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For context, modern humanity can travel to alternate versions of their planet through portals. They still only have our modern day technology and science. They stumble upon an Earth tens of million years ago that seems to have experienced a mass extinction. After quick analysis of fossils, they theorize this was likely this world's K–Pg extinction event. However, after more searching, they realize that something's wrong. The evidence for this is the lack of iridium or any corresponding rocks and metals of an asteroid impact. As indicated by the massive country-spanning crater, something must have done the planet in.

The truth is that a million years ago, some Lightning Deity struck the alternate Earth with a lightning bolt powerful enough that it mimicked the devastation of the asteroid impact, causing the mass extinction. The scientists involved don't know that.

My question is: What possible evidence could the lightning strike leave behind such that scientists would realize the mass extinction was caused by said lightning?

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    $\begingroup$ Lightening is a sudden spark between the upper atmosphere and the ground. The entire Earth as a sphere has a capacitance of less than a microFarad. You can scale up the effects of lightning, but this does not make physical sense. Let Zeus chuck antimatter and thunderbolts. $\endgroup$ Commented May 24 at 18:28
  • $\begingroup$ Very astute comment ! $\endgroup$
    – Fattie
    Commented May 24 at 22:59
  • $\begingroup$ Maybe having many small lightning (one hitting each animal) works better than a single big one? $\endgroup$ Commented May 25 at 21:56
  • $\begingroup$ @RichardKirk Hmm, you'd have to come up with some way that a lightning bolt would cause damage beyond the immediate area where it struck. But nuclear weapons and volcanoes do that, so the idea isn't inconceivable. $\endgroup$
    – Jay
    Commented May 26 at 9:57
  • $\begingroup$ How does a Lightning Deity cause lightning bolts to strike the Earth? Do they use some kind of Maxwell's demon to remove electrons from the ground and hold them in a cloud until they want to release the bolt, or is it a different mechanism? We can't say how the lightning bolt would or wouldn't be detected if we don't even know what it really is. (The fact that you have a Lightning Deity means we cannot assume that their "lightning bolt" is the same natural phenomenon that we know as a lightning bolt.) $\endgroup$
    – David K
    Commented May 26 at 16:57

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Being struck by one bolt with enough power to mimic an asteroid impact would probably leave a massive "Lichtenberg figure". I can only assume instead of scorch marks like below it would leave deformed terrain.

I don't know if this would leave like gaping fissures the size of canyons or cracks that would eventually be filled in by time though, but for your story you could probably just go with canyons created with this pattern stretching for miles to make it identifiable by scientists. I don't think there is anyway to determine how powerful the lightning bolt would have to be to do this, but a powerful enough one probably could.

https://d2w9rnfcy7mm78.cloudfront.net/12655447/original_2e58f2a037c746f57e54fcf6844aa319.jpg?1627352428?bc=0

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    $\begingroup$ Rather than canyons, I think the tracks would be glassed. Glass is incredibly stable even over geologic times (that's why we vitrify radioactive waste). Huge radiating glass tracks would be a pretty cool thing to discover! $\endgroup$ Commented May 24 at 17:37
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    $\begingroup$ Add-on to the comment by @thegreatemu : See fulgurites. $\endgroup$
    – njuffa
    Commented May 25 at 11:27
  • $\begingroup$ I would pay a lot for some extra-world special glass for my house. $\endgroup$
    – Olaf
    Commented May 27 at 12:32
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A lightning strike with an energy comparable to the Chicxulub ($1.15 × 10^{23}$ J) impact would not go unnoticed.

It would melt a large volume of rock, leaving behind something similar to the welding point of TIG welding, but on a larger scale. The shockwave caused by the rapid heating of the atmosphere involved would probably wipe out and scourge a good part of the planet, leaving noticeable marks, too.

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The debris from an asteroid impact come from both the impacting asteroid, and the planet; in fact, more of the planet's mass is thrown out into the plume than the asteroid.

This debris shoots up molten, then solidifies either during reentry or while in orbit (then melts again during reentry and solidifies again).

So you'll get these characteristic deep crust nodules raining down over the planet in a layer. That layer won't contain the excess iridium, but will have tektites and shocked quartz. Above that layer you'll find a different set of animals, a it represents an extinction event.

Now, building a fossil record on another planet is hard; finding fossils is mainly about looking all over the place and finding good spots. Depending on how different the planet is and how much effort you put into it, this is something that could take decades or centuries. So even knowing about the extinction event will take time.

The layer will, however, be present at a lot of spots. If they are used to doing geological studies on planets, they'll deploy highly automated core samplers at interesting locations to get a geological history of the planet in question. Characterizing planets geological history will in turn inform where you can look for commercially useful resources.

The extinction layer being caused by a high energy strike will be solved; then work will be done to characterize the impactor (was it a comet? asteroid? supervolcano?). Initially it would show up as a supervolcano (as the materials are all terrestrial).

Once the location of the strike is found, it won't look like a supervolcano. Vulcanologists will be studying it and trying to make sense of the event, as it won't match any of their models. They'll rewrite their models to match it: the more data they have of planets with supervolcanic events, the more anomalous this planet will be.

The impact will be far more violent than a supervolcano should be, will contain too much crust material and not enough magma. Its isotope and element ratio will match that of Earth's crust, which means if it is a comet or asteroid it is strangely similar to Earth's crust.

A lightning strike will produce a charge differential. This charge differential will dissipate away from the location of strike differently than the main explosion does. The violence of the impact will erase most of the evidence at the actual impact site, but at the edge of it evidence of super-charged flows of electricity (which in turn explode) could be found.

Excess electrons from the bolt would have interesting effects in the short term, but EM is so strong the planet would reach near neutrality as it expels excess electrons to the solar wind more than it would protons.

So most of the evidence would be negative - the location would be strange for a supervolcano, the lack of unusual element mix you'd expect from an astroid of comet hit, and the event being short and sharp like an impact. The remaining evidence would be non-impact related secondary explosions along paths created by the electrical charge spreading, which could be large enough to be etched into the geological record.

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For a more subtle indication, lightning produces a wide variety of chemical compounds in the atmosphere which then rain (very literally in most cases) down on the terrain and leaches into the soil and rocks. Some of these - like N2O2 - decompose quickly while others have more staying power. A lightning strike on the scale you're talking about would produce all sorts of weird chemicals as side effects.

A simple ice core would give you plenty of good evidence for what happened... on a physical level at least. Proving that a god did it would be a bit tougher.

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