Time to Revisit the Lunar Dust Problem?
(Visited 628 times, 1 visits today)FacebookTwitterPinterestSave分享0 How deep should lunar dust get over billions of years? Opinions have vacillated between extremes, but a new study might open up the debate again.Before the first soft landing on the moon by Surveyor 1 in 1966, scientists were quite worried about lunar dust being too deep to land on. The NASA-JPL page about Surveyor 1 explains:Before humans could take their first steps on the moon, that mysterious and forbidding surface had to be reconnoitered by robots. When President John Kennedy set a goal of landing astronauts on the lunar surface in 1961, little was known of that world, beyond what could be gleaned from observations by telescopes.We knew it was rocky, bleak and heavily cratered — how might these conditions affect the landing of a spacecraft there? Was the surface sufficiently solid to support the 33,500-pound Apollo lunar lander? Or was it so deeply covered in dust from billions of years of meteorite impacts, as some theorized, that the lunar module would simply sink out of sight, dooming the astronauts? These and a hundred other questions about the surface composition dogged mission planners, so a robot would make the dangerous journey first – the lunar lander from NASA’s Jet Propulsion Laboratory.Surveyor 1, 1966Dr Henry Richter, the instrument manager for the Explorer, Ranger and Surveyor missions, recalls lunar dust was a “real concern” in those years (personal communication). Scientists were relieved when the Surveyors landed successfully without sinking out of sight; even so, the Surveyors and Apollo lunar modules were equipped with wide footpads just in case. The Apollo astronauts were rather surprised to find the dust layer very shallow, just a few inches deep. Often, they could scrape bedrock with their boots.Many creationists used this fact to argue for a young moon. If the moon were 4.5 billion years old, they said, it should have accumulated great depths of dust. The fact that it did not suggested to them that the moon was not as old as claimed. The amount of dust accumulation, however, was later found to be based on flawed estimates of incoming dust, and perhaps by the assumption that particles would softly settle onto the lunar surface rather than slam in at high speeds, where it might melt and harden. Vitriolic critics lambasted the argument, and so many creationists sheepishly backed away from it – although, as we see from JPL’s quote, it was not just creationists who assumed great volumes of dust should be there. Snelling and Rush at ICR said in 1993,Unfortunately, attempted counter-responses by creationists have so far failed because of spurious arguments or faulty calculations. Thus, until new evidence is forthcoming, creationists should not continue to use the dust on the moon as evidence against an old age for the moon and the solar system.Wishing to use only the strongest arguments for youth, apologists like those at CMI have urged caution, listing the lunar dust argument among those that creationists should not use. “Nevertheless, as we have indicated before,” CMI continues, keeping the door slightly open, “creationists as well as evolutionists need to be prepared to re-examine arguments as new and better data emerges.”New and Better Data Are HereWhen particles slam into the moon, they “garden” the surface (regolith), by overturning layers and re-depositing them on the surface. Impacting bodies vary over 12 orders of magnitude, from nanometer-sized particles to large asteroids. Impactor size follows a power law, with big impacts being more rare than small ones. The last major mathematical model of regolith mixing (impact gardening) was done by Gault et al in 1974. Now, a new model by Costello, Ghent and Lucey, published in Icarus, has identified a major oversight in Gault’s model. While appreciative of the pioneering work on mixing done back then, Costello points out that Gault’s model only considered mixing due to primary impacts. What happens when secondaries are taken into account? [Note: primaries are original impacts; secondaries are fallback material launched from a primary impact.] Secondaries make a big difference!Our most important update is the inclusion of secondary impacts. Our calculations show that secondaries are necessary to produce the reworking rate inferred from the depth distribution of surface-correlated material in Apollo cores …. Overturn calculations that only consider the impact of primaries fail to describe observed reworking rates at all depths and timescales. We conclude that secondary impacts dominate mixing in the top meter of lunar regolith.We have reported before in these pages several times about the “impact” of secondary craters on crater count dating (e.g., 22 May 2012, 19 Oct 2015, 12 Oct 2016). One impact on Mars could launch a million secondaries, and some secondaries can travel between bodies, such as between Jupiter’s moons. So serious was the failure to account for secondary impacts, it rendered all previous calculations of surface dates based on crater counts questionable. Is a similar situation about to happen with Costello’s paper on the question of lunar dust accumulation?While primary impactors arrive at high speeds (20 km/sec) enough to melt rock, secondary impactors would tend to be smaller and drift down to the surface on ballistic paths. The astronauts were very familiar with the behavior of dust as they walked around and drove around in the rovers. They could see it float back down after being kicked up by their boots. And as we have reported, electrostatic forces can propel fine dust for long distances (10 Jan 2017, 28 Feb 2018). Continuing for millions and billions of years, would these processes not predict heavy accumulations of fine dust?Costello’s new model, which takes secondaries into account, finds better agreement with Apollo rock samples.Overturn due only to primary impacts is much too infrequent and shallow to produce the thorough mixing implied by the depth distribution of 26Al in the Apollo cores. It takes a flux of primary impactors hundreds of millions of years to reach 3 cm depth just once with 50% probability. The homogeneous distribution of 26Al suggests many more than one overturn event has occurred in less than a million years. The flux of secondary impacts appears to be much more effective, thoroughly reworking the regolith at 2–3 cm in less than a million years: a rate consistent with inferences from 26Al in the Apollo cores.This statement does not mean that the surface is a million years old. What it does mean is that earlier models significantly overlooked the effects of secondary impacts.Building on the core statistical concept presented by Gault et al. (1974) we present a generalized model that describes the rate and probability a point at depth experiences overturn as a function of time. By using material parameters consistent with lunar regolith and lunar impact flux, we calculate the rate and probability of overturn on the Moon. Compared to the overturn rate driven by the modern flux of primaries, overturn due to secondaries is in much better agreement with the Morris (1978) reworking rate and the depth-distribution of 26Al measured in Apollo cores. This is especially true at short timescales and shallow depths. Further, overturn due to secondaries better describes the rate at which surface features such as splotches rework the regolith and the rate at which cold spots and rays are reworked into the background. We conclude from these comparisons that secondaries are the dominant driver of overturn in the top meter of lunar regolith.Figure 9(c) in the paper shows what the new model predicts geologists would find in a one-meter drill core after one billion years. Everything down to a meter should show some evidence of reworking. Everything shallower than 50 centimeters should have been thoroughly reworked, being overturned 100 times. Everything shallower than 10 centimeters should be homogeneous, having been reworked at least 10,000 times! Multiply these values by 4.5 to get closer to the actual prediction old-agers would expect. Does that match what the Apollo astronauts actually found when they scraped hard rock with their boots?Even Costello’s new model is not complete. The estimates could be lower limits. Here are just a few of the uncertainties that still remain in this latest model, 49 years after Apollo 11:Superficially, calculations of overturn driven by micrometeorites could be improved by using the dust flux from studies of LADEE and LDEF data (e.g. Meshishnek et al., 1993; Horányi et al., 2015; Szalay and Horányi, 2016). More fundamentally, future incarnations of our model should include cratering laws and energy-partitioning that are designed specifically to describe micro-impacts. Another fundamental issue remains unaddressed in this treatment of micrometeorite overturn: the effects of micro-secondaries…. Evidence of mixing does not discriminate between primaries, secondaries, slumping, jetting or astronaut footprints. The depth-distribution of surface correlated materials observed in Apollo cores and the rate at which cold spots and rays disappear are the result of a complicated system of mixers. Determining the relative influence of each mixing driver is important for future modeling of regolith evolution. Here we have treated only one kind of regolith mixing: vertical excavation form cratering events. Because the mixing rates we predict with a flux of secondary impacts included are reasonable, one could argue that the vertical mixing of regolith is dominantly driven by secondary distal ejecta that produce secondary craters. Inferences about lateral transport and horizontal mixing are currently beyond the scope of this model; however, by better constraining the treatment of secondaries, we may be able to investigate mixing in three dimensions and compare our results to lateral mixing models (e.g Huang et al., 2017) in the future. The treatment of secondaries used in this work could be improved to first order by a piece-wise power law or polynomial re-casting of flux as well as a treatment of the velocity and impact angle distributions of secondary projectiles. Recall that in this work we crudely assume that all secondaries impact the lunar surface at 0.5 km s−1, the minimum in the range of maximum spall velocities….These and other shortcomings in the new model will require more analysis, the Costello team admits. The old model’s reliance on primaries alone, however, “casts a pall of uncertainty on the fundamental assumption” Gault used to model the distribution of material on the surface.Expect big celebrations on July 19, 2019We share this paper not to do the analysis ourselves, but to show that the door is open to reconsider lunar dust accumulation as evidence for a young moon. On the verge of the 50th Anniversary of Apollo, the time has come to “re-examine arguments as new and better data” have been provided. Perhaps the lunar dust argument will emerge stronger, and will drop off the list of arguments creationists should not use.We see that CMI left the door open a bit by responding to an earlier CEH entry (21 Nov 2013) by adding a footnote to Snelling and Rush’s detailed 1993 refutation of the moon-dust argument (which included some consideration of secondaries, although not as up-to-date as Costello’s paper). CMI’s footnote said on 3 July 2014, New NASA data has turned up that is said to have been on ‘long-lost’ tapes*, and shows a dust influx rate some ten times that of previous measurements. At face value it seems to raise the possibility of at least a partial revival of the moon dust argument. Given the very careful and detailed creationist analyses which led to its abandonment in the first place, and the other factors that could potentially affect these results (see this summary by a friend and ally), any reassessment would need to be similarly thorough and careful.We agree and hope that this latest entry will stimulate a new thorough and careful analysis.