What Evidence Do We Have for the Late Heavy Bombardment Period?
The Conventional Theory And Clues For It
When the solar system formed, it was a swirling disc full of debris that slowly grew into planetesimals, or what we can consider to be planet building blocks. About 4.6 billion years ago, those components began to glob together and form the planets, with one in particular called Theia impacting with us and eventually forming the moon. As the years rolled by, the number of planetesimals dwindled until none were left as they either merged together or were destroyed through impacts. Thus, even hits from objects in space started to dwindle as well. The LHBP is frequently seen as the last major upheaval in the solar system before everything settled down (more or less) after this settling down (Kruesi “When” 32).
The conventional idea is that the LHBP occurred 4.1 to 3.8 billion years ago. Much of the evidence for this comes from our celestial neighbor the moon. Why? Because its surface is like a cassette recorder. Anything that happens to it is preserved on its surface, while Earth has plate tectonics and erosion wiping away evidence of past events. By looking at the craters on the moon we can get an idea of the size and angle of impact. Looking at argon-40/argon-39 radioactive levels from moon rocks brought back by Apollo missions in the areas around the impacts, it indicated the time frame mentioned above, placing the LHBP as a post-lunar formation event. At the time of this conclusion, in 1974, the idea of the LHBP was not popular. Scientists argued that the team behind the study (Fouad Tera, Dimitri Papanastassiou, and Gerald Wasserberg) did not collect a diverse enough sample size to draw accurate conclusions. After all, what if their rocks all came from just one event? Lunar rocks brought back by Apollo astronauts come from areas of the moon that total just 4% of the total surface area, hardly a fair sampling. It was later shown that new impactors and lunar magnetism could also skew the argon readings, making them an unreliable dating gauge. More rocks from different areas would lead to better results. And after looking at known moon rocks that have fallen to Earth, they are all in the required timeframe for the LHBP and relatively agree with each other (Kruesi “When” 32-3, Packham).
As for the actual object that is colliding to form the crater, it is vaporized upon impact because of the energies involved. The vapor that results does condense into what we call sphericules, which fall back to the surface much like precipitation. They are usually in the millimeter to centimeter size range and can tell us details about the composition and violence of the impactor (Kruesi “A Longer”).
In fact, Earth has layers of sphericule that have become trapped in rock layers. Using geological dating techniques, we have found that the 14 known boundary layers have different subgroups. 4 of them are from 3.47-3.24 billion years ago, 7 are from 2.63-2.46 billion years ago, 1 is from 1.85 billion years ago, and 2 are rather recent, with one of them being the KT boundary aka the event that wiped out the dinosaurs (Kruesi “A Longer”).
The moon itself shows evidence all over its beaten surface for the LHBP. Surface studies show that the crust is fragmented - heavily - to the point that it allowed for an easier flow of magma to fill in certain craters that we see today. Gravity readings from the GRAIL probe showed this fracturing after surface anomalies were subtracted from the data and the trends of the patterns mime that of surface impacts seen. The grouping had to be close on a timescale to bring about the effects seen, hinting at a period of heavy bombardment (MIT).
Mainstream Ideas Overturned
It was during an analysis of these boundaries that Jay Melosh and Brandon Johnson (both from Purdue University) found some new clues that may revise out ideas behind the LHBP. In an April 25, 2012 issue of Science, they found that based on the size of other boundary layers, the LHBP likely caused the 1.85 billion-year boundary layer. They determined this by comparing the sphericules and noted that those from this layer resulted from massive impacts. This places the LHBP way later than previously thought (Ibid).
But it gets even better, folks. A separate study by William Bottke (from the Southwest Research institute in Boulder, Colorado) looked into why the LHBP was so long in the first place. When looking at the likely impactors, they seem to originate from a zone in the inner asteroid belt which no longer exists. According to the Nice Model, this is because an orbital shift between Uranus and Neptune caused objects to be thrown about. Using this model, it not only caused outer solar system objects to be thrown in but also inner ones as well, accounting for the missing impactors and also giving the LHBP a longer timeframe (Kruesi “A Longer,” Kruesi “When” 33, Choi).
Choi, Charles Q. “Asteroids Battered Young Earth Longer Than Thought.” Space.com. Purch, 25 Apr. 2012. Web. 16 Nov. 2016.
Kruesi, Liz. “A Longer Late Heavy Bombardment?” Astronomy Aug. 2012. Print.
---. “When Earth Felt Cosmic Rain.” Astronomy Nov. 2012: 32-3. Print.”
MIT. "Study finds barrage of small asteroids shattered Moon's upper crust." Astronomy.com. Kalmbach Publishing Co., 14 Sept. 2015. Web. 04 Sept. 2018.
Packham, Christopher. “Researchers Question Apollo-Era Evidence for the Late Heavy Bombardment.” Phys.org. ScienceX Network, 04 Oct. 2016. Web. 14 Nov. 2016.
© 2017 Leonard Kelley