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

New simulations show that Mercury and other unusually metal-rich objects in the solar system may be relics left behind by hit-and-run collisions in the early solar system. Credit: NASA/JPL/Caltech

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.

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."

Source: Arizona State University

Ancient volcanic explosions shed light on Mercury's origins

Measuring geological time: Two pyroclastic vents on the floor of Mercury’s Kipling crater, top, would likely not have survived the impact; they are more recent. The false color image of the same spot, bottom, marks pyroclastic material as brownish red. Credit: Image courtesy of Brown University

The surface of Mercury crackled with volcanic explosions for extended periods of the planet's history, according to a new analysis led by researchers at Brown University. The findings are surprising considering Mercury wasn't supposed to have explosive volcanism in the first place, and they could have implications for understanding how Mercury formed.

On Earth, volcanic explosions like the one that tore the lid off Mount St. Helens happen because our planet's interior is rich in volatiles -- water, carbon dioxide and other compounds with relatively low boiling points. As lava rises from the depths toward the surface, volatiles dissolved within it change phase from liquid to gas, expanding in the process. The pressure of that expansion can cause the crust above to burst like an overinflated balloon.

Mercury, however, was long thought to be bone dry when it comes to volatiles, and without volatiles there can't be explosive volcanism. But that view started to change in 2008, after NASA's MESSENGER spacecraft made its first flybys of Mercury. Those glimpses of the surface revealed deposits of pyroclastic ash -- the telltale signs of volcanic explosions -- peppering the planet's surface. It was a clue that at some point in its history Mercury's interior wasn't as bereft of volatiles as had been assumed.

What wasn't clear from those initial flybys was the timeframe over which those explosions occurred. Did Mercury's volatiles escape in a flurry of explosions early in the planet's history or has Mercury held on to its volatiles over a much longer period?

This latest work, available in online early view at the Journal of Geophysical Research: Planets, suggests the latter.

A team of researchers led by Tim Goudge, a graduate student in the Department of Geological Sciences at Brown, looked at 51 pyroclastic sites distributed across Mercury's surface. They used data from MESSENGER's cameras and spectrometers collected after the spacecraft entered orbit around Mercury in 2011. Compared with the data from the initial flybys, the orbital data provided a much more detailed view of the deposits and the source vents that spat them out.

The new MESSENGER data revealed that some of the vents have eroded to a much greater degree than others -- an indicator that the explosions didn't happen all at the same time.

"If [the explosions] happened over a brief period and then stopped, you'd expect all the vents to be degraded by approximately the same amount," Goudge said. "We don't see that; we see different degradation states. So the eruptions appear to have been taking place over an appreciable period of Mercury's history."

But just where that period of explosiveness fits into Mercury's geological history was another matter. To help figure that out, Goudge and his colleagues took advantage of the fact that most of the sites are located within impact craters. The age of each crater offers an important constraint in the age of the pyroclastic deposit inside it: The deposit has to be younger than its host crater. If the deposit had come first, it would have been obliterated by the impact that formed the crater. So the age of the crater provides an upper limit on how old the pyroclastic deposit can be.

As it happens, there's an established method for dating craters on Mercury. The rims and walls of craters become eroded and degraded over time, and the extent of that degradation can be used to get an approximate age of the crater.

Using that method, Goudge and his colleagues showed that some pyroclastic deposits are found in relatively young (geologically speaking) craters dated to between 3.5 and 1 billion years old. The finding helps rule out the possibility that all the pyroclastic activity happened shortly after Mercury's formation around 4.5 billion years ago.

"These ages tell us that Mercury didn't degas all of its volatiles very early," Goudge said. "It kept some of its volatiles around to more recent geological times."

The extent to which Mercury's volatiles stuck around could shed light on how the planet formed. Despite being the smallest planet in the solar system (since Pluto was demoted from the ranks of the planets), Mercury has an abnormally large iron core. That finding led to speculation the perhaps Mercury was once much larger, but had its outer layers removed -- either fried away by the nearby Sun or perhaps blasted away be a huge impact early in the planet's history. Either of those events, however, would likely have heated the outer parts of Mercury enough to remove volatiles very early in its history.

In light of this study and other data collected by MESSENGER showing traces of the volatiles sulfur, potassium, and sodium on Mercury's surface, both those scenarios seem increasingly unlikely.

"Together with other results that suggest the Moon may have had more volatiles than previously thought, this research is revolutionizing our thinking about the early history of the planets and satellites," said Jim Head, professor of geological sciences and a MESSENGER mission co-investigator. "These results define specific targets for future exploration of Mercury by orbiting and landed spacecraft."

Source: Brown University

Where do astronauts go when they need 'to go?'

NASA researchers sought to design a way to contain urine in the inevitable event that future astronauts would need 'to go' while wearing their spacesuits.

Alan Shepard became the first American to fly in space on May 5, 1961. Although NASA engineers had put considerable planning into his mission, dubbed Freedom 7, noticeably missing from this extensive preparation was a way for him to urinate in his spacesuit. 

During a lengthy launch delay, the inevitable happened, and Shepard's urine short-circuited his electronic biosensors. In less than a year, engineers had remedied this seeming oversight for John Glenn's Mercury orbital flight. The system developed for Glenn stood the test of time, remaining in use until the early days of the Space Shuttle program.

In a new article, Hunter Hollins of the National Air and Space Museum reviews the history of urine collection in space and the considerations necessary to accommodate this basic physiological function. That first successful urine collection device, used in 1962, has been on display at the National Air and Space Museum since 1976.

The new article, titled "Forgotten Hardware: How to Urinate in a Spacesuit," appears in the June 2013 edition of Advances in Physiology Education, a journal published by the American Physiological Society

No Need "To Go?"

Hollins writes that though the general public was interested in how astronauts would tackle taking care of this basic need in space (a letter stored in NASA's Historical Reference Collection from a Pennsylvania schoolgirl questioned where the first man in space would use the toilet), NASA's scientists and technicians seemed to ignored the problem before Shepard's mission. Combined with a lack of funding and little crosstalk between the organizations that would end up comprising NASA, scientists in the organization also assumed that the first astronauts would be able to "hold it" during their very short missions.

However, though Shepard's spaceflight was scheduled to last only 15 minutes, he spent eight hours in his spacesuit due to launch delays. During a four-hour stint on the launch pad, he relieved himself in the suit, damaging the electronic medical data sensors attached to his body.

After this understandable event, NASA researchers sought to design a way to contain urine in the inevitable event that future astronauts would need to go while wearing their spacesuits.

New Device a Relief for Astronauts

Working around the spacesuit itself was one barrier to successful urine collection. The pressure suits worn by astronauts help keep their occupants alive during spaceflight by ensuring that pressures inside stay within a healthy physiological range. However, the bulky, uncomfortable suits left little room for devices to capture urine.

The first iteration of urine collection devices proposed for space were in-dwelling catheters, a tube threaded through the penis to collect urine continuously from the bladder. However, such catheters are extremely uncomfortable and greatly increase the risk of infection.

After Gus Grissom's Mercury-Redstone 4 mission followed Shepard's in 1961 -- in which Grissom urinated between two pairs of rubber pants -- NASA researchers set about developing a more suitable urine collection device. They ended up basing theirs on the simple personal urinals already available at the time for people with medical problems, such as impaired bladder control, or those without access to public urinals, such as police officers on a long shift.

In the end, the resulting device resembled a condom made out of more durable materials and open on one end, with a tube connected to a storage container. On Glenn's Mercury-Atlas 6 mission, he voided a full bladder into the new device, confirming its utility.

Tweaks Still Necessary

Astronauts regularly used this type of device with minimal modifications until the early days of the Space Shuttle program, Hollins writes. However, those and modern urine collection devices still aren't perfect. Hollins notes that in a survey done in 2010, the majority of U.S. Air Force pilots flying high altitude spy planes reported problems with the urine collection devices they wore, including poor fit, leaking, and skin damage from extended contact with urine.

"It is the job of the engineer/physiologist to ensure that the man-machine interface promotes the health and safety of the human body," Hollins says.

Source: American Physiological Society (APS)

Mercury may have harbored an ancient magma ocean: Massive lava flows may have given rise to two distinct rock types

The First Solar Day. After its first Mercury solar day (176 Earth days) in orbit, MESSENGER has nearly completed two of its main global imaging campaigns: a monochrome map at 250 m/pixel and an eight-color, 1-km/pixel color map. Apart from small gaps, which will be filled in during the next solar day, these global maps now provide uniform lighting conditions ideal for assessing the form of Mercury’s surface features as well as the color and compositional variations across the planet. The orthographic views seen here, centered at 75° E longitude, are each mosaics of thousands of individual images. At right, images taken through the wide-angle camera filters at 1000, 750, and 430 nm wavelength are displayed in red, green, and blue, respectively.

By analyzing Mercury's rocky surface, scientists have been able to partially reconstruct the planet's history over billions of years. Now, drawing upon the chemical composition of rock features on the planet's surface, scientists at MIT have proposed that Mercury may have harbored a large, roiling ocean of magma very early in its history, shortly after its formation about 4.5 billion years ago.

The scientists analyzed data gathered by MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), a NASA probe that has orbited the planet since March 2011. Later that year, a group of scientists analyzed X-ray fluorescence data from the probe, and identified two distinct compositions of rocks on the planet's surface. The discovery unearthed a planetary puzzle: What geological processes could have given rise to such distinct surface compositions?

To answer that question, the MIT team used the compositional data to recreate the two rock types in the lab, and subjected each synthetic rock to high temperatures and pressures to simulate various geological processes. From their experiments, the scientists came up with only one phenomenon to explain the two compositions: a vast magma ocean that created two different layers of crystals, solidified, then eventually remelted into magma that then erupted onto Mercury's surface.

"The thing that's really amazing on Mercury is, this didn't happen yesterday," says Timothy Grove, a professor of geology at MIT. "The crust is probably more than 4 billion years old, so this magma ocean is a really ancient feature."

Grove, along with postdoc Bernard Charlier and Maria Zuber, the E.A. Griswold Professor of Geophysics and Planetary Science and now MIT's vice president for research, published the results in the journal Earth and Planetary Science Letters.

Making Mercury's rocks

MESSENGER entered Mercury's orbit during a period of intense solar-flare activity; as the solar system's innermost planet, Mercury takes the brunt of the sun's rays. The rocks on its surface reflect an intense fluorescent spectrum that scientists can measure with X-ray spectrometers to determine the chemical composition of surface materials.

As the spacecraft orbited the planet, an onboard X-ray spectrometer measured the X-ray radiation generated by Mercury's surface. In September 2011, the MESSENGER science team parsed these energy spectra into peaks, with each peak signifying a certain chemical element in the rocks. From this research, the group identified two main rock types on Mercury's surface.

Grove, Charlier and Zuber set out to find an explanation for the differences in rock compositions. The team translated the chemical element ratios into the corresponding building blocks that make up rocks, such as magnesium oxide, silicon dioxide and aluminum oxide. The researchers then consulted what Grove refers to as a "pantry of oxides" -- finely powdered chemicals -- to recreate the rocks in the lab.

"We just mix these together in the right proportions and we've got a synthetic copy of what's on the surface of Mercury," Grove says.

Crystals in the melt

The researchers then melted the samples of synthetic rock in a furnace, cranking the heat up and down to simulate geological processes that would cause crystals -- and eventually rocks -- to form in the melt.

"You can tell what would happen as the melt cools and crystals form and change the chemical composition of the remaining melted rock," Grove says. "The leftover melt changes composition."

After cooling the samples, the researchers picked out tiny crystals and melt pockets for analysis. The scientists initially looked for scenarios in which both original rock compositions might be related. For example, both rock types may have come from one region: One rock may have crystallized more than the other, creating distinct but related compositions.

But Grove found the two compositions were too different to have originated from the same region, and instead may have come from two separate regions within the planet. The easiest explanation for what created these distinct regions, Grove says, is a large magma ocean, which over time likely formed different compositions of crystals as it solidified. This molten ocean eventually remelted, spewing lava onto the surface of the planet in massive volcanic eruptions.

Grove estimates that this magma ocean likely existed very early in Mercury's existence -- possibly within the first 1 million to 10 million years -- and may have been created from the violent processes that formed the planet. As the solar nebula condensed, bits and pieces collided into larger chunks to form tiny, and then larger, planets. That process of colliding and accreting may produce enough energy to completely melt the planet -- a scenario that 

would make an early magma ocean very feasible.

"The acquisition of data by spacecraft must be combined with laboratory experiments," Charlier says. "Although these data are valuable by themselves, experimental studies on these compositions enable scientists to reach the next level in the interpretation of planetary evolution."

Larry Nittler, a staff scientist in the Department of Terrestrial Magnetism at the Carnegie Institution of Washington, led the research team that originally identified the two rock compositions from MESSENGER data. He says the MIT team's experimental results propose a very likely early history for Mercury.

"We're gradually filling in more blanks, and the story may well change, but this work sets up a framework for thinking about new data," says Nittler, who was not involved in the study. 

"It's a very important first step toward going from exciting data to real understanding."

This research was supported by a NASA cosmochemistry grant, a Marie Curie International Outgoing Fellowship, and the NASA MESSENGER mission.

Source: Massachusetts Institute of Technology