Asteroids from a Martian Mega Impact

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Like evidence left at a crime scene, the mineral olivine may be the clue that helps scientists piece together Mars’s possibly violent history. Could a long-ago giant impact have flung pieces of Mars throughout our inner solar system? Two researchers from the Tokyo Institute of Technology in Japan are on the case.

A Telltale Mineral

olivine

Olivine, a mineral that is common in Earth’s subsurface but weathers quickly on the surface. Olivine is a major component of Mars’s upper mantle. [Wilson44691]

Olivine is a major component of the Martian upper mantle, making up 60% of this region by weight. Intriguingly, olivine turns up in other places in our solar system too — for instance, in seven out of the nine known Mars Trojans (a group of asteroids of unknown origin that share Mars’s orbit), and in the rare A-type asteroids orbiting in the main asteroid belt.

How did these asteroids form, and why are they so olivine-rich? An interesting explanation has been postulated: perhaps this olivine all came from the same place — Mars — as the result of a mega impact billions of years ago.

Evidence for Impact

Mars bears plenty of signs pointing to a giant impact in its past. The northern and sourthern hemispheres of Mars look very different, a phenomenon referred to as the Mars hemisphere dichotomy. The impact of a Pluto-sized body could explain the smooth Borealis Basin that covers the northern 40% of Mars’s surface. 

Mars topography

This high-resolution topographic map of Mars reveals the dichotomy between its northern and sourthern hemispheres. The smooth region in the northern hemisphere, the Borealis basin, may have been formed when a giant object impacted Mars billions of years ago. [NASA/JPL/USGS]

Other evidence piles up: Mars’s orbit location, its rotation speed, the presence of its two moons — all could be neatly explained by a large impact around 4 billion years ago. Could such an impact have also strewn debris from Mars’s mantle across the solar system?

To test this theory, we need to determine if a mega impact is capable of producing enough ejecta — and with the appropriate compositions and orbits — to explain the Mars trojans and the A-type asteroids we observe. Tackling this problem, researchers Ryuki Hyodo and Hidenori Genda have performed numerical simulations to explore the ejecta from such a collision.

Distributing Debris

Hyodo and Genda examine the outcomes of a Mars mega impact using smoothed particle hydrodynamics simulations. They test different impactor masses, impactor speeds, angles of impact, and more to determine how these properties affect the properties of the Martian ejecta that result.

Mars mega-impact ejecta

Debris ejected in a Mars mega impact, at 20 hours post-impact. Blue particles are from the impactor, red particles are from Mars, yellow particles are clumps of >10 particles. [Hyodo & Genda 2018]

The authors find that a large amount of debris can be ejected from Mars during such an impact and distributed between ~0.5–3 AU in the solar system. Roughly 2% of this debris could originate from Mars’s olivine-rich, unmelted upper mantle — which could indeed be the source of the olivine-rich Mars Trojan asteroids and rare A-type asteroids.

How can we further explore this picture? Debris from a Mars mega impact would not just have been the source of new asteroids; the debris likely also collided with pre-existing asteroids — or even transferred to early Earth. Signatures of a Mars mega impact may therefore be recorded in main-belt asteroids or in meteorites found on Earth, providing tantalizing targets for future studies in the effort to map out Mars’s past.

Citation

Ryuki Hyodo and Hidenori Genda 2018 ApJL 856 L36. doi:10.3847/2041-8213/aab7f0

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