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Episode 61: Is Invoking Giant Impacts Special Pleading?

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Recap: Giant asteroid strikes have been used to explain a host of weird things about the solar system. The question is, is doing so a form of special pleading, or is there a good reason to blame these impact events?

A summary of answers to Puzzler from Episodes 59-60 will be discussed in episode 62.

Puzzler: There was no puzzler in this episode.

Q&A: This episode's question comes from Alan N. from Arizona but currently in Thailand who asks: "There's a new Mars meteorite in the news recently: the Black Beauty. My question is: how do we know these rocks come from Mars? Why not Venus or the asteroid belt? And since I assume the scientists know what they are taking about, what kind of activity ejected these rocks? I don't know what the escape velocity of Mars is but it must require substantial energy?"

For those who don't know, in early January this year, a recently found meteorite was announced to be determined that it came from Mars, and it was unlike any of the other Martian meteorites that we have. It was both a different composition and different age from everything else in our collection of roughly 100 Martian meteorites.

So how do we know they came from Mars. It's not like they have a "Made on Mars" sticker attached. What we do is we sample gases trapped inside of the meteorite and figure out what proportion of what is in the tiny bubbles inside the rock.

Once we had landers on Mars, we could directly sample its atmosphere. When the atmosphere was found to be very similar to that gas in some meteorites, the conclusion was that was their likely origin. There are some additional indicators, all dealing with chemistry, but the gas is the main one.

What's neat about this meteorite is that the gases inside don't really match Mars' atmosphere, at least not today. So, why do we think it came from Mars?

Part of it is that the chemistry does match the rock chemistry found by the recent Mars rovers; in fact, it matches the chemistry better than a lot of the other meteorites we're pretty sure came from Mars. Another part of it is process of elimination. The meteorite has an abnormally high amount of water in it. Ten times what the other martian meteorites have, but it's not contamination from Earth.

So the question then is, where could it have come from? Its trapped gases are sorta like Mars', the chemistry is sorta like some of the rocks we've seen on Mars, and it has 6,000 parts per million of water in it - more than other Mars meteorites that we have.

It's through process of elimination that we get that it came from Mars. It's almost certainly not from Earth because of the LOW water content relative to Earth rocks and other chemical indicators. We can't see how it could be from Mercury because there shouldn't be any water in its rocks. It's 2.1 billion years old, but it was only in space for a few million years and landed on Earth, and models for Venus show that its runaway greenhouse effect would've gotten rid of all the water long before that. Comets on the other hand would have way too much water. And asteroids shouldn't have that much water, either.

With Mars, it probably had that much water at some point, we just don't have examples ... but from the process of elimination, what's left is Mars. It doesn't fit with anything else, but it does sorta fit with our few observations of Mars, so the conclusion for now is that the meteorite is most likely from Mars.

To answer the final question of how it got here, we return to what this entire episode has been about -- impacts. Mars has roughly 385,000 craters 1 km and larger, and millions that are smaller. An impact event carries a huge amount of energy, and that can launch objects off the planet. That's how we have Martian meteorites and lunar meteorites. We also have meteorites from Vesta. We may even have some from Venus or Mercury - we just don't know yet because we don't have anything like atmospheric samples to link them for Venus, and we may never know for Mercury.

Additional Materials:

  • Logical Fallacies / Critical Thinking Terms addressed in this episode: Special Pleading.
  • Relevant Posts on my "Exposing PseudoAstronomy" Blog


Claim: The basic claim in this episode is that astronomers invoke giant asteroid or comet impacts to explain a lot of weird solar system things. When I discussed this as a possibility to explain Venus' weird spin in my anniversary episode on Velikovsky, a comment I got on my blog was to the effect that the person enjoys reading mainstream scientists' excuses for things that seem even more silly to them than the pseudoscientists'.

So, I thought I'd take an episode and talk about whether our invoking an asteroid impact to explain things was our own way of special pleading in lack of any evidence or something that's been supposedly disproven. I'm going to talk about four main cases in this episode, some of which I've talked about before, but this time in the exclusive context of the asteroid impact ideas.

Background - Solar System Formation

Returning to the format I initially set out for this podcast program, I'm going to first take you through background information on the solar system's history and dynamics.

The nebular hypothesis is currently our best model for how solar systems form. It's often ridiculed by creationists and other pseudoscientists, but that's a different episode. As an aside, did anyone watch Alton Brown's "Good Eats" TV show? I keep feeling like I'm sounding like him because he'd always bring something up but then say that'd be a topic for a future episode.

...Anyway, the very basic explanation of the nebular hypothesis is that a very expansive cloud of dust and gas was perturbed by almost anything and a tiny area now had enough density to start to collapse under its own gravity. This kept going and a star was born with a family of planets that originated from other, smaller clumps throughout the local region. The planets were built up by smaller chunks which were built by smaller ones and so on back to the original dust grains. After the star is born, it's very energetic, throwing off a lot of ultraviolet radiation, and it effectively destroys the nebula in a local region so that no more planet formation can happen nearby.

You're left with a star, planets, and innumerable smaller chunks of material that didn't get swept up and impact and become part of the other planets in time. If they're rocky or metaly, they're asteroids. If they're icy, they're comets.

Most objects have generally roughly nicely circular orbits around the star. Some don't. Others will get perturbed by a large planet's gravity and get set on a new orbit. It takes awhile to do what we call "clear out the solar system of debris," and you really only have four choices: Impact the star, impact a planet, get shot into a stable orbit, or be ejected from the system entirely.

That's a crash course through solar system formation that usually takes either one 80-minute class or two 50-minute class periods to get through -- I know this 'cause I've taught it. It's all based on fairly basic physics that was figured out centuries ago -- indeed, the nebular hypothesis was first formulated a few hundred years ago. As with all models, it's been modified over the years in its specifics, especially when we started to find exoplanets, and as we use telescopes like Hubble to observe star-forming regions and very young stellar systems. But, but the very basic idea is still the same: Early on, you're left with a lot of junk floating around, and a lot of that junk is big.

Background - Cratering Chronology

Another piece of background information is the cratering history of the solar system. From the nebular hypothesis, one can formulate a hypothesis about what the cratering rate should have been throughout the history of the solar system. If you start with a lot of stuff left over from planet formation, you're probably going to have a lot of it impacting planets early on. And getting ejected from the system or sent into the sun. As more stuff is ejected, destroyed, or falls into a stable orbit, the cratering rate is going to get less and less until it reaches something that's fairly steady, only based on stuff that occasionally gets knocked out of the relatively stable asteroid belt, or the stray comet.

With that hypothesis, what do we see? For those who've been with the podcast for at least 7 months and remember episode 40, you already know the answer. The short-short version is that we can count how many craters of a certain size there are on a given surface. Like, say, a lava flood plane on the Moon, better known as a mare. If we then have a rock from that surface, such as, say, from the Apollo astronauts or Luna robotic craft, we can use radiometric techniques to date the rock and be able to say that a surface with that many craters is that old, and it should be that old as long as it has that many craters regardless of where we are on the Moon. And you can use various scaling laws to get to other planets. And if you want the longer version of that, head back to Episode 40.

Once everything is said and done, the results from Apollo really show just that -- the impact rate was thousands - if not tens to hundreds of thousands - of times 4 billion years ago compared with what it is now.

The point in going through these two bits of background are to demonstrate that we have two consistent stories based on a lot of observational data. They paint a picture of a very violent early solar system where big things smashed into other big things very frequently.

With that context, we'll now look at four specific examples of invoking asteroid impacts to explain weird things and decide whether it's special pleading.

Uranus' Tilt

The first is the tilt of the planet Uranus. Earth's orbit around the sun is defined as the plane of the solar system. All the planets, the asteroids, and most comets are near the plane of the solar system. Almost all the planets' rotations are about an axis that's somewhat perpendicular to the plane of the solar system, such that the planets' equators are on the plane.

The fundamentals of the nebular hypothesis for planetary formation suggest that this should be the case based on conservation of angular momentum -- everything should be in the same plane and the spins should be the same way. Remember that 'cause it's important for our third item.

One of the big exceptions is the ice giant planet Uranus, 7th planet from the sun, and a featureless blue blob as imaged by Voyager 2 and if anyone's seen it through a backyard telescope. Or most any telescope on Earth. Uranus does orbit in the plane of the solar system - that's not an issue. What's weird is that it's tilted such that its equator, not its spin axis, is roughly perpendicular to its orbital path. In other words, it spins on its side.

The consensus model to explain this is that Uranus was struck by something roughly Earth size, early on in the solar system, and that whacked it over like if you smack someone upside the head, it's going to cause their head to tilt. Not that you should try this demo at home, work, school, nor anyone else. But watch a movie and see it.

A possible alternative that some have suggested is a dance between Jupiter and Saturn, called the Nice model, around 4 billion years ago that could have transferred some energy to Uranus and caused it to tip over. I think that's a pretty minority opinion and, as I said, almost all the literature you'll read on this says it was whacked.

Is this special pleading? I would say not. We have a weird anomaly, we know from observations that have confirmed hypotheses that big stuff was flying through the solar system early on, and if that happened, it can fairly easily explain Uranus' tilt.

Earth's Moon

The second solar system anomaly is Earth's moon. For more on that, refer back to episode 53. The bottom-line point from that episode was that there are several different models to build Earth's weird moon. All of them have problems, some of them pretty fatal. The BEST model that fits the MOST data we have about the Moon supports the idea of the Big Splash only some 10s of millions of years after Earth formed where a Mars-sized object smashed into Earth at an angle and the Moon was formed from the orbiting debris.

Is it special pleading? No. Again, we know that this stuff was going on, and while it's not perhaps the most likely nor the simplest explanation, it's the one that best fits the data which I discussed in episode 53 in much more detail.

Venus' Spin

Going back to the problem with Uranus, we see this again with Venus for the third invoking of an asteroid strike to explain something weird. Venus does spin with its axis roughly perpendicular to the plane of its orbit. The problem is that Venus is rotating in the wrong direction.

This can't happen under the physics that explains nebular collapse. You have to all spin in roughly the same direction. We don't know how to form Venus spinning the opposite way as everything else. Since we can't form it that way, either something is very wrong with a hypothesis that explains most other observations, or it was modified after it formed.

And, as with Uranus, the best way and the most likely way given the possibilities that we know of to modify it is with a giant impact. Something whacked Venus real good and tipped it completely over. And, as with Uranus, there are some models that can explain it dynamically with tidal forces from the sun acting over time to flip it over.

Is it special pleading? Again, I would say probably not. For the reasons I said before: It's more likely that it was modified to get to its current condition, as opposed to forming that way. The more likely of the two ways that we know to tip it over is by it being struck by a large object.

To get back to the feedback I had gotten, what about Velikovsky's idea that Venus was spat from Jupiter and settled into its current orbit and spin after traipsing through the solar system for a few thousand years? The problem with that is everything about it, as I explained in episode 46 on Velikovsky. Nothing that we know about physics, chemistry, nor history support Velkovsky's Venus claims. But, what we know about solar system formation can support Venus being struck by a large object a few billion years ago.

Mercury's Giant Core or Lack of Crust and Mantle

For the fourth and final mystery, we go to the planet Mercury, closest known planet to the star that I like to call W-359, but most people call the Sun. It's been known at least since 1974 with the first flyby mission of Mariner 10 that Mercury has an abnormally large and heavy core, very likely iron. Since we think that most stuff in the inner solar system formed out of the same stuff that the majority of asteroids are - known as chondritic or rocky asteroids - Mercury's composition should have been similar. Earth's core is also large, at around a half of the diameter of the planet. That can be explained by the formation of the Moon by a giant impact.

But, on Mercury, Mariner 10 results showed the core was not about half the radius, but at least 75% the radius, or about 40% the volume versus Earth's core being only 17% the volume. The latest results from the MESSENGER mission in orbit of Mercury suggest the core is about 85% the radius, or about 60% the volume.

The nebular hypothesis on its own can't easily form Mercury like that. So, either it needs to be thrown out, modifications made, or Mercury was altered in some way after it formed. As such, there are three possibilities that have been proposed to explain it.

The first, is yet again, Mercury got whacked. The whacking model has Mercury starting at about 2x its current mass, and something 1/6 its mass - several hundred kilometers across - smacked it dead-on. This would have stripped away a lot of the original crust and mantle, leaving it with an abnormally large core relative to what was left as a planet.

The second model is that Mercury formed when the sun was throwing birthing temper tantrums and its temperature was abnormally high so that it may have been as high as 10,000 K at Mercury. This would have vaporized a lot of the surface rock and the subsequent rock-vapor atmosphere would then have been stripped away by the young sun's solar wind.

A third model suggests that the solar nebula - before the sun had really been birthed and everything was still forming - caused more drag than we had previously though early on closer to the sun. Just like if you have a strong undertoe in the ocean, lighter stuff - like rock - will get carried away, while the heavier stuff, like a lead ball, would stay put and continue to gather more material that's heavy like itself. As the nebula thinned as stuff was gathered up by the growing protostar, Mercury could then start to gather lighter stuff because of less drag.

All these models are hypotheses of what may have happened to Mercury based on a post-formation idea that works with our other models, early solar system idea that doesn't not work with other models, and a third idea that's a small modification to the nebular model that doesn't change anything else and is within parts of the model that have large uncertainties.

From these, you can make predictions about what each would do to the surface, as in, predictions about different things that we should see on the present-day surface if each one is correct.

The first two both predict that Mercury's surface should not have elements that are easily vaporized -- stuff that's light-weight, in other words. The third one is different in that it has no ban on easily vaporized elements.

MESSENGER can sense surface composition. What it found was higher-than-expected amounts of potassium and sulfur, but of which are easily vaporized. Before that result, I would have argued the impact idea was the most likely. But with these new data, they indicate that whatever caused Mercury to have such a large core, or form without a large crust and mantle, couldn't have removed those elements. Unless you can find a way to get them back after they were removed, which is even an extra layer of complexity.

And we don't need that extra layer. As I said at the beginning, the nebular model is very good in its basics of explaining how solar systems form, and it's backed up by a lot of observations. But the details of it do continue to get modified. Adding extra physical processes like gas drag has only been able to be done recently due to better computers and lab experiments. These added layers of complexity don't show we were wrong before, but that we didn't have the whole story. It's how science works, we add extra layers as we learn more about the physics involved and have more constraints.

And that's what Mercury does: It adds another constraint and based on these latest data, it shows that a process that was previously thought to probably have a minor role, may not have been so minor.

So, is invoking a giant impact to explain Mercury an example of special pleading? No, because the new data show it's unlikely to be the model that best explains all the observables. Since special pleading means that you continue to make excuses for something after it's been shown to be wrong, this is not a case of special pleading because most now accept that it probably is NOT the explanation.


So there you have it -- four examples of weird solar system anomalies that people explain with big impact events. I would say that none of them are examples of special pleading. They're examples of invoking a process that we know went on to explain a feature that we know it can explain. And, in the case of Mercury, it's a process that's no longer in favor based on the new data.

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