Lunar Complexity Challenges Simple Theories
Tomorrow is “International Observe the Moon Night” according to Space.com, stimulating laypeople and astronomy neophytes to get outdoors to look at the moon with telescopes, binoculars, and the naked eye while the last-quarter moon is in convenient position for evening viewing. Humans have contemplated the moon for millennia on such evenings and imagined all sorts of tales for explaining its appearance. Now that the Space Age has given us samples and detailed measurements, do scientists know what they are looking at? Yes and no. The current spacecraft, NASA’s Lunar Reconnaissance Orbiter (LRO), now past its first year of operation, has shown our moon to be a more complex body than previously thought.
Popular science articles and TV shows speak of the history of our moon as a foregone conclusion: formation from an impact with earth 4.5 billion years ago, a Late Heavy Bombardment 3.9 billion years ago from large impactors from the asteroid belt, formation of the maria (seas) of lava, smaller craters from near-Earth impactors, and ongoing space weathering and regolith formation as the moon cooled down into the body we see today. The realities defy such simple tales. Some observations seem to support the story, but as is common in scientific discovery, new observations throw curveballs at simplistic theories. This is evident from three new LRO papers published in today’s issue of Science.1,2,3 “For the first time we’re actually detecting how complex the lunar surface is,” one of the lead authors, Benjamin Greenhagen of JPL, remarked. “It’s a bit of a paradigm shift.”
Popular science sites were awash in headlines like “NASA’s LRO Exposes Moon’s Complex, Turbulent Youth” (Jet Propulsion Lab), “New Type of Moon Volcano Discovered” (National Geographic), and “Moon’s Face Reveals Extreme Cosmic Abuse” (Live Science). While Science Daily published an optimistic headline “Moon’s Craters Give New Clues to Early Solar System Bombardment,” New Scientist focused on controversy with, “Crater map rekindles debate over moon impacts.” Everyone agreed that the LRO findings make explanations for the moon’s origin more complex.
Head et al led the trio of papers with an analysis of 5,184 large craters (> 20km diameter) as measured by LRO’s Lunar Orbiter Laser Altimeter (LOLA).1 This instrument has provided the best bias-free data on crater topography, allowing unprecedented mapping of large craters with less sampling error. They produced size-frequency distribution plots for craters from all over the moon. Given a host of assumptions about secondary cratering, crater obliteration by impactors, and prehistoric volcanism, their crater-count data seemed to support the idea of two distinguishable populations of impactors – in favor of the Late Heavy Bombardment (LHB) hypothesis of large impactors 3.9 billion years ago, and smaller impactors in later eras. The LHB idea began when samples from the Apollo missions gave similar radiometric ages. Planetary scientists envisioned some kind of disruption of Jupiter and Saturn that sent main-belt asteroids careening toward the earth and moon at that time with a different population of near-earth objects forming craters later (see Wikipedia description). The LHB has even been extended to account for crater densities on the Earth, Mars and other parts of the solar system.
The hypothesis, however, is admittedly controversial, the authors agreed: “Controversy has long existed concerning the nature of the impactor population bombarding inner planetary body surfaces throughout solar system history.” Some allege the Apollo samples gave similar dates because they were all related to Mare Imbrium. New Scientist quoted a German researcher countering that the “distribution of crater sizes may instead be due to local surface processes that can cover up craters, such as lava flows and ejected debris from impacts.” Craters can cover up data in complicated ways, he alleged, “making it impossible to equate the size distribution of craters with the size distribution of impactors.” He told New Scientist, “Don’t take [the crater counts] at face value. You have to apply corrections.” But which corrections are the correct ones? Each correction rests on assumptions.
The second and third papers, by Greenhagen et al2 and Glotch et al,3 announced the rather surprising detection of volcanic silicate minerals on the moon (see National Geographic and JPL articles). They used LRO’s infrared “Diviner” instrument to indirectly detect the composition of lunar rocks at numerous representative swaths from orbit. Most of the moon’s surface has been known to contain basalt and anorthosite, simpler products of standard volcanic magma eruptions. To get “highly silicic compositions on the moon,” though suggested in earlier observations, was unexpected,3 because the circumstances for obtaining these “evolved lithologies” are more complex. Perhaps the silicates didn’t mix well with liquid magmas and became concentrated, or perhaps underlying basaltic magma melted the silicates and pushed them up onto the surface. Either way, the scientists found intrusive and extrusive silicates at scattered locations on both sides of the moon, complicating any explanations for their formation. One of the sites is the crater Aristarchus, known for many observations of transient lunar phenomena. It also suggests, according to Glotch et al, that “magmatic processes capable of producing highly evolved compositions occurred over an extended period of time” – perhaps 500 million years, about 1/9 of alleged lunar history. No evidence of fresh mantle material, though was detected, even though the Orientale Basin impactor should have penetrated the mantle.
These findings from LRO should keep the theorists busy for awhile. They may not be able to invoke eccentric orbits in the moon’s history, though (sometimes theorists invoke heat from circularization of a highly eccentric orbit to explain internal heating, surface and internal tides, and geological effects at the surface). A paper in press at Icarus4 by Matija Cuk of Harvard, however, criticizes earlier attempts to infer a highly eccentric orbit for the early moon based on its shape. After calculating numerous factors (despin rates, effects of impacts on spin, resonances and such), and analyzing arguments for the eccentric orbit hypothesis, Cuk concluded, “there is currently no compelling evidence for a significantly higher past lunar eccentricity.” If the moon entered existence in a relatively circular orbit (its eccentricity today is e=0.055), that makes the already-improbable impact hypothesis for its origin require even more special conditions.
More discoveries continue to show our nearest celestial neighbor to be full of surprises. New Scientist showed a photo of natural bridges on the moon that, we are quickly told, “probably formed as a result of an impact in the last billion years” but have survived without collapsing all that time, even though they are not sure they would support the weight of an astronaut. And last month, an article on PhysOrg showed a new map of the Schrodinger Basin taken by LRO, revealing a complex patchwork of very old and very young features, including recent volcanic activity, all mixed up in one crater alleged to be 3.8 billion years old.
These reports provide plenty to ponder on International Observe the Moon Night.
1. Head, Fassett et al, “Global Distribution of Large Lunar Craters: Implications for Resurfacing and Impactor Populations,” Science, 17 September 2010: Vol. 329. no. 5998, pp. 1504-1507, DOI: 10.1126/science.1195050.
2. Greenhagen, Lucey et al, “Global Silicate Mineralogy of the Moon from the Diviner Lunar Radiometer,” Science, 17 September 2010: Vol. 329. no. 5998, pp. 1507-1509, DOI: 10.1126/science.1192196.
3. Glotch, Lucey et al, “Highly Silicic Compositions on the Moon,” Science, 17 September 2010: Vol. 329. no. 5998, pp. 1510-1513, DOI: 10.1126/science.1192148.
4. Matija Cuk, “Lunar Shape Does Not Record a Past Eccentric Orbit,” Icarus (article in press, accepted manuscript, available online 09/15/2010), DOI:10.1016/j.icarus.2010.08.027.
Are planetary scientists coming closer to “the truth” about our moon’s origin? The data are certainly getting better. But infrared maps and crater size-distribution statistics do not interpret themselves. They need to be incorporated into a paradigm (interpretive framework with its ancillary assumptions). That paradigm has shifted again, according to Greenhagen.
The paradigm has not shifted to revolution status, evidently. Scientists are at the stage of admitting more anomalies in the current paradigm. According to Thomas Kuhn, this is a routine part of normal science. Only when the anomalies accumulate to the point of unwieldiness, or younger scientists enter the field with maverick ideas, can the paradigm get replaced by a new paradigm in a “scientific revolution.”
The current paradigm includes a time framework and numerous unproveable assumptions. Entrenched assumptions currently include the Age of the Solar System (A.S.S.) of 4.5 billion years (the Law of the Misdeeds and the Perversions that cannot be altered), the reliability of radiometric dating along with its copious assumptions about initial conditions, the hypothesis the moon formed when a Mars-size body impacted the Earth, the Late Heavy Bombardment, the reliability of crater count dating, and more. Interpretations made about the moon (our nearest neighbor) are often extrapolated to other bodies in the solar system.
The moon, though, is a very complex body. There are big differences between the maria and the cratered highlands, some of which approach crater saturation (which implies that any history before saturation has been erased). There are old-looking things and young-looking things side by side. There are differences in surface composition, texture, regolith depth, and elevation. There is evidence of ongoing activity today (08/23/2010), including young-looking lava flows (11/12/2008), recent cratering (05/21/2008), and transient lunar phenomena (07/12/2007) witnessed for decades by amateurs. Why isn’t the moon stone cold dead after 4.5 billion years (01/25/2009)? How could gas and lava get to the surface now (11/09/2006)? Why is the far side of the moon so different from the near side? Why does the terrain differ so much from place to place?
To handle the anomalies, a host of special conditions and one-time events need to be invoked (02/19/2007). The Late Heavy Bombardment (LHB) itself is an ad hoc proposition, specifying unique conditions that presumably sent main-belt asteroids toward the earth once but never again (a Jupiter-Saturn tango? Uranus and Neptune passing by on their way out? a unique swarm of comets?). One must not be fooled into thinking that giving a speculative idea a name like LHB somehow confers plausibility on it. The impact hypothesis for the moon’s origin is even more highly contrived, requiring just-right conditions for the impactor’s size, speed, composition and angle of attack (01/26/2007, 11/04/2002). Divination must be used (not with LRO’s Diviner infrared instrument, but the occult kind) to envision histories forever buried beneath craters and the consequences of unobserved secondary and tertiary craters. Head et al assumed certain patterns from secondary craters, but how do they know that large secondaries were not launched into orbit, to land with a bang centuries or millennia later? Tweaking assumptions can have drastic effects on conclusions.
Bro, can youse paradigm? Scientists are often oblivious to the assumptions they spend on paradigms. Within the guild, everything seems intuitively obvious. Anomalies are mere puzzles that will be solved within the consensus, they glibly presume (03/31/2005). What is needed is thinking outside the box, if for no other reason that to study the soundness of the box. Why, for instance, must scientists take a bottom-up strategy for origins? Is it somehow more blessed to explain everything up from particles to people, from hydrogen to high-tech, than from top-down design?
Top-down thinking certainly has the Second Law of Thermodynamics in its favor. For an example of outside-the-box, top-down thinking about the moon, you might want to examine the new updated edition of Our Created Moon by Whitcomb and DeYoung, available on Amazon.com. It explores the design purposes of the moon as well as analyzing flaws in secular origin theories. Other top-down analyses of the moon include Tas Walker’s analysis of fault scarps (9/2/2010 at CMI), an analysis of transient lunar phenomena by Dr. Don DeYoung in Journal of Creation April 2003 (free access to PDF file), videos on “Our Created Moon” by Dr. DeYoung viewable on Answers in Genesis, and an article by DeYoung on ICR comparing bottom-up and and top-down views on the origin of the moon.
Regardless of world view, everyone should take a look at our amazing moon this weekend for “International Observe the Moon Night” (see Space.com). Think about not only its beauty and rarity (05/04/2006), but also another powerful reality: without that moon, we could not exist (07/13/2008, 12/24/2008).