September 8, 2014 | David F. Coppedge

Planets Don't Fit Evolutionary Models

Secular planetary scientists are surprised by almost every object they observe in the solar system.  Their models cannot reproduce our system of planets.

One of the most common words in solar system science is “mystery.”  We’ll look at some examples first, then see whether current models are able to account for observations.

Mercury magnetic mystery:  The word “mystery” is front and center in Astrobiology Magazine‘s headline, “Mysteries of a magnetic Mercury.”  An embedded video clip about findings from the MESSENGER mission explains how the planet’s molten core is larger than expected: “Many scientists thought the interior might have cooled to a solid because of the planet’s small size.”  The core inferred from instruments is not only large, extending to 85% of Mercury’s radius, but has unique characteristics: “Mercury’s core is different from any other planetary core in the solar system.”  It doesn’t work the same way as Earth’s, either.  As could be expected, though, the NASA site took the opportunity to tease readers about life, even though “Mercury is not a location in the Solar System where life is thought to be possible.”  UCLA‘s Hao Cao calls Mercury’s magnetic field “bizarre” and “peculiar.”  His research “implies that planets have multiple ways of generating a magnetic field.”  But with that admission goes hope of identifying a unified model.  The press release highlights the unexpected nature of the findings:

Within Earth’s core, iron turns from a liquid to a solid at the inner boundary of the planet’s liquid outer core; this results in a solid inner part and liquid outer part. The solid inner core is growing, and this growth provides the energy that generates Earth’s magnetic field. Many assumed, incorrectly, that Mercury would be similar.

“Hao’s breakthrough is in understanding how Mercury is different from the Earth so we could understand Mercury’s strongly hemispherical magnetic field,” said Russell, a co-author of the research and a professor in the UCLA College’s department of Earth, planetary and space sciences. “We had figured out how the Earth works, and Mercury is another terrestrial, rocky planet with an iron core, so we thought it would work the same way. But it’s not working the same way.

Re-write the Earth textbooks:  Time to re-write the textbooks about Earth history again, PhysOrg claims.  Now evolutionary planetologists figure that oxygen appeared a full 60 million years earlier than previously thought.

Mars shouldn’t exist:  Another Astrobiology Magazine article claims that computer models have serious problems forming a Mars-sized planet; it turns up in only 5% of runs.  The headline announces this as a “Planetary Mystery.”  Mars is a “rare planet,” Elizabeth Howell writes.  Nor is the problem new: “The formation of Mars is a long-standing problem,” one planetologist remarked.

Europa: the dream is alive:  It must be funding time at NASA, because discovery of possible “plate tectonics” on Jupiter’s icy moon may promote a planned orbital mission there.  “The groundswell of enthusiasm [for a proposed mission] is likely to be bolstered by the latest big news, reported on 7 September, that there may be giant plates of ice shuffling around on Europa — much as plates of rock do on Earth,” Nature News says.   Science Now pointed out that Europa may be “actively recycling its surface.”  The short article notes, “The result would make Europa the only known body in the solar system besides Earth with plate tectonics.”  It’s not clear, though, why a relatively small moon should have share this distinction with Earth when other moons and planets of all sizes do not.  Ignoring that question, both articles concentrated on the implications for life on Europa.  Meanwhile, at Space.com, Mike Wall is puzzled why the giant geysers at Europa reported in December have mysteriously disappeared.

Stretching Saturn’s rings:  Planetary scientists are still trying to stretch the age of Saturn’s rings.  Nature reported that low dust measurements might allow them to conclude the rings are as old as the solar system: the rate of incoming dust, thought to be a prime agent of “dirtying” the rings, appears much smaller than expected.  That, of course, measures the dust rate now, not as it might have been in the past.  It’s a breath of hope, though, for modelers worried about the young-rings problem.  One of them is Jeff Cuzzi, who decades ago pointed out several reasons why the rings must be young.  The article notes that there are “still other arguments for why Saturn’s rings could be young,” but then quotes Cuzzi’s response to the dust measurements, “But I’m more inclined to believe the old-rings model now than before.”

Titanic mysteries:  Saturn’s giant moon Titan continues to be a focus of rapt attention as Cassini’s radar maps and images accumulate to form a clearer picture.  To explain the problems with Titan’s liquids, though, some scientists are venturing into proposing unobservable phenomena.  “While most of the liquid in the lakes is thought to be replenished by rainfall from clouds in the moon’s atmosphere, the cycling of liquid throughout Titan’s crust and atmosphere is still not well understood.”  A new idea proposes “subsurface reservoirs” to replenish the observed liquid lakes.  The article scores a high perhapsimaybecouldness index:

“We knew that a significant fraction of the lakes on Titan’s surface might be connected with hidden bodies of liquid beneath Titan’s crust, but we just didn’t know how they would interact”, says Mousis. “Now, we’ve modelled the moon’s interior in great detail, and have a better idea of what these hidden lakes or oceans could be like.”

More storytelling is found in an Icarus paper that wove tales about nitrogen lakes when methane depleted one or more times in the unobservable past.  In the proposals, “may have” is the operative phrase.  “Thus, we can speculate that a paleo-nitrogen cycle may explain the erosion and the age of Titan’s surface, and may have produced some of the present valley networks and shorelines.”  Why, it might even happen again, the authors speculate.

Another paper about Titan on Icarus considered what complex organic compounds would be expected to dissolve in the methane lakes over time: “Titan lakes will saturate in benzene from direct airfall in geologic timescales,” they said.  That’s not what measurements have found: the lakes are primarily methane and ethane, the simplest hydrocarbons.  See New Scientist for summary and positivist spin.

An article on Astrobiology Magazine, drunk with speculation about life on Titan, leaked a mystery: “The negative ions were a complete surprise.”  Building on that surprise, astrobiologists speculated that the ions could have promoted the formation of tholins (hydrocarbon tars), which would have sunk to the bottom of the frigid, murky lakes.  Whether that’s a good place to look for the origin of life seems a matter of faith.

Heating the squeeze:  On Icarus, Isamu Matsuyama found a tweak that could increase the heat flow on Saturn’s active moon Enceladus, but only after admitting “Enceladus’ dissipated energy flux due to the obliquity tide is smaller than the observed value by many orders of magnitude.”  He found a “resonant response to the eccentricity tide” that “can be large enough to explain Enceladus’ observed heat flow” according to his model.  Whether it actually does remains to be tested by other modelers.

Shocking chondrules:  The mysterious meteorite parts known as chondrules are shocking in more ways than one.  They are surprising because of their proportions of volatile elements.  For another, modelers hoped to explain them with “shock models: i.e., “processing of dust in shock waves in protoplanetary disks,” say Stammler and Dullemond in Icarus. “In this model, the dust and the chondrule precursors are overrun by shock waves, which heat them up by frictional heating and thermal exchange with the gas.”  Too bad; doesn’t work.  Back to the drawing board:

In this paper we reanalyze the nebular shock model of chondrule formation and focus on the downstream boundary condition. We show that for large-scale plane-parallel chondrule-melting shocks the postshock equilibrium temperature is too high to avoid volatile loss. Even if we include radiative cooling in lateral directions out of the disk plane into our model (thereby breaking strict plane-parallel geometry) we find that for a realistic vertical extent of the solar nebula disk the temperature decline is not fast enough. On the other hand, if we assume that the shock is entirely optically thin so that particles can radiate freely, the cooling rates are too high to produce the observed chondrules textures. Global nebular shocks are therefore problematic as the primary sources of chondrules.

Nothing works:  The article on Mars by Elizabeth Howell referred to above (Astrobiology Magazine) points out general problems with theories of the solar system’s origin.   The best models fail to account for Mars and other features of our family of planets.  This bears on the philosophical question, “Are the orbits and sizes of the planets a natural byproduct of the formation, or are there features that happened because of rare events?”  Unpredictable “rare events” are generally shunned in science, because they venture close to the Stuff Happens Law rather than providing understanding of nature’s processes.  Howell provides a glimpse into the modeling lab where nothing works out as expected:

For that reason, the model started with the assumption that Jupiter and Saturn existed when the Inner Solar System was still being formed. The researchers ran two sets of 50 simulations — one with Jupiter and Saturn close to the eccentric orbits that they have today, and one with Jupiter and Saturn in more circular orbits.

“We formed two to six Inner Solar System planets in our simulations, usually,” Fischer says. “We do see something that looks like a Venus analog. Mercury is much harder. We see maybe one good analog out of all the simulations. That’s something, that really no simulations are producing Mercury, so there’s probably something wrong in the way of thinking about that.

Fischer acknowledged that the results may tell scientists that forming Mars and Mercury are low-probability eventspossible, but rare. Or, the simulation may show scientists that the assumptions they have about the Solar System need to be revisited. These are matters that will need to be addressed in future research, she said.

Mercury and Mars are not the only problematical planets.  Howell lists other worries about the formation of Earth and the gas giants, raising big questions about what scientists know about planets, and how our solar system fits into the universe.

Understanding how the Solar System is put together also has implications for life beyond Earth. If it is, indeed, rare for certain planets to form, this could also make it rarer for life to exist in the Universe. So far, Fischer said, scientists haven’t been able to replicate the Solar System in models.

“There’s a lot of discussion over the philosophy of how to approach this, whether you look for the Solar System as the most likely outcome, or whether you should look for conditions that can produce the Solar System. Currently, there’s no configuration that has been proposed that forms the Solar System most of the time.”

So despite the textbooks, TV documentaries and confident-sounding planetarium programs, “mystery” is the nature of the Real World.   Some planetary scientists remain optimistic about the growing field of exoplanet studies (as exemplified in a review on PNAS by planet-hunter Geoffrey Marcy).  But there’s very little out there that looks anything like our solar system.  For instance, “planets one- to four-times the radius of Earth, a population missing in the solar system, are common.”   That’s a far cry from earlier expectations that other stellar planetary systems would resemble ours. “With all this progress, textbooks are being rewritten yearly,” Marcy said, implying that much of what adults were taught is wrong.

The old Norwegian comedian Yogi Yorgesson used to sing “I’m glad I’m a happy nincompoop.”  In the lyrics (1950), he said he would gather with his nincompoop friends, and they would “laugh about how much we don’t know.”  We are certainly not going to call planetary scientists nincompoops.  Most of them are highly intelligent people.  They’re not laughing about all the things they don’t know; they are worrying about them, like Yogi said: “Smart people worry when the day is done, but nincompoops have all the fun.”  So we ask: given the number of things about which planetary scientists and nincompoops don’t know, who is better off?

 

 

 

The negative ions were a complete surprise
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