Ancient, massive asteroid impact could explain Martian geological mysteries

July 18, 2017 by Ula Chrobak, University of Colorado at Boulder
Credit: University of Colorado at Boulder

The origin and nature of Mars is mysterious. It has geologically distinct hemispheres, with smooth lowlands in the north and cratered, high-elevation terrain in the south. The red planet also has two small oddly-shaped oblong moons and a composition that sets it apart from that of the Earth.

New research by University of Colorado Boulder professor Stephen Mojzsis outlines a likely cause for these mysterious features of Mars: a colossal impact with a large asteroid early in the planet's history. This asteroid - about the size of Ceres, one of the largest asteroids in the Solar System - smashed into Mars, ripped off a chunk of the northern hemisphere and left behind a legacy of metallic elements in the planet's interior. The crash also created a ring of rocky debris around Mars that may have later clumped together to form its moons, Phobos and Deimos.

The study appeared online in the journal Geophysical Research Letters, a publication of the American Geophysical Union, in June.

"We showed in this paper—that from dynamics and from geochemistry—that we could explain these three unique features of Mars," said Mojzsis, a professor in CU Boulder's Department of Geological Sciences. "This solution is elegant, in the sense that it solves three interesting and outstanding problems about how Mars came to be."

Astronomers have long wondered about these features. Over 30 years ago, scientists proposed a large asteroid impact to explain the disparate elevations of Mars' northern and southern hemispheres; the theory became known as the "single impact hypothesis." Other scientists have suggested that erosion, plate tectonics or ancient oceans could have sculpted the distinct landscapes. Support for the single impact hypothesis has grown in recent years, supported by computer simulations of giant impacts.

Mojzsis thought that by studying Mars' metallic element inventory, he might be able to better understand its mysteries. He teamed up with Ramon Brasser, an astronomer at the Earth-Life Science Institute at the Tokyo Institute of Technology in Japan, to dig in.

The team studied samples from Martian meteorites and realized that an overabundance of rare metals—such as platinum, osmium and iridium—in the planet's mantle required an explanation. Such elements are normally captured in the metallic cores of rocky worlds, and their existence hinted that Mars had been pelted by asteroids throughout its early history. By modeling how a large object such as an asteroid would have left behind such elements, Mojzsis and Brasser explored the likelihood that a colossal impact could account for this metal inventory.

A global false-color topographic view of Mars from the Mars Orbiter Laser Altimeter (MOLA) experiment. The spatial resolution is about 15 kilometers at the equator and less at higher latitudes, with a vertical accuracy of less than 5 meters. The figure illustrates topographic features associated with resurfacing of the northern hemisphere lowlands in the vicinity of the Utopia impact basin (at the near-center of the image in blue). Credit: MOLA Science Team

The two scientists first estimated the amount of these elements from Martian meteorites, and deduced that the metals account for about 0.8 percent of Mars' mass. Then, they used impact simulations with different-sized asteroids striking Mars to see which size asteroid accumulated the metals at the rate they expected in the early solar system.

Based on their analysis, Mars' metals are best explained by a massive meteorite collision about 4.43 billion years ago, followed by a long history of smaller impacts. In their computer simulations, an impact by an asteroid at least 1,200 kilometers (745 miles) across was needed to deposit enough of the elements. An impact of this size also could have wildly changed the crust of Mars, creating its distinctive hemispheres.

In fact, Mojzsis said, the crust of the appears to be somewhat younger than the ancient southern highlands, which would agree with their findings.

"The surprising part is how well it fit into our understanding of the dynamics of planet formation," said Mojzsis, referring to the theoretical impact. "Such a large impact event elegantly fits in to what we understand from that formative time."

Such an impact would also be expected to have generated a ring of material around Mars that later coalesced into Phobos and Deimos; this explains in part why those moons are made of a mix of native and non-Martian material.

In the future, Mojzsis will use CU Boulder's collection of Martian meteorites to further understand Mars' mineralogy and what it can tell us about a possible asteroid impact. Such an impact should have initially created patchy clumps of asteroid material and native Martian rock. Over time, the two material reservoirs became mixed. By looking at meteorites of different ages, Mojzsis can see if there's further evidence for this mixing pattern and, therefore, potentially provide further support for a primordial collision.

"Good theories make predictions," said Mojzsis, referring to how the theory may predict how Mars' makeup. By studying meteorites from Mars and linking them with planet-formation models, he hopes to better our understanding of how massive, ancient asteroids radically changed the in its earliest days.

Explore further: Team discovers lull in Mars' giant impact history

More information: R. Brasser et al, A colossal impact enriched Mars' mantle with noble metals, Geophysical Research Letters (2017). DOI: 10.1002/2017GL074002

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Osiris1
not rated yet Jul 19, 2017
Who knows, maybe a primordial earth hit mars and ended its habitability yet transferred its life to our world. Like we were a lifeboat for martian life. Kinda like an O'Henry story.
Urgelt
not rated yet Jul 19, 2017
Whew. These guys are making some massive assumptions. Martian meteorites on Earth are possibly not an accurate way to estimate rare metals in the Martian crust; and their conclusion that a really huge portion of the crust is rare metals seems risky.

The problem is contamination. It's not necessarily obvious, from studying a meteorite fragment on Earth, how much of that fragment originated with the Martian crust and how much originated with the impact object. The proportions of minerals in the fragments might not line up with the crust's.

I like the impact hypothesis anyway, but I wouldn't take this one study to the bank and expect interest on it.
Shootist
not rated yet Jul 19, 2017
Whew. These guys are making some massive assumptions. Martian meteorites on Earth are possibly not an accurate way to estimate rare metals in the Martian crust; and their conclusion that a really huge portion of the crust is rare metals seems risky.

The problem is contamination. It's not necessarily obvious, from studying a meteorite fragment on Earth, how much of that fragment originated with the Martian crust and how much originated with the impact object. The proportions of minerals in the fragments might not line up with the crust's.

I like the impact hypothesis anyway, but I wouldn't take this one study to the bank and expect interest on it.


It is pretty obvious that Borealis Basin (the Northern lowlands) is the result of an impact or impacts that significantly predated the basins formed at Hellas and Utopia during the Terminal Lunar Cataclysm at ~4.1Gy to ~3.8Gy. This is another attempt to fit what is observed in a workable model.
rrwillsj
1 / 5 (1) Jul 19, 2017
Leaves me wondering why the hypothesized Martian biosphere had such a small effect upon the aresology?
Urgelt
not rated yet Jul 19, 2017
Eh? Shootist, I don't take issue with the existence of impact craters on Mars or their importance in shaping its surface.

I take issue with the idea that the mineral isotopic ratios and abundances in martian meteorite fragments collected on Earth are representative of isotopic ratios and abundances in the Martian crust.

Why would we think that? There are *two* sources of isotopes in any fragment: the colliding object and Mars.

And yes, there are also two sources of isotopic ratios in the Martian crust: pristine Mars, collapsed from the original dust cloud; and bombardments. But there is no reason to think that the isotopic signatures of Martian meteorites on Earth are representative of the Mars crust's ratios and abundances. That's a hell of an assumption to make, and it leads to a very risky conclusion. .08% of the Martian crust is rare metals? Permit me to doubt it.
Shootist
not rated yet Jul 25, 2017
@Urgelt Taken at face value the mineral assay of Martian impact fragment found on Earth contain more of elements 75-76-77 than can otherwise be explained, yes?

Upon inspection, It is fairly obvious to determine whether a Martian impact fragment found on Earth had melted, mixed and reformed during the process that removed it from Mars.

It would take a statistician, far more skilled than I, to tell us whether that unmelted, unmixed, unreformed, fragment that made it to Earth wasn't actually a piece of the Martian crust, but a stray bit of space rock knocked off the Martian surface by another stray bit of space rock. Otherwise, these fragments cannot be anything other than a random sample of Mars' crust.

Do note Mars' crust has never be subsumed by tectonics and reformed, at least not since before 4.1Gy, and is, in fact, mostly unconsolidated rubble to a depth of 60 km.

Is that where you're headed with this?
Urgelt
not rated yet Jul 25, 2017
There are reasons to doubt that Mars' crust contains such a heavy concentration of rare metals. When it formed, it was hot; heavy metals sink. The absence of tectonic processes for over 4.1 billion years should tell you something: there hasn't been very much recycling of sub-mantel material to the crust.

It's not so easy to examine a melted and cooled meteorite fragment and say, definitely, which parts of it originated with the Martian crust. Possibly, it isn't impossible. But it's a hell of a challenge.

The authors of this study made no great effort to do that. They simply sampled, measured and concluded. But under the conclusion is an enormous assumption: that Martian meteorite fragments contain isotopes in the ratios found in the Martian crust, and that different ratios from meteorites - which we are sure exist, looking at meteorite fragments that are *not* from Mars - aren't affecting their measurements.

It's a foolish assumption.

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