NASA Astronomy Picture of the Day 2010-8

Venus' Once Molten Surface

If you could look across Venus with radar eyes, what might you see? This computer reconstruction of the surface of Venus was created from data from the Magellan spacecraft. Magellan orbited Venus and used radar to map our neighboring planet's surface between 1990 and 1994. Magellan found many interesting surface features, including the large circular domes, typically 25-kilometers across, that are depicted above. Volcanism is thought to have created the domes, although the precise mechanism remains unknown. Venus' surface is so hot and hostile that no surface probe has lasted more than a few minutes.

Prometheus Creating Saturn Ring Streamers

What's causing those strange dark streaks in the rings of Saturn? Prometheus. Specifically, an orbital dance involving Saturn's moon Prometheus keeps creating unusual light and dark streamers in the F-Ring of Saturn. Now Prometheus orbits Saturn just inside the thin F-ring, but ventures into its inner edge about every 15 hours. Prometheus' gravity then pulls the closest ring particles toward the 80-km moon. The result is not only a stream of bright ring particles but also a dark ribbon where ring particles used to be. Since Prometheus orbits faster than the ring particles, the icy moon pulls out a new streamer every pass. Above, several streamers or kinks are visible at once. The above photograph was taken in June by the robotic Cassini Spacecraft orbiting Saturn. The oblong moon Prometheus is visible on the far left.

The Planet and the Radio Dish

What planet is this? Although seemingly something out of The Little Prince, the planet is actually Earth. More specifically, it is a small part of the Earth incorporated into a four image stereographic "Little Planet " projection. The central fisheye image points down, while the surrounding wide-angle images were taken at a 30 degree tilt and added digitally later. Earth-anchored items surrounding the image center include green grass, dark shadows, and trees near and far. At the image top ("noon" if the planet were a clock) is the well-lit Parkes Radio Telescope dish in New South Wales, Australia. The surrounding sky contains many jewels of the night including the Moon at 9 pm, the plane of our Milky Way Galaxy at 1:30 pm and 7 pm, and the Small Magellanic Cloud galaxy at 5 pm. A full field interactive version of this scene can be found here.

Eclipse Shadow Cone Over Patagonia

Sometimes, during a total eclipse of the Sun, a strange shadow of darkness can be seen stretching off into the distance. Called shadow cones, they are visible because the Earth's atmosphere is not completely transparent, scattering sunlight and hence appearing blue during the day. Shadow cones are particularly dramatic for eclipses near the horizon, as geometry creates a long corridor of sun-blocked air. Visible above is a shadow cone caught during a sunset total solar eclipse visible last month from Patagonia, Argentina. The eclipsed Sun itself still appears bright around the edges of the Moon because of light from the surrounding corona. A few minutes later, the Moon began to move away from the Sun as both set behind distant Andes mountains.

M8: The Lagoon Nebula

This beautiful cosmic cloud is a popular stop on telescopic tours of the constellation Sagittarius. Eighteenth century cosmic tourist Charles Messier cataloged the bright nebula as M8. Modern day astronomers recognize the Lagoon Nebula as an active stellar nursery about 5,000 light-years distant, in the direction of the center of our Milky Way Galaxy. Remarkable features can be traced through this sharp picture, showing off the Lagoon's filaments of glowing gas and dark dust clouds. Twisting near the center of the Lagoon, the bright hourglass shape is the turbulent result of extreme stellar winds and intense starlight. The alluring view is a color composite of both broad and narrow band images captured while M8 was high in dark, Chilean skies. It records the Lagoon with a bluer hue than typically represented in images dominated by the red light of the region's hydrogen emission. At the nebula's estimated distance, the picture spans about 30 light-years.

The Not So Quiet Sun

After a long solar minimum, the Sun is no longer so quiet. On August 1, this extreme ultraviolet snapshot of the Sun from the Solar Dynamics Observatory captured a complex burst of activity playing across the Sun's northern hemisphere. The false-color image shows the hot solar plasma at temperatures ranging from 1 to 2 million kelvins. Along with the erupting filaments and prominences, a small(!) solar flare spawned in the active region at the left was accompanied by a coronal mass ejection (CME), a billion-ton cloud of energetic particles headed for planet Earth. Making the 93 million mile trip in only two days, the CME impacted Earth's magnetosphere, triggering a geomagnetic storm and both northern and southern auroral displays.

Rainbow at Sunset

Where is the Sun when you see a rainbow? Behind you, of course. But you can see both a rainbow and the Sun (far right) side by side in this graceful panorama recorded on July 28. The cloudy sunset view covers a full 360 degrees around the horizon, composed using 20 individual images taken from an observatory on the outskirts of Potsdam, Germany. The rainbow itself is produced by sunlight internally reflected in rain drops from the direction opposite the Sun back toward the observer. As the sunlight passes through the drops, from air to water and back to air again, longer wavelengths (redder colors) are refracted or bent less than shorter wavelengths (bluer colors), separating the sunlight into the colors of the rainbow. This sharp picture captures the full, bright, primary rainbow arc as well as more subtle effects. You can see a partial, dimmer, secondary rainbow arc above and left of the primary, and faint arcs just inside the primary rainbow called supernumerary rainbows.

Two Hours Before Neptune

Two hours before closest approach to Neptune in 1989, the Voyager 2 robot spacecraft snapped this picture. Clearly visible for the first time were long light-colored cirrus-type clouds floating high in Neptune's atmosphere. Shadows of these clouds can even be seen on lower cloud decks. Most of Neptune's atmosphere is made of hydrogen and helium, which is invisible. Neptune's blue color therefore comes from smaller amounts of atmospheric methane, which preferentially absorbs red light. Neptune has the fastest winds in the Solar System, with gusts reaching 2000 kilometers per hour. Speculation holds that diamonds may be created in the dense hot conditions that exist under the cloud tops of Uranus and Neptune.

IRAS 05437+2502: An Enigmatic Star Cloud from Hubble

What's lighting up nebula IRAS 05437+2502? No one is sure. Particularly enigmatic is the bright upside-down V that defines the upper edge of this floating mountain of interstellar dust, visible near the image center. In general, this ghost-like nebula involves a small star forming region filled with dark dust that was first noted in images taken by the IRAS satellite in infrared light in 1983. Shown above is a spectacular, recently released image from the Hubble Space Telescope that, although showing many new details, has not uncovered a clear cause of the bright sharp arc. One hypothesis holds that the glowing arc was created by a massive star that somehow attained a high velocity and has now left the nebula. Small, faint IRAS 05437+2502 spans only 1/18th of a full moon toward the constellation of the Bull (Taurus).

The Sand Dunes of Titan

Why do some sand dunes on Titan appear backwards? Central Titan, it turns out, is covered by sand, some of which appears strange. Images from the Cassini spacecraft currently orbiting Saturn have uncovered long rows of huge sand dunes near Titan's equator that rise as high as 300 meters. Shadows indicate that most dune shapes are created by wind blowing from the west. The problem is, the typical wind at Titan's equator blows from the east. One recent hypothesis that might solve this grainy conundrum posits that the only winds strong enough to move sand and create dunes occur during rare equinoxes and blow strongly from the west. The above images show a radar swath of Titan's equatorial sand dunes at the top, while similar sand dunes that formed in Namibia on Earth at the bottom. Why central Titan is even covered by so much sand is still being investigated.

Crepuscular Rays Over Lake Michigan

What could cause such rays of dark? Dark sky rays were caught in spectacular fashion earlier last month from Pentwater, Michigan, USA, looking west over Lake Michigan. The cause is something surprisingly familiar: shadows. Clouds near the horizon can block sunlight from reflecting off air, making columns outward from the Sun appear unusually dark. Cloud shadows can be thought of as the complement of the more commonly highlighted crepuscular rays, also visible above, where sunlight pours though cloud holes. Sometimes, on the opposite side of the sky, anticrepuscular rays can also be seen.

Perseid Prelude

ch August, as planet Earth swings through dust trailing along the orbit of periodic comet Swift-Tuttle, skygazers can enjoy the Perseid Meteor Shower. The shower should build to its peak now, best seen from later tonight after moonset, until dawn tomorrow morning when Earth moves through the denser part of the wide dust trail. But shower meteors have been spotted for many days, like this bright Perseid streaking through skies near Lake Balaton, Hungary on August 8. In the foreground is the region's Church of St. Andrew ruin, with bright Jupiter dominating the sky to its right. Two galaxies lie in the background of the wide-angle, 3 frame panorama; our own Milky Way's luminous arc, and the faint smudge of the more distant Andromeda Galaxy just above the ruin's leftmost wall. If you watch for Perseid meteors tonight, be sure and check out the early evening sky show too, featuring bright planets and a young crescent Moon near the western horizon after sunset.

Arp 286: Trio in Virgo

A remarkable telescopic composition in yellow and blue, this scene features a trio of interacting galaxies almost 90 million light-years away, toward the constellation Virgo. On the left, two, spiky, foreground Milky Way stars echo the trio galaxy hues, a reminder that stars in our own galaxy are like those in the distant island universes. Predominately yellow, with sweeping spiral arms and dust lanes, NGC 5566 is enormous, about 150,000 light-years across. Just below it lies small, blue NGC 5569. Near center, the third galaxy, NGC 5560, is multicolored and apparently stretched and distorted by its interaction with NGC 5566. The galaxy trio is also included in Halton Arp's 1966 Atlas of Peculiar Galaxies as Arp 286. Of course, such cosmic interactions are now appreciated as a common part of the evolution of galaxies.

Night of the Perseids

On the night of August 12, from moonset until dawn was a good time to see meteors. Enthusiasts watched as comet dust rained on planet Earth, streaking through dark skies during the annual Perseid Meteor Shower. Anticipating the shower approaching its peak, astronomer Marco Verstraaten recorded a series of exposures capturing meteors over a period of 6 hours using a wide angle lens from a not-so-dark site in the Netherlands. Combining them still produced this dramatic night sky view with many colorful meteor streaks. The starry backdrop includes the Milky Way and even the faint Andromeda Galaxy, right of center. Although the comet dust particles are traveling parallel to each other, the shower meteors clearly seem to radiate from a spot on the sky in the eponymous constellation Perseus. The radiant effect is due to perspective, as the parallel tracks appear to converge at a distance. Bright stars in Perseus extend into the gap between the foreground trees.

Layered Hills in Arabia Terra on Mars

Why are some hills on Mars so layered? The answer is still under investigation. Clearly, dark windblown sand surrounds outcropping of light sedimentary rock across the floor of crater Arabia Terra. The light rock clearly appears structured into many layers, the lowest of which is likely very old. Although the dark sand forms dunes, rippled dunes of lighter colored sand are easier to see surrounding the stepped mesas. Blown sand possibly itself eroded once-larger mesas into the layered hills. Most of the layered shelves are wide enough to drive a truck around. The above image, showing an area about 3 kilometers across, was taken in 2003 October by the now defunct Mars Global Surveyor spacecraft orbiting Mars.

Meteors Over Quebec

Meteors streaked through the sky above many of Earth's cities last week, but nobody was hurt, and no damage has been reported. The assault from space appeared to originate from someplace in the constellation of Perseus, and included millions of small projectiles hurtling toward Earth at over 200,000 kilometers per hour. Pictured above, people gathered at ASTROLab du Mont-Megantic in southern Quebec, Canada gazed helplessly toward the sky during a similar event last year as they themselves were unable to stop the meteor onslaughts. Fortunately, Earth's defense, consisting of a planet-wide blanket of air over 100-kilometers thick, obliterated the attacking projectiles by using friction generated by their own speed to heat them into disintegration. The large triangle in the foreground, although impressive in appearance, was not part of the Earth's meteor defense system. The space attack was expected as part of the annual Perseids meteor shower as the Earth passed through sand-sized debris left over from the sun-orbiting Comet Swift-Tuttle.

NGC 4755: A Jewel Box of Stars

The great variety of star colors in this open cluster underlies its name: The Jewel Box. One of the bright central stars is a red supergiant, in contrast to the many blue stars that surround it. The cluster, also known as Kappa Crucis contains just over 100 stars, and is about 10 million years old. Open clusters are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters. This Jewel Box lies about 6,400 light-years away, so the light that we see today was emitted from the cluster before even the Great Pyramids in Egypt were built. The Jewel Box, pictured above, spans about 20 light-years, and can be seen with binoculars towards the southern constellation of the cross (Crux).

Crescent Moon and Planets Over Portugal

Sometimes, it's fun to share the sky. Although it might appear that the two sky enthusiasts on the ridge are sharing only a crescent moon between them, three bright planets also stand before them. The brightest point in the sky is the planet Venus, while reddish Mars floats above it, and Saturn shines off to its right. In the foreground are picturesque clumps of sand of the beach at Costa da Caparica, before the reflecting waters of the Atlantic Ocean. The evening picked to planet gaze was a particularly good one as the Perseids meteor shower was also reaching its peak. Fortunately, this evening sky was not unique to Almada, Portugal last Thursday, but visible to sky enthusiasts the world over last week. These planets will all remain visible at sunset this week, although Venus will sink to the horizon as it closes in on the setting Sun.

Pelican Nebula Close-up

The prominent ridge of emission featured in this vivid skyscape is designated IC 5067. Part of a larger emission nebula with a distinctive shape, popularly called The Pelican Nebula, the ridge spans about 10 light-years and follows the curve of the cosmic pelican's head and neck. Fantastic, dark shapes inhabiting the view are clouds of cool gas and dust sculpted by energetic radiation from hot, massive stars. But stars are also forming within the dark shapes. In fact, twin jets emerging from the tip of the central, dark tendril are the telltale signs of an embedded protostar cataloged as Herbig-Haro 555. The Pelican Nebula itself, also known as IC 5070, is about 2,000 light-years away. To find it, look northeast of bright star Deneb in the high flying constellation Cygnus.

NGC 1365: Majestic Island Universe

Barred spiral galaxy NGC 1365 is truly a majestic island universe some 200,000 light-years across. Located a mere 60 million light-years away toward the chemical constellation Fornax, NGC 1365 is a dominant member of the well-studied Fornax galaxy cluster. This impressively sharp color image shows intense star forming regions at the ends of the bar and along the spiral arms, and details of dust lanes cutting across the galaxy's bright core. At the core lies a supermassive black hole. Astronomers think NGC 1365's prominent bar plays a crucial role in the galaxy's evolution, drawing gas and dust into a star-forming maelstrom and ultimately feeding material into the central black hole.

Perseid Storm

Storms on the distant horizon and comet dust raining through the heavens above are combined in this alluring nightscape. The scene was recorded in the early hours of August 13 from the Keota Star Party site on the Pawnee National Grasslands of northeastern Colorado, USA. Looking east across the prairie, the composite of 8 consecutive exposures each 30 seconds long captures the flash of lightning and a bright Perseid meteor. On the right, even the clouds can't block the light from brilliant planet Jupiter, whose mythological namesake knew how to handle both lightning bolts and meteors. Of course, this meteor's streak points back toward the shower's radiant in the heroic constellation Perseus, sharing a starry background that includes the Pleiades star cluster poised above the storm clouds. Just above the bright meteor lies the faint Andromeda Galaxy.

Hoag's Object: A Strange Ring Galaxy

Is this one galaxy or two? This question came to light in 1950 when astronomer Art Hoag chanced upon this unusual extragalactic object. On the outside is a ring dominated by bright blue stars, while near the center lies a ball of much redder stars that are likely much older. Between the two is a gap that appears almost completely dark. How Hoag's Object formed remains unknown, although similar objects have now been identified and collectively labeled as a form of ring galaxy. Genesis hypotheses include a galaxy collision billions of years ago and the gravitational effect of a central bar that has since vanished. The above photo taken by the Hubble Space Telescope in July 2001 reveals unprecedented details of Hoag's Object and may yield a better understanding. Hoag's Object spans about 100,000 light years and lies about 600 million light years away toward the constellation of the Snake (Serpens). Coincidentally, visible in the gap (at about one o'clock) is yet another ring galaxy that likely lies far in the distance.

A Milky Way Shadow at Loch Ard Gorge

Have you ever seen the Milky Way's glow create shadows? To do so, conditions need to be just right. First and foremost, the sky must be relatively clear of clouds so that the long band of the Milky Way's central disk can be seen. The surroundings must be very near to completely dark, with no bright artificial lights visible anywhere. Next, the Moon cannot be anywhere above the horizon, or its glow will dominate the landscape. Last, the shadows can best be caught on long camera exposures. In the above image taken in Port Campbell National Park, Victoria, Australia, seven 15-second images of the ground and de-rotated sky were digitally added to bring up the needed light and detail. In the foreground lies Loch Ard Gorge, named after a ship that tragically ran aground in 1878. The two rocks pictured are the remnants of a collapsed arch and are named Tom and Eva after the only two people who survived that Loch Ard ship wreck. A close inspection of the water just before the rocks will show reflections and shadows in light thrown by our Milky Way galaxy. Low clouds are visible moving through the serene scene in this movie.

Galaxy Cluster Abell 1689 Magnifies the Dark Universe

What's the matter with this cluster of galaxies? To find out what forms matter takes in the Abell 1689 cluster requires not only deep images from telescopes like the Hubble Space Telescope, but detailed computer modeling as well. To start, almost every fuzzy yellow patch in the above image is an entire galaxy. A close inspection, however, shows that many background galaxies are strangely magnified and distorted into long curving arcs by the gravitational lens deflections of the cluster. Computer analyses of the placement and smoothness of these arcs indicate that in addition to the matter in the galaxies you can see, the cluster must also contain a significant amount of dark matter such as the model digitally superposed in purple. Now Abell 1689 remains enigmatic because the arcs are so numerous and diverse that no single dark matter model has emerged that can explain them all and still remain consistent with dark matter models needed to constrain their motion. Still, the detailed information available from clusters of galaxies like Abell 1689 gives hope that one day full solutions will be found that will not only fully reveal the dark matter in clusters, but also reveal the amounts of dark energy in the universe needed to lie along the line of sight to the distant arcs. Students and Teachers: Free video online Astro 101 course

M27: Not a Comet

While hunting for comets in the skies above 18th century France, astronomer Charles Messier diligently kept a list of the things he encountered that were definitely not comets. This is number 27 on his now famous not-a-comet list. In fact, 21st century astronomers would identify it as a planetary nebula, but it's not a planet either, even though it may appear round and planet-like in a small telescope. Messier 27 (M27) is an excellent example of a gaseous emission nebula created as a sun-like star runs out of nuclear fuel in its core. The nebula forms as the star's outer layers are expelled into space, with a visible glow generated by atoms excited by the dying star's intense but invisible ultraviolet light. Known by the popular name of the Dumbbell Nebula, the beautifully symmetric interstellar gas cloud is over 2.5 light-years across and about 1,200 light-years away in the constellation Vulpecula. This impressive color composite highlights details within the well-studied central region and fainter, seldom imaged features in the nebula's outer halo. It includes narrowband images recorded using filters sensitive to emission from oxygen atoms, shown in blue-green hues, and hydrogen atoms in red.

Brighter Than Mars

ven though you may have just read an email claiming Mars will be incredibly bright tonight, the brightest star on the horizon is not Mars. From central Iran on August 24th, the brightest star in this twilight desert skyview is Venus, aka the Evening Star. But a bright Mars is in the picture, just above and right of more brilliant Venus. Despite claims in the internet's annually returning Mars Hoax that Mars will be as big and bright as the Full Moon, this celestial scenario is very similar to the western sky you can see tonight. Along with Mars, the still beautiful vista includes Spica, alpha star of the constellation Virgo, above and left of Venus. Farther right of Venus, Saturn peeks through the sunset's fading glow just above the clouds. Near the opposite horizon, the Full Moon illuminating the desert is about 400,000 times brighter than Mars.

Hole in the Sun

This ominous, dark shape sprawling across the face of the Sun is a coronal hole -- a low density region extending above the surface where the solar magnetic field opens freely into interplanetary space. Studied extensively from space since the 1960s in ultraviolet and x-ray light, coronal holes are known to be the source of the high-speed solar wind, atoms and electrons which flow outward along the open magnetic field lines. During periods of low activity, coronal holes typically cover regions just above the Sun's poles. But this extensive coronal hole dominated the Sun's northern hemisphere earlier this week, captured here in extreme ultraviolet light by cameras onboard the Solar Dynamics Observatory. The solar wind streaming from this coronal hole triggered auroral displays on planet Earth.

The Local Fluff

The stars are not alone. In the disk of our Milky Way Galaxy about 10 percent of visible matter is in the form of gas, called the interstellar medium (ISM). The ISM is not uniform, and shows patchiness even near our Sun. It can be quite difficult to detect the local ISM because it is so tenuous and emits so little light. This mostly hydrogen gas, however, absorbs some very specific colors that can be detected in the light of the nearest stars. A working map of the local ISM within 10 light-years based on recent observations is shown above. These observations show that our Sun is moving through a Local Interstellar Cloud as this cloud flows outwards from the Scorpius-Centaurus Association star forming region. Our Sun may exit the Local Interstellar Cloud, also called the Local Fluff, during the next 10,000 years. Much remains unknown about the local ISM, including details of its distribution, its origin, and how it affects the Sun and the Earth.

Comanche Outcrop on Mars Indicates Hospitable Past

Could life once have survived on Mars? Today, neither animal nor plant life from Earth could survive for very long on Mars because at least one key ingredient -- liquid water -- is essentially absent on the red planet's rusty surface. Although evidence from the martian rovers indicates that long ago Mars might once have had liquid water on its surface, that water might also have been too acidic for familiar life forms to thrive. Recently, however, a newly detailed analysis of an unusual outcropping of rock and soil chanced upon in 2005 by the robotic Spirit rover has uncovered a clue indicating that not all of Mars was always so acidic. The mound in question, dubbed Comanche Outcrop and visible near the top of the above image, appears to contain unusually high concentrations of elements such as magnesium iron carbonate. The above image is shown in colors exaggerated to highlight the differences in composition. Since these carbonates dissolve in acid, the persistence of these mounds indicates that water perhaps less acidic and more favorable for life might have once flowed across Mars. More detailed analyses and searches for other signs will surely continue.

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