A Hell Of A Time During The Hadean

Back at the beginning of Earth's existence there was a time known as the Hadean Eon—Hadean as in Hades, or hell. The history of the Hadean Earth (~4.0–4.5 billion years ago) is poorly understood because rocks from that time have not survived. The oldest known rocks are little older than ~3.8 billion years. A new paper in the journal Nature attempts to shed some new light on the least known part of Earth's distant past. Researchers have long speculated about the conditions on Earth in the first 500 million years after the planet's formation, some 4.5 billion years ago. The researchers report that, according to their model, the early Earth is likely to have been hit by up to four asteroids, each capable of snuffing out fledgling life and completely resurfacing the planet.

The Hadean is the first geologic eon of Earth and lies before the Archean. It began with the formation of the Earth roughly 4.5 billion years ago and ended as defined by the ICS (International Commission on Stratigraphy) 4,000 million years ago. Impact scars on Mars, the Moon and Mercury suggest that the entire inner Solar System was bombarded by asteroids during that time. But melting and reforming of Earth's crust has wiped clean any sign of early impacts, making it difficult to work out how often asteroids hit or how big they were. In the somewhat innocuously titled “Widespread mixing and burial of Earth’s Hadean crust by asteroid impacts,” Simone Marchi et al. used the Moon's crater record to develop a computer model of Earth's bombardment history. Using a Monte Carlo simulation they created a reconstruction of collisions between the infant Earth and the assorted junk swirling around the Solar System more than 4 billion years ago. They repeated the process ~5,000 times and used the results to provide new insights into the chaotic conditions present on the rock that was to become our home. More details are given in the paper:

Here we assess the early Earth’s impact history by rescaling a recent estimate of the lunar impact flux4 to Earth. The advantages of this approach are numerous. First, the Moon provides a much clearer record of the early impact history of the Earth–Moon system8. Moreover, the lunar cratering record provides an absolute impactor flux that is independent of assumptions made by terrestrial planet formation models. The rescaling was done transforming lunar craters into a projectile flux, with the flux used to estimate the number of terrestrial impactors taking place in intervals of 25 Myr between 3.5 and 4.5 Gyr ago. For the purpose of our work we assume that the Moon-forming impact was at ~4.5 Gyr ago, but our results are insensitive to the exact timing. The sizes of the projectiles were randomly drawn from an assumed impactor size–frequency distribution (SFD) with a shape similar to that of large main-belt asteroids. The cut-off of this population was varied for different time intervals. Impactors striking after ~4.15 Gyr ago, the putative starting time of the late heavy bombardment (LHB) were given a maximum cut-off of 1,000 km, roughly corresponding to the largest present-day asteroid Ceres. The impactor SFD before ~4.15 Gyr ago is assumed to have larger left-over planetesimals. We assumed a similar impactor SFD with a cut-off threshold at 4,000 km, which was extrapolated from the SFD of inner and central main-belt asteroids ranging between a few hundred kilometres to 1,000 km

The new terrestrial bombardment model reveals the role of impacts on the geological evolution of the Hadean Earth, in particular the mixing, burial and melting of the uppermost layers. “To model these effects quantitatively, we performed a suite of impact simulations with the Simplified Arbitrary Lagrangian Eulerian shock-physics code (iSALE), and computed the resulting excavation cavity size, excavation volume and depth, and the volume of target melt,” the author's state. “We varied the target temperatures and considered impactor diameters ranging from 15 to 4,000 km with a range of impact velocities. ”

Reportedly, a key process is the impact-generated mixing as a result of the excavation and collapse of large transient cavities in the lithosphere. Before ~4.4 Gyr ago, up to 60–70% of the Earth’s surface was reworked to a median depth of 20 km. This is illustrated in the figure below, taken from the paper. The four upper maps are projections of the cumulative record of craters at different times. Each circle indicates the final crater size estimated from the transient cavity size from our simulations and a conservative estimate for the transient-to-final crater size scaling. The four lower maps include melt extrusion on the surface.

The main physical data we have from this era come from ancient submillimetre zircon grains. Some of these zircons, from western Australia, date back 4.2–4.3 billion years, a time when both Earth and the Moon were being pummelled by wave after wave of extraterrestrial bodies. “We find that the surface of the Hadean Earth was widely reprocessed by impacts through mixing and burial by impact-generated melt,” the paper states. “This model may explain the age distribution of Hadean zircons and the absence of early terrestrial rocks.” This implies that the new results are consistent with known physical evidence, always a good thing for a new theory.

Marchi et al. were able to estimate the total volume of asteroids that hit Earth and the Moon using measurements of the abundance of elements such as gold and osmium in their crusts. If these relatively heavy elements had been present when the Moon was violently separated from Earth 4.5 billion years ago, they would have sunk to respective cores of the surviving planet and moon. This means that any traces of these elements in today's crusts and mantles must have been delivered by asteroid impacts more recently. By comparing the relative abundances of the two elements in Earth and Moon rocks, the team was able to estimate the number of very large impacts that might have hit Earth.

Statistically speaking, the model suggests that the biggest asteroids to hit Earth would have been as large as 3,000 kilometers across. Between one and four would have been 1,000 kilometers wide or larger, it predicts, with a total of three to seven exceeding 500 kilometers in width. The most recent of these would have hit around 4.2–4.3 billion years ago.

The Hadean was a little bit of hell on Earth.

As reported in an accompanying article, John Valley, a geochemist at the University of Wisconsin–Madison, says that the study does not prove that Earth was uninhabitable 4.3 billion years ago. He points out that the effects of each impact would have been short-lived, leaving habitable spells between blasts, and that the impacts would not necessarily have melted the entire surface of the planet. Life could have started multiple times, or persisted in rocks more than 1 kilometer below the oceans, he adds.

The bottom line on this new simulation is that life, in its earliest incarnations, may have been killed off as many as four times during the Hadean. Existing oceans would have repeatedly boiled away into steam as a result of large collisions as late as about 4 billion years ago (now that would be real global warming). So now, when asked when life on Earth began, the proper reply is to ask “which time?” Perhaps more troubling is to ask if this time, our time, is the last time life had to start anew on planet Earth?

It is well known that there have been several large extinction events since life drug itself out of the oceans and laid claim to Earth's continents. Best known, of course, is the death of the dinosaurs 66 million years ago—a calamity for the dinosaurs but a happy occurrence for us mammals. Another new study suggests that timing is everything when it comes to extinction causing asteroid collisions.

Stephen Brusatte, a palaeontologist at the University of Edinburgh, and his colleagues describe a new take on the famous extinction in Biological Reviews. They claim that, just before a large asteroid slammed into the Earth 66 million years ago, the diversity of plant-eating dinosaur species declined slightly. That minor shift may have been enough to doom all dinosaurs when the bolide hit. The already stressed populations of plant eaters died off. No plant eaters, no meat eaters, no dinosaurs.

A bit of global cooling and an asteroid, and it's no more dinosaurs.

Bad luck, old boy. Though scientists say that it was definitely the asteroid that finished off the dinosaurs it was a colder shift in climate that made them vulnerable. That's right, without climate change—in this case global cooling—we might have dinosaurs walking the planet today, and humanity may not have evolved. “The asteroid hit at a particularly bad time,” says Brusatte, lead author of the paper. “If it had hit a few million years earlier or later, dinosaurs probably would have been much better equipped to survive.”

Of course, we mammals didn't let a good extinction event go to waste. Just remember that it was a combination of a colder climate and cosmic bad luck that did the dinosaurs in, giving us a shot at the ecological big time. When someone starts blathering on about climate change you should also remember that the Pleistocene Ice Age is an anomalously cold time for our planet. If anything, a bit of global warming would probably improve conditions for life here on Earth. What should we do? Keep an eye out for asteroids and think warm thoughts.

Be safe, enjoy the interglacial and stay skeptical.