Aerial photo of Barringer Crater. (Credit: NASA)

When you look up at the moon (using the naked eye or a telescope), the most noticeable features will likely be the craters that litter the surface. Furthermore, have you ever compared pictures of Mercury and Luna? In most instances, it's a struggle to tell the difference between the two! What about cratering on most of Jupiter and Saturn's satellites? What about all of the asteroids? Clearly, all of the objects in our solar system have survived some serious poundings  over the years.

For us on Earth, cratering is far less commonplace. This is not because our home planet is some special anti-asteroid location (if that were the case, there would be no moon!), but because there are various internal mechanisms that keep the surface of Earth fresh, to some degree. Said mechanisms aren't present on Mercury, Luna, or any of the other heavily cratered moons in our solar system. This, in turn, complicates matters pertaining to asteroid impact events; including the most notable:

Illustration By: Mark Garlick

The most well-known event, called the Cretaceous–Paleogene extinction event, took place some 65 million years ago. Given the long stretch of time between then and now, it's difficult to pinpoint exactly what wiped most of the dinosaurs out. Regardless, several potential scenarios have been put forth over the years, which include a global ice age, a super-volcano eruption, or the favored theory: an asteroid impact. Over the past several years, this one gained more and more traction until it was confirmed (to the best of our knowledge, anyway). The bigger questions for us to ask are "why" and "how can we prevent something like this from happening again?" Before those questions can be answered, we must take a closer look at the evidence.

By looking at cratering records on Earth, the moon and other planetary and non-planetary objects in our solar system, we've discovered that there appears to be a periodic cycle between major cratering events. Depending on the source publication, this cycle is anywhere between 26 to 35 million years.

Two theories have been proposed over the years to explain this noted cycle, one of them places the blame on an object called Nemesis! Unlike Luke Skywalker's home planet (dubbed Tatooine) from the Star Wars series, we only have one star lighting up a large fraction of the sky. Let's assume for a moment that that isn't the case, that our sun has a little brother, one that is far, far away from Earth and only comes somewhat close to us every 30 million years. Nemesis (the cute pet name given to this theoretical world) would only have to get close enough to perturb the orbit of some of the comets and asteroids within the Oort Cloud, sending them barreling inward, arriving in the inner solar system.

The Orbit of Nemesis (Credit: Space.com)

As far as Nemesis is concerned, it stands to reason that if it did exist, we would be able to spot it when we look up into the night sky (I mean, at such a distance, it should still be one of the most noticeable stars in the sky. Right?) This seemingly contradictory observation can be done away with it one assumes Nemesis to be a brown dwarf. Stars of this type are much dimmer than their larger counterparts, but still large enough to exert gravitational influence on other objects. Despite this, Nemesis has been somewhat debunked, as astronomers think the orbit of such a star would be highly unstable, thus rendering the cycle less absolute.

The second main theory posits that the motion of our solar system through the galactic disk of the Milky Way, an action that takes 225 million years to complete per orbit, could somewhat account for this perceived cycle, but like the one above, this one comes with its own set of problems.

Since neither theory offers a completely sound explanation, many astronomers are still looking for a viable cause, which brings us to the newest theory (it seems to be a better fit, but it also makes some assumptions).

Based on observational evidence, scientists think that the Milky Way, along with every other galaxy, Is surrounded by a halo of dark matter (our own planet could be enshrouded in one that is so large, it would look like Jupiter at optical wavelengths). The halo essentially helps keep our galaxy from flying apart, but we can't actually observe it, we can only see its gravitational influence on other objects.

Building on that, the authors of this paper not only suggest that dark matter plays a role in periodic mass extinction events, but that there could be not one, but two types of dark matter: the familiar kind, called weakly interacting massive particles (WIMPS) and another, less common, kind. This type, called "partially interacting dark matter" (PIDM), would essentially interact through a mechanism similar to electromagnetism and would emit "dark photons" as a byproduct of the interaction. Just having a small fraction of this kind of dark matter could lead to the formation of another dark matter halo that would encompass the visible disk of our galaxy.

The authors of this paper (and the fathers of this hypothesis) believe that the additional gravitational influence of the dark matter halo found around the central disk could play a role in the comet perturbation cycle. Under this scenario, when our solar system passes through the region in which the majority of dark matter resides, it may receive an added 'push' that sends objects in the Oort Cloud flying our way.

Image Credit: Paul Paladin (Source)

It is one of the more unique theories that I've read on this conundrum and it does hinge on the idea that a different type of dark matter emits dark photons. In this sounds completely bizarre, keep in mind that others are suggesting that there is a near invisible sun hiding out in our own backyard (Furthermore, we also know definitively that the universe is weirder than we can even imagine).

Whatever the reason, it should be said that the scientists can't even agree on whether or not such a cycle even exists (though there is solid geographical and fossil evidence to suggest that there is a burst of larger asteroids that pummel Earth every 26 to 35 million years). I'm sure we'll see lots of criticism about the paper in the coming months.

Read the full paper here. Similarly, without the moon, our planet would be a much different, more unstable place. See our article on what would happen if the moon broke apart.


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