Rare metals on Mars and Earth implicate colossal impacts

March 16, 2018 by Amanda Doyle, Astrobio.net
The surface features of the northern and southern hemispheres of Mars are very different. In this topographic map, the northern hemisphere (shown in blue) is mostly smooth lowlands and has experienced extensive volcanism. The southern hemisphere (in orange) has an older, cratered highland surface. This dichotomy could have been caused by a giant impact. Credit: University of Arizona/LPL/SwRI

New research has revealed that a giant impact on Mars more than four billion years ago would explain the unusual amount of "iron loving" elements in the Red Planet.

Planets form as small dust grains stick together and agglomerate with other grains, leading to bigger bodies termed "planetesimals." These planetesimals continue to collide with each other and are either ejected from the solar system, gobbled up by the sun, or form a planet. This is not the end of the story, as planets continue to accrete material well after they have formed. This process is known as late accretion, and it occurs as leftover fragments of planet formation rain down on the young planets.

Planetary scientist Ramon Brasser of the Tokyo Institute of Technology and geologist Stephen Mojzsis of the University of Colorado, Boulder took a closer look at a colossal impact during Mars' late accretion that could explain the unusual amount of rare metallic elements in Mars' , which is the layer below the planet's crust. Their recently published paper, "A colossal impact enriched Mars' mantle with noble metals," appeared in the journal Geophysical Research Letters.

When proto- accrete enough material, metals such as iron and nickel begin to separate and sink to form the . This explains why Earth's core is mainly composed of iron, and it is expected that elements that readily bond with iron should also mainly exist in the core. Examples of such 'iron loving' elements, known as siderophiles, are gold, platinum and iridium, to name a few. Just like Mars, however, there are more siderophiles in the Earth's mantle than would be expected by the process of core formation.

"High pressure experiments indicate that these metals should not be in the mantle. These metals don't like being dissolved in silicate and instead they prefer to sink through the mantle into the Earth's core," Brasser tells Astrobiology Magazine. "The fact that we do have them at all means that they must have arrived after the core and the mantle separated, when it became much more difficult for these metals to reach the core."

A 2016 paper by Brasser and colleagues conclusively showed that a giant impact is the best explanation for Earth's high siderophile element abundance.

The amount of siderophiles accumulated during late accretion should be proportional to the 'gravitational cross section' of the planet. This cross section is effectively the cross hairs that an impactor 'sees' as it approaches a target planet. The gravitational cross section extends beyond the planet itself, as the world's gravity will direct an object towards it even when the object was not on a direct collision course. This process is called gravitational focusing.

The earlier paper showed that Earth has more siderophiles in the mantle than it should, even according to the gravitational cross section theory. The scientists explained this by showing that an impact of a lunar-sized body on the Earth (in addition to the event that formed the moon) would have enriched the mantle with enough siderophiles to explain the current value.

An early giant impact

Analysis of Martian meteorites show that Mars accreted another 0.8 percent by mass (weight percent, or wt percent) of material via late accretion. In the new paper, Brasser and Mojzsis show that for Mars to have amended its mass by about 0.8 wt percent in a single impact event required a body at least 1,200 kilometers in diameter.

They further argue that such an impact ought to have occurred some time between 4.5 and 4.4 billion years ago. Studies of zircon crystals in ancient Martian meteorites can be used to date the formation of the Martian crust to before 4.4 billion years ago. As such, a giant impact should have caused widespread crustal melting and such a catastrophic event must have occurred before the evidence for the oldest crust. If the impact occurred as early in the planet's history as 4.5 billion years ago, then the siderophiles should have been stripped away during core formation. This history provides firm bookend constraints on when the impact happened.

Understanding late accretion is not just important for explaining the siderophile abundance, but also for placing an upper limit on the age of Earth's biosphere.

"During each impact, a small bit of Earth's crust is locally melted," says Brasser. "When the accretion is very intense, almost all of Earth's crust is molten. As the accretion intensity decreases, the amount of crustal melting also decreases. We argue that the earliest time you could form a biosphere is when the is low enough so that less than 50 percent of the crust is molten at any given time."

The surface of Mars also has an unusual dichotomy, which could be explained by a giant impact. The southern hemisphere exists as an ancient cratered terrain, and the northern hemisphere appears younger and smoother and was influenced by extensive volcanism. A giant might also have created the Martian moons, Deimos and Phobos, although an alternative theory is that the highly porous Phobos could be a captured asteroid.

Explore further: Collisions after moon formation remodeled early Earth

More information: A colossal impact enriched Mars' mantle with noble metals, arxiv.org/abs/1706.02014

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1.8 / 5 (6) Mar 16, 2018
Wow, more bad news for those intrepid pioneers who hope to find Earth-size planets that can be rehabilitated for an Earth biosphere.

That the original Earth having a crust with the assortment of elements needed to establish life, was a fluke. Even if mass, stable orbit, stable star, lots of water. Without the primitive chemical soup, not much chance to sustain even the basic requirements for life.

From a moral standpoint this has it's benefits. The Wanna-be-a-Pizarro Brother crowd is all too ready to embrace genocide and extermination of Alien biospheres as an entitlement.

Interstellar colonization is looking a lot more difficult and improbable of success. It certainly is going to require a lot more co-operative organization and supporting infrastructure than the mobocracy of 'Rugged Individualists" would ever be willing to accept.
3.7 / 5 (3) Mar 16, 2018
....your point being, again? It's hard to see anything behind all your despise.
4 / 5 (4) Mar 16, 2018
Wow, more bad news for those intrepid pioneers who hope to find Earth-size planets that can be rehabilitated for an Earth blah blah blah...

On the contrary, the fact that anyone ever found a meteorite from Mars so that they could investigate the zircon crystals inside has to tell you that these impacts happen all the time. Because the chances of someone picking up a rock that just happens to be the one rock from Mars are infinitesimally small. If it happens to be one of billions of such rocks all over Earth, then the odds are good.

What would be strange and improbable is to find a planetary system where such impacts do not occur. The only way you get a planet to form is by having a dense disk of material swirling at high speed. And that disk is not going to collapse into one clump, it will form trillions of clumps that then run into each other over billions of years. It is messy, so everything will happen eventually.
2.3 / 5 (3) Mar 16, 2018
dnatwork, I agree with your surmise that across this vast Cosmos, many improbabilities are possible. Or, should that be assume instead of surmise?

We are wishfully presuming that Earth type Living Worlds will occur close enough to detect. And running a timeline parallel to our world's 5 billion years.

Well, here's hoping. No matter how slight the possibility.
Mark Thomas
3 / 5 (2) Mar 17, 2018
"Wow, more bad news for those intrepid pioneers who hope to find Earth-size planets that can be rehabilitated for an Earth biosphere."

Rrwillsj, you are making little to no sense at all here. My wild guess is the impacts like the Earth endured when forming the moon are a little less common, but still part of a continuum of giant impacts that are a normal part of late accretion. This article highlights the fact that Mars also likely suffered a giant impact like Earth, so again, giant impacts appear to be somewhat normal and expected. What that has to do with the rest of your rambling comment is unclear.
1 / 5 (5) Mar 17, 2018
Electric discharge could be the answer here, transmutation of matter in plasma chamber.
2 / 5 (4) Mar 17, 2018
MT, all we are observing in planetary systems are random collections of worlds. Without order or coherence or predictability.

Just cosmic level disappointments.

Succinct enough for you MT?
Captain Stumpy
4 / 5 (4) Mar 17, 2018
MT, all we are observing in planetary systems are random collections of worlds. Without order or coherence or predictability.

Just cosmic level disappointments.

and burying your head in sand makes it hard to see the coastline

viewing 100 worlds in a potential "beach" of billions and calling it "Just cosmic level disappointment" is like viewing 1 dead burned person in a bus wreck and proclaiming that no other person on Earth can be alive and unburnt
Mark Thomas
3.7 / 5 (3) Mar 17, 2018
"MT, all we are observing in planetary systems are random collections of worlds."

Why would you expect much more than that given the instrumentation we have?

"Without order or coherence or predictability."

Maybe once we know more we will be able to establish some level of predictability. For example, what are the most likely outcomes for G2 stars from 4-5 billion years old.

"Just cosmic level disappointments."

LOL! I don't know what you were hoping for, but it is a little premature to declare the universe a "cosmic level disappointment" just yet. For all we know, it may be the greatest break humanity ever caught that nobody is likely to be close.
3 / 5 (2) Mar 18, 2018
Before you all go announcing "Undeniable Certain Victory! (Immediately... Imminently... Perhaps Soon...
Sooner Than Eventually... Okay, One of These Days? Years? Generations?)"

I would suggest you tone it down to quietly reflective acceptance of verifiable realism.

Cause you might be ridiculed for organizing a victory parade. When the closest thing you have to a working spaceship? Is a putter-cart pulled, bunting clad float. Draped with "Honorary" politicians. Celebrating their over payed awesomeness by waving at the passing spectators.
3 / 5 (2) Mar 18, 2018
It is interesting to see things coming full circle. For ages it was thought that Earth was unique, then as Astronomy got better it was realized that there are trillions of planetary systems out there and it was assumed that life in general and even intelligent life must be everywhere. Now we're seeing that stable planetary systems like ours along with a lot of other key factors are driving the probability of intelligent life ever lower.

My take is that there must be other intelligent life but the likelihood of two intelligent races being close enough to interact is really, really low.
Mark Thomas
1 / 5 (1) Mar 18, 2018
My take is that there must be other intelligent life but the likelihood of two intelligent races being close enough to interact is really, really low.

Sounds like as good a guess as any and I think you are probably right. My guess is "intelligent" life might be relatively close, maybe swimming in some alien sea less than 100 light years from here. But as you pointed out, intelligent life capable of communicating and/or traveling through interstellar space, that is probably a whole different story.

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