Saturday, July 28, 2012

A new 'hit-and-run' Giant Impact scenario

Figure 1a: Five snapshots from the 30° impact angle and 1.30vesc impact velocity case (cC06) showing cuts through the impact plane. Colour coded is the type and origin of the material. Dark and light blue indicate target and impactor iron; Red and orange show corresponding silicate material. The far right shows the situation at the time of impact. At 0.52h, it can be seen how the impactor ploughs deep through the targets mantle and pushes considerable amount of target material into orbit. A spiral arm of material forms and gravitationally collapses into fragments. The outer portions of the arm mainly consist of impactor silicates and escapes due to having retained a velocity well above escape velocity. The silicate fragments further inward are stronger decelerated and enter eccentric orbits around the target. The impactor's iron core also looses much of its angular momentum to the outer parts of the spiral arm and re-impacts the proto-Earth. -  Figure 1b: The origin of the disk material highlighted, half a collisional timescale ( (Rimp + Rtar) / vimp ) after impact. In the grazing reference case (cA08), the majority of the proto-lunar disk originates from a spill-over of the impactor. In the head-on cases (cC01, fB06, iA10), much more material comes from the target mantle, being pushed out into orbit by the impactor core. Colours are identical to figure 1. Turquoise on the right shows water ice for the icy impactor case iA10.
Reufer, Meier, Benz & Wieler
Universität Bern
Eidgenössische Technische Hochschule Zürich
Lund University, Sölvegatan

The formation of the Moon from the debris of a slow and grazing giant impact of a Mars-sized impactor on the proto-Earth (Cameron & Ward 1976, Canup & Asphaug 2001) is widely accepted today. We present an alternative scenario with a hit-and-run collision (Asphaug 2010) with a fractionally increased impact velocity and a steeper impact angle.

Hydrodynamical simulations have identified a slow, grazing impact in being able to reproduce the Moon's iron deficiency and the angular momentum of the Earth-Moon-system. But in this canonical scenario, the Moon forms predominantly from impactor material, thus contradicting the Moon's close geochemical similarity to Earth. Furthermore, due to the slow impact velocity, only limited heat input is provided for the aftermath of the collision. Post-impact mechanisms (Pahlevan & Stevenson 2007) required to match the impact scenario with the compositional observations, depend on the thermal conditions in the post-impact debris disk. We show that a new class of hit and-run collisions with higher impact velocities and a steeper impact angles is also capable of forming a post-impact debris disk from which the Earth's Moon can later form, but leads to a much hotter post-impact debris disk. Furthermore, the ratio of target body material in the debris disk is considerably larger, compared to the canonical scenario. This new class of impacts was previously rejected due to the limited resolutions 26 of early simulations (Benz 1989).

View the (pdf) Icaris manuscript, at arXiv 1207.5224
Figure 2: Comparing post-impact temperatures of the proto-Earth between the grazing reference simulation left (cA08) and the head-on case on the right (cC06). Color coded is temperature in K in logged scale. The initial average temperature before the impact inside the target mantle is ~2000K.

"While the Moon has an iron core like Earth, it does not have the same fraction of iron - and computer models supporting the Theia impact idea show just the same thing

"However, the ratio of the Earth's and the Moon's oxygen isotopes is nearly identical, and not all scientists agree on how that may have come about.

"Confounding the issue further, scientists reporting in Nature Geoscience in March said that a fresh analysis of lunar samples taken by the Apollo missions showed that the Moon and the Earth shared an uncannily similar isotope ratio of the metal titanium."

Moon formation: Was it a 'hit and run' accident?
BBC News, Science & Environment, July 27, 2012

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