Forming Dense Metal Planets like Mercury is Probably Pretty Difficult and Rare in the Universe

The planet Mercury, the closet planet to our Sun, is something of an exercise in extremes. Its days last longer than its years and at any given time, its sun-facing side is scorching hot while its dark side is freezing cold. It is also one of the least understood planets in our Solar System. While it is a terrestrial (i.e. rocky) planet like Earth, Venus and Mars, it has a significantly higher iron-to-rock ratio than the others.
Continue reading “Forming Dense Metal Planets like Mercury is Probably Pretty Difficult and Rare in the Universe”

Was There a Time When the Moon was Habitable?

To put it simply, the Earth’s Moon is a dry, airless place where nothing lives. Aside from concentrations of ice that exist in permanently-shaded craters in the polar regions, the only water on the moon is believed to exist beneath the surface. What little atmosphere there is consists of elements released from the interior (some of which are radioactive) and helium-4 and neon, which are contributed by solar wind.

However, astronomers have theorized that there may have been a time when the Moon might have been inhabitable. According to a new study by an astrophysicist and an Earth and planetary scientist, the Moon may have had two early “windows” for habitability in the past. These took place roughly 4 billion years ago (after the Moon formed) and during the peak in lunar volcanic activity (ca. 3.5 billion years ago).

The study – which recently appeared in the journal Astrobiology under the title “Was There an Early Habitability Window for Earth’s Moon?“- was produced by Dirk Schulze-Makuch and Ian A. Crawford. Whereas Schulze-Makucha is a professor of astrophysics at Washington State University (WSU) and the Technical University Berlin (TUB), Crawford is a professor of planetary science and astrobiology at Birkbeck College, University of London.

For the sake of their study, Schulze-Makuch and Crawford drew on the results of several recent space missions and analyses of lunar rock and soil samples – which indicated that the Moon is not as dry as previously thought. They also drew on recent studies of the products of lunar volcanism, which indicate that the lunar interior contains more water than previously thought and that the lunar mantle may even be as comparably water-rich as Earth’s upper mantle.

From this, they concluded that conditions on the lunar surface were sufficient to support simple lifeforms during two periods in the past. The first was roughly 4 billion years ago, when the Moon began to form from a debris disk caused by an impact between a Mars-sized object (named Theia) and Earth – aka. the Giant Impact Hypothesis. The second occurred 3.5 billion years ago when the Moon was at the peak of its volcanic activity.

At both times, planetary scientists think the Moon was releasing considerable amounts of superheated volatile gasses from its interior, which would include water vapor. This outgassing could have formed pools of liquid water on the lunar surface and an atmosphere dense enough to keep it there for millions of years. The early Moon is also believed to have had its own magnetic field, which would have protected lifeforms on the surface from deadly solar radiation.

As Schulze-Makuch said in a recent interview with Astriobiology Magazine:

“If liquid water and a significant atmosphere were present on the early Moon for long periods of time, we think the lunar surface would have been at least transiently habitable.”

A map showing the permanently shadowed regions (blue) that cover about 3 percent of the moon’s south pole. Credit: NASA Goddard/LRO mission

Schulze-Makuch and Crawford’s work draws on data from recent space missions and analyses of lunar rock and soil samples that show the Moon is more watery than scientists gave it credit for. These include India’s first lunar mission, Chandrayaan I, which created a high-resolution chemical and mineralogical map of the lunar surface in 2009, which confirmed the presence of water molecules in the soil.

In that same year, NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS) mission crashed a rocket stage into the Cabeus cater near the moon’s south pole and confirmed evidence of water in the resulting plume of debris. And in 2013, the Lunar Reconnaissance Orbiter created a detailed map of the southern polar region that showed abundant concentrations of water.

Additionally, ongoing examinations of the lunar rocks returned by the Apollo astronauts and studies of lunar volcanic deposits have provided strong evidence that there is a large amount of water in the lunar mantle that is thought to have been deposited very early on in the Moon’s formation. As for how the life got there, that remains a bit of an open question.

Schulze-Makuch and Crawford believe that it may have originated much as it did on Earth, but that the more likely scenario is that it was brought from Earth by meteorites. Essentially, the earliest evidence for life on Earth indicates that cyanobacteria existed on our planet 3.5 to 3.8 billion years ago. This coincides with the Late Heavy Bombardment, when the Solar System was experiencing frequent and giant meteorite impacts.

It is possible that during the Late Heavy Bombardment, pieces of Earth bearing simple organisms could have been ejected and landed on a “transiently habitable” Moon. Credit: NASA

So basically, it is possible that large impacts could have blasted off pieces of the Earth’s surface, which contained simple organisms like cyanobacteria. These chunks could have then reached the Moon and landed on its surface, seeding it with basic lifeforms that would have been capable of surviving in the lunar environment. As Schulze-Makuch said:

“It looks very much like the Moon was habitable at this time. There could have actually been microbes thriving in water pools on the Moon until the surface became dry and dead.”

Looking ahead, there are several missions that are scheduled to explore the lunar surface. These include India’s Chandrayaan-2, a rover and sample analysis mission, and China’s Chang’e 4 and Chang’e 5 rovers – which will explore the southern polar region and conduct a sample return mission, receptively. NASA and Roscosmos also plan to send multiple missions to the Moon in the coming years to map it’s mineralogy, water deposits, and radiation environment.

Some of these missions may be able to obtain samples from volcanic deposits that correspond to the period of heightened volcanic activity that took place 3.5 billion years ago for signs of water and biomarkers. In the meantime, experiments could be conducted on Earth or aboard the ISS to simulate lunar environments to see if microorganisms could survive under the conditions that are predicted to have existed at these times.

If successful, these sample return missions and experiments could indicate that the Moon itself was once a habitable environment. And, with the right kind of geoengineering (aka. terraforming), maybe it could be habitable again someday!

Further Reading: Astrobiology Magazine, Astrobiology

New Insights Into What Might Have Smashed Uranus Over Onto its Side

Uranus

The gas/ice giant Uranus has long been a source of mystery to astronomers. In addition to presenting some thermal anomalies and a magnetic field that is off-center, the planet is also unique in that it is the only one in the Solar System to rotate on its side. With an axial tilt of 98°, the planet experiences radical seasons and a day-night cycle at the poles where a single day and night last 42 years each.

Thanks to a new study led by researchers from Durham University, the reason for these mysteries may finally have been found. With the help of NASA researchers and multiple scientific organizations, the team conducted simulations that indicated how Uranus may have suffered a massive impact in its past. Not only would this account for the planet’s extreme tilt and magnetic field, it would also explain why the planet’s outer atmosphere is so cold.

The study, “Consequences of Giant Impacts on Early Uranus for Rotation, Internal Structure, Debris, and Atmospheric Erosion“, recently appeared in The Astrophysical Journal. The study was led by Jacob Kegerreis, a PhD researcher from Durham University’s Institute for Computational Cosmology, and included members from the Bay Area Environmental Research (BAER) Institute, NASA’s Ames Research Center, the Los Alamos National Laboratory, Descartes Labs, the University of Washington and UC Santa Cruz.

Artist’s impression of the huge impact that is believed have formed the Moon roughly 4.5 billion years ago. Credit: Joe Tucciarone

For the sake of their study, which was funded by the Science and Technology Facilities Council, The Royal Society, NASA and the Los Alamos National Laboratory, the team ran the first high-resolution computer simulations of how massive collisions with Uranus would affect the planet’s evolution. As Kegerries explained in a recent Durham University press release:

“Uranus spins on its side, with its axis pointing almost at right angles to those of all the other planets in the solar system. This was almost certainly caused by a giant impact, but we know very little about how this actually happened and how else such a violent event affected the planet.”

To determine how a giant impact would affect Uranus, the team conducted a suite of smoothed particle hydrodynamics (SPH) simulations, which were also used in the past to model the giant impact that led to the formation of the Moon (aka. the Giant Impact Theory). All told, the team ran more than 50 different impact scenarios using a high-powered computer to see if it would recreate the conditions that shaped Uranus.

In the end, the simulations confirmed that Uranus’ tilted position was caused by a collision with a massive object (between two and three Earth masses) that took place roughly 4 billion years ago – i.e. during the formation of the Solar System. This was consistent with a previous study that indicated that an impact with a young proto-planet made of rock and ice could have been responsible for Uranus’ axial tilt.

Composite image of Uranus by Voyager 2 and two different observations made by Hubble — one for the ring and one for the auroras. Credit: NASA/ESA

“Our findings confirm that the most likely outcome was that the young Uranus was involved in a cataclysmic collision with an object twice the mass of Earth, if not larger, knocking it on to its side and setting in process the events that helped create the planet we see today,” said Kegerries.

In addition, the simulations answered a fundamental questions about Uranus that was raised in response to previous studies. Essentially, scientists have wondered how Uranus could retain its atmosphere after a violent collision, which would have theoretically blown off its out layers of hydrogen and helium gas. According to the team’s simulations, this was most likely because the impact struck a grazing a blow on Uranus.

This would have been enough to alter Uranus’ tilt, but was not strong enough to remove its outer atmosphere. In addition, their simulations indicated that the impact could have jettisoned rock and ice into orbit around the planet. This could then have coalesced to form the planet’s inner satellites and altered the rotation of any pre-existing moons already in orbit around Uranus.

Last, but not least, the simulations offered a possible explanation for how Uranus got its off-center magnetic field and its thermal anomalies. In short, the impact could have created molten ice and lopsided lumps of rock inside the planet (thus accounting for its magnetic field). It could have also created a thin shell of debris near the edge of the planet’s ice layer which would have trapped internal heat, which could explain why Uranus’ outer atmosphere experiences extremely cold temperatures of -216 °C (-357 °F).

This artist’s conception shows the Uranus-sized exoplanet Kepler-421b, which orbits an orange, type K star about 1,000 light-years from Earth. Credit: David A. Aguilar (CfA)

Beyond helping astronomers to understand Uranus, one of the least-understood planets in the Solar System, the study also has implications when it comes to the study of exoplanets. So far, most of the planets discovered in other star systems have been comparable in size and mass to Uranus. As such, the researchers hope their findings will shed light on these planet’s chemical compositions and explain how they evolved.

As Dr. Luis Teodoro – of the BAER Institute and NASA Ames Research Center – and one of the co-authors on the paper, said, “All the evidence points to giant impacts being frequent during planet formation, and with this kind of research we are now gaining more insight into their effect on potentially habitable exoplanets.”

In the coming years, additional missions are planned to study the outer Solar System and the giant planets. These studies will not only help astronomers understand how our Solar System evolved, they could also tell us what role gas giants play when it comes to habitability.

Further Reading: Durham University, The Astrophysical Journal

How Many of Earth’s Moons Crashed Back Into the Planet?

For decades, scientists have pondered how Earth acquired its only satellite, the Moon. Whereas some have argued that it formed from material lost by Earth due to centrifugal force, or was captured by Earth’s gravity, the most widely accepted theory is that the Moon formed roughly 4.5 billion years ago when a Mars-sized object (named Theia) collided with a proto-Earth (aka. the Giant Impact Hypothesis).

However, since the proto-Earth experienced many giant-impacts, several moons are expected to have formed in orbit around it over time. The question thus arises, what happened to these moons? Raising this very question, a team an international team of scientist conducted a study in which they suggest that these “moonlets” could have eventually crashed back into Earth, leaving only the one we see today.

The study, titled “Moonfalls: Collisions between the Earth and its past moons“, recently appeared online and has been accepted for publication in the Monthly Notices of the Royal Astronomical Society. The study was led by Uri Malamud, a postdoctoral fellow from the Technion Israeli Institute of Technology, and included members from the University of Tübingen, Germany, and the University of Vienna.

For the sake of their study, Dr. Malamud and his colleagues – Prof. Hagai B. Perets,  Dr. Christoph Schafer and Mr. Christoph Burger (a PhD student) – considered what would happen if Earth, in its earliest form, had experienced multiple giant impacts that predated the collision with Theia. Each of these impacts would have had the potential to form a sub-Lunar mass “moonlet” that would have interacted gravitationally with the proto-Earth, as well as any possible previously-formed moonlets.

Ultimately, this would have resulted in moonlet-moonlet mergers, the moonlets being ejected from Earth’s orbit, or the moonlets falling to Earth. In the end, Dr. Malamud and his colleagues chose to investigate this latter possibility, as it has not been previously explored by scientists. What’s more, this possibility could have a drastic impact on Earth’s geological history and evolution. As Malamud indicated to Universe Today via email:

“In the current understanding of planet formation the late stages of terrestrial planet growth were through many giant collisions between planetary embryos. Such collisions form significant debris disks, which in turn can become moons. As we suggested and emphasized in this and our previous papers, given the rates of such collisions and the evolution of the moons – the existence of multiple moons and their mutual interactions will lead to moonfalls. It is an inherent, inescapable part of the current planet formation theory.”

However, because Earth is a geologically active planet, and because its thick atmosphere leads to natural weathering and erosion, the surface changes drastically with time. As such, it is always difficult to determine the effects of events that happened during the earliest periods of Earth – i.e. the Hadean Eon, which began 4.6 billion years ago with the formation of the Earth and ended 4 billion years ago.

Artist’s conception of asteroids or comets bearing water to a proto-Earth. Credit: Harvard-Smithsonian Center for Astrophysics

To test whether or not multiple impacts could have taken place during this Eon, resulting in moonlets that eventually fell to Earth, the team conducted a series of smooth particle hydrodynamical (SPH) simulations. They also considered a range of moonlet masses, collision impact-angles and initial proto-Earth rotation rates. Basically, if moonlets did fall to Earth in the past, it would have altered the rotation rate of the proto-Earth, resulting in its current sidereal rotation period of 23 hours, 56 minutes, and 4.1 seconds.

In the end, they found evidence that while direct impacts from large objects were not likely that a number of grazing tidal-collisions could have taken place. These would have caused material and debris to be thrown up into the atmosphere that would have formed small moonlets that would have then interacted with each other. As Malamud explained:

“Our results however do show that in the case of a moonfall, the distribution of the material from the moonfall is not even on the Earth, and therefore such collisions can give rise to asymmetries and composition inhomogeneities. As we discuss in the paper, there are actually possible evidence for the latter – moonfalls can potentially explain the isotopic heterogeneities in highly siderophile elements in terrestrial rocks. In principle a moon collisions may also produce a large scale structure on the Earth, and we speculated that such an effect could have contributed to the formation of Earth’s earliest super-continent. This aspect, however, is more speculative, and it is difficult to directly confirm, given the geological evolution of the Earth since those early times.”

This study effectively extends the current and widely-popular Giant Impact Hypothesis. In accordance with this theory, the Moon formed during the first 10 to 100 million years of the Solar System, when the terrestrial planets were still forming. During the final stages of this period, these planets (Mercury, Venus, Earth and Mars) are believed to have grown mainly through impacts with large planetary embryos.

Image credit: Lynette Cook for Gemini Observatory/AURA
An artist’s depiction of two colliding rocky bodies. Such a collision is the most likely source for the warm dust in the HD 131488 system. Credit: Lynette Cook for Gemini Observatory/AURA

Since that time, the Moon is believed to have evolved due to mutual Earth and Moon tides, migrating outwards to its current location, where it has been ever since. However, this paradigm does not consider impacts that took place before the arrival of Theia and the formation of Earth’s only satellite. As a result, Dr. Malamud and his colleagues assert that it is disconnected from the wider picture of terrestrial planet formation.

By taking into account potential collisions that predate the formation of the Moon, they claim, scientist could have a more complete picture of how both the Earth and the Moon evolved over time. These findings could also have implications when it comes to the study of other Solar planets and moons. As Dr. Malamud indicated, there is already compelling evidence that large-scale collisions affected the evolution of planets and moons.

“On other planets we do see evidence for very large impacts that produced a planet scale topographic features, such as the so-called Mars dichotomy and possibly the dichotomy of Charon’s surface,” he said. “These had to arise from large scale impacts, but small enough as to make sub-global planet features. Moonfalls are natural progenitors of such impacts, but one cannot exclude some other large impacts by asteroids which could produce similar effects.”

There’s also the possibility of such collisions happening in the distant future. According to current estimates of its migration, Mars’ moon Phobos will eventually crash into the surface of the planet. While small compared to the impacts that would have created moonlets and the Moon around Earth, this eventual collision is direct evidence that moonfalls took place in the past and will again in the future.

In short, the history of the early Solar System was violent and cataclysmic, with a great deal of creation resulting from powerful collisions. By having a more complete picture of how these impact events affected the evolution of the terrestrial planets, we may gain new insight into how life-bearing planets formed. This, in turn, could help us track down such planets in extra-solar systems.

Further Reading: arXiv

Through The Nuclear Looking Glass: The Moon & The Bomb

For centuries, scientists have been attempting to explain how the Moon formed. Whereas some have argued that it formed from material lost by Earth due to centrifugal force, others asserted that a preformed Moon was captured by Earth’s gravity. In recent decades, the most widely-accepted theory has been the Giant-impact hypothesis, which states that the Moon formed after the Earth was struck by a Mars-sized object (named Theia) 4.5 billion years ago.

According to a new study by an international team of researchers, the key to proving which theory is correct may come from the first nuclear tests conducted here on Earth, some 70 years ago. After examining samples of radioactive glass obtained from the Trinity test site in New Mexico (where the first atomic bomb was detonated), they determined that samples of Moon rocks showed a similar depletion of volatile elements.

The study was led by James Day – a professor of geoscience at the Scripps Institution of Oceanography at the University of California, San Diego. Along with his colleagues – who hail from the Paris Institute of Earth Physics, the McDonnell Center for the Space Sciences, and NASA’s Johnson Space Center – they examined samples of glass retrieved from the Trinity test site to determine their chemical compositions.

A frame of the ‘Trinity’ fireball, taken .025 seconds after the detonation of the atomic bomb. Credit: US Govt. Defense Threat Reduction Agency

This glass, known as trinite, was created when the plutonium bomb was detonated at the Trinity test site in 1945 as part of the Manhattan Project. To a distance of 350 meters (1,100 feet) from ground zero, arkosic sand (which is primarily composed of quartz grains and feldspar) was converted to green-colored glass by the extreme heat and pressure caused by the massive explosion.

For years, scientists have been studying these glass deposits, which they determined was the result of sand being sucked up into the explosion, and then rained down as molten liquid onto the surface. When Day and his colleagues examined it, they noted that samples of the glass were depleted of zinc and other volatile elements – which are known to evaporate under extreme heat and pressure – depending on how far they were from ground zero.

According to their study, which was published in Science Advances on February 8th, 2017, samples of trinite that were obtained between 10 and 250 meters (30 to 800 feet) from the blast site were depleted of these elements far more than samples that were taken from farther away. In addition, the isotopes of zinc that remained were heavier and less-reactive than in others.

They then compared these results to studies performed on lunar rocks, which showed a similar depletion of volatile elements. From this, they determined that similar heat and pressure conditions existed at one time on the Moon which caused these elements to evaporate. This is consistent with the theory that a massive impact took place in the past that turned the Moon’s surface into an ocean of magma.

A huge impact may have formed the Moon, but other large impacts could have determined the makeup of Earth and other planetary bodies. Image Credit: Joe Tucciarone

As Day explained in a UC San Diego press release:

“The results show that evaporation at high temperatures, similar to those at the beginning of planet formation, leads to the loss of volatile elements and to enrichment in heavy isotopes in the left over materials from the event. This has been conventional wisdom, but now we have experimental evidence to show it.”

While the predominant theory since the 1980s has been the Giant impact hypothesis, the debate has been ongoing and subject to new findings. For example, back in January of 2017, a new study published in Nature Geoscience – which was led by by Raluca Rufu of the Weizmann Institute of Science in Rehovot, Israel – indicated that the Moon may have been the result of many smaller collisions.

Using computer simulations, the Weizmann team found that multiple small impacts could have formed many moonlets around Earth which would have then coalesced to create the Moon. But by showing that volatile elements undergo the same kinds of reactions to heat and pressure, regardless of where the reaction takes place, Day and his colleagues have offered some solid evidence that points towards a single impact event.

This study is just the latest in a series that is helping Earth scientists to put constraints on when and how the Moon formed, which are also helping us to get a better understanding of the history of the Solar System and its formation.

Further Reading: Science Advances, UCSD

Isotopic Evidence of the Moon’s Violent Origins

Artist’s impression of an impact of two planet-sized worlds (NASA/JPL-Caltech)

Scientists have uncovered a history of violence hidden within lunar rocks, further evidence that our large, lovely Moon was born of a cataclysmic collision between worlds billions of years ago.

Using samples gathered during several Apollo missions as well as a lunar meteorite that had fallen to Earth (and using Martian meteorites as comparisons) researchers have observed a marked depletion in lunar rocks of lighter isotopes, including those of zinc — a telltale element that can be “a powerful tracer of the volatile histories of planets.”

The research utilized an advanced mass spectroscopy instrument to measure the ratios of specific isotopes present in the lunar samples. The spectrometer’s high level of precision allows for data not possible even five years ago.

Scientists have been looking for this kind of sorting by mass, called isotopic fractionation, since the Apollo missions first brought Moon rocks to Earth in the 1970s, and Frédéric Moynier, PhD, assistant professor of Earth and Planetary Sciences at Washington University in St. Louis — together with PhD student, Randal Paniello, and colleague James Day of the Scripps Institution of Oceanography — are the first to find it.

The team’s findings support a now-widely-accepted hypothesis — called the Giant Impact Theory, first suggested by PSI scientists William K. Hartmann and Donald Davis in 1975 — that the Moon was created from a collision between early Earth and a Mars-sized protoplanet about 4.5 billion years ago. The effects of the impact eventually formed the Moon and changed the evolution of our planet forever — possibly even proving crucial to the development of life on Earth.

(What would a catastrophic event like that have looked like? Probably something like this:)

Read more: What’s the Moon Made Of? Earth, Most Likely.

“This is compelling evidence of extreme volatile depletion of the moon,” said Scripps researcher James Day, a member of the team. “How do you remove all of the volatiles from a planet, or in this case a planetary body? You require some kind of wholesale melting event of the moon to provide the heat necessary to evaporate the zinc.”

In the team’s paper, published in the October 18 issue of Nature, the researchers suggest that the only way for such lunar volatiles to be absent on such a large scale would be evaporation resulting from a massive impact event.

“When a rock is melted and then evaporated, the light isotopes enter the vapor phase faster than the heavy isotopes, so you end up with a vapor enriched in the light isotopes and a solid residue enriched in the heavier isotopes. If you lose the vapor, the residue will be enriched in the heavy isotopes compared to the starting material,” explains Moynier.

The fact that similar isotopic fractionation has been found in lunar samples gathered from many different locations indicates a widespread global event, and not something limited to any specific regional effect.

The next step is finding out why Earth’s crust doesn’t show an absence of similar volatiles, an investigation that may lead to clues to where Earth’s surface water came from.

“Where did all the water on Earth come from?” asked Day. “This is a very important question because if we are looking for life on other planets we have to recognize that similar conditions are probably required. So understanding how planets obtain such conditions is critical for understanding how life ultimately occurs on a planet.”

“The work also has implications for the origin of the Earth,”  adds Moynier, “because the origin of the Moon was a big part of the origin of the Earth.”

Read more on the Washington University news release and at the UC San Diego news center.

Inset image: Cross-polarized transmitted-light image of a lunar rock. Photo by James Day, Scripps/UCSD