Bizarre Features

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“According to the team’s modeling, the waves near the point where the asteroid struck would have been approximately 300 metres tall. When they hit the coast they wouldn’t be quite that big, but would still have reached 75 or 80 metres, Costard says.

“The most probable source of the tsunami, the scientists concluded, is a 60-kilometre impact crater located about 1000 kilometres off the putative coast. But it is also possible that the deposits could have been produced by the combined results of two independent impacts, represented by smaller craters closer to the shore.

“The new research also explains bizarre features known as thumbprint terrain, on the seaward ends of some of the tsunami deposits. Composed of curving, concentric ridges 10 to 20 metres high, these look for all the world like the ridges in a fingerprint.

“The explanation, Clifford says, starts with the fact that the tsunami would have come in two pulses. The first would have been produced when the asteroid hit, shoving tremendous amounts of water out of its way. The second would have occurred when water rushed back into the resulting depression from all sides.

“The onrushing water would have crashed together in the centre of the impact depression in a giant splash, then rebounded outward in a second tsunami, even larger than the first.

“And that’s just the beginning of the story. When the first wave, a few minutes ahead of the second, hit the shoreline, part of it would have been reflected back out to sea. There, it would have met the oncoming second wave, where the turbulence would have caused sediment to be dropped in patterns exactly like the enigmatic thumbprint terrains.”

Text: Mars may have experienced a giant tsunamiCosmos Magazine.

Image: The Great Wave Off Kanagawa By Hokusai, Supercoloring version.

A Hole in the Atmosphere

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“Obviously, something […] big hitting the Earth is going to hit with a lot of energy. […] This is the energy one million tons of dynamite would release if it was exploded and is the energy unit used for nuclear explosions. The largest yield of a thermonuclear warhead is around 50–100 megatons. The kinetic energy of the falling object is converted to the explosion when it hits. The 10-kilometer object produces an explosion of 6 × 107 megatons of TNT (equivalent to an earthquake of magnitude 12.4 on the Richter scale). The 1-kilometer object produces a milder explosion of “only” 6 × 104 megatons (equivalent to an earthquake of magnitude 9.4 on the Richter scale).

“On its way to the impact, the asteroid pushes aside the air in front of it creating a hole in the atmosphere. The atmosphere above the impact site is removed for several tens of seconds. Before the surrounding air can rush back in to fill the gap, material from the impact: vaporized asteroid, crustal material, and ocean water (if it lands in the ocean), escapes through the hole and follows a ballistic flight back down. Within two minutes after impact, about 105 cubic kilometers of ejecta (1013 tons) is lofted to about 100 kilometers. If the asteroid hits the ocean, the surrounding water returning over the the hot crater floor is vaporized (a large enough impact will break through to the hot lithosphere and maybe the even hotter asthenosphere), sending more water vapor into the air as well as causing huge steam explosions that greatly compound the effect of the initial impact explosion.

“There will be a crater regardless of where it lands. The diameter of the crater in kilometers is = 0.765 × (energy of impact in megatons TNT). Plugging in the typical impact values, you get a 150-kilometer diameter crater for the 10-kilometer asteroid and a 20-kilometer diameter crater for the 1-kilometer asteroid. The initial blast would also produce shifting of the crust along fault lines.

The oceans cover about 75% of the Earth’s surface, so it is likely the asteroid will hit an ocean. The amount of water in the ocean is nowhere near large enough to “cushion” the asteroid. The asteroid will push the water aside and hit the ocean floor to create a large crater. The water pushed aside will form a huge tidal wave, a tsunami. […] What this means is that a 10-km asteroid hitting any deep point in the Pacific (the largest ocean) produces a megatsunami along the entire Pacific Rim.

“Some values for the height of the tsunami at different distances from the impact site are given in the following table. The heights are given for the two typical asteroids, a 10-kilometer and a 1-kilometer asteroid.

Distance (in km) 10 km asteroid 1 km asteroid
300 1.3 km 42 m
1000 540 m 18 m
3000 250 m 8 m
10000 100 m 3 m

“The steam blasts from the water at the crater site rushing back over the hot crater floor will also produce tsunamis following the initial impact tsunami and crustal shifting as a result of the initial impact would produce other tsunamis—a complex train of tsunamis would be created from the initial impact (something not usually shown in disaster movies).”

Text: Nick Strobel, Effects of an Asteroid Impact on Earth, Astronomynotes.com

Image: “The Chelyabinsk event of 15 Feb. 2013, having an energy equivalent to 500 kilotons of TNT, was the largest well-documented meteor event since the Tunguska event of 1908. Analysis of the Chelyabinsk object’s in-atmosphere trajectory from video records has found that its orbit was similar to the orbit of the 2-km-diameter Near-Earth Asteroid 86039 (1999 NC43).There is a possibility that the Chelyabinsk object was created through a collision between asteroid 86039 and another asteroid. It is also very interesting that the Cheyabinsk Asteroid approached from the Sun’s direction, making it essentially undetectable in telescopes” – Analysis of Chelyabinsk meteor and resultant air burst appears in Nature