Through a combination of data analysis and numerical modeling work, researchers have found a record of the ancient Moon-forming giant impact observable in stony meteorites. Their work will appear in the April 2015 issue of the Journal Science. The work was done by NASA Solar System Exploration Research Virtual Institute (SSERVI) researchers led by Principal Investigator Bill Bottke of the Institute for the Science of Exploration Targets (ISET) team at the Southwest Research Institute and included Tim Swindle, director of the University of Arizona’s Lunar and Planetary Laboratory.
The inner Solar System’s biggest known collision was the Moon-forming giant impact between a large protoplanet and the proto-Earth. The timing of this giant impact, however, is uncertain, with the ages of the most ancient lunar samples returned by the Apollo astronauts still being debated. Numerical simulations of the giant impact indicate this event not only created a disk of debris near Earth that formed the Moon, but it also ejected huge amounts of debris completely out of the Earth-Moon system. The fate of this material, comprising as much as several percent of an Earth mass, has not been closely examined until recently. However, it is likely some of it blasted main belt asteroids, with a record plausibly left behind in their near-surface rocks. Collisions on these asteroids in more recent times delivered these shocked remnants to Earth, which scientists have now used to date the age of the Moon.
The research indicates numerous kilometer-sized fragments from the giant impact struck main belt asteroids at much higher velocities than typical main belt collisions, heating the surface and leaving behind a permanent record of the impact event. Evidence that the giant impact produced a large number of kilometer-sized fragments can be inferred from laboratory and numerical impact experiments, the ancient lunar impact record itself, and the numbers and sizes of fragments produced by major main belt asteroid collisions.
Once the team concluded that pieces of the Moon-forming impact hit main belt asteroids and left a record of shock heating events in some meteorites, they set out to deduce both the timing and the relative magnitude of the bombardment. By modeling the evolution of giant impact debris over time and fitting the results to ancient impact heat signatures in stony meteorites, the team was able to infer the Moon formed about 4.47 billion years ago, in agreement with many previous estimates. The most ancient Solar System materials found in meteorites are about one hundred million years older than this age.
Insights into the last stages of planet formation in the inner solar system can be gleaned from these impact signatures. For example, the team is exploring how they can be used to place new constraints on how many asteroid-like bodies still existed in the inner Solar System in the aftermath of planet formation. They can also help researchers deduce the earliest bombardment history of ancient bodies like Vesta, one of the targets of NASA’s Dawn mission and a main belt asteroid whose fragments were delivered to Earth in the form of meteorites. It is even possible that tiny remnants of the Moon-forming impactor or proto-Earth might still be found within meteorites that show signs of shock heating by giant impact debris. This would allow scientists to explore for the first time the unknown primordial nature of our homeworld.
Co-author Swindle, who specializes in finding the times when meteorites or lunar samples were involved in large collisions, said: “Bill Bottke had the idea of looking at the asteroid belt to see what effect a Moon-forming giant impact would have, and realized that you would expect a lot of collisions in the period shortly after that.
“Here at LPL, we had been determining ages of impact events that affected meteorites, and when we got together, we found that our data matched his predictions,” he added. “It’s a great example of taking advantage of groups that work in two different specialties – orbital dynamics and chronology – and combining their expertise.”
Intriguingly, some debris may have also returned to hit the Earth and Moon after remaining in solar orbit over timescales ranging from tens of thousands of years to 400 million years.
“The importance of giant impact ejecta returning to strike the Moon could also play an intriguing role in the earliest phase of lunar bombardment,” said Bottke, who is an alumnus of the University of Arizona’s Lunar and Planetary Laboratory. “This research is helping to refine our time scales for ‘what happened when’ on other worlds in the Solar System.”
Yvonne Pendleton, Director of the NASA SSERVI Institute, notes: “This is an excellent example of the power of multidisciplinary science. By linking studies of the Moon, of main belt asteroids, and of meteorites that fall to Earth, we gain a better understanding of the earliest history of our Solar System.”
The above story is based on materials provided by University of Arizona. Note: Materials may be edited for content and length.
Mercury as seen by the Messenger probe. Signs of water and organic material have now been picked up © Nasa
Nasa’s Messenger spacecraft has uncovered evidence that not only does water ice exist on the surface of the planet Mercury, but in many places this ice appears to be covered in a 10cm-thick layer of soot-like organic material.1,2 Although the discovery of water ice on the closest planet to the sun was not entirely unexpected, the discovery of organic material was and suggests that some interesting chemistry might be taking place on the planet’s inhospitable surface.
Radar observations of Mercury from Earth had already detected bright, highly reflective regions at the planet’s poles, which astronomers thought could well be water ice originally brought by comets. Orbiting at a distance of just 36 million miles from the sun, compared with Earth’s 93 million miles, Mercury’s daytime surface temperature can reach 400°C. In permanent shadows within craters at the planet’s poles, however, the temperature probably doesn’t rise much above -170°C, allowing water ice to exist more or less indefinitely.3
Confirming whether water ice exists on Mercury’ surface is one of the main objectives of Nasa’s Messenger (MErcury Surface, Space ENvironment, GEochemistry and Ranging) spacecraft, which was launched in 2004 and went into orbit around the planet in 2011. To this end, its suite of analytical instruments includes the Mercury Laser Altimeter (MLA) and a neutron spectrometer. The MLA can indicate the presence of ice based on its reflection of near-infrared light, while the neutron spectrometer can detect the hydrogen in ice based on the effect it has on the energy level of neutrons blasted off the surface of Mercury by cosmic rays.
In a trio of papers, a group of US and French scientists now report their analysis of the data generated by the MLA and the neutron spectrometer, which reveals that the situation on Mercury is actually more complicated than originally supposed.
When the MLA surveyed Mercury’s north pole, it found bright, reflective regions indicative of surface water ice, but it also found more numerous non-reflective regions that were darker than Mercury’s normal rocky surface. Furthermore, the reflective regions tended to be at cold, high latitudes near the pole, whereas the dark regions tended to be at lower latitudes where the temperature should be too high for surface ice to exist long term.
The neutron spectrometer was unable to resolve individual bright and dark regions. Overall, however, its data indicated the existence of a thick, hydrogen-rich material covered in a thin, less hydrogen-rich layer around Mercury’s north pole.
Putting these data together, the scientists conclude that the bright regions are pure water ice, while the dark regions consist of water ice covered in a 10cm-thick layer of organic material. This explains why the dark regions are found at warmer latitudes, because the organic material provides a thermal blanket that prevents the ice from subliming. At higher latitudes, however, it’s cold enough in the craters for ice to exist without such a blanket.
‘MLA’s discovery of dark material in those permanently shadowed craters was a big surprise,’ group member David Paige at the University of California, Los Angeles, tells Chemistry World. ‘We had expected that Mercury’s subsurface ice deposits were covered by regular Mercury soil, but when this material turned out to be extremely dark, then it got us thinking about what it could possibly be.’
The scientists think that the water and organic material were originally delivered to Mercury’s surface by comets, with both eventually migrating to the cold polar regions where they persisted in craters. Here, powered by high-energy particles in the solar wind, carbon- and nitrogen-containing molecules in the ice undergo chemical reactions, producing increasingly complex, polymer-like organic compounds that form an insulating layer on top of the ice. Determining the precise chemical composition of this layer will require future missions to Mercury, says Paige.
Mark Sephton, professor of organic geochemistry and meteorites at Imperial College London, UK, says the finding of water and organic material on Mercury is ‘an exciting development’ with some important implications for both planetary development and the origin of life. ‘The recognition that planetary surfaces can accumulate volatile materials has implications for theories of how atmospheres and oceans can be formed,’ he explains. ‘Moreover, evidence of a relatively passive surface preserving layers of primitive materials suggests the potential for planetary and lunar records of solar system evolution. Lastly, the presence of organic matter and water reminds us that the raw materials of life are efficiently delivered to many planetary surfaces in the solar system.’
1 D A Paige et al, Science, 2012, DOI: 10.1126/science.1231106
2 G A Neumann et al, Science, 2012, DOI: 10.1126/science.1229764
3 D J Lawrence et al, Science, 2012, DOI: 10.1126/science.1229953
Editors note: Full article can be found here.