September 6, 2016 | David F. Coppedge

Why Astronomers Hammer Planets

Secular planetary scientists have a skeleton key that unlocks any planetary mystery: the BFH.

A “BFH” (big freaking hammer) is the jocular acronym for the most useful tool in a repairman’s toolkit. When something won’t budge, hit it with the BFH. If it breaks, it needed fixing anyway. That’s the thinking of the less nuanced problem-solver. The problem with a BFH is that when it’s your only tool, you tend to see everything as a nail.

The BFH for the astrobiologist or secular planetologist is the “impact hypothesis.” Since design is ruled out by the so-called rules of science (a.k.a. methodological naturalism), unusual features that may appear designed must be explained only by appealing to unguided physical processes. Here are some recent examples of the impact hypothesis in use.

Phos for us: One essential element for life on earth is phosphorus. Unfortunately, it tends to be locked up in rocks. NASA’s Astrobiology Magazine sets up the problem in order to justify wielding the BFH:

Phosphorus is one of life’s most vital components, but often goes unheralded. It helps form the backbone of the long chains of nucleotides that create RNA and DNA; it is part of the phospholipids in cell membranes; and is a building block of the coenzyme used as an energy carrier in cells, adenosine triphosphate (ATP).

Yet the majority of phosphorus on Earth is found in the form of inert phosphates that are insoluble in water and are generally unable to react with organic molecules. This appears at odds with phosphorus’ ubiquity in biochemistry, so how did phosphorus end up being critical to life?

That’s why Keith Cooper teased with his headline, “Did meteorites bring life’s phosphorus to Earth?” He brings in a new theory about how meteorites bearing a mineral called shreibersite may have seeded the earth with a form of phosphorus that was available for the RNA World. So here comes the hammer blow: “Meteorites that crashed onto Earth billions of years ago may have provided the phosphorous [sic] essential to the biological systems of terrestrial life.”

Another hypothesis stated on PhysOrg tries to make phosphorus available via Miller-type spark-discharge processes that “could” have produced urea as an intermediate. This article resurrects the Darwin “warm little pond” meme. The scenario relies on special concentrations of salts and ions in specific locales without intelligent lab researchers adding them as needed. Nothing was said about keeping the phosphate ions from getting locked up in rocks again, once conditions change.

Carbon for us: Another essential element for life is carbon. Once again, Astrobiology Magazine appeals to the BFH for special delivery of this vital ingredient; only this time, the hammer is really big: “Earth’s carbon points to planetary smashup.” The impactor delivering carbon was as big as the planet Mercury, astrobiologists at Rice University are claiming. “Research by Rice University Earth scientists suggests that virtually all of Earth’s life-giving carbon could have come from a collision about 4.4 billion years ago between Earth and an embryonic planet similar to Mercury.” This must have been after the Mars-sized BFH that formed Earth’s moon. Or maybe the Rice guys didn’t think about that. Anyway, here is the setup:

“One popular idea has been that volatile elements like carbon, sulfur, nitrogen and hydrogen were added after Earth’s core finished forming,” said Li, who is now a staff scientist at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. “Any of those elements that fell to Earth in meteorites and comets more than about 100 million years after the solar system formed could have avoided the intense heat of the magma ocean that covered Earth up to that point.

The problem with that idea is that while it can account for the abundance of many of these elements, there are no known meteorites that would produce the ratio of volatile elements in the silicate portion of our planet,” Li said.

The BBC News promoted this story, too. A Mercury-sized impactor solves the sulfur abundance puzzle at the same time, they claim. But does the proposed solution explain where the impactor got its carbon and sulfur?

“One scenario that explains the carbon-to-sulfur ratio and carbon abundance is that an embryonic planet like Mercury, which had already formed a silicon-rich core, collided with and was absorbed by Earth,” Dasgupta said. “Because it’s a massive body, the dynamics could work in a way that the core of that planet would go directly to the core of our planet, and the carbon-rich mantle would mix with Earth’s mantle.

While focused on the dynamics, it seems they just moved the source of those two elements to another location. But why would another planetesimal have lots of carbon and sulfur? Why would meteorites be enriched in available phosphorus? Where did they get it? One thing is clear: life needs lots of carbon, sulfur and phosphorus, and Earth has just the right amounts.

Vesta luck: The BFH “scenario” works all over the solar system, not just at Earth. The asteroid Vesta, for instance (visited by the Dawn Spacecraft, 2/09/15) has unusual amounts of the mineral olivine. Where did it come from? At Icarus, planetary scientists wielded the BFH once again: “Olivine on Vesta as exogenous contaminants brought by impacts: Constraints from modeling Vesta’s collisional history and from impact simulations.” Here’s the justification for the scenario:

The survival of asteroid Vesta during the violent early history of the Solar System is a pivotal constraint on theories of planetary formation. Particularly important from this perspective is the amount of olivine excavated from the vestan mantle by impacts, as this constrains both the interior structure of Vesta and the number of major impacts the asteroid suffered during its life. The NASA Dawn mission revealed that olivine is present on Vesta’s surface in limited quantities, concentrated in small patches at a handful of sites not associated with the two large impact basins Rheasilvia and Veneneia. The first detections were interpreted as the result of the excavation of endogenous olivine, even if the depth at which the detected olivine originated was a matter of debate. Later works raised instead the possibility that the olivine had an exogenous origin, based on the geologic and spectral features of the deposits. In this work, we quantitatively explore the proposed scenario of a exogenous origin for the detected vestan olivine to investigate whether its presence on Vesta can be explained as a natural outcome of the collisional history of the asteroid over the last one or more billion years.

Once again, though, the theory has to explain how the A-type and S-type asteroids got their olivine. Presumably most of the asteroids in the asteroid belt would be expected to have the same olivine proportions. There doesn’t seem to be much talk about olivine at Dawn’s current target, the largest asteroid Ceres. We’ll examine the latest suite of papers about Ceres in a future post.

Impacts have certainly occurred on most bodies of the solar system, as evident from the numerous craters everywhere. When impacts need to be finely tuned to produce a desired result, however, the impact “scenario” begins looking suspicious.

To work effectively, the BFH requires use of another tool in the planetary scientist’s toolkit. That’s the Magic Wand that speaks, in a Sagan-like tone, saying “billions and billions of years.”

If you haven’t yet seen Spike Psarris’s classic DVD about the solar system, What You Aren’t Being Told About Astronomy: Volume I, Our Created Solar System, you’ll enjoy the fun he makes of the impact hypothesis and how secular astronomers use it to explain anything and everything.

 

 

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