Crescent Enceladus

Evoking the haunting beauty of Earth's Moon, a crescent Enceladus appears in the skies around Saturn.

Lit terrain seen here is on the Saturn-facing hemisphere of Enceladus. North on Enceladus is up and rotated 25 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on May 1, 2013.

The view was acquired at a distance of approximately 329,000 miles (530,000 kilometers) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 138 degrees. Image scale is 2 miles (3 kilometers) per pixel.

Photo credit: NASA/JPL-Caltech/Space Science Institute

The Missing Waves of Titan

One of the most shocking discoveries of the past 10 years is how much the landscape of Saturn's moon Titan resembles Earth. Like our own blue planet, the surface of Titan is dotted with lakes and seas; it has river channels, islands, mud, rain clouds and maybe even rainbows. The giant moon is undeniably wet.

The "water" on Titan is not, however, H2O. With a surface temperature dipping 290 degrees F below zero, Titan is far too cold for liquid water. Instead, researchers believe the fluid that sculpts Titan is an unknown mixture of methane, ethane, and other hard-to-freeze hydrocarbons.

The idea that Titan is a wet world with its own alien waters is widely accepted by planetary scientists. Nothing else can account for the observations: NASA's Cassini spacecraft has flown by Titan more than 90 times since 2004, pinging the Moon with radar and mapping its lakes and seas. ESA's Huygens probe parachuted to the surface of Titan in 2005, descending through humid clouds and actually landing in moist soil.

Yet something has been bothering Alex Hayes, a planetary scientist on the Cassini radar team at Cornell University.

If Titan is really so wet, he wonders, "Where are all the waves?"

Here on Earth, bodies of water are rarely still. Breezes blowing across the surface cause waves to ripple and break; raindrops striking sea surfaces also provide some roughness. Yet on Titan, the lakes are eerily smooth, with no discernible wave action down to the millimeter scale, according to radar data from Cassini.

"We know there is wind on Titan," says Hayes. "The moon's magnificent sand dunes [prove] it."

Add to that the low gravity of Titan—only 1/7th that of Earth—which offers so little resistance to wave motion, and you have a real puzzle.

Researchers have toyed with several explanations. Perhaps the lakes are frozen. Hayes thinks that is unlikely, however, "because we see evidence of rainfall and surface temperatures well above the melting point of methane." Or maybe the lakes are covered with a tar-like substance that damps wave motion. "We can't yet rule that out," he adds.

The answer might be found in the results of a study Hayes and colleagues published in the July 2013 online edition of the journal Icarus. Taking into account the gravity of Titan, the low viscosity of liquid hydrocarbons, the density of Titan's atmosphere, and other factors, they calculated how fast wind on Titan would have to blow to stir up waves: A walking-pace breeze of only 1 to 2 mph should do the trick.

This suggests a third possibility: the winds just haven’t been blowing hard enough. Since Cassini reached Saturn in 2004, Titan’s northern hemisphere (where most of the lakes are located) has been locked in the grip of winter. Cold heavy air barely stirs, and seldom reaches the threshold for wave-making.

But now the seasons are changing. In August 2009 the sun crossed Titan’s equator heading north. Summer is coming, bringing light, heat and wind to Titan's lake country.

"According to [climate models], winds will pick up as we approach the solstice in 2017 and should be strong enough for waves," he says.

If waves appear, Cassini should be able to detect them. Radar reflections from wavy lake surfaces can tell researchers a great deal. Wave dimensions, for instance, may reveal the viscosity of the underlying fluid and, thus, its chemical composition. Also, wave speeds would track the speed of the overlying winds, providing an independent check of Titan climate models.

Hayes is excited about "bringing oceanography to another world. All we need now," he says, "are some rough seas."

Video credit: NASA

Mimas and Pandora

Mimas and Pandora remind us of how different they are when they appear together as in this Cassini spacecraft image. Although they are both moons of Saturn, Pandora's small size means that it lacks sufficient gravity to pull itself into a round shape like its larger sibling, Mimas.

Researchers believe that the elongated shape of Pandora (50 miles, or 81 kilometers across) may hold clues to how it and other moons near the rings formed.

This view looks toward the anti-Saturn hemisphere of Mimas (246 miles, or 396 kilometers across). North on Mimas is up and rotated 28 degrees to the right. The image was taken in blue light with the Cassini spacecraft narrow-angle camera on May 14, 2013.

The view was acquired at a distance of approximately 690,000 miles (1.1 million kilometers) from Mimas. Image scale is 4 miles (7 kilometers) per pixel. Pandora was at a distance of 731,000 miles (1.2 million kilometers) when this image was taken. Image scale on Pandora is 4 miles (7 kilometers) per pixel.

Photo credit: NASA/JPL-Caltech/Space Science Institute

The Earth, by Cassini

In this rare image taken on July 19, 2013, the wide-angle camera on NASA's Cassini spacecraft has captured Saturn's rings and our planet Earth and its moon in the same frame. It is only one footprint in a mosaic of 33 footprints covering the entire Saturn ring system (including Saturn itself). At each footprint, images were taken in different spectral filters for a total of 323 images: some were taken for scientific purposes and some to produce a natural color mosaic. This is the only wide-angle footprint that has the Earth-moon system in it.

The dark side of Saturn, its bright limb, the main rings, the F ring, and the G and E rings are clearly seen; the limb of Saturn and the F ring are overexposed. The "breaks" in the brightness of Saturn's limb are due to the shadows of the rings on the globe of Saturn, preventing sunlight from shining through the atmosphere in those regions. The E and G rings have been brightened for better visibility.

Earth, which is 898 million miles (1.44 billion kilometers) away in this image, appears as a blue dot at center right; the moon can be seen as a fainter protrusion off its right side. An arrow indicates their location in the annotated version. (The two are clearly seen as separate objects in the accompanying narrow angle frame: PIA14949.) The other bright dots nearby are stars.

This is only the third time ever that Earth has been imaged from the outer solar system. The acquisition of this image, along with the accompanying composite narrow- and wide-angle image of Earth and the moon and the full mosaic from which both are taken, marked the first time that inhabitants of Earth knew in advance that their planet was being imaged. That opportunity allowed people around the world to join together in social events to celebrate the occasion.

This view looks toward the unilluminated side of the rings from about 20 degrees below the ring plane.

Images taken using red, green and blue spectral filters were combined to create this natural color view. The images were obtained with the Cassini spacecraft wide-angle camera on July 19, 2013 at a distance of approximately 753,000 miles (1.212 million kilometers) from Saturn, and approximately 898.414 million miles (1.445858 billion kilometers) from Earth. Image scale on Saturn is 43 miles (69 kilometers) per pixel; image scale on the Earth is 53,820 miles (86,620 kilometers) per pixel. The illuminated areas of neither Earth nor the Moon are resolved here. Consequently, the size of each "dot" is the same size that a point of light of comparable brightness would have in the wide-angle camera.

Photo credit: NASA/JPL-Caltech/Space Science Institute

Note: For more information, see NASA Releases Images of Earth by Distant Spacecraft, PIA17170: One Special Day in the Life of Planet Earth -- Close-Up (which clearly shows the Earth and Moon as two separate objects), PIA17038: Two Views of Home (which shows the Earth taken by both Cassini, around Saturn, and Messenger, around Mercury, on the same day), PIA14949: One Special Day in the Life of Planet Earth, and Pale Blue Dot: Distant Spacecraft Photograph Earth.  Also, previously, Cassini Probe to Take Photo of Earth From Deep Space.

Saturn's Rings

The shadows of Saturn's rings edge ever farther southward as Saturn creeps towards southern winter (or northern summer). Saturn is now almost exactly halfway between its equinox (August 2009) and southern winter solstice (in May 2017).

At equinox, the rings' shadows appeared as a thin line at Saturn's equator. See PIA11667.

This view is centered on an area at 22 degrees south latitude on Saturn. The image was taken with the Cassini spacecraft wide-angle camera on May 6, 2013 using a spectral filter sensitive to wavelengths of near-infrared light centered at 752 nanometers.

The view was acquired at a distance of approximately 813,000 miles (1.3 million kilometers) from Saturn. Image scale is 48 miles (78 kilometers) per pixel.

Photo credit: NASA/JPL-Caltech/Space Science Institute

Pan and Waves in Saturn's A-Ring

The shepherd moon Pan orbits Saturn in the Encke gap while the A ring surrounding the gap displays wave features created by interactions between the ring particles and Saturnian moons.

Pan (17 miles, or 28 kilometers across) maintains the Encke gap through gravitational interactions with ring particles. The wave features in the A ring are generated through interactions between ring particles and moons such as Pan.

This view looks toward the sunlit side of the rings from about 34 degrees above the ringplane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on May 11, 2013.

The view was obtained at a distance of approximately 240,000 miles (386,000 kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 111 degrees. Image scale is 1 mile (2 kilometers) per pixel.

Photo credit: NASA/JPL-Caltech/Space Science Institute

Janus

The Cassini spacecraft catches a glimpse of Janus, an irregularly shaped moon. Lacking sufficient gravity to pull itself into a round shape, Janus has had its lumpy primordial shape only slightly modified by impacts since its formation.

See PIA10447 and PIA12714 for higher resolution views of Janus (111 miles, or 179 kilometers across). See PIA08170 and PIA08348 to learn about how Janus periodically swaps orbits with Epimetheus.

This view looks toward the trailing hemisphere of Janus. North on Janus is up and rotated 44 degrees to the left. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 28, 2013.

The view was obtained at a distance of approximately 780,000 miles (1.3 million kilometers) from Janus and at a Sun-Janus-spacecraft, or phase, angle of 77 degrees. Image scale is 5 miles (7 kilometers) per pixel.

Photo credit: NASA/JPL-Caltech/Space Science Institute