Reexamining a Cataclysm

Image of Earth's Moon centered on the Orientale Basin taken by Galileo Spacecraft.

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One of the legacies of the Apollo program is the rare lunar samples it returned. These samples (along with meteorites that originated from the moon and even one from Mars) can be radiometrically dated, and together they paint a picture a cataclysmic time in the history of our solar system. Over a period of time some 3.8 to 4.1 billion years ago, the moon underwent a fierce period of impacts that was the origin of most of the craters we see today. Paired with the “Nice model” (named after the French university where it was developed, not because it was pleasant in any way), which describes the migration of planets to their current orbits, it is widely held that the migration of Jupiter or one of the other gas giants migrations during this period, caused a shower of asteroids or comets to rain down upon the inner solar system in a time known as the “Late Heavy Bombardment” (LHB).

A new paper by astronomers from Harvard and the University of British Columbia disagrees with this picture. In 2005, Strom et al. published a paper in Science which analyzed the frequency of craters of various sizes on the lunar highlands, Mars, and Mercury (since these are the only rocky bodies in the inner solar system without sufficient erosion to wash away their cratering history). When comparing relatively young surfaces which had been more recently resurfaced to older ones from the Late Heavy Bombardment area, is that there were two separate, but characteristic curves. The one for the LHB era revealed a crater frequency peaking at craters near 100 km (62 miles) in diameter and dropping off rapidly to lower diameters. Meanwhile, the younger surfaces showed a nearly even amount of craters of all sizes measurable. Additionally, the LHB impacts were an order of magnitude more common than the newer ones.

The Strom et al. took this as evidence that two different populations of impactors were at work. The LHB era, they called Population I. The more recent, they called Population II. What they noticed was the current size distribution of main belt asteroids (MBAs) was “virtually identical to the Population 1 projectile size distribution”. Additionally, since the size distribution of the MBA is the same today, this indicated that the process which sent these bodies our way didn’t discriminate based on size, which would weed out that size and alter the distribution we observed today. This ruled out processes such as the Yarkovsky effect but agreed with the gravitational shove as a large body would move through the region. The inverse of this (that a process was selecting rocks to chuck our way based on size) would be indicative of Strom’s Population II objects.

However, in this paper recently uploaded to arXiv, Cuk et al. argue that the dates of many of the regions investigated by Strom et al. cannot be reliably dated and therefore, cannot be used to investigate the nature of the LHB. They suggest that only the Imbrium and Orientale basins, which have their formation dates precisely known from rocks retrieved by Apollo missions, can be used to accurately describe the cratering history during this period.

With this assumption, Cuk’s group reexamined the frequency of crater sizes for just these basins. When this was plotted for these two groups, they found that the power law they used to fit the data had “an index of -1.9 or -2 rather than -1.2 or -1.3 (like the modern asteroid belt)”. As such, they claim, “theoretical models producing the lunar cataclysm by gravitational ejection of main-belt asteroids are seriously challenged.”

Although they call into question Strom et al.’s model, they cannot propose a new one. They suggest some causes that are unlikely, such as comets (which have too low of impact probabilities). One solution they mention is that the population of the asteroid belt has evolved since the LHB which would account for the differences. Regardless, they conclude that this question is more open ended than previously expected and that more work will need to be done to understand this cataclysm.

K-T Boundary

Chicxulub Crater

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What killed the dinosaurs? That’s a question that has puzzled paleontologists since dinosaurs were first discovered. Maybe the global climate changed, maybe they were killed by disease, volcanoes, or the rise of mammals. But in the last few decades, a new theory has arisen; an asteroid strike millions of years ago drastically changed the Earth’s environment. It was this event that pushed the dinosaurs over the edge into extinction. What’s the evidence for this asteroid impact? A thin dark line found in layers of sediment around the world; evidence that something devastating happened to the planet 65 million years ago. This line is known as the K-T boundary.

What is the K-T boundary? K is actually the traditional abbreviation for the Cretaceous period, and T is the abbreviation for the Tertiary period. So the K-T boundary is the point in between the Cretaceous and Tertiary periods. Geologists have dated this period to about 65.5 million years ago.

When physicist Luis Alvarez and geologist Walter Alvarez studied the K-T boundary around the world, they found that it had a much higher concentration of iridium than normal – between 30-130 times the amount of iridium you would expect. Iridium is rare on Earth because it sank down into the center of the planet as it formed, but iridium can still be found in large concentrations in asteroids. When they compared the concentrations of iridium in the K-T boundary, they found it matched the levels found in meteorites.

The researchers were even able to estimate what kind of asteroid must have impacted the Earth 65.5 million years ago to throw up such a consistent layer of debris around the entire planet. They estimated that the impactor must have been about 10 km in diameter, and release the energy equivalent of 100 trillion tons of TNT.

When that asteroid struck the Earth 65.5 million years ago, it destroyed a region thousands of kilometers across, but also threw up a dust cloud that obscured sunlight for years. That blocked photosynthesis in plants – the base of the food chain – and eventually starved out the dinosaurs.

Researchers now think that the asteroid strike that created the K-T boundary was probably the Chicxulub Crater. This is a massive impact crater buried under Chicxulub on the coast of Yucatan, Mexico. The crater measures 180 kilometers across, and occurred about 65 million years ago.

Geologists aren’t completely in agreement about the connection between the Chicxulub impact and the extinction of the dinosaurs. Some believe that other catastrophic events might have helped push the dinosaurs over the edge, such as massive volcanism, or a series of impact events.

We have written many articles about the K-T boundary for Universe Today. Here’s an article about how the dinosaurs probably weren’t wiped out by a single asteroid, and here’s an article about how asteroids and volcanoes might have done the trick.

Here’s more information from the USGS, and an article from NASA.

We have recorded an episode of Astronomy Cast all about asteroid impacts. Listen to it here: Episode 29: Asteroids Make Bad Neighbors.

Reference:
USGS