To Mars or Bust - A Planetary Embryo
(Originally posted May 16, 2017 on Blogger)
INTRO - Going Against the Grain
Welcome to part 4 in my ongoing series, To Mars or Bust, a series meant to give critical attention to missions intending to send humans to colonize and ultimately terraform the red planet. As I've written in previous blogs, the critiques found in this series are directed at what I see as naïve colonization and planet engineering expectations.
I do support missions to Mars that can protect our pioneers en route, on planet, and as they return home to Earth. I've written this in previous blogs, but as the blogs in this series are fast becoming my most read with an international audience from just over 20 countries around the world, I feel I should reemphasize previous clarifications of my stance. I'm a big fan of Mars, I'm just not a fan of sending people there to "live" (die). With that said, let's look at one of the driving themes behind the near global push to send humans to permanently live on Mars: the theme that Mars is "Earth-like".
PERSPECTIVE - Checking Baggage at the Intellectual Door
In the grand scheme of things, Mars is quite different from Earth. And I think its marked differences are fascinating and should be appreciated for what they are, rather than to be either ignored or forced to fit "Earth-like" predispositions held by so many people these days. Conclusions that geological features on Mars are alluvial despite the fact many of those same features could be aeolian is an example of going into a study with a preconceived notion of what the conclusion should be.
Preconceived conclusions have no place in science. Ideas of what ought to be do certainly exist in the minds of scientists prior to the start of their respective research—it's what fuels their desires to seek answers—but once research has begun, the dedicated scientist abandons expectations in order to follow the evidence wherever it takes leads; even if that evidence takes them down the path of what might seem an inglorious dead end.
But as any resilient mind would attest, dead ends can be revelations in and of themselves if for no other reason than they end a line of incorrect thinking. It's at that terminus where we can embark on a whole new path of critical thought which can and has lead to glorious scientific revolutions; paradigm shifts that have reshaped our ever-growing perspective of the world and universe within which we all live.
It's important we embark on intellectual journeys without the burdensome baggage of tightly-held preconceptions of where we want our journey to lead or where we think it should lead. If we do, then we risk taking mere jaunts to the nearby murky borders of ideology. Crossing such dreary boundaries is matter of blind conviction, which is no hallmark of the genuinely inquisitive.
Let's put all we have learned about Mars aside for a moment, and start from the beginning so that we can appreciate just how singular it is from Earth and the other terrestrial planets of our solar system. Much of what I wish to convey in this blog is new information that it seems many folks are yet unaware of. I think that's a travesty on the part of those whose jobs it is to convey new and incredible discoveries to those like yourselves who genuinely care to know.
Put on your seatbelts my friends, because contrary to popular belief, I contend there is no planet like Mars, and that it exists as an extraordinarily unique celestial body worthy of honest appreciation for what it is, and not for what we might want it to be.
The Early Solar System
Long before our Sun and subsequent solar system formed, our corner of the Milky Way was home to a lonely molecular cloud of interstellar hydrogen gas, and dust.
About 4.6 billion years ago, that molecular cloud in the Orion Spur of the Milky Way began to collapse on itself; possibly triggered by the compressional forces of a nearby supernova. As its mass collapsed, a disk of in-spiraling gas and dust took form. In time, a critical point of unfathomably-high temperature and pressure was reached. It was at this point when the collapsing hydrogen gas began to fuse into helium, and our proto-Sun ignited to life. The young Sun continued to gorge itself upon the gas and dust spinning around it, consuming 99.8% of all that was left.
The 0.2% of material left over continued to spin around the new and mighty star as a primitive solar disk from which our planets would eventually form. But contrary to long-held belief, not all the rocky inner planets we know today initially arose from that left-over disk. Before Mercury, Venus, Earth, or Mars, there was likely a first generation of massive terrestrial planets now long gone.
We have learned from both the Doppler velocity surveys, as well as the Kepler mission, that stellar systems hosting Earth-sized terrestrial planets with Earth-like orbital periods are not the norm. Most systems have massive 'super-Earths' tidally locked in short-period, tight orbits close to their parent star. Imagine a rocky planet twice the mass of Earth with an orbit as close or closer to the Sun than Mercury. As can be seen in the plot below, this large planet, short-period orbit scenario is nearly ubiquitous:
So why is our solar system unique? Why do we have smaller Earth-sized planets with longer-period orbits at distances from the Sun that prevent them from becoming tidally locked?
Turns out that our solar system wasn't so different after all. At least in its early years.
Mercury, Venus, Earth (we'll get to Mars shortly) are now believed to be a second generation of planets made up of left overs of left overs. It is theorized that our solar system may have once hosted super-Earths in tight orbital periods around our Sun, much as is observed elsewhere in our galaxy. But a massive outer intruder changed all that.
Jupiter, the largest, most massive of all the planets once migrated into the inner solar system as close as Mars is today. The havoc it caused as it crossed the asteroid belt has forever changed the nature of the inner system. Jupiter formed within a few million years (Haisch et al., 2001) from what was a gas-dominated protoplanetary disk.
It is believed that Jupiter initially formed at a distance of 3.5 astronomical units (AUs). For reference, one AU is the average distance from the Sun to Earth (~150 million km). The still growing young Jupiter was influenced by gas-driven migration of its orbit early on (Armitage, 2007). As such, Jupiter began to migrate towards the inner solar system, where it is believed there existed a first generation of massive terrestrial planets. Jupiter migrated as close as ~1.5 AU (about where Mars is today). This inward advance might have continued had Jupiter not captured another gas giant—Saturn—in orbital resonance. Once captured, Jupiter reversed course and began to migrate outward, settling at ~5.2 AU from the Sun, where it remains in stable orbit to this day. But its reversal was too late, the fate of the first-gen inner planets had been sealed.
Jupiter's inward-then-outward migration saw the Goliathan planet cross the asteroid belt twice. On its inward migration it sent 10 to 100 km-diameter planetesimals into a collisional cascade that ultimately drove the massive inner planets into the Sun; their rocky material forever lost into the thermonuclear abyss (Batygin and Laughlin, 2015). As their orbits decayed, the once mighty planets were torn apart under the gravitational force of the Sun. Imagine seeing the ground beneath your feet being ripped asunder, dragged weightlessly into a missing sky to join a train of debris spiraling ever-closer to the surface of the Sun.
The terrestrial planets we see today (Earth included), had yet to form. With the mighty inner planets forever gone, all that was left in the protoplanetary disk were planetary embryos. Over time these embryos collided and coalesced, combining their masses to form larger and larger bodies (1,000 to 5,000 km in diameter). Ultimately, Mercury, Venus, and Earth formed. (We'll get to Mars shortly.)
The last planetary embryo to collide and coalesce with Earth was a, ahem, Mars-sized embryo (see Theia impact hypothesis for more).
Could Mars be a planetary embryo that escaped collision and subsequent merging with other embryos to form a planet? Never having grown larger because Jupiter effectively evicted most of the protoplanetary debris that would have otherwise been available to the red planet?
Size alone isn't enough to determine this, but the fact that Mars is compositionally unique from the other terrestrial planets has certainly raised eyebrows over the years.
Earth and Venus accreted heterogeneously, which is to say that the material from which they formed coalesced relatively slowly over time in layers like an onion. Each layer compositionally unique from the other; their composition dictated by existing temperatures and pressures in the primitive solar disk at the time of their accretion. Over time, temperatures and pressures within the disk change; hence the heterogeneous layered composition of the inner planets.
Mars on the other hand, accreted homogeneously, which is to say it lacks the layered composition of the other terrestrial planets. For this to have happened, Mars would have had to accrete relatively quickly. To assess this rapid accretion hypothesis, two research scientists looked at elemental and isotopic ratios of Martian meteorites, and determined that Mars did indeed form very quickly; on the order of only 2 to 4 million years (Dauphas and Pourmand, 2011), as compared to 10-100 million years for Earth.
According to Dauphas and Pourmand, Mars accreted quickly enough that the radioactive decay of the unstable isotope, aluminum-26 (717,000-year half-life) turned the Martian surface into a massive magma ocean.
Current planet formation theory predicts that Mars formed near Venus and Earth, then migrated outward to its present location at ~1.524 AU. This theory stands on shaky ground now that we have elemental and isotopic evidence revealing Mars' composition is unlike that of the other three inner planets.
The most recent studies indicate that Mars never formed near the inner planets, and has only ever existed far beyond the terrestrial feeding zone in the material-starved region cleared out by the inward migration of Jupiter (Brasser et al., 2017). Though generally unknown to the public (and media), it is widely accepted across academic circles that Mars is a stranded planetary embryo that never developed into a fully-grown planet (ibid.). Mars, like the dwarf-planet Ceres, formed in the asteroid belt well beyond Earth's accretion zone.
What's more, is that our Sun was ~30% less luminous in Mars' early years as illustrated in the graph below:
A fainter Sun would have shifted the so-called "habitable zone" inward, likely placing Venus well within its boundaries, and possibly leaving Mars well beyond them. It is further predicted that while Mars’ is quite different from Earth, "...Venus formed close enough to our planet that it is expected to have a nearly identical composition from common building blocks" (ibid.). But that's another blog my friends. Perhaps a part five.
With all this said, I want to point out a theme I focused on in my blog on Pluto's "demotion"; Mars' unique and unexpected history, its unique and unusual composition, and its relatively small size are not to be considered downgrades to an otherwise fascinating place in our solar system. If anything, these characteristics are what make Mars and other celestial bodies, all the more intriguing. It's ok to be different. It's when something (or someone) is forced to fit a mold that ill-defines it (or them) that it isn't ok.
As always, thanks for reading.