Dasgupta's group previously investigated how carbon would have fared on the early, molten Earth: Even if the carbon didn't vaporize into space, it would have been drawn into the metallic core after bonding with iron-rich alloys,Dasgupta said.

Researchers have postulated that carbon and other elements now present on Earth came from meteorites or comets hitting the planet.

"One popular idea has been that volatile elements like carbon, sulfur, nitrogen and hydrogen were added after Earth's core finished forming," the study's lead author, Yuan Li, now a staff scientist at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, said in the statement. "Any of those elements that fell to Earth in meteorites and comets more than about 100 million years after the solar system formed could have avoided the intense heat of the magma ocean that covered Earth up to that point.

But the real-life proportions of elements on Earth don't seem to match up with comets and meteorites: "The problem with that idea is that while it can account for the abundance of many of these elements, there are no known meteorites that would produce the ratio of volatile elements in the silicate portion of our planet," he added, referring to a main component of rocks in Earth's mantle, the 1,800-mile-deep (2,890 kilometers) soup of magma and rock between the crust and core.

The researchers' experimental setup re-created the high-temperature and high-pressure interior of Earth by squeezing rocks with hydraulic presses. This method can emulate conditions about 250 miles (400 km) below Earth's surface, in the planet's mantle. The investigators decided to test whether the elements silicon or sulfur — found in the cores of Mercury and Mars, respectively — had any impact on the amount of carbon that would be bound up within Earth's core.

The scientists found that when sulfur bonded with iron under intense pressure — like that in Earth's core — it kept carbon from bonding with the molecules in the core as strongly, the researchers said in the statement. (In the paper,published in the journal Nature Geoscience on Sept. 5, the scientists said that "in reduced or sulfur-rich bodies, carbon is expelled from the segregating core.") In that scenario, the carbon could remain higher up, in Earth's mantle, and be available later on for the development of life instead of being locked away, the study said.

The researchers then compared the concentrations of carbon that emerged during the experiments with the concentrations of elements in Earth's actual mantle.

"One scenario that explains the carbon-to-sulfur ratio and carbon abundance is that an embryonic planet like Mercury, which had already formed a silicon-rich core, collided with and was absorbed by Earth," Dasgupta said. "Because it's a massive body, the dynamics could work in a way that the core of that planet would go directly to the core of our planet, and the carbon-rich mantle would mix with Earth's mantle.

"In this paper, we focused on carbon and sulfur," he said. "Much more work will need to be done to reconcile all of the volatile elements, but at least in terms of the carbon-sulfur abundances and the carbon-sulfur ratio, we find this scenario could explain Earth's present carbon and sulfur budgets."

Email Sarah Lewin at slewin@space.com or follow her @SarahExplains. Follow us @Spacedotcom, Facebook and Google+. Original article onSpace.com.

Virgin Galactic's SpaceShipTwo vehicles are designed to carry two pilots and six passengers on launches into suborbital space. The spacecraft cannot orbit the Earth, but it's expected to provide passengers several minutes of weightlessness and expansive views of the Earth from space before re-entering the atmosphere and gliding to a runway landing. Virgin Galactic is selling tickets for the experience at $250,000 per seat.

Thursday's test flight took off from the Mojave Air and Space Port, with the VMS Eve carrying the VSS Unity to an altitude of about 50,000 feet (15,000 meters). That's the target altitude for the separation between a carrier plane and SpaceShipTwo during an actual launch. From takeoff to landing, the test flight lasted 3 hours and 43 minutes.

"Today's flight test is going well," Virgin Galacticrepresentatives wrote on Twitter during the flight. "It'll be longer than a typical mission, in part to test what happens when Unity gets very cold."

The test flight also marked another milestone for Virgin Galactic, Branson added.

"Any time a new vehicle is tested in the air is a big deal," Branson wrote. "The significance of yesterday's flight is that it is the first flight of a spaceship built entirely by The Spaceship Company and operated by Virgin Galactic." 

Virgin Galactic unveiled the VSS Unity spacecraftearlier this year, on Feb. 19. The spacecraft was already under construction by The Spaceship Company — a Mojave, California-based subsidiary of Virgin Galactic — at the time of the October 2014 accident.

The Enterprise was in the midst of a rocket-powered test flight when it broke apart over the Mojave Desert, killing co-pilot Michael Alsbury and seriously injuring pilot Peter Siebold. An investigation into the crash by the U.S. National Transportation Safety Board found that Alsbury deployed the space plane's "feathering" re-entry system — twin tail booms designed to rotate upward during descent to increase drag and stability — too early. Virgin Galactic and The Spaceship Company subsequently built new safeguards into VSS Unity to prevent the type of accident that doomed Enterprise. 

The memory of that 2014 accident seemed to be fresh on the collective mind of the Virgin Galactic team, based on the company's statements.

"Today's an emotional day for us all. So grateful to our families, customers, & friends for sticking with us," Virgin Galactic representatives wrote on Twitter Thursday afternoon. "Couldn't open space without you."

According to Virgin Galactic, Thursday's captive carry test flight is the first in a rigorous series of upcoming trials for VSS Unity. But the company stressed that it will only move forward after a thorough analysis of this week's test. 

"With this flight in the books, our team will now analyze a mountain of flight data, learning what worked well and what could be improved for our next flight test," Virgin Galactic representatives wrote. "Only when that analysis is done, along with detailed vehicle inspections, some already-planned work, and potentially more captive carry flights, will we be ready to move into the next phase of test flight."

Email Tariq Malik at tmalik@space.com or follow him @tariqjmalik and Google+. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

New life-forms?

The Nankai Trough is located near the superhot border of two clashing tectonic plates, where underwater volcanoes bubble like cauldrons. The samples that researchers will collect may reach temperatures of 266 degrees Fahrenheit (130 degrees Celsius). Gathering such hot samples in other areas would require drilling to about 2.5 miles (4 km) below the seafloor, rather than the 0.75 miles currently planned, the researchers said.

"We may discover some unknown life-forms that can survive or adapt to the extremely challenging deep and hot sedimentary environment," Fumio Inagaki, the expedition's co-chief scientist and a researcher at the Japan Agency for Marine-Earth Science and Technology, told Live Science. "The nature and extent of the deep sub-seafloor biosphere is still largely unknown."

Analyzing such deep-sea microbes could help scientists inthe search for alien life. "We have already known, based on previous DNA-based studies, that most microbes living in marine subsurface sediments are evolutionarily distinct from known life in the Earth's surface biosphere," Inagaki said. "In this sense, we may have already seen alien life in the different world on our planet," since they are distinct from life on the surface, he noted.

"Our study will provide some clues to understand habitability of deep life not only below the ocean floor of our planet, but also inside of other celestial bodies," Inagaki added. A deeper understanding of how life might live in extreme places on Earth could shed light on whether or not it could exist in similarly extreme places elsewhere.

A globular, single-celled organism called Geogemma barossii tolerates the highest temperatures of any catalogued life-form on Earth. The microbe was discovered in 2003, in deep-sea hydrothermal vents at the Juan de Fuca Ridge off the northwest U.S. coast, where it withstands temperatures of about 250 degrees F (120 degrees C). The new samples that scientists hope to collect as part of the International Ocean Discovery Program's (IODP) Expedition 370 may yield microbes that break the current record for the hottest temperature at which life on Earth can exist. [Axial Seamount: Images of an Erupting Undersea Volcano]

Like a trip to outer space

Chikyu is the world's largest scientific research ship, measuring 689 feet long, 125 feet wide and 393 feet high (210 by 38 by 120 m), Inagaki said. A system of thrusters and GPS sensors also makes the ship quite stable, "enabling the vessel to keep at the same position for deep drilling under strong currents," he added.

Indeed, an ordinary research vessel wouldn't suffice for such an extreme mission.

"This expedition is as complex as a mission to outer space might be," Kai-Uwe Hinrichs, a researcher at the University of Bremen in Germany and lead author of the scientific proposal behind the expedition, said in a statement.

"It requires the technology to 'land' the coring bit on the right spot in over 4-kilometer-deep water, drill through ancient ocean sediments to collect samples far below the ocean floor, bring them back onboard intact, then transport them by helicopter to the super-clean geomicrobiology laboratory to ensure no contamination," Hinrichs added. "Like a space mission, this expedition is fraught with complexity, danger and vast opportunity for discovery."

After the samples are collected, they will be sent on a 1-hour helicopter ride to a superclean lab onshore (to prevent contamination), where scientists will investigate them further. The researchers will analyze the qualities of the sediments and count minuscule and sparse cells, looking for as few as 100 cells per 0.06 cubic inches (1 cubic centimeter), or about the equivalent of 100 sand grains floating in an Olympic-size swimming pool.

"Looking for life in core samples is like looking for a needle in a haystack," Yuki Morono, the expedition's co-chief scientist and a researcher at the Kochi Institute for Core Sample Research in Japan, said in the statement. "At the surface, the sediments are teeming with microbial cells, but in samples from deeper in the core, the cells become far more sparse."

The aim of the expedition is to define the limits to deep-dwelling life by exploring the so-called biotic fringe, the boundary where current knowledge predicts that no living cells persist. The mission will examine whether this boundary is sharp, diffuse or exists at all.

"We have the extraordinary opportunity to explore the depth at which sediments and rocks become too hot for life, even for microbes that can live at temperatures greater than 85 degrees C [185 degrees F]," Verena Heuer, the expedition's co-chief scientist and a researcher at the University of Bremen in Germany, said in the statement.

"The gradual increase in temperature from approximately 30 degrees C to 130 degrees C [86 to 266 degrees F] in the sampled sediments will give us the opportunity to explore how microbial life changes with increasing temperatures and ultimately ceases to exist," Heuer added. "Is this lower boundary of the sub-seafloor's habitable zone like a rigid brick wall, or is it like a leaky fence?"

The researchers will also use DNA-sequencing technology to determine the ancestry of deep-sea microbes and how they adapt to such extreme environments. During the cruise, they will share their experiences via blogs and the Expedition 370 website.

The public is also invited to guess the heat limit of deep life via an online contest.

Original article on Live Science.

And the thing is, it didn't detect anything. That non-discovery is what's causing physicists to rethink how they think about dark matter.

Holding up galaxies

Dark matter is an answer to a nearly century-old problem. In the early 1930s, shortly after astronomers realized that the universe consisted of countless galaxies, scientists turned their attention to understanding the dynamics of how stars orbited within the galaxies — essentially, how galaxies rotate. Dutch astronomer Jan Oort applied Newton's laws of motion and gravity to the observed matter in our own Milky Way and found that our galaxy rotated faster than he had calculated. It appeared that the Milky Way had twice the mass that astronomers had estimated. Of course, this was in an era in which precise galactic astronomy was coming into existence and a disagreement between calculation and measurement of only a factor of two was considered excellent agreement.

However, shortly after Oort's measurement, Bulgarian-Swiss astronomer Fritz Zwicky was studying the Coma Cluster, a large group of more than a thousand galaxies that were bound together by their mutual gravity in an enormous structure. When he measured the velocity of the galaxies, he found that they, too, were moving far too fast for gravity's gentle tug to hold them together. By all rights, the cluster should have torn itself apart. But it didn't. He calculated that the cluster held 400 times more matter than could be seen by ordinary telescopes. Modern measurements have reduced that number, but the discrepancy still is thought to be a factor of 100. Zwicky proposed that there was a type of unseen matter holding together the cluster that he called "dunkle Materie," or dark matter. [TED-Ex Talk: How Does Dark Matter Explain a Star's Speed?]

In the 1970s, astronomer Vera Rubin was trying to find a noncontroversial topic to study when she turned her attention to the rotation curves of galaxies. This is a measure of the orbital speed of stars in galaxies as a function of their distance from the center. She found that the measurements agreed with predictions very well at the center of the galaxy and even approaching the periphery. But, in the very outskirts of the galaxy, stars were orbiting far faster than could be accommodated by the known laws of physics and the observed matter. It looked like Zwicky's dark matter might be showing up in the interiors of galaxies as well. (Clearly, Rubin failed epically in her attempt to find a noncontroversial topic.)

Over the decades, several ideas have been proposed to explain an extensive array of astronomical mysteries, from the possibility that Newton's laws of motion might not apply when accelerations become small, to the idea that both Newton and Einstein were wrong aboutgravity. These hypotheses have not survived rigorous testing. Another idea was that perhaps there exist types of matter in the universe that do not emit electromagnetic energy … this was Zwicky's dark matter.

But, even here, there were many possibilities. The most plausible option was that the universe was home to a menagerie of black holes, brown dwarfs, rogue planets and other dark objects that consisted of the same kind of ordinary matter that makes up the visible component of the universe. These objects are massive and compact and cold enough that they don’t emit light like stars.  Some objects like these were found, but not enough to solve the mystery.  And so, astronomical surveys in the 1990s completely ruled out this idea, too. Taking a page from Sherlock Holmes in “The Sign of Four,” in which he said, “When you have ruled out the impossible, whatever remains, however improbable, must be the truth,” scientists have been forced to conclude that a new form of invisible dark matter permeates the universe. Perhaps even more surprising, there appear to be five times more dark matter than ordinary matter.

The properties of dark matter

We have never directly observed dark matter, but we know a great deal about what it must be: It must be massive (because it affects the rotation of galaxies); it must be electrically neutral (because we can't see it); it must be different from ordinary matter (because we see no evidence for it interacting with matter in the usual ways); and it must be stable (because it has existed since the dawn of the universe). These properties are unequivocal.

However, we don't know exactly what it is. In the most popular generic theory, the dark-matter particle is called a WIMP, for weakly interacting massive particle. WIMPs are kind of like heavy neutrons (but definitely not neutrons), with a mass of 10 to 100 times heavier than a proton. They were created in great quantities during the Big Bang and a small relic remainder persists to this day.

When cosmologists add the WIMP idea to their ideas of the Big Bang, they can calculate how it would interact. They find that in the earliest stages of the universe, WIMPs were a dominant form of matter, but, as the universe expands and cools and the interaction rate drops to zero, the relic amount of WIMPs is about five times as massive as ordinary matter. Combined with the fact that the WIMP idea can also explain a pressing theoretical mystery of why the Higgs boson particle has such a small mass, scientists call this the "WIMP miracle," in view of the fact that the WIMP idea seems to answer so many questions.

A dark non-discovery

So this brings us back to the LUX experiment. It is simply the latest and most powerful experiment designed to detect WIMPs. The idea is that the detector will sit underground for a long time, say a year or more, and a rare WIMP will bump into a xenon atom and be detected. (The detector is buried underground to protect it from cosmic rays, which would bump into xenon atoms and fake a dark matter detection. Being half a mile or a mile underground stops nearly all cosmic rays.)

On Aug. 27, the LUX researchers announced their results. They saw no evidence for dark-matter WIMPs.

So that's not actually that surprising. There have been dozens of experiments that have looked for dark matter and found nothing. In this sense, LUX has joined a venerable group. But LUX is also far more capable. It was expected to have a better shot at finding WIMPs than any other apparatus.

But it didn't.

So does that mean that the WIMP idea is dead? No, not really. Dark matter detectors are optimized to be most sensitive to certain masses, kind of like how a particular microphone might pick up tenor voices better than they hear soprano or bass ones.  LUX was optimized to find a WIMP with a mass of a few tens of times heavier than a proton. In fact, it ruled out a WIMP having a mass 50 times of a proton with impressive rejection. However, LUX doesn't do so well at detecting WIMPs with a mass below 10 times that of a proton.  And, if WIMPs exist and have a mass of 1,000 times heavier than a proton, this is also not LUX's optimum range for investigation. So there remains a range of masses where a WIMP might exist.

Axions, dark life and dark charge

Even so, WIMPs were simply the most popular idea for dark matter. There are other hypotheses. One hypothesis points to a particle called a sterile neutrino, which is a cousin of the more familiar neutrino generated in nuclear reactions. In fact, neutrinos from the biggest nearby nuclear reactor (the sun) are constantly barraging the Earth.  Unlike regular neutrinos, which are ultralight and interact via the weak nuclear force, sterile neutrinos are heavy and don't experience the weak force. It is the heavy mass and non-interacting qualities that make the sterile neutrino an ideal dark-matter candidate.

Another possible dark-matter particle, the axion, was proposed in 1977 as a way to ensure that the strong nuclear force treated matter and antimatter on equal footing (so as agree with observations). The axion is a very light, but still massive, hypothetical particle. The LUX detector is not designed to study axions.

Then, of course, there is the even more creative hypothesis, which suggests that dark matter isn't a single, neutral, non-interacting particle. After all, ordinary matter is pretty complicated. At the quantum scale, we have quarks and leptons and four forces. At the macro scale, we have you and me and sugar and stars and volcanoes and all the various ways they interact. Ordinary matter has all sorts of interactions and constituents.  Why not dark matter? [7 Strange Facts About Quarks]

Under this hypothesis, dark matter might have a "dark charge," or dark matter's form of electrical charge. In the same way that dark matter doesn't experience dark charge, maybe ordinary matter doesn't experience dark charge. This dark charge would interact with dark photons, or the dark twins of ordinary light particles.

And, even more daring, perhaps dark matter experiences many forces and there is an entire complex dark sector, with dark atoms and dark molecules and perhaps even dark life. We are able to set constraints on these possible dark interactions; for instance, we know enough to rule out dark stars and planets, but dark asteroids are possible. Again, LUX is silent on these new ideas.

It's not fair to say that the LUX measurement leads to a crisis in particle physics and cosmology. But it certainly gives scientists pause and suggests that maybe we should take another look at this WIMP thing. Maybe other ideas need to be revisited.  On the other hand, scientists who want to continue to pursue the WIMP idea still have something to look forward to as technology advances. LUX uses a third of a ton of liquid xenon. In 10 or 15 years, scientists are planning to build detectors that might contain 100 tons, providing even more chances of capturing that rare WIMP interaction. These are heady times to be a dark-matter scientist.

But, in the end, we still don't know. We just know that the capabilities of LUX are good enough that maybe it's time to broaden our thinking. In the words of the rock band Buffalo Springfield, "There's something happening here. What it is ain't exactly clear..."

Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook, Twitter and Google+. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Live Science.

Defending Earth

After the sun formed, leftover dust and rock circled the newborn star in a disk. The material smashed together, building planetary embryos. Some bits of debris hurled down on the young planets, while some material was cast out of the solar system forever. Over time, the fiery rain slowed as the population of dust and rock dwindled.

As life evolved on Earth, bits of remaining debris occasionally crashed down — at times, wiping out plants and animal species. While most of the largest rocks have already collided with planets or have left the solar system, enough leftover material remains to pepper our planet. 

"Over a hundred tons of material fall to Earth every day," Johnson said. 

Most of that is dust, but occasionally, a solid chunk of rock makes it to the ground (a meteorite). Those meteorites can be chunks that have broken off of asteroids, but they also may be debris that was once part of a planet or the moon and was blown off in an early collision. For example, Earth is covered in meteorites that were once part of the moon and Mars — places humans have been able to study up close with various probes, rovers and (in the case of the moon) human missions. 

But asteroids are more challenging. The sample of Bennu that scientists are aiming to return in the OSIRIS-REx mission will help scientists better understand the composition of other asteroids and space debris if they were to prove threatening.

During the OSIRIS-REx mission, scientists also will study how the sun affects the orbital paths of small rocky bodies. As sunlight strikes a rotating asteroid, the radiating heat provides a tiny thrust that can change the orbit of the rock — a process known as the Yarkovsky effect. 

"We can't just determine where an asteroid is going to go just by orbital dynamics," said Alex Young, a heliophysics scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We have to account for this Yarkovsky effect."

Observing how the sun ever so slightly redirects Bennu will help scientists to improve their models of other asteroids and their potential to impact Earth.

On top of all that, guiding OSIRIS-REx into orbit around Bennu will help NASA learn more about manipulating a spacecraft around small bodies.

"NASA's gotten very skilled at going to Mars, Saturn, through multibody systems," said NASA Chief Scientist Ellen Stofan. But smaller objects are different, and maneuvering OSIRIS-REx is a step toward landing humans on asteroids and the moons of Mars, she said.

Shifting orbits

Debris from the early solar system still floats between the planets. While most of the largest objects have been identified, NASA estimates that only half of the rocks the size of Bennu have been spotted.

"We've got to find them first before we can do anything about them," Johnson said.

For that, planetary defense researchers turn to NASA's NEOWISE, a repurposed telescope mission to identify near-Earth objects. The instrument has helped scientists to build a catalog of rocks near the planet, some passing closer than satellites.

Observation isn't the only tool for planetary defense. In partnership with the European Space Agency, NASA is working on the Double Asteroid Redirection Test (DART). The two agencies plan to send a spacecraft to rendezvous with an asteroid and its moon. By deliberately crashing into the tiny moonlet, DART will shift its course by "a fraction of a percent"according to NASA. The mission will test the ability of the space agencies to deflect an asteroid years before it collides with Earth.

Johnson described DART as "a kinetic impactor that would impact the moon of Didymos, which we affectionately call Didymoon."

Another upcoming project is planning to bring a boulder-size asteroid sample to the moon, where astronauts can remove samples to carry back to Earth. After traveling to an asteroid and selecting a hunk to take home, the Asteroid Redirect Mission (ARM) will attempt to change the path of the asteroid by using its mass as what Johnson called "a natural tug rope." 

"Gravity will pull the asteroid off its natural orbit," he said.

Asteroid Basics: 

Asteroids are fascinating for lots of reasons. They contain a variety of valuable resources and slam into our planet on a regular basis, occasionally snuffing out most of Earth's lifeforms. How much do you know about space rocks?

Of course, NASA doesn't have a mission ready to deflect incoming danger. Johnson told Space.com that it will take somewhere from five to 10 years to launch a mission that will effectively deflect a specific threat.

Earth-threatening asteroids aren't prolific in the solar system. Johnson said that serious impacts that might be worth deflecting happen only about once every couple of centuries.

"It's not something I worry about a lot on a regular basis," he said.

But that doesn't mean we should become complacent. 

"Since we don't know everything that's out there, it's something that could happen almost any day," he said. 

He suggested that the best way to avert disaster is to improve the ability to identify potentially hazardous objects. The sooner large, dangerous objects are identified, the more time humans on Earth have to take the necessary steps.

"We need to improve our capabilities to detect and find them further out in space so we have time to prepare," he said.

Follow Nola Taylor Redd on Twitter @NolaTRedd and Google+. Follow us at@Spacedotcom, Facebook or Google+. Originally published on Space.com.