Cometary and asteroid impacts can be far more complex occurrences than simply one rock hitting
another.
There can be circumstances where not just one impact occurs, but a string of them in
succession.
This is due to how comets and asteroids are constructed, typically being loose conglomerates
of material leftover from the formation of the solar system.
This allows them to be easily tidally disrupted by a gravitational force and come apart.
We saw this process happen with comet shoemaker-levy 9, which broke into pieces before plunging
into jupiter forming multiple impact points.
But we see evidence for this elsewhere.
Known as catenae, they are crater chains on the surfaces of planets and moons that clearly
show that the body that formed them had broken up before impact.
During the Voyager missions, a number of cases of this were noted on both Callisto and Ganymede.
Identifying true catenae can be tricky, certain volcanic features on worlds can mimic the
effect, this is particularly the case for Mars which was once highly and regularly volcanic.
As an aside, the profile of Mars’ volcanism has very much been in flux as of late.
We used to think that the planet was dead, with a cooled core due to a lack of a magnetic
field.
But then hints that something was still going on there volcanically kept surfacing.
One was seismic measurements done by the Mars InSight lander in 2018, which showed seismic
activity consistent with volcanism, which has very distinctive seismic signatures.
Indeed, one of the ways scientists predict impending volcanic eruptions is through the
detection of a seismic feature called a harmonic tremor, which in a nutshell is the motion
of lava moving upward from deep below causing the surrounding rock to vibrate.
Another hint was in a Mars-linked meteorite that showed signs of movement in the planet’s
mantle just half a billion years ago, and even more recent evidence of volcanic deposits
only 50,000 years ago.
Humans were walking earth at the time volcanic activity was still going on there.
But it still may not be over.
A very strange gravitational anomaly is observed in Mars’ Elysium Planitia region, which
seems to indicate moving magma.
Whether this means that Mars could someday see an eruption or even the formation of a
volcanic crater chain remains unknown, but probably unlikely.
But it does have implications for life, in that internal heating could be sustaining
subsurface lakes of liquid water on Mars.
It might also eventually have implications on Mars colonization, if there is indeed a
useable geothermal energy source on that world.
There’s also another type of cratering event only slightly related, observed on the moon.
Here you have a huge impact crater surrounded by secondary craters radiating out, which
can both be from material ejected from the crater or from volcanism set off by the impact.
But there’s one body where we may have also observed a crater chain.
It’s earth, and it involves a group of features known as the 38th parallel structures.
It’s still unclear whether this feature represents a multiple crater alignment, but
there are candidates within it that do in fact appear to be buried impact craters.
They cluster along the 38th parallel and comprise seven potential craters stretching about 700
kilometers or 430 miles across Kansas, Missouri and Southern illinois.
Two are almost certainly impact events, and the idea goes that sometime during the late
Mississippian to early Pennsylvanian periods, sometime around 320 million years ago, it’s
possible that a multiple impact strike hit earth.
Two of these are very likely impact events, known as the Crooked Creek and Decaturville
structures.
These are in my home state of Missouri, and I’ve been to both.
After this long, there isn’t much to see.
The craters are buried, but you can find a geological oddity called shatter cones that
are created in impact events.
Both of these events happened about the same time, but in geologic time scales they still
could have been millions of years apart, as much as ten million years leeway.
Some geologists are skeptical of the other features however, one of the Hick’s Dome
in Illlinois, appears to be volcanic in nature and there are associated volcanic rocks.
There’s also the question of Earth’s gravitational field.
Jupiter tore Shoemaker-Levy apart, but Jupiter is enormous compared to earth.
Earth’s weaker gravity may not be able to easily rip apart an object before it falls
into the atmosphere.
At the same time however, evidence for serial impacts is found on the moon, which has an
even weaker gravitational field.
But there is also the question of the impactor that caused the extinction of the dinosaurs.
It may have been more than one impact.
This involves the recent discovery of an undersea feature in the Atlantic Ocean off the coast
of Guinea.
Known as the Nadir Crater after the nearby Nadir seamount, this feature is circular and
about 8.5 kilometers in diameter.
It’s suspected, but not guaranteed to be an impact crater, more work needs to be done
to determine it including drilling for rock samples, but what’s interesting is that
it shows evidence to have formed about 66 million years ago, right around the same time
as the extinction of the dinosaurs and the Cretacious-Paleogene boundary.
If that’s the case, simulations suggest an asteroid about 400 meters across impacting.
This is different from the Yucatan impactor in that it was, that was many times bigger,
as much as 81 kilometers in size.
But Nadir would have produced tsunamis and environmental impacts.
It’s going to be difficult to prove, but it may represent a binary asteroid to the
Yucatan Impactor.
Binary asteroids are not rare, so it is plausible that two objects hit that day, creating an
even bigger calamity for the dinosaurs than we had realized.
But at the same time, impacts of the size of Nadir are far more common on average than
huge ones like that at the Yucatan, so it seems unlikely that the two are linked.
But through core drilling it may be possible to find a chemical link.
The thing is, with the Chicxulub impactor, it was almost certainly an asteroid originating
from the other main belt.
Recent work has identified plentiful amounts in the impact crater of the metal Iridium,
which is heavily linked to asteroids due to analyses of asteroidal material in meteorite
collections, but also sample returns.
The cometary possibility for the origin of the object is waning, though still not completely
settled.
But what’s interesting here is that we may be among the last members of the last species
on planet earth to have ever faced extinction by an asteroid.
This was always a great filter threat, that civilizations never reach a stage of development
that they can mitigate asteroid impacts and that everyone simply goes the way of the dinosaur
and thus the great silence.
But we may have recently put this potential great filter to bed.
This involves one of NASA’s recent successes.
Known as the DART mission, or Double Asteroid Redirection Test, the results are clear.
The impactor moved the target asteroid significantly.
The target asteroid, dimorphos which is part of a double asteroid system, had an orbital
period of 11 hours and 55 minutes around it’s parent asteroid, Didymos.
Now, it’s 32 minutes shorter after the impact, and they’ve gotten the margin of error on
the measurement down to less than 2 minutes, so clearly the asteroid redirect was a success.
The original prediction incidentally was more on the order of seconds, if they could observe
a change of 73 seconds or more, then the mission would have been defined as successful.
So the actual number of 32 minutes is staggeringly larger than expected.
This ushers in the age of planetary defense and that if we can implement a quick reaction
detection and mitigation system, which seems likely in the coming years, then we’ve dodged
the ball that was formerly a very hard great filter.
This would also mean that from the time a civilization reaches space to the time it
becomes capable of asteroid mitigation is very short, in our case only about 70 years,
which is a drop in the bucket of geologic time as far as dangerous asteroid impacts
go.
We’re also getting better at detecting smaller asteroids before the enter earth’s atmosphere
promising a day where incidents like Chelyabinsk can be prepared for ahead of time, and people
will know to stay away from windows during the event.
That incident injured a great number of people, mostly from shattered glass lacerations.
It may not be too long where we can avoid that.
The next step for scientists is to study the ejecta from the impact.
It’s important to figure out just why this asteroid deflected so readily, and that probably
has to do with the physics and the composition of the asteroid’s surface.
No two asteroids are going to be exactly the same, and while Dimorphos and Didymus do not
pose a threat to earth, ones that could should be characterized as to their composition so
we can best predict what will happen during a deflection.
This work will be done by analyzing an Italian cubesat released to follow DART and photograph
the impact from a safe distance away and collect data, which it successfully did.
And in about four years the European Space Agency will send a probe known as Hera to
take a detailed look at the aftermath in this asteroid system furthering our understanding
of exactly what happened.
Interestingly however, there may be another way to specifically deflect dangerous comets
that isn’t quite as dramatic as an impactor.
Comets can hit earth as much as asteroids can and there are even events possible worse
than asteroids, in that hails of comets can be sent into the inner solar system by things
like stellar encounters that could create a veritable bombardment of the inner planets.
But comets are also pristine things, or they tend to be, where they contain volatile ices
leftover from the beginnings of the solar system.
We have observed comets deflect and change their trajectories due to outgassing as they
get near the sun and start jetting material.
This is simple heating from the sun doing that, and it’s possible that in the future
using cometary outgassing to deflect them might be an option in certain situations.
You basically send a giant heater to the outer solar system and nudge comets around far before
they become a threat.
So in the future, we may have a varied and wide ranging tool kit to situationally react
to and deflect dangerous objects of all manner of composition.
So the coming age of asteroid danger mitigation is upon us, we are soon to be the last humans
to ever have existed under an asteroid threat.
In future generations, it may not be a problem.
Or maybe this is all premature.
I leave you with this.
We have only just discovered evidence of interstellar asteroids and comets passing through the solar
system at significantly higher speeds than normal solar system material.
We also know from trajectories captured in satellite data that objects of interstellar
origin, meteorites traveling at great speed, have recently impacted earth.
We’ve never seen evidence of a major interstellar object impacting a body in the solar system,
but we don’t really know what the rules are here.
Can environments in the galaxy, such as in proximity to a black hole or supernova or
some other unusual circumstance, accelerate asteroids to extreme speeds, perhaps even
some fraction of relativistic speeds?
We don’t know, but if such things are out there, it may be that there is nothing we
could do about such an impact.
Thanks for listening!
I am futurist and science fiction author John Michael Godier currently unable to imagine
Didymos is too happy with us.
There it was, minding its own business traveling with its little buddy Dimorphos and wham here
come the humans.
Is it any wonder aliens don’t talk to us and be sure to check out my books at your
favorite online book retailer and subscribe to my channels for regular, in-depth explorations
into the interesting, weird and unknown aspects of this amazing universe in which we live.