A Chaotic Early Stage Of Evolution

Scientists broadly agree on the necessity of a chaotic early stage of evolution of the giant planets to explain the overall architecture of the current Solar System. What remains intensely debated are details such as the number and trigger of heavy bombardments, and most importantly, their timing. A later instability, however, could have had potentially devastating effects on the newly formed Earth. The effects of the timing of the heavy bombardment also extend to other terrestrial planets. For instance, scientists think that Mars formed much earlier than the Earth, within only a few million years. As a consequence, Mars could have been pummeled by even very early heavy bombardments. Narrowing down the timing of the heavy bombardment phases is key to studying these early collisions, but it poses a formidable challenge to scientists. The events we are picturing here took place billions of years ago. The orbital architecture of the Solar System provides some evidence for at least one phase of major reshuffling, but limited information in regard to the timing. Furthermore, the dynamical evolution of the early Solar System is not well understood, leaving the possibility for a wide range of outcomes. It would not be possible, without information from witnesses, to reconstruct the details of the crash, such as the incoming directions of the cars and their speeds. The same is true for planetary migration, because the dynamical evolution described above is chaotic.

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Scientists refer to chaotic evolution not to indicate lack of order, but rather to convey that the orbital evolution of a system of planets is highly dependent on the initial conditions, and cannot be precisely computed, not even with the most powerful computers. Astronomers use complex numerical tools to track the future orbital evolution of the planets, under the influence of the Sun’s gravity and that of all the other planets. What they find is intriguing. The forward evolution shows rapid modifications to the shape and inclination with respect to the ecliptic of the orbits of the terrestrial planets. The shape is measured by the eccentricity of the orbital ellipse, how elongated it is, or to be precise, the ratio of the distance between the two foci and the major axis. For the Earth and Venus, these variations are relatively small, of the order of 20 percent at most. The eccentricity of the Earth’s orbit is about 0.017, and it can oscillate between about 0.013 and 0.021, over a timescale of the order of 10 million years. The orbits of Mercury and Mars, by contrast, can show drastic swings, up to 70 percent or more of their current eccentricity and inclination. In extreme cases, Mercury’s orbit could expand to bring it in close proximity with Venus.

The Circle Game

When this happens in the models, either Mercury collides with Venus, or it is ejected from the Solar System. A similar evolution is also expected for Mars. They may inspire science fiction, but such computations do hold a critical piece of information. While these simulations concern the future evolution of the terrestrial planets, similar arguments hold true for their past. Chaos makes it impossible to reconstruct precisely the past orbits of the planets, including the extent of migration and the timing of the instabilities. Scientists are left struggling to find additional data in order to narrow down the range of possibilities. For this, they literally turn their attention from the architecture of the heavens down to the ground beneath their feet, looking for old rocks which may have witnessed the clash of the giant planets. Rocks can hold a detailed record of how they formed and the conditions they have experienced. Particular minerals form within certain ranges of temperature and pressure. Their atomic lattices can contain or trap elements which may provide further insights on their formation, including timing. How can a rock be dated? Imagine a bucket in the rain. One could estimate the time elapsed since the bucket was exposed in the rain by simply measuring the amount of water caught.

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In fact, the water mass is proportional to time lapse multiplied by the number of raindrops in the unit time, assuming steady rainfall. The same concept applies to rocks, but scientists count atoms instead of raindrops. Atoms of a particular chemical element, defined by the number of protons, can vary in the number of their neutrons, resulting in different isotopes. This happens in a timescale just shy of 4.5 billion years. So measuring the relative proportion of 206Pb to 238U accumulated in a mineral provides us with a measure of the time elapsed since the mineral was formed. Different minerals can host different and multiple atomic clocks, which could provide useful constraints to pin down early Solar System evolution. But we need to find rocks that are likely to be contemporaneous with or predate the rearrangement of planets in the early Solar System. The Earth would be the ideal place to look, given the easy accessibility and abundance of samples. Luckily for us, the nearest celestial object, our Moon, provides an excellent starting point in the quest for rocks 4 billion years old and more. Some surmised the Moon might have been intensely active geologically, with volcanoes spouting large volumes of lava to produce the darker terrains. The most fervent supporters of an active Moon even suggested the Moon could be volcanically active now. Others were wilder in their speculations, and postulated the past existence of copious lunar water. Much less contentious was the observation that the lunar surface is covered in countless cavities, as they were called by Galileo. If these cavities were produced by impacts, as surmised by Gilbert in 1892, could they then inform us about ancient heavy bombardments? What would have appeared a fantastic dream to these early lunar observers became a tangible reality from the late 1950s, with the advent of the space exploration era. The exploration of the Moon was an astonishing feat of engineering fueled by the rivalry of two political superpowers. Science did not play any significant role in deciding these events, but surely benefited greatly from them. This bounty generated detailed scientific investigations and debates worldwide.