Ceres: The tiny world where volcanoes erupt ice


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Dawn Science Team NASA/JPL-Caltech/GSFC

Dawn's Framing Camera looks down on the fractured summit of Ahuna Mons, tallest mountain on dwarf planet Ceres. The cracks on top suggest Ahuna grew by inflation: icy freezing water pushed up inside the mountain, making a dome. (This image the following one have the same scale orientation, are taken from the <em>Science</em> paper.)

Dawn Science Team NASA/JPL-Caltech/GSFC

Researchers draped a digital terrain model over a shaded image of Ahuna, placing contour lines at 100 meter (330 foot) elevation intervals. Key spot elevations are shown in meters.

Dawn Science Team NASA/JPL-Caltech/GSFC

Ahuna Mons is a volcano that rises 13,000 feet high spreads 11 miles wide at its base. This would be impressive for a volcano on Earth. But Ahuna Mons stands on Ceres, a dwarf planet less than 600 miles wide that orbits the Sun between Mars Jupiter. Even stranger, Ahuna Mons isn’t built from lava the way terrestrial volcanoes are — it’s built from ice. “Ahuna is the one true ‘mountain’ on Ceres,” said David Williams, associate research professor in Arizona State University’s School of Earth Space Exploration. “After studying it closely, we interpret it as a dome raised by cryovolcanism.”

This is a form of low-temperature volcanic activity, where molten ice — water, usually mixed with salts or ammonia — replaces the molten silicate rock erupted by terrestrial volcanoes. Giant mountain Ahuna is a volcanic dome built from repeated eruptions of freezing salty water.

Williams is part of a team of scientists working with NASA’s Dawn mission who have published papers in the journal Science this week. His specialty is volcanism, that drew him to the puzzle of Ahuna Mons.

“Ahuna is truly unique, being the only mountain of its kind on Ceres,” he said. “It shows nothing to indicate a tectonic formation, so that led us to consider cryovolcanism as a method for its origin.”

Dawn scientist Ottaviano Ruesch, of NASA’s Goddard Space Flight Center, Greenbelt, Maryland, is the lead author on the Science paper about Ceres volcanism. He says, “This is the only known example of a cryovolcano that potentially formed from a salty mud mix, which formed in the geologically recent past.”

Williams explained that “Ahuna has only a few craters on its surface, which points to an age of just couple hundred million years at most.”

According to the Dawn team, the implications of Ahuna Mons being volcanic in origin are enormous. It confirms that although Ceres’ surface temperature averages almost -40° (Celsius or Fahrenheit; the scales converge at this temperature), its interior has kept warm enough for liquid water or brines to exist for a relatively long period. And this has allowed volcanic activity at the surface in recent geological time.

Ahuna Mons is not the only place where icy volcanism happens on Ceres. Dawn’s instruments have spotted features that point to cryovolcanic activity that resurfaces areas rather than building tall structures. Numerous craters, for example, show floors that appear flatter than impacts by meteorites would leave them, so perhaps they have been flooded from below. In addition, such flat-floored craters often show cracks suggesting that icy “magma” has pushed them upward, then subsided.

A few places on Ceres exhibit a geo-museum of features. “Occator Crater has several bright spots on its floor,” said Williams. “The central spot contains what looks like a cryovolcanic dome, rich in sodium carbonates.” Other bright spots, he says, occur over fractures that suggest venting of water vapor mixed with bright salts.

“As the vapor has boiled away,” he explained, “it leaves the bright 1salts carbonate minerals behind. ”

Looking inside

Although volcanic-related features appear across the surface of Ceres, for scientists perhaps the most interesting aspect is what these features say about the interior of the dwarf world. Dawn observations suggest that Ceres has an outer shell that’s not purely ice or rock, but rather a mixture of both.

Recently, Williams was involved in research that discovered that large impact craters are missing, presumably erased by internal heat, but smaller craters are preserved. “This shows that Ceres’ crust has a variable composition — it’s weak at large scales but strong at smaller scales,” he said. “It has also evolved geologically.”

In the big picture, said Williams, “Ceres appears differentiated internally, with a core a complex crust made of 30 to 40 percent water ice mixed with silicate rock salts.” And perhaps pockets of brine still exist in its interior.

“We need to continue studying the data to better understthe interior structure of Ceres,” said Williams.

Ceres is the second port of call for the Dawn mission, which was launched in 2007 visited another asteroid, Vesta, from 2011 to 2012. The spacecraft arrived at Ceres in March 2015. It carries a suite of cameras, spectrometers, gamma-ray neutron detectors. These were built to image, map, measure the shape surface materials of Ceres, they collect information to help scientists understthe history of these small worlds what they can tell us of the solar system’s birth.

NASA plans for Dawn to continue orbiting Ceres collecting data for another year or so. The dwarf planet is slowly moving toward its closest approach to the Sun, called perihelion, which will come in April 2018. Scientists expect that the growing solar warmth will produce some detectable changes in Ceres’ surface or maybe even trigger volcanic activity.

“We hope that by observing Ceres as it approaches perihelion, we might see some active venting. This would be an ideal way to end the mission,” said Williams.


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Images from Sun’s edge reveal origins of solar wind


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NASA’s Goddard Space Flight Center/Lisa Poje

Views of the solar wind from NASA's STEREO spacecraft (left) after computer processing (right). Scientists used an algorithm to dim the appearance of bright stars dust in images of the faint solar wind. This innovation enabled them to see the transition from the corona to the solar wind. It also gives us the first video of the solar wind itself in a previously unmapped region.

data from Craig DeForest, SwRI

Computer-processed data of the solar wind is shown.

data from Craig DeForest, SwRI

Ever since the 1950s discovery of the solar wind – the constant flow of charged particles from the sun – there’s been a stark disconnect between this outpouring the sun itself. As it approaches Earth, the solar wind is gusty turbulent. But near the sun where it originates, this wind is structured in distinct rays, much like a child’s simple drawing of the sun. The details of the transition from defined rays in the corona, the sun’s upper atmosphere, to the solar wind have been, until now, a mystery. Using NASA’s Solar Terrestrial Relations Observatory, or STEREO, scientists have for the first time imaged the edge of the sun described that transition, where the solar wind starts. Defining the details of this boundary helps us learn more about our solar neighborhood, which is bathed throughout by solar material – a space environment that we must understto safely explore beyond our planet. A paper on the findings was published in The Astrophysical Journal on Sept. 1, 2016.

“Now we have a global picture of solar wind evolution,” said Nicholeen Viall, a co-author of the paper a solar scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “This is really going to change our understanding of how the space environment develops.”

Both near Earth far past Pluto, our space environment is dominated by activity on the sun. The sun its atmosphere are made of plasma – a mix of positively negatively charged particles which have separated at extremely high temperatures, that both carries travels along magnetic field lines. Material from the corona streams out into space, filling the solar system with the solar wind.

But scientists found that as the plasma travels further away from the sun, things change: The sun begins to lose magnetic control, forming the boundary that defines the outer corona – the very edge of the sun.

“As you go farther from the sun, the magnetic field strength drops faster than the pressure of the material does,” said Craig DeForest, lead author of the paper a solar physicist at the Southwest Research Institute in Boulder, Colorado. “Eventually, the material starts to act more like a gas, less like a magnetically structured plasma.”

The breakup of the rays is similar to the way water shoots out from a squirt gun. First, the water is a smooth unified stream, but it eventually breaks up into droplets, then smaller drops eventually a fine, misty spray. The images in this study capture the plasma at the same stage where a stream of water gradually disintegrates into droplets.

Before this study, scientists hypothesized that magnetic forces were instrumental to shaping the edge of the corona. However, the effect has never previously been observed because the images are so challenging to process. Twenty million miles from the sun, the solar wind plasma is tenuous, contains free-floating electrons which scatter sunlight. This means they can be seen, but they are very faint require careful processing.

In order to resolve the transition zone, scientists had to separate the faint features of the solar wind from the background noise light sources over 100 times brighter: the background stars, stray light from the sun itself even dust in the inner solar system. In a way, these images were hiding in plain sight.

Images of the corona fading into the solar wind are crucial pieces of the puzzle to understanding the whole sun, from its core to the edge of the heliosphere, the region of the sun’s vast influence. With a global perspective, scientists can better understthe large-scale physics at this critical region, which affect not only our planet, but also the entire solar system.

Such observations from the STEREO mission – which launched in 2006 – also help inform the next generation of sun-watchers. In 2018, NASA is scheduled to launch the Solar Probe Plus mission, which will fly into the sun’s corona, collecting more valuable information on the origin evolution of the solar wind.

STEREO is the third mission in NASA Heliophysics Division’s Solar Terrestrial Probes program, which is managed by Goddard for the Science Mission Directorate, in Washington, D.C.


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Discovery one-ups Tatooine, finds twin stars hosting three giant exoplanets


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Illustration is courtesy of Timothy Rodigas.

A team of Carnegie scientists has discovered three giant planets in a binary star system composed of stellar ”twins” that are also effectively siblings of our Sun. One star hosts two planets the other hosts the third. The system represents the smallest-separation binary in which both stars host planets that has ever been observed. The findings, which may help explain the influence that giant planets like Jupiter have over a solar system’s architecture, have been accepted for publication in The Astronomical Journal. New discoveries coming from the study of exoplanetary systems will show us where on the continuum of ordinary to unique our own Solar System’s layout falls. So far, planet hunters have revealed populations of planets that are very different from what we see in our Solar System. The most-common exoplanets detected are so-called super-Earths, which are larger than our planet but smaller than Neptune or Uranus. Given current statistics, Jupiter-sized planets seem fairly rare–having been detected only around a small percentage of stars.

This is of interest because Jupiter’s gravitational pull was likely a huge influence on our Solar System’s architecture during its formative period. So the scarcity of Jupiter-like planets could explain why our home system is different from all the others found to date.

The new discovery from the Carnegie team is the first exoplanet detection made based solely on data from the Planet Finder Spectrograph–developed by Carnegie scientists mounted on the Magellan Clay Telescopes at Carnegie’s Las Campanas Observatory. PFS is able to find large planets with long-duration orbits or orbits that are very elliptical rather than circular, including the new trio of planets discovered in this `”twin'” star study. This special capability comes from the long observing baseline of PFS; it has been taking observations for six years.

Led by Johanna Teske, the team included a number of Carnegie scientists from both the Department of Terrestrial Magnetism in Washington, DC, the Carnegie Observatories in Pasadena, CA, as well as Steve Vogt of the University of California Santa Cruz.

“We are trying to figure out if giant planets like Jupiter often have long and, or eccentric orbits,” Teske explained. “If this is the case, it would be an important clue to figuring out the process by which our Solar System formed, might help us understwhere habitable planets are likely to be found.”

The twin stars studied by the group are called HD 133131A HD 133131B. The former hosts two moderately eccentric planets, one of which is, at a minimum, about 1 a half times Jupiter’s mass the other of which is, at a minimum, just over half Jupiter’s mass. The latter hosts one moderately eccentric planet with a mass at least 2.5 times Jupiter’s.

The two stars themselves are separated by only 360 astronomical units (AU). One AU is the distance between the Earth the Sun. This is extremely close for twin stars with detected planets orbiting the individual stars. The next-closest binary system that hosts planets is comprised of two stars that are about 1,000 AU apart.

The system is even more unusual because both stars are “metal poor,” meaning that most of their mass is hydrogen helium, as opposed to other elements like iron or oxygen. Most stars that host giant planets are “metal rich.” Only six other metal-poor binary star systems with exoplanets have ever been found, making this discovery especially intriguing.

Adding to the intrigue, Teske used very precise analysis to reveal that the stars are not actually identical “twins” as previously thought, but have slightly different chemical compositions, making them more like the stellar equivalent of fraternal twins.

This could indicate that one star swallowed some baby planets early in its life, changing its composition slightly. Alternatively, the gravitational forces of the detected giant planets that remained may have had a strong effect on fully-formed small planets, flinging them in towards the star or out into space.

“The probability of finding a system with all these components was extremely small, so these results will serve as an important benchmark for understanding planet formation, especially in binary systems,” Teske explained.


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Anomalous grooves on Martian moon Phobos explained by impacts


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ESA/Mars Express, modified by Nayak & Asphaug

Some of the mysterious grooves on the surface of Mars’ moon Phobos are the result of debris ejected by impacts eventually falling back onto the surface to form linear chains of craters, according to a new study. One set of grooves on Phobos are thought to be stress fractures resulting from the tidal pull of Mars. The new study, published August 19 in Nature Communications, addresses another set of grooves that do not fit that explanation.

“These grooves cut across the tidal fields, so they require another mechanism. If we put the two together, we can explain most if not all of the grooves on Phobos,” said first author Michael Nayak, a graduate student in Earth planetary sciences at UC Santa Cruz.

Phobos is an unusual satellite, orbiting closer to its planet than any other moon in the solar system, with an orbital period of just 7 hours. Small heavily cratered, with a lumpy nonspherical shape, it is only 9,000 kilometers from the surface of Mars (the distance from San Francisco to New York back) is slowly spiraling inward toward the planet. Phobos appears to have a weak interior structure covered by an elastic shell, allowing it to be deformed by tidal forces without breaking apart.

Coauthor Erik Asphaug, a planetary scientist at Arizona State University professor emeritus at UC Santa Cruz, has been studying Phobos for many years. Recent computer simulations by him NASA planetary scientist Terry Hurford showed how tidal stresses can cause fracturing linear grooves in the surface layer. Although this idea was first proposed in the 1970s, the existence of so many grooves with the wrong orientation for such stress fractures had remained unexplained.

Nayak developed computer simulations showing how those anomalous grooves could result from impacts. Material ejected from the surface by an impact easily escapes the weak gravity of Phobos. But the debris remains in orbit around Mars, most of it moving either just slower or just faster than the orbital velocity of Phobos, within a few orbits it gets recaptured falls back onto the surface of the moon.

Nayak’s simulations enabled him to track in precise detail the fate of the ejected debris. He found that recaptured debris creates distinctive linear impact patterns that match the characteristics of the anomalous grooves chains of craters that cut across the tidal stress fractures on Phobos.

“A lot of stuff gets kicked up, floats for a couple of orbits, then gets recollected falls back in a linear chain before it has a chance to be pulled apart disassociated by Mars’ gravity,” Nayak said. “The controlling factor is where the impact occurs, that determines where the debris falls back.”

The researchers used their model to match a linear chain of small craters on Phobos to its primary source crater. They simulated an impact at the 2.6-kilometer crater called Grildrig, near the moon’s north pole, found that the pattern resulting from ejected debris falling back onto the surface in the model was a very close match to the actual crater chain observed on Phobos.

With its low mass close orbit around Mars, Phobos is so unusual that it may be the only place in the solar system where this phenomenon occurs, Nayak said.


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Planet Nine could spell doom for solar system


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University of Warwick

The solar system could be thrown into disaster when the sun dies if the mysterious ‘Planet Nine’ exists, according to research from the University of Warwick. Dr Dimitri Veras in the Department of Physics has discovered that the presence of Planet Nine – the hypothetical planet which may exist in the outer Solar System – could cause the elimination of at least one of the giant planets after the sun dies, hurling them out into interstellar space through a sort of ‘pinball’ effect.

When the sun starts to die in around seven billion years, it will blow away half of its own mass inflate itself — swallowing the Earth — before fading into an ember known as a white dwarf. This mass ejection will push Jupiter, Saturn, Uranus Neptune out to what was assumed a safe distance.

However, Dr. Veras has discovered that the existence of Planet Nine could rewrite this happy end-ing. He found that Planet Nine might not be pushed out in the same way, in fact might instead be thrust inward into a death dance with the solar system’s four known giant planets — most notably Uranus Neptune. The most likely result is ejection from the solar system, forever.

Using a unique code that can simulate the death of planetary systems, Dr. Veras has mapped nu-merous different positions where a ‘Planet Nine’ could change the fate of the solar system. The further away the more massive the planet is, the higher the chance that the solar system will experience a violent future.

This discovery could shed light on planetary architectures in different solar systems. Almost half of existing white dwarfs contain rock, a potential signature of the debris generated from a similarly calamitous fate in other systems with distant “Planet Nines” of their own.

In effect, the future death of our sun could explain the evolution of other planetary systems.

Dr. Veras explains the danger that Planet Nine could create: “The existence of a distant massive planet could fundamentally change the fate of the solar system. Uranus Neptune in particular may no longer be safe from the death throes of the Sun. The fate of the solar system would depend on the mass orbital properties of Planet Nine, if it exists.”

“The future of the Sun may be foreshadowed by white dwarfs that are ‘polluted’ by rocky debris. Planet Nine could act as a catalyst for the pollution. The Sun’s future identity as a white dwarf that could be ‘polluted’ by rocky debris may reflect current observations of other white dwarfs throughout the Milky Way,” Dr Veras adds.

The paper ‘The fates of solar system analogues with one additional distant planet’ will be published in the Monthly Notices of the Royal Astronomical Society.


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