© Charles Chandler
The section on the Titius-Bode Law
mentioned the possibility that the Asteroid Belt between Mars & Jupiter is the debris from a celestial collision, with Ceres as the largest surviving remnant of a planet. The rest of the debris would have since fallen into the Sun, or impacted other planets & moons, perhaps in what is known as the Late Heavy Bombardment, thought to have occurred 4.1~3.8 billion years ago.
The impacts seem to have remelted the crusts of the Earth, the Moon, and of Mars. The latter two have roughly similar topographies, with highlands that are heavily cratered from the LHB, but also with vast expanses of flood basalts that are sparsely cratered. This means that the "flooding" occurred during the LHB. The mares are believed to have been formed by volcanic megaflows, but it would be an odd coincidence for both the Moon and Mars to undergo the same internal process at roughly the same time, despite their differences. And for this to have occurred by chance, just when both were being subjected to the same dramatic external influence, is too many coincidences — it's far more likely that the LHB caused the mares. The thermalization of the impacts would have provided a lot of heat. At the same time, the Sun might have been flaring violently from infalling debris, and thus projecting a lot more heat onto the surfaces of the planets and moons. This might have caused generalized melting, and the highlands might have been chunks of solids floating in seas of magma.
It's then no coincidence that the Earth's crust got remelted at the same time. If it got the same number and size of impacts per surface area as the Moon and Mars, the energy from the bombardment would have been in excess of 1029 joules. The only difference was that the remelting was more thorough. Rocks pre-dating the LHB have been found in the highlands on both the Moon and on Mars, but not on Earth.
If the Earth's crust was totally remelted, why didn't the continents slump more? The continents are made of granite, while the oceanic crust is basalt. Granite is lighter than basalt, so the continents "float" like icebergs in the basaltic sea. But if the continents were completely remelted by the LHB, they should have flowed out over the basalt, leaving a thin layer (~14.90 km deep) of granite wrapping all of the way around the world. It's possible that the crust didn't get quite hot enough to become that runny during the LHB.
But this reveals an even more fundamental question — why didn't the granite settle out long before the LHB, when the Earth was first forming? Pretty much all of the models of the Earth assume that when it first condensed, it was completely molten. It immediately began cooling by radiative heat loss (i.e., photons). The molten rock would have been good at conducting heat to the surface, but bad at radiating that heat into space, so it would have taken a long time for a crust to solidify. In the meantime, all of the lighter chemicals had plenty of time to bubble up to the surface. The granite would have quickly found its way to the top, and it would have leveled off into a relatively flat layer on top of the basalt. Later, when the surface temperature dropped below the boiling point of water, the steam in the atmosphere would have condensed, forming the oceans. But if the granite layer had been perfectly flat, the oceans should have covered the entire globe — no dry land. So how did the granite get consolidated into continents?
Comparison of the sizes of the Earth and the proposed granite impacter.
It's possible that the continental granite didn't get distributed during the molten stage, because the granite simply wasn't present yet — it might have arrived later, during the LHB
. In other words, the continents might be what's left of an asteroid. Heat from the impact would have remelted everything, but perhaps not so completely that the granite could pancake all of the way around the globe. Rather, it settled into the original supercontinent. The continental granites have a volume of 7.58 × 109
This is just 0.69% the volume of the Earth (1.08 × 1012
), while being 1/3 the volume of the Moon (2.20 × 1010
), and 20 times the volume of Ceres (4.21 × 108
). So the impacter would have been small by planetary standards, but larger than any extant asteroid. (See Figure 1
.) And this would explain the granular nature of the continental granites. Had they been molten for a long time and had they cooled slowly, they'd have the texture of the gabbros and cumulates in the oceanic crust. Instead, granites are more like shock crystals, with lots of chemical differentiation, but without the fractional crystallization that invariably occurs when magma cools slowly in the presence of gravity.
It's also possible that the Earth's oceans arrived at the same time, answering another long-standing question in geophysics. During the Earth's molten phase, water would have bubbled up to the surface even faster than granite. If the surface temperature was just 100 K greater than it is now, the water would have boiled off. This would have created a volume of water vapor 1000 times greater than all of the nitrogen & oxygen in the current atmosphere.2
With only 63% the mass of that N2
mix, the H2
O would have floated above the troposphere. In the stratosphere, the water vapor would have been atomized by UV radiation, reducing it to monoatomic oxygen & hydrogen. The oxygen would have been heavy enough to remain gravitationally bound to the Earth, but the hydrogen would have drifted off into space, leaving us with a lot more oxygen than nitrogen in the atmosphere, and no water. This begs the question of how the water stuck around long enough for the Earth to cool. One possibility is that it didn't — rather, it arrived later, after the crust had already cooled below 373 K. For this reason, scientists started looking for extra-terrestrial water sources, initially favoring comets. But the deuterium in cometary ices is a poor match to the Earth's water,3
while the ices found in asteroids are an excellent match.4
So asteroids are the likelier source of the Earth's water.
Getting asteroids to bring both
the granites & the water to Earth is not a more difficult order to fill — it's the easiest. C-type asteroids can contain up to 22% water,5
and 75% of all asteroids are this type,6:316-335
Besides the water, asteroids are 78+% rocky matter that can be shock melted to produce granites. The proposed impacter would have had a volume of 8.97 × 109
, with 85% granite and 15% water, which would be normal for an asteroid.
In conclusion, it seems that thinking of ourselves as Earthlings might be an oversimplification — the stuff of which we are made might have originally condensed into Ceres. An impact shattered our former planet 4.1 billion years ago. But a big chunk of debris found a new host here on Earth. The granite created an island in the middle of its own meltwater. The shallow seas around the island provided a breeding ground for new life forms, who were our ancestors. So while life as we know it couldn't have developed on Ceres, because all of the water was frozen, and while the Earth didn't have any water of its own, because it had all boiled off, a big chunk of ice transported from Ceres to Earth made liquid oceans possible. And the donation of a large amount of granite made dry land and shallow seas possible. Therein, life found a way.
1. Schubert, G.; Sandwell, D. (1989): Crustal volumes of the continents and of oceanic and continental submarine plateaus. Earth and Planetary Science Letters, 92: 234-246 ⇧
2. Perlman, H. (2016): How Much Water is on and in the Earth. USGS Water Science School ⇧
3. Altwegg, K. et al. (2015): 67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio. Science, 347 (6220): 1261952 ⇧
4. Morbidelli, A. et al. (2000): Source regions and time scales for the delivery of water to Earth. Meteoritics and Planetary Science, 35: 1309-1320 ⇧
5. Shaw, S. (2012): C-Type Asteroids. Astronomy Source ⇧
6. Binzel, R. P.; Gehrels, T.; Matthews, M. S. (ed.) (1989): Asteroids II. Tucson: University of Arizona Press ⇧
7. Carey, B. (2005): Largest Asteroid Might Contain More Fresh Water than Earth. space.com ⇧