Impacts and Planetary Evolution

14 April 2010

The Destructive Aspect of Impacts

The fossil records suggest the occurrence of a number of mass extinction events during the Phanerozoic eon (starting ~ 550 Mill. years ago) of which the most profound occurred some 250 Mill. years ago and is related to the P/Tr boundary. Although the trigger of these extinctions is a subject of debate, they are most probably due to large impacts, volcanic eruptions, or a combination of both. We concentrate our investigations on impacts. Depending on the size of the corresponding asteroid or comet, impacts are able to destroy large parts of an evolved biosphere, or can even sterilize the whole planet. An example is the impact of a ~10 km body which formed the Chicxulub crater in Mexico and likely caused the K/T (Cretaceous-Tertiary) mass extinction 65 million years ago. Complex hydrodynamic simulations of impact events and shock wave experiments play an important role in verifying the credibility of impact events as causes of mass extinctions..

The Constructive Aspect of Impacts

The formation of life requires the existence of water, chemical compounds containing at least carbon and nitrogen (CN-compounds), and the availability of energy. If a planet has been formed close to its star, it mainly consists of refractory material and water and CN-compounds are largely depleted on the planet’s surface. However, after its formation the planetary mantle contained volatile substances which could have degassed. The energy for an efficient degassing originated from

  1. the accretion of planetesimal bodies and planet embryos,
  2. the decay of relatively short-lived radioactive elements (such as 26Al),
  3. the segregation of the Earth core, and
  4. the Moon-forming impact with a Mars-sized planetary embryo.

In order to understand the significance of impacts as a constructive phenomenon we look to the earliest history of the solar nebula (~ 4.6 billion years ago). The terrestrial planets formed in two accretion phases. During the first accretion phase, between 40 and 60 Million years ago, anembryonic proto-Earth was produced from planetesimals(*) located in the close vicinity of the Earth’s orbit. The planetesimals could have been as large as the Moon. During the late accretion phase, between 30 and 50 million years ago, large planetary bodies originating in the outer main asteroid belt were incorporated into the proto-Earth,bringing with them water and organic substances that enriched the mantle of the Earth with volatile matter.

Due to the high impact energy of the planetesimals and planetary embryos the Earth was formed as a fireball with a magma layer. The heavy elements (mainly iron and nickel) accumulated in the centre of the Earth and formed its core,transforming gravitational energy to thermal energy and causing a further heating of the planet. At the end of the late accretion phase the Moon-forming impact with a Mars-sized body occurred. On the Earth’s surface a deep magma ocean of about 100-500 km formed. The outgassing of this ocean produced a nearly neutral atmosphere consisting of H2O vapour and CO2. In the course of a few million years the magma ocean cooled and a planetary crust was formed. Subsequently, the atmospheric water vapour condensed and a shallow water ocean covered the crust. The blanketing effect of the CO2, an atmospheric greenhouse gas, inhibited a strong cooling of the planet’s surface.

Thus two of the main important preconditions for the formation of life were fulfilled: (i) the existence of water and (ii) the availability of sufficient energy. However, organic matter, especially with CN compounds, in an adequate concentration were probably missing at that time. The vital organic ingredients may have been delivered during a heavy bombardment by asteroids and comets around 3.9 billion years ago (the so-called Late Heavy Bombardment), evidence for which is found in the lunar cratering record. At that time a huge number of celestial bodies (about 1021 kg material) impacted on the Earth. A fraction of the impactors contained water and organic material, which was evaporated, mixed with dust on the Earth’s surface and flung into the atmosphere. It can be assumed that a complex dust chemistry evolved and produced prebiotic molecules necessary for the formation of life.

Mathematical simulations and experiments

In the Department of Asteroids and Comets mathematical simulations of the hydrodynamic and chemical processes in the solar nebula are carried out to explore the possible composition of planetesimals and estimate the amount of volatile material within them. In addition, impact simulations are performed in order to calculate the different energy input fractions into the planetary atmosphere, the planetary surface and the impactor.

The energy of an impact is dissipated in the formation of a vapour plume and acrater (e.g. the Barringer Crater in Arizona). The vapour plume together with the ejected dust can cause large scale changes in the environmental conditions on the planet. A combination of the simulated impact data with atmospheric dynamic and chemical modelling is used to simulate the plume and its evolution. In this way both destructive and constructive facets of impacts can be quantitatively analysed. On the one hand, the environmental changes may cause a mass extinction if the impactor was large enough and a biosphere has already formed, e.g. due to the formation of an aerosol layer in the stratosphere which blocks the sunlight. On the other hand, the effects could have improved the conditions for the formation of complex organic molecules during the Late Heavy Bombardment as described in the previous section.

In addition to mathematical models the results of shock experiments are studied. Rocks and minerals suffering an impact experience pressures between 5 to 50 GPa. Experiments are performed at the Ernst-Mach Institut für Kurzzeitdynamik in Freiburg to understand the relation between pressure and temperature in shocked material. Furthermore, the influence of impacts on organic matter (e.g. polycyclic aromatic carbohydrates or fullerenes) and micro-organisms is investigated.

The combination of mathematical simulations and experimental results is a powerful approach to a greater understanding of the significance of impacts for the formation and evolution of a planetary biosphere.

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