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 seed grants fuel citizen engagement

July 7, 2014

Researchers in the College of Public Programs’ Center for Policy Informatics (CPI) have been awarded two $10,000 seed grants by Arizona State University’s Office of the Vice President for Entrepreneurship and Innovation under the Citizen Science and Engagement grant program.

Justin Longo and Dara Wald will be working on "Crowdsourcing the Next Great Citizen Science Idea," and Wald will also collaborate with Monica Elser on "Citizen Science to Forecast the Future of a Desert City." Justin Longo and Dara Wald Download Full Image

Both projects bring together two key components of ASU’s Citizen Science and Citizen Engagement program: public participation in research and a mentor network.

"Collaborating on the research experience provides valuable knowledge and skills to students, our faculty and the community," says Mitzi Montoya, vice president and university dean for entrepreneurship and innovation. "Our Citizen Science and Citizen Engagement programs give high school students an opportunity to access and participate in research at the university level, and it gives our researchers the opportunity to generate new ideas and perspectives from new partners."

In "Crowdsourcing the Next Great Citizen Science Idea," Longo and Wald, both postdoctoral fellows, will be using a challenge platform to crowdsource ideas for citizen science projects – asking students across Arizona and throughout the country to submit ideas. The guiding principle of this effort – crowdsourcing the next great citizen science project – is that truly novel innovations are more likely to result from distributed approaches because diverse participants bring unique perspectives to the problem.

“This project is a great opportunity for student teams – whether in high schools or at the university level – to imagine new ways to think about citizen science. Through the challenge platform, the team with the most innovative proposal will work with researchers at ASU to bring their idea to life,” says Longo.

“The past five years have seen the idea of citizen science grow dramatically, especially when the Internet is used as the means to get people involved. Projects like the Zooniverse host a range of citizen science projects, from classifying galaxies to transcribing First World War soldiers’ diaries. Internet-based citizen science projects have a few things in common, but they generally take advantage of some things that people still do better than machines, such as reasoning, pattern recognition, interpretation and valuing. And, good citizen science projects have tasks that are short, simple and fun,” he adds.

The Citizen Science challenge launch will coincide with the start of the 2014-2015 academic year later this summer.

The second grant was awarded to Wald, along with Monica Elser, education manager of ASU’s Global Institute of Sustainability, to develop the project "Citizen Science to Forecast the Future of a Desert City." This project will build on current research efforts at CPI and ASU’s Decision Center for a Desert City (DCDC), an interdisciplinary research center funded by the National Science Foundation (NSF).

"Citizen Science to Forecast the Future of a Desert City" aims to develop a web-based water-reporting site, MyFuturePhoenix, to engage citizens in water management efforts. Initially, high school students will track, classify and analyze personal water use data. Once connected to WaterSim, a water policy and management simulation model developed by DCDC, participants will be able to visualize the collective effect of water use decisions made today on the Phoenix area of the future for the year 2050.

“Developing a tool that collects self-reported data will help citizens understand how their choices influence their community today and how they will influence their community in the future,” says Wald.

She adds, “Despite significant advances in knowledge, the availability, quantity, validity and reliability of water use data, especially for mid-sized cities and at the household level across Arizona, is constrained by government resources and policy choices. This citizen science initiative has the potential to contribute data that is currently not available for university researchers, and to collect much more data than one agency or researcher could collect alone.”

“These projects are representative of our belief that students can prepare for lives of service by engaging in problem-solving,” says Jonathan Koppell, dean of the College of Public Programs. “By engaging young students in the process of finding solutions, we show them that at any age, they can be involved in the innovations that make our communities stronger.”

Ongoing projects at Decision Center for a Desert City include social, behavioral and economic studies to inform water sustainability and urban climate change adaptation. The Center for Policy Informatics is working to create mechanisms that bring expertise into governance discussions in an effort to support social decision-making and public participation.

Heather Beshears

director marketing and communications, College of Public Service and Community Solutions