Solar System Archives

June 18, 2009December 24, 2015 by Brian Ventrudo

NASA IBEX Spacecraft Detects Neutral Hydrogen Bouncing Off Moon

NASA's Interstellar Boundary Explorer has made the first detection of neutral atoms coming from the Moon (background image). The color-coded data toward the bottom shows the neutral particles and geometry measured at the Moon on Dec. 3, 2008.

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NASA’s Interstellar Boundary Explorer (IBEX) spacecraft has made the first observations of fast hydrogen atoms coming from the moon, following decades of speculation and searching for their existence. Launched last October, the IBEX has a mission to image and map the dynamic interactions caused by the hot solar wind slamming into the cold expanse of space. But as the IBEX team commissioned the spacecraft, they discovered the stream of neutral hydrogen atoms which are caused by the solar wind scattering off the moon’s surface.

The detector which made the discovery, called IBEX-Hi, was designed and built by the Southwest Research Institute and Los Alamos National Labs to measure particles moving at speeds of 0.5 million to 2.5 million miles an hour.

“Just after we got IBEX-Hi turned on, the moon happened to pass right through its field of view, and there they were,” says Dr. David J. McComas, IBEX principal investigator and assistant vice president of the SwRI Space Science and Engineering Division, where the IBEX-Hi particle detector was primarily built. “The instrument lit up with a clear signal of the neutral atoms being detected as they backscattered from the moon.”

The solar wind, the supersonic stream of charged particles that flows out from the sun, moves out into space in every direction at speeds of about a million mph. The Earth’s strong magnetic field shields our planet from the solar wind. The moon, with its relatively weak magnetic field, has no such protection, causing the solar wind to slam onto the moon’s sunward side.

From its vantage point in high earth orbit, IBEX sees about half of the moon — one quarter of it is dark and faces the nightside (away from the sun), while the other quarter faces the dayside (toward the sun). Solar wind particles impact only the dayside, where most of them are embedded in the lunar surface, while some scatter off in different directions. The scattered ones mostly become neutral atoms in this reflection process by picking up electrons from the lunar surface.

The IBEX team estimates that only about 10 percent of the solar wind ions reflect off the sunward side of the moon as neutral atoms, while the remaining 90 percent are embedded in the lunar surface. Characteristics of the lunar surface, such as dust, craters and rocks, play a role in determining the percentage of particles that become embedded and the percentage of neutral particles, as well as their direction of travel, that scatter.

McComas says the results also shed light on the “recycling” process undertaken by particles throughout the solar system and beyond. The solar wind and other charged particles impact dust and larger objects as they travel through space, where they backscatter and are reprocessed as neutral atoms. These atoms can travel long distances before they are stripped of their electrons and become ions and the complicated process begins again.

The combined scattering and neutralization processes now observed at the moon have implications for interactions with objects across the solar system, such as asteroids, Kuiper Belt objects and other moons. The plasma-surface interactions occurring within protostellar nebula, the region of space that forms around planets and stars — as well as exoplanets, planets around other stars — also can be inferred.

IBEX’s primary mission is to observe and map the complex interactions occurring at the edge of the solar system, where the million miles per hour solar wind runs into the interstellar material from the rest of the galaxy. The spacecraft carries the most sensitive neutral atom detectors ever flown in space, enabling researchers to not only measure particle energy, but also to make precise images of where they are coming from.

And the spacecraft is just getting started. Towards the end of the summer, the team will release the spacecraft’s first all-sky map showing the energetic processes occurring at the edge of the solar system. The team will not comment until the image is complete, but McComas hints, “It doesn’t look like any of the models.”

The research was published recently in the journal Geophysical Research Letters.

Source: Southwest Research Institute

June 10, 2009December 24, 2015 by Anne Minard

Wild Little Mercury to Cause Interplanetary Smashup? Maybe.

Artistic design : J Vidal-Madjar; planet textures from NASA; copyright: IMCCE-CNRS

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The chaotic evolution of the planetary orbits in the Solar System could cause a close approach or even a collision within the next 5 billion years, according to a paper in this week’s issue of Nature.

The odds are small, but that didn’t stop NASA from releasing a series of really fun “what-if” images (below) …

Mercury is the wild card, according to co-authors Jacques Laskar and Mickael Gastineau of the Paris Observatory. If its orbit elongates, our puniest neighbor could throw the whole block in peril.

Because its orbit resonates with that of Jupiter, Mercury could become the planet gone wild (eccentric in astronomy speak), colliding with Venus.

The chance is slim, the authors point out — around 1 percent. But the finding — revealed through thousands of computer simulations — was a surpise.

“More surprisingly, in one of these high-eccentricity solutions, a subsequent decrease in Mercury’s eccentricity induces a transfer of angular momentum from the giant planets that destabilizes all the terrestrial planets,” the authors write, “with possible collisions of Mercury, Mars or Venus with the Earth.”

Gregory Laughlin, an astronomer at the University of California Santa Cruz who wrote an accompanying editorial about the new paper, couched it as “a note of definite cheer” in the midst of “a seemingly endless torrent of baleful economic and environmental news.” Indeed, there’s a 99 percent chance that the planets will not engage in a destructive round of planetary billiards, and that’s a good thing.

“With 99 percent certainty, we can rely on the clockwork of the celestial rhythm — but with the remaining 1 percent we are afforded a vicarious thrill of danger,” he writes.

Presumably inspired by that vicarious thrill, NASA teamed up with space artist J. Vidal-Madjar to craft the following smash-up images, which were provided by Nature. Enjoy!

June 3, 2009December 24, 2015 by Brian Ventrudo

How Magnetic Tornadoes Might Regenerate Mercury’s Atmosphere

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Compared to Earth, Mercury doesn’t have much of an atmosphere. The smallest rocky planet has weak surface gravity, only 38% that of Earth. And the scorching-hot daytime surface temperatures of 800 degrees Fahrenheit (approximately 450 degrees Celsius) should have boiled away any trace of Mercury’s atmosphere long ago. Yet recent flybys of the MESSENGER spacecraft clearly revealed Mercury somehow retains a thin layer of gas near its surface. Where does this atmosphere come from?

“Mercury’s atmosphere is so thin, it would have vanished long ago unless something was replenishing it,” says Dr. James A. Slavin of NASA’s Goddard Space Flight Center, Greenbelt, Md., a co-investigator on NASA’s MESSENGER mission to Mercury.

The solar wind may well be the culprit. A thin gas of electrically charged particles called a plasma, the solar wind blows constantly from the surface of the sun at some 250 to 370 miles per second (about 400 to 600 kilometers/second). According to Slavin, that’s fast enough to blast off the surface of Mercury through a process called “sputtering”, according to Slavin. Some sputtered atoms stay close enough to the surface to serve as a tenuous yet measurable atmosphere.

But there’s a catch – Mercury’s magnetic field gets in the way. MESSENGER’s first flyby on January 14, 2008, confirmed that the planet has a global magnetic field, as first discovered by the Mariner 10 spacecraft during its flybys of the planet in 1974 and 1975. Just as on Earth, the magnetic field should deflect charged particles away from the planet’s surface. However, global magnetic fields are leaky shields and, under the right conditions, they are known to develop holes through which the solar wind can hit the surface.

During its second flyby of the planet on October 6, 2008, MESSENGER discovered that Mercury’s magnetic field can be extremely leaky indeed. The spacecraft encountered magnetic “tornadoes” – twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space – that were up to 500 miles wide or a third of the radius of the planet.

“These ‘tornadoes’ form when magnetic fields carried by the solar wind connect to Mercury’s magnetic field,” said Slavin. “As the solar wind blows past Mercury’s field, these joined magnetic fields are carried with it and twist up into vortex-like structures. These twisted magnetic flux tubes, technically known as flux transfer events, form open windows in the planet’s magnetic shield through which the solar wind may enter and directly impact Mercury’s surface.”

Venus, Earth, and even Mars have thick atmospheres compared to Mercury, so the solar wind never makes it to the surface of these planets, even if there is no global magnetic field in the way, as is the case for Venus and Mars. Instead, it hits the upper atmosphere of these worlds, where it has the opposite effect to that on Mercury, gradually stripping away atmospheric gas as it blows by.

The process of linking interplanetary and planetary magnetic fields, called magnetic reconnection, is common throughout the cosmos. It occurs in Earth’s magnetic field, where it generates magnetic tornadoes as well. However, the MESSENGER observations show the reconnection rate is ten times higher at Mercury.

“Mercury’s proximity to the sun only accounts for about a third of the reconnection rate we see,” said Slavin. “It will be exciting to see what’s special about Mercury to explain the rest. We’ll get more clues from MESSENGER’s third flyby on September 29, 2009, and when we get into orbit in March 2011.”

Slavin’s MESSENGER research was funded by NASA and is the subject of a paper that appeared in the journal Science on May 1, 2009.

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as Principal Investigator. The Johns Hopkins University Applied Physics Laboratory, Laurel, Md., built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.

Source: NASA

April 2, 2009December 24, 2015 by Anne Minard

Titan (Weirdness) is More Than Meets The Eye

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Think Titan looks pretty round?

Not quite, according to new data released today by the Cassini radar team — and slight irregularities in the shape of the bizarre moon may account for the concentration of lakes at the highest latitudes, among other perplexing features.

The radar image above, obtained by Cassini’s radar instrument during a near-polar flyby in 2007, shows a big island smack in the middle of one of the larger lakes imaged on Saturn’s moon Titan. The island is about 90 kilometers (62 miles) by 150 kilometers (93 miles) across, about the size of Kodiak Island in Alaska or the Big Island of Hawaii. The image is centered at about 79 north degrees north (north is left) and 310 degrees west, adding weight to the theory that most of Titan’s lakes occur near the poles.

Titan is an intriguing object partly because its climate cycles are reminiscent of Earth’s, but tend to rely on hydrocarbons like methane and ethane instead of water — which couldn’t exist in a liquid state at temperatures hundreds of degrees below zero. Methane and ethane fill the air with a smoggy haze that rains down as ash. Sometimes it’s washed away by hydrocarbons that flow like gasoline and collect in black lakes with surfaces as smooth as glass.

Cassini has been orbiting Saturn for four years, observing Titan periodically with multiple radar instruments. A research team led by Howard Zebker, a geophysicist at Stanford University, has been using the radar data to estimate the surface elevation. Combined, two instruments — a nadir-pointing radar altimeter and a multiple-beam synthetic aperture radar (SAR) imaging system — measure the time delay of the altimeter echoes and the precise radar beam angles to points on the surface.

“These techniques show that the poles of Titan lie at lower elevations than the equator, and that the topography also varies longitudinally,” the authors report in today’s Science Express..

“If we posit that the lakes are surface expressions of a more or less continuous liquid organic ‘water table,’ then the lower elevations of the poles could lead to the observed preponderance of lakes at high latitudes,” they add. In other words, the lower elevations of poles may make them the only places where any continuous, liquid “water table” would be close enough to the moon’s surface to appear as lakes.

Titan’s overall shape, they suggest, might be that a sphere slightly flattened at the top and bottom. The exact mechanisms behind the oblate shape are unclear. Titan is also elongated toward Saturn, due to the tides raised by Saturn’s gravity.

Source: The paper appears online at the S cience Express website. More Titan images are available at the Cassini website.

March 31, 2009December 24, 2015 by Anne Minard

Fresh Mars Impact Craters Blast Ice Onto Surface

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Fresh impact craters on Mars have revealed more evidence of stable ice that’s been hiding just beneath the surface all along, say scientists working on images sent back by the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter.

The craters appeared sometime between January and September of last year, in areas that had been previously imaged without them prior to January 2008. The impacts served as “natural probes” to excavate evidence that gets to a long-standing question about ice on Mars: where is it stable, and where is it residual, in the process of sublimating away?

Study leader Shane Byrne, of the University of Arizona’s Lunar and Planetary Laboratory, presented the new set of images last week, at the Lunar and Planetary Science Conference in Texas.

Computer models agree that Mars contains stable high-latitude ice, but researchers have encountered difficulty deciding the quantity and geographical boundary of the ice, partly because they can’t see it beneath the surface and partly because pinning down an indirect measure of sub-surface ice — a long-term, global average water vapor concentration in the atmosphere — has proved challenging.

mars-ejecta2 — Credit: NASA/JPL/University of Arizona

The new craters are a significant clue, because they hit not in the high latitudes where planetary scientists are fairly certain about stable ice, but in the mid-latitudes where the ice’s reach is unclear.

“Here we report on natural probes of the Martian subsurface which have ‘detected’ ice in this critical mid-latitude zone,” wrote Byrne and his co-authors.

“In five such cases (with latitudes spanning 43.3° to 55.6° N), these impacts have excavated bright material which in High Resolution Imaging Science Experiment (HiRISE) data have a brightness and color consistent with water ice.”

Each of the five new craters is a few meters in diameter, several decimeters deep and with associated bright material a few meters across, the authors report. Four of them showed no spectral evidence of water ice. But one proved a jackpot.

“Spectra from this site show clear water ice absorption features at 1.25, 1.5 and 2 ?m,” the team reported. Exposed surface ice is not expected to be stable at the latitudes, and the team has already noticed shrinkage and fading.

Based on atmospheric water vapor data, even stable underground ice isn’t expected to be widespread at the mid-latitudes where the reservoir was found: “Thus the ground ice exposed here is probably in the process of retreat from a previously larger extent,” the authors wrote.

Source: LPSC proceedings

March 26, 2009December 24, 2015 by Anne Minard

New Views of Spring on Mars

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New high-resolution images taken last month of Mars’ south polar region are revealing signs of spring that are decidedly Martian.

The image above features a spider trough network left behind as seasonal dry ice caps have sublimated away in the warmer temperatures. It’s part of a new series of images released this week by the University of Arizona’s High Resolution Imaging Experiment, or HiRISE, aboard NASA’s Mars Reconnaissance Orbiter.

See more information and photos below.

The gas beneath the ice cap can flow in the same places year after year, eroding troughs in the surface of the planet.

“What happens on Mars, we think, is that as the seasonal ice cap thins from the bottom, gas underneath the cap builds up pressure,” said HiRISE deputy principal investigator Candice J. Hansen-Koharcheck of the NASA Jet Propulsion Laboratory in Pasadena, California.

“And where gas under the ice finds a weak spot or a crack, it will flow out of the opening, often carrying a little dust from the surface below.”

The next HiRISE image shows how dust that has been carried to the surface by gas jetting through the ice cap is blown about by prevailing winds before settling in fan-shaped deposits atop the ice cap. Varying orientations suggest that as the ice layer thins, a set of gas jets becomes active, they die down, then further away another set starts up at a later time with a different prevailing wind direction.

co2-jets — NASA/JPL/University of Arizona

Many jets appear to be active at the same time since numerous fans are all deposited in the same direction: this next, closer image is an example of such an occurrence.

co2-jets-closeup — Credit: NASA/JPL/University of Arizona

This southern hemisphere crater has gullies on its north and northeast walls. Gullies are proposed to be carved by liquid water originating from the subsurface or melting ice/snow on the surface.

Dark dunes are visible on the crater floor. Lighter, smaller dunes rim the south side of the crater floor. The entire scene, pictured below, has a pitted texture, suggesting that ground ice was once present in this region. When ground ice sublimates (goes from a solid directly to a gas), it leaves behind empty spaces in the soil that turn into pits as the remaining overlying soil collapses to fill them.

The full set of new HiRISE Mars images is here. Check out all the downloadable formats and sizes, with some even designed to fit an iPhone screen!

Source: Lori Stiles, at the University of Arizona

March 23, 2009December 24, 2015 by Anne Minard

Moonshadows on Saturn’s Rings Are Harbingers of Spring

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Moonshadows on Saturn’s rings are foretelling the planet’s equinox, when the sun will be exactly aligned with the planet’s equator and rings — and then will shift north from the southern hemisphere, kickstarting northern spring.

NASA’s Cassini spacecraft has captured, for the first time, the tell-tale moonshadows – sort of like groundhogs on Earth.

moonshadow2 — Click to play the short movie. Credit: NASA/JPL/Space Science Institute

The image above is a still from a movie, from Cassini’s hour-long observation of the shadow of the small moon Epimetheus.

Like Earth and most of the other planets, Saturn’s spin axis is tilted relative to its motion around the sun. So the sun, seen from Saturn, cycles from the southern hemisphere to the north and back again. A full sweep of seasonal changes on Saturn and its rings and moons takes a Saturnian year, equal to 29.5 Earth years. Thus, about every 15 Earth years, or half-Saturn-year, the sun passes through the plane containing the planet’s rings.

During these times, the shadows of the planet’s rings fall in the equatorial region on the planet. And the shadows of Saturn’s moons external to the rings, especially those whose orbits are inclined with respect to the equator, begin to intersect the planet’s rings. When this occurs, the equinox period has essentially begun, and any vertical protuberances within the rings, including small embedded moons and narrow vertical warps in the rings, will also cast shadows on the rings. At exactly the moment of equinox, the shadows of the rings on the planet will be confined to a thin line around Saturn’s equator and the rings themselves will go dark, being illuminated only on their edge. The next equinox on Saturn, when the sun will pass from south to north, is Aug. 11, 2009.

Because of these unique illumination circumstances, Cassini imaging scientists have been eager to observe the planet and its rings around the time of equinox. Cassini’s first extended mission, which began on July 1, 2008, was intended to gather observations during this time. Hence its name: Cassini Equinox Mission.

More than just pretty pictures, the observations could reveal any deviations across the rings from a perfectly flat wafer-like disk. Saturn’s ring system is wide, spanning hundreds of thousands of miles or kilometers. But the main inner rings (called A, B and C) are perhaps only 10 meters (30 feet) thick, and they are sometimes obscured from view inside thicker outer rings.

“We hope that such images will help us measure any vertical warping in the A and B rings,” said John Weiss, an imaging team associate from the Space Science Institute in Boulder, Colorado. “Because we know how big the moons are, and where they are in their orbits around Saturn when they cast these shadows, we have all the information we need to infer any substantial vertical structure that might be present.”

On Jan. 8, Epimetheus, a small moon 113 kilometers (70 miles) across, was the first moon observed casting a shadow onto the outer edge of the A ring. Next Pan, 30 kilometers (20 miles) across and orbiting within the rings, was caught casting a shadow on the A ring on Feb. 12. Eventually, more moons will cast shadows on the rings and all shadows will grow longer as exact equinox approaches.

Source: Cassini Imaging Central Laboratory for Operations (CICLOPS)

March 19, 2009December 24, 2015 by Ian O'Neill

The Sun as a White Dwarf Star

Dusty debris around an old white dwarf star (NASA)

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What will happen to all the inner planets, dwarf planets, gas giants and asteroids in the Solar System when the Sun turns into a white dwarf? This question is currently being pondered by a NASA researcher who is building a model of how our Solar System might evolve as our Sun loses mass, violently turning into an electron-degenerate star. It turns out that Dr. John Debes work has some very interesting implications. As we use more precise techniques to observe existing white dwarf stars with the dusty remains of the rocky bodies that used to orbit them, the results of Debes’ model could be used as a comparison to see if any existing white dwarf stars resemble how our Sun might look in 4-5 billion years time…

A comparison of the Sun in its yellow dwarf phase and red giant phase

Today, our Sun is a healthy yellow dwarf star. If you want to be precise, it is a “G V star”. This yellow dwarf will happily burn 600 million tonnes of hydrogen per second in its core for 10 billion years, generating the light that is required to make our planet habitable. The Sun is approximately half-way through this hydrogen burning phase, so it’s OK, things aren’t going to change (for the Sun at least) for a long time yet.

But what happens then? What happens in 4-5 billion years when the supply of hydrogen runs out in the core? Although our Sun isn’t massive enough to entertain the thought of going out in a blaze of supernova glory, it will still go through an exciting, yet terrifying death. After evolving through the hydrogen-burning phase, the Sun will puff up into a huge red giant star as the hydrogen fuel becomes scarce, expanding 200 times the size it is now, probably swallowing the Earth. Helium, and then progressively heavier elements will be fused in and around the core. The Sun will never fuse carbon however, instead it will shed its outer layers forming a planetary nebula.

Once things calm down, a small sparkling jewel of a white dwarf star will remain. This tiny remnant will have a mass of around half that of our present Sun, but will be the size of the Earth. Needless to say, white dwarfs are very dense, intense gravitational pull countered not by fusion in the core (like all Main Sequence stars), but by electron degeneracy pressure.

Relative sizes of IK Pegasi A (left), IK Pegasi B (lower center; a white dwarf) and the Sun (NASA)

When the Solar System reaches this phase in its evolution, what will it look like? What will become of the asteroids, gas giants, moons and rocky planets? I was very fortunate to chat with astrophysicist Dr John Debes, from NASA’s Goddard Space Flight Center, at January’s American Astronomical Society (AAS) conference in Long Beach (California) who is developing an n-body code simulating an evolving Solar System.

After the Sun has stopped hydrogen fusion in its core, it loses mass as it sheds its outer layers after the red giant phase and subsequent planetary nebula formation. It is estimated that the Sun will lose about 50% of its mass during this time, naturally affecting the Solar System as a whole. As the Sun loses mass, the outer planets (such as Jupiter) will drift outwards, increasing their orbital radii. In the simulation, Debes is very careful to ensure there is a gradual reduction in solar mass to ensure stability in the simulation.

What we are left with is an old Solar System, where little is left of the inner planets (it is likely that anything within the orbit of the Earth will have been swallowed by the Sun as it expanded through the red giant phase). Although the future white dwarf Solar System will seem very alien to present day, some things won’t change. Jupiter’s orbit might have receded with the drop in solar mass, it will remain a planetary heavyweight, causing disruption in asteroid orbits. Using known asteroid data, the motion of these chunks of rocks are allowed to evolve, and over millions of years, they may get thrown out of the Solar System, or more interestingly, pushed closer to the white dwarf. Once the whole system has settled down, resonances in the asteroid belt will become amplified; Kirkwood Gaps (caused by gravitational resonance with Jupiter) will widen, and according to Debes’ simulations, the edges of these gaps will become perturbed even more, making more asteroids available to be tidally disrupted and shredded to dust.

Artists concept of shredded asteroid around white dwarf (NASA/JPL-Caltech)

The AAS conference was full of amazing research into white dwarf observations. The reason for this is that there are many white dwarf candidates out there with dusty metallic absorption lines. This means that there used to be rocky bodies orbiting these stars, but became pulverised (by tidal shear) for astronomers to analyse. These white dwarf systems can give us a clue as to what mechanisms could be supplying the white dwarfs with dusty material, even giving us a glimpse into the future of our Solar System.

“We have a physical picture for the link between planetary systems and dusty white dwarfs,” Debes said when describing his model in relation to the mysterious dusty white dwarf observations. “Dusty white dwarfs are truly a mystery! We think we know what might be going on, but we don’t have a smoking gun yet.”

However, Debes is getting close to finding a possible smoking gun, he’s basing his model on some of the key characteristics of these ancient dusty remnants to see what the Solar System could look like in billions of years time.

So, where does this dust come from? As the asteroid orbits are perturbed by Jupiter, they may get close enough to be tidally disrupted. Get too close and they will get shredded by the gravitational shear created by the steep tidal radius of the compact white dwarf. The asteroid dust then settles into the white dwarf. The presence of this dust has a very obvious signature in the absorption lines of spectroscopic data, allowing researchers to infer an accretion rate for metal-rich white dwarfs. In Debes’ model, he has set the upper limit to 10¹⁶ g/year and a lower limit to 10¹³ g/year, consistent with observed estimates.

Spectra of G29-38. Could this resemble the spectra of the Sun after turning into a white dwarf? (NASA/Spitzer)

In his evolved Solar System model, Jupiter’s gravity controls this accretion rate, pushing asteroids toward the white dwarf and, by using a powerful supercomputer to track the perturbations and eventual shredding of known asteroids, there may be an opportunity to arrive at a profound conclusion. Debes is able to use his model to compare observations of known dusty white dwarfs with the simulated outcome of the Solar System. With reference to previous studies (in particularly Koester & Wilken, 2006 in the journal Astronomy & Astrophysics), Debes has found some similar white dwarf “Suns”.

“For G29-38, the canonical dusty white dwarf, they [Koester & Wilken] estimate a total mass of 0.55 solar masses–about what people believe the mass that our own sun will have remaining when it becomes a white dwarf,” Debes added. “But mass estimates are a bit uncertain–I’ve seen estimates ranging from 0.55-0.7 solar masses for this particular white dwarf.”

The Su<span>n's future? The whit</span>e dwarf G29-38 (NASA) — The Sun's future? The white dwarf G29-38 (NASA)

“Another good candidate is a DAZ [a metal-rich white dwarf] called WD 1257+278, which does not show dust but is spot on with the mass expected for the Sun–0.54 M_Sun,” said Debes. “Its accretion rate is also consistent with my model predictions so far assuming an asteroid belt mass and characteristic perturbation timescale that I found in my simulations.”

Debes is continuing to make his model more and more sophisticated, but already the results are promising. Most exciting is that we may already be observing white dwarfs, like G29-38 or WD 1257+278, giving us a tantalizing glimpse of what our Solar System will look like when the Sun becomes a white dwarf star, ripping apart any remaining asteroids and planets as they stray too close to the Sun’s tidal shear. However, it also raises the question: if white dwarfs like G29-38 are being fed by the remains of tidally-blended asteroids, are there massive planets shepherding asteroids in these white dwarf systems too?

March 2, 2009December 24, 2015 by Anne Minard

Pluto’s Atmosphere Boasts Methane, Warmer Temps

Artist’s impression of how the surface of Pluto might look, if patches of pure methane rest on the surface. At the distance of Pluto, the Sun appears about 1,000 times fainter than on Earth. Credit: ESO

Pluto is certainly frigid, but new research has revealed its atmosphere is a bit warmer.

Astronomers using the European Southern Observatory’s Very Large Telescope have found unexpectedly large amounts of methane in Pluto’s atmosphere, which evidently helps it stay about 40 degrees warmer than the dwarf planet’s surface. The atmosphere warms to -180 degrees Celsius (-356 degrees Fahrenheit), compared to a surface that’s usually -220 degrees Celsius (-428 degrees Fahrenheit).

“With lots of methane in the atmosphere, it becomes clear why Pluto’s atmosphere is so warm,” said Emmanuel Lellouch of the Observatoire de Paris in France. Lellouch is lead author of the paper reporting the results, which is in press at the journal Astronomy and Astrophysics.

Pluto, which is about a fifth the size of Earth, is composed primarily of rock and ice and orbits about 40 times further from the Sun than the Earth.

It has been known since the 1980s that Pluto also has a thin, tenuous atmosphere. Abundant nitrogen, along with traces of methane and probably carbon monoxide, are held to the surface by an atmospheric pressure only about one hundred thousandth of that on Earth, or about 0.015 millibars. As Pluto moves away from the Sun, during its 248 year-long orbit, its atmosphere gradually freezes and falls to the ground. In periods when it is closer to the Sun — as it is now — the temperature of Pluto’s solid surface increases, causing the ice to sublimate into gas.

Until recently, only the upper parts of the atmosphere of Pluto could be studied. By observing stellar occultations, a phenomenon that occurs when a Solar System body blocks the light from a background star, astronomers were able to demonstrate that Pluto’s upper atmosphere was some 50 degrees warmer than the surface. Those observations couldn’t shed any light on the atmospheric temperature and pressure near Pluto’s surface. But unique, new observations made with the CRyogenic InfraRed Echelle Spectrograph (CRIRES), attached to ESO’s Very Large Telescope, have now revealed that the atmosphere as a whole, not just the upper atmosphere, has a mean temperature much less frigid than the surface.

Usually, air near the surface of the Earth is warmer than the air above it, largely because the atmosphere is heated from below as solar radiation warms the Earth’s surface, which, in turn, warms the layer of the atmosphere directly above it. Under certain conditions, this situation is inverted so that the air is colder near the surface of the Earth. Meteorologists call this an inversion layer, and it can cause smog build-up.

Most, if not all, of Pluto’s atmosphere is thus undergoing a temperature inversion: the temperature is higher, the higher in the atmosphere you look. The change is about 3 to 15 degrees per kilometer (.62 miles). On Earth, under normal circumstances, the temperature decreases through the atmosphere by about 6 degrees per kilometer.

The reason why Pluto’s surface is so cold is linked to the existence of Pluto’s atmosphere, and is due to the sublimation of the surface ice; much like sweat cools the body as it evaporates from the surface of the skin, this sublimation has a cooling effect on the surface of Pluto.

The CRIRES observations also indicate that methane is the second most common gas in Pluto’s atmosphere, representing half a percent of the molecules. “We were able to show that these quantities of methane play a crucial role in the heating processes in the atmosphere and can explain the elevated atmospheric temperature,” said Lellouch.

Two different models can explain the properties of Pluto’s atmosphere. In the first, the astronomers assume that Pluto’s surface is covered with a thin layer of methane, which will inhibit the sublimation of the nitrogen frost. The second scenario invokes the existence of pure methane patches on the surface.

“Discriminating between the two will require further study of Pluto as it moves away from the Sun,” says Lellouch. “And of course, NASA’s New Horizons space probe will also provide us with more clues when it reaches the dwarf planet in 2015.”

LEAD IMAGE CAPTION: Artist’s impression of how the surface of Pluto might look, if patches of pure methane rest on the surface. At the distance of Pluto, the Sun appears about 1,000 times fainter than on Earth. Credit: ESO

Source: ESO

February 25, 2009December 24, 2015 by Anne Minard

Jupiter, Saturn Plowed Through Asteroids, Study Says

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When Mars and Jupiter migrated to their present orbits around 4 billion years ago, they left scars in the asteroids belt that are still visible today.

The evidence is unveiled in a new paper in this week’s issue of the journal Nature, by planetary scientists David Minton and Renu Malhotra from the University of Arizona in Tucson.

The asteroid belt has long been known to harbor gaps, called Kirkwood gaps, in distinct locations. Some of these gaps correspond to unstable zones, where the modern-day gravitational influence of Jupiter and Saturn eject asteroids. But for the first time, Minton and Malhotra have noticed that some clearings don’t fit the bill.

“What we found was that many regions are depleted in asteroids relative to other regions, not just in the previously known Kirkwood gaps that are explained by the current planetary orbits,” Minton wrote in an email. In an editorial accompanying the paper, author Kevin Walsh added, “Qualitatively, it looks as if a snow plough were driven through the main asteroid belt, kicking out asteroids along the way and slowing to a stop at the inner edge of the belt.”

Walsh hails from the Observatoire de la Côte d’Azur in France. In his News and Views piece, he explains that the known Kirkwood gaps, discovered by Daniel Kirkwood in 1867, “correspond to the location of orbital resonances with Jupiter — that is, of orbits whose periods are integer ratios of Jupiter’s orbital period.” For example, if an asteroid orbited the Sun three times for every time Jupiter did, it would be in a 3:1 orbital resonance with the planet, he wrote. Objects in resonance with a giant planet have inherently unstable orbits, and are likely to be ejected from the solar system. When planets migrated, astronomers believe objects in resonance with them also shifted, affecting different parts of the asteroid belt at different times.

“Thus, if nothing has completely reshaped the asteroid belt since the planets settled into their current orbits, signatures of past planetary orbital migration may still remain,” Walsh wrote. And that’s exactly what Minton and Malhotra sought.

The asteroid belt easily gave up its secrets, showing the lingering evidence of planetary billiards on the inner edge of the asteroid belt and at the outer edge of each Kirkwood gap. The new finding, based on computer models, lends additional support to the theory that the giant planets — Jupiter, Saturn, Uranus and Neptune — formed twice as close to the sun as they are now and in a tighter configuration, and moved slowly outward.

“The orbit of Pluto and other Kuiper belt objects that are trapped in [orbits that resonate] with Neptune can be explained by the outward migration of Neptune,” Minton and Malhotra write in the new study. “The exchange of angular momentum between planetesimals and the four giant planets caused the orbital migration of the giant planets until the outer planetesimal disk was depleted.” Planetesimals are rocky and icy objects left over from planet formation.

“As Jupiter and Saturn migrated,” the authors continue, they wreaked havoc on the young asteroid belt, “exciting asteroids into terrestrial planet-crossing orbits, thereby greatly depleting the asteroid belt population and perhaps also causing a late heavy bombardment in the inner Solar System.”

The late heavy bombardment is proposed to have occurred about 3.9 billion years ago, or 600 million years after the birth of the Solar System, and it’s believed to account for many of the Moon’s oldest craters. Walsh said a reasonable next step, to corroborate the theory about the newly described clearings in the asteroid belt, is to link them chronologically with the bombardment.

LEAD PHOTO CAPTION: Artist’s depiction of the asteroid belt between Mars and Jupiter. Credit: David Minton and Renu Malhotra

Source: Nature