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. 

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 Hubble 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). 

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 clouds-tops of Uranus and Neptune. 

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.

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. 

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. 

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. 

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. 

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. 

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.