The planetary formation of our solar system hasn't always appeared as it does today. Like most planetary systems, our gas giants formed closer to the Sun, but their orbits grew further from away over time. By studying our moon and samples from it, scientists have discovered evidence of The Late Bombardment theory where the gas giants' travel disturbed asteroid orbits and caused them to repeatedly strike the Earth and Moon.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
In this episode, we're going to talk about history, namely the early history of our inner solar system as revealed by samples from our moon.
Let's begin by recapping that early history.
Evidence suggests our Earth formed 4.6 billion years ago, taking less than 100 million years to go from a portion of the gas nebula surrounding our early sun to a planet pretty similar in size and composition to what we have today. Not long after, the Earth was hit by a giant Mars-sized object — now named Theia — with the debris forming our moon. So by 4.5 billion years ago, the Earth-Moon system was pretty much set, but in the outer solar system, planets were still moving around.
The gas planets were closer in than they are today, but moving outward. Jupiter at about 4 AU, — or 4 times the current Earth-Sun distance — Saturn at about 5.5 AU, and Uranus at about 7 AU. As they neared their current orbits — just under 4 billion years ago — their gravities disrupted the orbits of many asteroids near our current asteroid belt and comets from the outer solar system.
In a theory called the Late Heavy Bombardment, that disruption was a period where the Earth and Moon were pummeled by asteroids and comets, delivering some fraction of our water, and perhaps even prebiotic materials. In the Late Heavy Bombardment theory, this pummeling lasted less than 200 million years — a very brief period given the age of our solar system.
So how did this theory come about?
To begin, it's important to remember how our solar system formed. Our early, early solar system was a disk of hot gas around our proto-sun, itself enshrouded in a cocoon of gas. As the gas cooled, dust condensed out, then rocks, then boulders, then planetesimals and finally planets. Naturally, during the stage where trillions upon trillions of rocks and icy rocks formed, not all of those would later be incorporated into planets. So you would expect the solar system to have huge amounts of asteroids and comets, but of course every time one of those impacted a planet, there was one less floating around. And while there are still thousands of asteroids in the inner solar system around and in-between planets, it's expected that most would be pretty quickly swept up or ejected by gravity in those early days. The same would be true in the outer solar system between Jupiter and Neptune.
Beyond Neptune, where no major planets existed, there are billions upon billions of asteroids and comets — extending roughly 25% of the distance to our neighboring stars. However, early Neptune was likely at the same distance as today's Saturn. So as it moved roughly three times farther away, it scattered thousands to millions of those asteroids and comets, with many coming to the inner solar system. The late heavy bombardment theory suggests that scattering happened about half a billion years after the Earth and Moon completed forming.
Why would that be?
The answer is based on evidence, and because our Earth's surface is so dynamic and active, that evidence comes from the Moon. Once our Moon solidified, very little occurred to change its surface other than craters caused by impacts. If you look at our Moon, the dark regions — or mare — tend to be circular, and that's because the lava filled in massive craters. Those impacts also threw up rocks into space, and we have nearly 800 meteorites originating from our Moon. But we also have several hundred pounds of rocks and dust returned by our Apollo astronauts early in the 70s, and examination of those rocks found that most of the large impacts occurred during a short period.
Thus, the late heavy bombardment theory.
In 2024, China's Chang'e-6 mission landed on the far side of the Moon and returned about four pounds of material for study. The advantage of the Moon's far side is that it lacks the later volcanic activity, the darker regions we see on our side of the Moon.
In an article published in Science Advances by Dr. Wang-feng Zhang and collaborators of the Chinese Academy of Sciences, they find a much longer, smoother period of early cratering, roughly in agreement with lunar meteorites. This suggests not an intense period of bombardment, but as the gas planets migrated outwards, they continually disrupted asteroid orbits, sending a stream rather than a burst of objects into our region of the solar system.
Further investigations using materials from the Moon's backside rather than our near side could lead to a better understanding of our solar system and how materials were delivered to our Earth.