ASU News

Planet Mercury a result of early hit-and-run collisions

July 6, 2014

Planet Mercury’s unusual metal-rich composition has been a longstanding puzzle in planetary science. According to a study published online in Nature Geoscience July 6, Mercury and other unusually metal-rich objects in the solar system may be relics left behind by collisions in the early solar system that built the other planets.

The origin of planet Mercury has been a difficult question in planetary science because its composition is very different from that of the other terrestrial planets and the moon. This small, innermost planet has more than twice the fraction of metallic iron of any other terrestrial planet. Its iron core makes up about 65 percent of Mercury’s total mass; Earth’s core, by comparison, is just 32 percent of its mass. Mercury Download Full Image

How do we get Venus, Earth and Mars to be mostly "chondritic" (having a more-or-less Earth-like bulk composition) while Mercury is such an anomaly? For Arizona State University professor Erik Asphaug, understanding how such a planet accumulated from the dust, ice and gas in the early solar nebula is a key science question.

There have been a number of failed hypotheses for Mercury’s formation. None of them until now has been able to explain how Mercury lost its mantle while retaining significant levels of volatiles (easily vaporized elements or compounds, such as water, lead and sulfur). Mercury has substantially more volatiles than the moon does, leading scientists to think its formation could have had nothing to do with a giant impact ripping off the mantle, which has been a common popular explanation.

To explain the mystery of Mercury’s metal-rich composition, ASU’s Asphaug and Andreas Reufer of the University of Bern have developed a new hypothesis involving hit-and-run collisions, where proto-Mercury loses half its mantle in a grazing blow into a larger planet (proto-Venus or proto-Earth). One or more hit-and-run collisions could have potentially stripped away proto-Mercury’s mantle without an intense shock, leaving behind a mostly-iron body and satisfying a number of the major puzzles of planetary formation – including the retention of volatiles – in a process that can also explain the absence of shock features in many of the mantle-stripped meteorites.

Asphaug and Reufer have developed a statistical scenario for how planets merge and grow based on the common notion that Mars and Mercury are the last two relics of an original population of maybe 20 bodies that mostly accreted to form Venus and Earth. These last two planets lucked out.

“How did they luck out? Mars, by missing out on most of the action – not colliding into any larger body since its formation – and Mercury, by hitting the larger planets in a glancing blow each time, failing to accrete,” explains Asphaug, who is a professor in ASU’s School of Earth and Space Exploration. “It’s like landing heads two or three times in a row – lucky, but not crazy lucky. In fact, about one in 10 lucky.”

By and large, dynamical modelers have rejected the notion that hit-and-run survivors can be important because they will eventually be accreted by the same larger body they originally ran into. Their argument is that it is very unlikely for a hit-and-run relic to survive this final accretion onto the target body.

“The surprising result we have shown is that hit-and-run relics not only can exist in rare cases, but that survivors of repeated hit-and-run incidents can dominate the surviving population. That is, the average unaccreted body will have been subject to more than one hit-and-run collision,” explains Asphaug. “We propose one or two of these hit-and-run collisions can explain Mercury’s massive metallic core and very thin rocky mantle.”

According to Reufer, who performed the computer modeling for the study, “Giant collisions put the final touches on our planets. Only recently have we started to understand how profound and deep those final touches can be.

“The implication of the dynamical scenario explains, at long last, where the ‘missing mantle’ of Mercury is – it’s on Venus or the Earth, the hit-and-run targets that won the sweep-up,” says Asphaug.

Disrupted formation

The duo’s modelling has revealed a fundamental problem with an idea implicit to modern theories of planet formation: that protoplanets grow efficiently into ever larger bodies, merging whenever they collide.

Instead, disruption occurs even while the protoplanets are growing.

“Protoplanets do merge and grow, overall, because otherwise there would not be planets,” says Asphaug. “But planet formation is actually a very messy, very lossy process, and when you take that into account, it’s not at all surprising that the ‘scraps,’ like Mercury and Mars, and the asteroids are so diverse.”

These simulations are of great relevance to meteoritics, which, just like Mercury’s missing mantle, faces questions like: Where’s all the stripped mantle rock that got removed from these early core-forming planetesimals? Where are the olivine meteorites that correspond to the dozens or hundreds of iron meteorite parent bodies?

“It’s not missing – it's inside the mantles of the planets, ultimately,” explains Asphaug. “It got gobbled up by the larger growing planetary bodies in every hit-and-run series of encounters.”

The School of Earth and Space Exploration in an academic unit in ASU's College of Liberal Arts and Sciences.

Nikki Cassis

marketing and communications director, School of Earth and Space Exploration

ASU News

ASU math professor earns international Marie Curie fellowship

July 7, 2014

An Arizona State University professor is traveling to London to help develop a qualitative theory for describing the long-term, unusual and hard-to-predict behavior in random processes that arise in physics, biology, finance and other areas of science.

Vladislav Vysotsky, an assistant professor in the School of Mathematical and Statistical Sciences within the College of Liberal Arts and Sciences, has won a prestigious Marie Curie Actions International Incoming Fellowship by the European Commission. Top-class researchers from outside Europe are selected for the fellowship to encourage research cooperation between Europe and other parts of the world. portrait of ASU assistant professor Vladislav Vysotsky Download Full Image

Vysotsky will begin his two-year fellowship project this fall at the mathematics department of Imperial College London, which is consistently ranked as one of the top universities in the world. He will work with professor Alex Mijatovic, an expert in probability theory and its applications in financial mathematics and co-head of Imperial Probability, an association of researchers across scientific disciplines at Imperial College with a common interest in probability.

“We are interested in similar types of problems, and the basis of our collaboration is the complementary nature of our skills,” says Vysotsky. “I am an expert in theoretical probability and discrete-time stochastic processes, while professor Mijatovic specializes in continuous-time processes with jumps, and has well-established relations with financial market practitioners.”

Their research project aims to have an impact on significant practical and theoretical problems of much interest to physicists, mathematicians, financial risk managers and others who are interested in evaluating the risk of random events.

Vysotsky is looking forward not only to working with Mijatovic, but also establishing collaborations with other probabilists and specialists in other areas of mathematics, including the famous group on dynamical systems at Imperial. Another benefit of the location is that London is ideally connected with the world-leading centers of probability theory, including University of Warwick in the UK and Universities of Pierre et Marie Curie and Paris-Sud in Paris.

Vykotsky earned a master of science in mathematics and a doctoral degree in probability theory from St. Petersburg State University, Russia, and was a visiting assistant professor at the University of Delaware.

Rhonda Olson

Marketing and Communications, School of Mathematical and Statistical Sciences