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JacobAldridge | 2 years ago

I was only 12 when Shoemaker-Levy banged into Jupiter in 1994, so wiser minds or those with better memories may correct me.

At the time, the leading theory for what killed the dinosaurs was still quite terrestrial - volcanos and climate change.

There was increasing evidence for the meteorite impact theory, but a big block was “Space is big, outside the early formation of the Solar System comets and asteroids don’t just slam into planets”.

Then comet Shoemaker-Levy showed us that they actually do, perhaps still quite frequently, with Jupiter playing an imperfect shield for Earth. It was one of the last roadblocks to the now-widely accepted impact theory (still not ‘solved’ of course, and perhaps only part of the extinction puzzle).

Dinosaurs were back in the zeitgeist thanks to Jurassic Park (1993), but Shoemaker-Levy and the impact theory gave us the 1998 twin movies Armageddon and (the better of the two, imho) Deep Impact.

discuss

order

dylan604|2 years ago

Was that really the leading theory you were taught? If you were 12, I'm <10 years older, and even then I distinctly remember the asteroid theory being taught as what wiped out the dinosaurs. Climate change is kind of given after that, but the asteroid was taught as the cause.

There's been a few "big one" theories I've heard about. The impending California earthquake is a popular one, but I'm familiar with super volcanoes and asteroids too from childhood.

davesque|2 years ago

Yeah, I remember the same. I was 13 in 1994 and I remember the impact theory being taught as the cause well before the Shoemaker-Levy impact.

njarboe|2 years ago

Alvarez et al., 1980 [1] was the paper that convinced most geologists that the extinction was caused by an asteroid impact [source, PhD in Earth Science]. They found a layer of clay with very high iridium concentration at the K-T boundary in multiple locations. Some asteroids have high iridium concentrations relative to the Earth's crust so an impact of a large one would leave this signal all over the Earth.

Wikipedia has an interesting timeline of theories for the K-T extinction (now called the K-P which is not as cool a name), but strangely does has a link to this paper[1].

[1]https://www.science.org/doi/abs/10.1126/science.208.4448.109... [2]https://en.wikipedia.org/wiki/Timeline_of_Cretaceous%E2%80%9...

Paper's Abstract Platinum metals are depleted in the earth's crust relative to their cosmic abundance; concentrations of these elements in deep-sea sediments may thus indicate influxes of extraterrestrial material. Deep-sea limestones exposed in Italy, Denmark, and New Zealand show iridium increases of about 30, 160, and 20 times, respectively, above the background level at precisely the time of the Cretaceous-Tertiary extinctions, 65 million years ago. Reasons are given to indicate that this iridium is of extraterrestrial origin, but did not come from a nearby supernova. A hypothesis is suggested which accounts for the extinctions and the iridium observations. Impact of a large earth-crossing asteroid would inject about 60 times the object's mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record. One prediction of this hypothesis has been verified: the chemical composition of the boundary clay, which is thought to come from the stratospheric dust, is markedly different from that of clay mixed with the Cretaceous and Tertiary limestones, which are chemically similar to each other. Four different independent estimates of the diameter of the asteroid give values that lie in the range 10 ± 4 kilometers.

dylan604|2 years ago

Thinking back, I remember my dad talking about the asteroid killing the dinosaurs. However, my dad had not been in school for a long time by 1980. He didn't have a subscription to any science type magazines, and clearly no internet. So I'm now curious how he kept up. My dad wasn't one to read the paper at home, and he didn't have an office job affording him time to read at work. Obviously people talk at work, but it now has me thinking.

I'm guessing the killer asteroid was a theory for some time, and this 1980 paper was just the thing like you said.

Archelaos|2 years ago

A single event like the Shoemaker-Levy 9 impact does not tell us much about frequencies on its own. Besides, the original width of Shoemaker-Levy 9 was estimated to have been between 1.5 to 2 km.[1] The Chicxulub meteorite was estimated to have been between 10 and 80 km wide.[2] And it happened around 65 million years ago. So frequencies then and frequencies now might be quite different. That meteorite impacts happen from time to time on earth was nothing controversial. The question was, whether there had been a (very rare) hugh impact, large enough at the time of the Cretaceous–Paleogene extinction event and whether this was the only cause for all those extinctions. This is a very complex scenario and several of its details are still debated today.[3]

The most important evidence that an impact was the main cause of the extinction event was provided by the discovery of the Chicxulub crater in 1990/91. However, the investigation of this and other evidence is still ongoing. Contrary to popular belief, scientific debates of this magnitude are not resolved by a single ingenious theory or observation. It is the hard work of many, many people over years and decades that gradually changes and refines the web of belief of a scientific community.

[1] https://solarsystem.nasa.gov/asteroids-comets-and-meteors/co...

[2] https://arxiv.org/abs/1403.6391

[3] Wikipedia offers summaries of some alternative hypotheses, showing how complex the arguments are: https://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_e...