-Fada, Chad, AfricaN 17° 25', E 21° 45'14< 345
Janisjarvi, RussiaN 61° 58', E 30° 55'14700 ± 5
Aorounga, Chad, AfricaN 19° 6', E 19° 15'12.6< 345
Serra da Cangalha, BrazilS 8° 5', W 46° 52'12< 300
Bosumtwi, GhanaN 6° 30', W 1° 25'10.51.07
Arkenu-1 and Arkenu-2, LibyaN 22° 4', E 23° 45'10 / 6.8< 140
Rock Elm, Wisconsin, U.S.A.N 44° 43', W 92° 14'6< 505
Roter Kamm, NamibiaS 27° 46', E 16° 18'2.53.7 ± 0.3
Tswaing, South AfricaS 25° 24', E 28° 5'1.130.220
Amguid, AlgeriaN 26° 5', E 4° 23'0.45< 0.1
Veevers, Western AustraliaS 22° 58', E 125° 22'0.08< 1
 

Google Earth crater gallery

 Amguid in Algeria  [click to start slide show]

Effects of an impact

How much damage can these impacting bodies cause? Years ago, I asked Trevor Gould, who runs the Southern African Meteorite Recovery program, to write up an item for MNASSA about such impactors. He noted that a small body hitting Durban would vapourize the surroundings and produce a heat wave that was 100°C as far away as Johannesburg. That's impressive.

As the impacting body hurtles through the atmosphere, intense pressure, and thus heat, is generated. The peak pressure produced by the impact of a stony body at 25 km/s is 9 million atmospheres!

Craters that are larger than about 1km across, which is 90% of the known Earth craters, generate so much pressure (and thus heat) upon impact that the impactor is completely melted, and even vapourized. We're talking 9 million atmospheres for a body that moves in at 24km/s, folks!

If the crater is larger than about 20km (which is a quarter of the known examples), we are in a different league. Such impacts can generate sufficient debris that we're talking nuclear winter – darkening skies and temperate fluctuations. This class of event is expected once ever three million years or so.

On the other hand, if the impactor whacks into the ocean (more likely, since 71% of the Earth's surface is watery), there won't be a crater, but there will be a tsunami. Calculations show that a 400-metre rock striking anywhere in the Atlantic will cause flooding on both sides of the ocean, with wave runups towering over 60 metres!

On the Moon, there's no water or atmosphere to hide or erode the craters, so one can get a pretty good idea of what craters look like, by turning binoculars or a telescope onto our neighbour. Probably the most striking thing about them (and about most of the craters on Earth), is that they are round. With that in mind, let's try a cool experiment.

Do it yourself

The aim of this simple but effective experiment is to demonstrate how a crater is formed after an impact. You will need: a packet of baking flour, some cocoa powder, a shallow dish about A4-sized, and a packet of Whispers, chocolate-coated hazelnuts, or any round chocolate, really. Chocolate is essential in this experiment, because it tastes better.

Using a sieve, distribute the flour into the shallow dish, so that it's about 2cm thick and nice and fluffy. Sprinkle over a layer of dark cocoa so that the white flour is totally covered. The dark cocoa represents the upper-most layers of the ground, and the white flour, the deeper strata.

Taste a few of the chocolates until you're satisfied they are OK. Then select one, and simply drop it into the dish. A height of about 2 metres is very dramatic; much lower will also do. Have some more chocolates while you observe the excellent crater you have just made.

You'll notice the outer edge has been thrust upward above ground-level, and seen from above is perfectly symmetrical. And there is lot of ejecta (white flour) scattered all about, which has been dug out from beneath the surface.

Now select a second missile, and this time throw it at the dish, much like a bad darts player. The missile will impact at an angle, with pretty results. You'll notice that the resulting crater is not round, but elongated in the direction of the missile's track. The ejecta, also, is distributed unevenly, predominantly in a forward-direction. Eat the remaining chocolates.

I also tried other "meteorites": a small, very light chess piece, as well as a Ferrero Rocher. Even the small chess piece, dropped from only 25cm, makes a splat. The Ferrero Rocher made intricate scalloped patterns along the inside edge of the excavated crater.

Thinking about the experiment

From the experiment, you've seen that straight-down impacts leave circular craters, whereas impacts at an angle create elongated craters. Now recall that lunar craters (and those on Earth) are all round. This suggests that they were formed by vertical impact events.

However, this is incredibly unlikely. Before meeting up with Earth, the space-rocks were whizzing about the solar system in all directions, at basically random angles to the Earth, and only a small fraction would be on a head-on path. So what is going on?

One clue comes from examining the craters made in the experiment. Notice that the diameter of the crater is, at most, three times the diameter of the impactor. In the case of real craters, the resulting crater diameter is much larger than the impacting body. The famous Barringer Crater in Arizona, USA, is 1.2km across, yet the impacting body was only 50 metres across. Our own Vredefort crater, 300km across, was formed by a 10km impactor. If we simply scale the experiment, a 2cm chocolate missile should have made a half-metre wide crater.

The missing link, explaining why small impacting bodies are able to make such huge craters, is that their speed at impact is much higher than what you or I can achieve by throwing or dropping. The energy of motion (kinetic energy) at impact depends on the mass of the impactor, but much more so on the velocity. As an equation: E = ½ m × v 2. If the mass m doubles, the energy E doubles; but if the velocity v doubles, the energy will be four times greater (notice the v 2 in the equation). And in the case of space rocks, typical speeds will be 90,000 km per hour – compare this to a cricket speed-bowler who at best can manage 162 km per hour!

In our experiment, the impactor simply pushed the material out of its way, transfering its momentum to the flour and cocoa. But a meteoritic impactor has so much energy at ground-zero that it's almost like a nucular bomb going off, generating a spherical shock that travels through the surface, melting, vapourizing and generally stirring things up pretty effectively.

And on that explosive note, I'm off to go looking for the one Ferrero Rocher that survived.

Links

  1. Google Earth homepage
  2. Earth Impact Database, Planetary & Space Science Centre
  3. Small selection of crater images with captions
  4. Traces of Catastrophe (e-book, Lunar and Planetary Institute)

nothing more to see. please move along.