Mercury - A 1st-Generation Relic
Mercury isn't much bigger than our moon, but it's considerably more dense. In fact, Mercury is the second densest terrestrial planet in our solar system behind Earth.
What makes it dense, is also what makes Mercury an anomaly: metallic iron. Mercury contains roughly twice the fraction of metallic iron than Venus, Earth, or Mars, and this is a curious thing because we'd expect it to be much less given the fact it accreted from the same circumstellar disc of dust and gas as the other rocky planets.
And though we would expect planets that formed closer to the Sun to have a larger proportion of heavier elements such as metallic iron (due to the effects of gravity towards the dense solar center), a near doubling of a heavy element like iron doesn't compute.
Before the planets formed, our Sun was surrounded by a rotating disk of dense gas and dust, with an elemental distribution pattern dictated by the effects of gravity. The center of the system would have the most mass, and therefore the strongest gravitational effect. This would cause a large fraction of the disk's heavier elements to migrate radially closer to the Sun, with relatively higher fractions of lighter elements further out. This is one reason why the denser rocky planets with iron-rich cores formed nearer the Sun, and the lighter element gas and ice giants of hydrogen, and helium formed further out. (Jupiter contains an exotic liquid metallic hydrogen mass which I discuss in an older blog post, "Jupiter's Liquid Metallic Hydrogen".)
Generally speaking, the total iron mass fractions of Venus, Earth, and Mars make sense. They have been calculated to be 0.35, 0.38, and 0.26 respectively (Reynolds and Summers, 1969). So we might expect Mercury to hypothetically have say 0.45 let's say. I made that up merely as a conceptual reference. However, Mercury's total iron mass fraction is a whopping 0.68 (Ibid.).
This near doubling of iron is simply too much. It's an anomaly in that there is no logical way to explain why there is such a jump in iron between Venus and Mercury given calculations on how elements would have distributed in the early solar system. The transition should be smoother than this.
It gets stranger. Planetary scientists have also been perplexed by the fact Mercury's core is about 85% of the planet's radius (over 60% of the planet's volume). As a ratio to planet size and volume, Mercury's core is considerably larger and more voluminous than any of the other terrestrial planets. Again, this is anomalous.
It's as if Mercury was once a much bigger planet. And it may have been. Mercury's crust is uniquely thin. So thin in fact, that scientists believe the planet was stripped of a substantial portion of its mass at some point in the early solar system. If so, then that might explain the unusually large core, because a larger core would be more fitting of a larger planet.
The question then, is to ask how Mercury lost its mass. Hypotheses have been put forth suggesting it may have been struck by a larger planetary body at some point in the early solar system. A violent collision like that would certainly strip Mercury of much of its original mantle material, but this hypothesis doesn't explain why Mercury retains a significant fraction of volatiles.
Volatiles are elements and compounds that can be easily vaporized by energetic events such as impacts from massive celestial bodies. Examples of volatiles include such elements as water, sulfur, and lead to name a few. These would have been vaporized if Mercury were struck by a larger planetary body, yet Mercury has retained them.
One possible way Mercury's mantle could be stripped, yet retain a significant fraction of volatiles, would be if the planet was grazed by ultra-low-angle blows with another planet or planets. Just such a hypothesis was published (Asphaug & Reufer, 2014).
The authors suggest that one or more ultra-low-angle blows would have stripped about a third of Mercury's mantle at a time without inducing an intense volatile-vaporizing shock. Of course some volatiles would be lost, but not in relatively significant amounts.
The stripped mantle masses would have accreted to the larger target body, with Mercury being the left-over surviving projectile mass. Glancing blows may have been possible given the hypothesis that the early solar system may have been host to as many as 20 Mars-sized bodies.
But to have ultra-low angle impacts, two bodies would have to be in similar orbits, with one slightly offset from the other just enough that they could skid against each other as they pass. Or at least that's how I envision it, but I could be wrong here.
One problem with this theory is that Mercury may have had a more eccentric orbit during the early solar system. A highly-eccentric orbit might set Mercury up for a higher-angle collision that would generate a volatile-vaporizing shock. To explain the possibility that Mercury may have initially had a more eccentric orbit, we have to look at yet another anomalous characteristic of that planet: its liquid core.
Curiously, Mercury still has a liquid (or at least mostly liquid) core which is unexpected for such a small planet. Scientists expected a less-insulated core to have long since cooled and solidified.
They were first tipped off that Mercury might have a liquid core back in 1974/'75, when Mariner-10 made three close approaches to the planet and detected a weak, but existent magnetic field. Just as is the case with Earth (and Jupiter), magnetic fields generally require an internal dynamo such as a rotating liquid core.
To confirm the liquid core hypothesis, scientists at JPL pointed their Goldstone 70-meter antenna at the small planet, and fired a powerful radar signal. The echo return(s) allowed scientists to calculate Mercury's spin rate to an accuracy of one-one-thousandth of a percent. That sort of accuracy in measurement revealed the planet's spin had tiny 'twists' to it. These 'twists' were twice as pronounced as would be expected from a planet with a solid core. The consensus is that Mercury's core is liquid, or at least mostly so.
Planetary scientists at JPL proposed in 2007 that Mercury's core may have kept from cooling and solidifying because it contains sulfur (a volatile no less). Sulfur would lower the core's melting temperature, thereby allowing it to remain in liquid phase longer than it would have otherwise.
Sulfur could have been obtained via radial mixing; a type of mixing that combines elements acquired from both near and far from the Sun, radially along the circumstellar disk of gas and dust in the early 'days' of our solar system.
As discussed above, elements in the circumsolar disk would have proportioned themselves across the disk's radius according to density, with high fractions of heavier elements nearer the Sun, and lighter elements further out. Sulfur is a relatively light element, and so to gain a larger fraction of it, Mercury would have had to acquire it further out along the circumsolar disk.
I believe this would have required Mercury to have had a highly-eccentric orbit capable of extending further out into the disk in order to accrete sulfur. Today, Mercury has the most eccentric orbit of the 4 inner rocky planets, though I'd imagine this is far more tame than it might have been about 4 billion years ago.
To set Mercury up for an ultra-low-angle grazing blow, the colliding body or bodies would have likely had similarly eccentric orbits, or so I'd imagine. But some dynamical modellers believe such grazing blows would have been rare. Asphaug & Reufer (2014) disagree, and have suggested a one-in-ten chance. However, critcs believe that even if such blows were statistically probable, such glancing blows would ultimately result in accretion into a single body. The jury is still out on this debate.
Be that as it may, couple the fact the planet has an unusually massive core, with the fact Mercury has clearly lost a substantial portion of its mantle, and we have strong evidence that Mercury was once a considerably larger planet. Perhaps on par with Venus, and Earth.
And while scientists are still working towards a consensus as to how Mercury lost so much of its mantle, the anomalous high iron content remains to be addressed. As discussed above, we'd expect a higher iron content in Mercury given it accreted closest to the Sun, but not nearly as high as it is.
This scenario suggests that Mercury is a relic of an original population of inner terrestrial planets all of which would have had proportionally higher fractions of iron at a time when there was more of it available in the circumstellar disk with which to accrete. Earth and Venus may well have been two of the hypothesized 20 Mars-sized bodies at that time; not yet fully formed, and not yet with stable orbits. In fact, in a 2014 interview with National Geographic, Asphaug stated, "The missing mantle of Mercury is maybe right beneath our feet." Which is to say, the protoplanetary embryo that would later become Earth, may have been what grazed and accreted some of Mercury's mantle.
If so, then that would be incredibly fascinating in that we can learn a bit about Mercury from our own mantle material. Another one of the hypothesized 20 or so Mars-sized planetary bodies would have been Mars itself. Mars however, escaped accretion with large bodies, and remained as it is today; a planetary embryo. As such, Mars too is a first-generation relic of the earliest 'days' of our solar system.
An Earth-sized Mercury in relatively close orbit to the Sun may not have been the innermost planet at that time. There may have been others, and they may have been super-Earths. To understand why this may have been the case, we have to look beyond our solar system to other stellar systems in our galaxy.
OUR SOLAR SYSTEM - AN ANOMALY AMONG STELLAR SYSTEMS
Based on findings from the Doppler Velocity Surveys, as well as the Keppler mission, it appears stellar systems hosting Earth-and-Venus-sized terrestrial planets is not the norm. In other words, our solar system is quite unique in this regard. Not only are Earth-sized terrestrial planets not the norm, but terrestrial planets with large orbits like Earth are not the norm either. Most rocky planets out there have very tight orbits, much closer to their host star than Mercury is to our Sun.
As more and more exoplanets are confirmed (over 2,600 at the time of this writing), the trend appears to be that the most common configuration for stellar systems is to have tidally-locked super-Earths in short-period, tight orbits around their host star. We're talking orbits of just a few days or weeks, or even hours as is the case with a planet in the TRAPPIST system. Mercury's orbit is a marathon by comparison, with an orbital period of 88 days.
A "super-Earth" is an extrasolar planet with a mass higher than Earth's, but substantially below the mass of our solar system's ice giants (Uranus, or Neptune). As Wikipedia warns, the term "super-Earth" is in reference to an exoplanet's mass only, and makes no claim to said planet's habitability.
Below is a plot of confirmed exoplanets based on their respective radii, and orbital periods to their host stars. We quickly see that most exoplanets discovered so far are considerably larger than Earth, with considerably shorter orbital periods. I've added Earth to the plot for reference:
As can be seen from the plot above, most exoplanets are more massive than Mercury, Venus, Earth, or Mars, and have orbital periods far shorter than any of our terrestrial planets. This makes our inner solar system set up an anomaly in and of itself. Why are our terrestrial planets so small, and with such large orbits by comparison to other systems?
Before we attempt to address that question, I suppose it is important we realize that our current methods for detecting exoplanets tend to favor the discovery of massive, tight-orbit planets. I explain this unintentional bias in my blog post, "The Basics of Inferring Exoplanet Properties" for those interested. That link opens in a new window so you don't lose your place here.
In recent years, scientists have begun to wonder why our solar system is so different in that it lacks super-Earths in sub-Mercury orbits. One hypothesis suggests that our solar system actually did have sub-Mercury super-Earths in tidal lock with the Sun, just like we see so frequently across our neck of the Milky Way.
If this was the case, then we must ask what might have happened to these super-Earths. Their fates may have been disintegration and ultimate vaporization into the Sun's thermonuclear abyss. To understand how this might have happened we must revisi the Grand Tack Hypothesis, which I first discussed in my blog post, "To Mars or Bust - A Planetary Embryo". You're welcome to visit that link to learn more about the hypothesis, or read a general synopsis of it below.
THE GRAND TACK HYPOTHESIS
To discuss this hypothesis, we're going to step away from early Mercury and the hypothesized sub-Mercury-tidally-locked-super-Earths for a minute, and turn our attention to Jupiter.
The Grand Tack Hypothesis suggests Jupiter formed within a few million years from a gas-dominated region of the proto-planetary disk about 3.5 AU from the Sun (Haische et al., 2001). Remember, the higher fraction of lighter elements like helium and hydrogen would be radially further out in this region and beyond.
The keen reader will note that 3.5 AU is 1.7 AU closer to the Sun than where Jupiter's orbit is today. And this is correct. During the early 'days' of our solar system, Jupiter is believed to have formed at 3.5 AU, which was near the frost line; the radial distance from the proto-Sun at which volatiles condensed to solid particles.
According to the hypothesis, after eating up all the available material in its radial portion of the protoplanetary disk, Jupiter experienced an inward gas-driven migration (a Type II migration) toward the inner solar system, gathering up yet more material as it continued its inward trek to about 1.5 AU (Armitage, 2007); the distance from the Sun at which Mars resides today.
I am only guessing here, but this is perhaps where Jupiter garnered a substantial portion of its iron. I'll even go so far as to say Jupiter may have a small rocky core because of its inward trek. But take that with a grain of salt, because I'm certainly not qualified to make such statements with any authority. It just seems like a logical possibility to me.
While the original hypothesis stated Jupiter made it all the way in to 1.5 AU, more recent computer simulations suggest the planet would have reversed course at 2.0 AU. These more recent models suggest a reversal at 1.5 AU would have resulted in the largest terrestrial planet forming near the orbit of Venus, rather than Earth's. Models reversing Jupiter's migration at 2.0 AU result in a system most similar to what we see today (Brasser et al., 2016).
At any rate, at its inward-most point, Jupiter captured the still-growing gas giant, Saturn (itself experiencing a Type I migration at the time), in an orbital resonance. This gravitationally guided Jupiter outward to 5.2 AU from the Sun, where it remains to this day.
So what's the big deal other than Jupiter migrated in, then reversed course and migrated back out? This possibly happened at a time when Earth and Venus were still figuring things out as planetary embryos.
Interestingly, had Jupiter not captured Saturn in orbital resonance, it would have likely continued its inward trek, potentially settling into a very tight orbit around the Sun, and becoming what planetary scientists call a hot Jupiter. Hot Jupiters, as it turns out, are about as ubiquitous across our galaxy as super-Earths!
So in the instant Saturn was captured, our solar system took a tangent from being like most the rest of the stellar systems out there. We also avoided looking like other systems in that the hypothesized super-Earths (or super-Earth singular) were driven into the Sun by Jupiter's migration.
Its migration would have resonantly captured and gravitationally transported 10 to 100+ km diameter planetesimals inward towards the Sun. This inward migration would have been a bit chaotic, resulting in bodies crossing orbits. Orbit crossings just like intersections where everyone is on their cell phones rather than watching the road, are recipes for collisions.
If the specific energy of any collisions between colliding bodies exceeded a certain critical value characteristic of catastrophic disruption, breakup will ensue rather than accretion (Batygin and Laughlin, 2015).
The collisional cascade initiated by Jupiter's resonant shepherding of planetesimals would have broken that larger material up into smaller and smaller pieces. Batygin and Laughlin refer to this as resonantly-forced collisional grinding, whereupon debris becomes small enough to feel the effects of aerodynamic drag from the circumsolar disk, and that drag results in an inward drift.
Just as an inward-migrating Jupiter captures and shepherds planetesimals inward, those inward-migrating planetesimals will capture any sub-Mercury orbit super-Earths. If the total mass of that inward-migrating material is significant enough, they will gravitationally guide those tight-orbit first generation super-planets into the thermonuclear abyss: the Sun.
What's left is a mass-depleted (this includes iron!), gas-starved disk from which a 2nd generation of planets formed; those being Venus, Earth, and possibly Mars. Though Mars, as I stated above, may be a surviving first-generation planetary embryo. It's certainly a planetary embryo, but it's possible it is a relic from the earliest 'days' of our solar system. A time when Earth, and Venus were still embryos themselves.
The mass-depletion of course, can be attributed to the vaporized super-Earths who took a substantial portion of material with them into the Sun. Mercury, with its larger orbital period, seems to have survived, suffering only (possibly) glancing mantle-stripping blows.
If any of this is true, then it might explain why Mercury has such a high metallic iron fraction, while Venus and Earth do not. Mercury formed in a "meatier" disk, whereas Venus and Earth were left to accrete from a rather anemic disk.
Again, take grains of salt my friends. As far as I'm aware, none of this is consensus among planetary scientists.
An interesting side note, with regard to Jupiter's Grand Tack: we can determine which of Jupiter's moons formed prior to that planet's inward migration, and which formed after by looking at their atmospheres, or lack thereof.
Moon that formed prior to Jupiter's inward migration, like Ganymede, and Callisto, had their atmospheres stripped when they were dragged inward with Jupiter towards the Sun. Moons that formed after Jupiter's inward jaunt like T̶i̶t̶a̶n Io for example, have retained some atmosphere (Heller, Marleau, and Pudritz, 2015).
How much of what has been written here is true is yet to be determined by greater minds than my own. I anxiously await what new discoveries will be made in the coming years.
As always, thanks for reading..