Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. The larger moon, Phobos, is indeed seen to be a cratered, asteroid-like object in this stunning color image from the Mars Express spacecraft, recorded at a resolution of about seven meters per pixel. But Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon - that gravitational tidal forces are dragging it down. In 100 million years or so it will likely crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.
The robot Cassini spacecraft now orbiting Saturn swooped past the sponge-textured moon in late 2005 and took an image of unprecedented detail. That image, shown above in false color, shows a remarkable world strewn with strange craters and a generally odd surface. The slight differences in color likely show differences in surface composition. At the bottom of most craters lies some type of unknown dark material. Inspection of the image shows bright features indicating that the dark material might be only tens of meters thick in some places. Hyperion is about 250 kilometers across, rotates chaotically, and has a density so low that it might house a vast system of caverns inside
Jupiter's moon Io. Two sulfurous eruptions are visible on Jupiter's volcanic moon Io in this color composite image from the robotic Galileo spacecraft that orbited Jupiter from 1995 to 2003. At the image top, over Io's limb, a bluish plume rises about 140 kilometers above the surface of a volcanic caldera known as Pillan Patera. In the image middle, near the night/day shadow line, the ring shaped Prometheus plume is seen rising about 75 kilometers above Io while casting a shadow below the volcanic vent. Named for the Greek god who gave mortals fire, the Prometheus plume is visible in every image ever made of the region dating back to the Voyager flybys of 1979 - presenting the possibility that this plume has been continuously active for at least 18 years. The above digitally sharpened image was originally recorded in 1997 on June 28 from a distance of about 600,000 kilometers.
Locked in synchronous rotation, the Moon always presents its well-known near side to Earth. But from lunar orbit, Apollo astronauts also grew to know the Moon's far side. This sharp picture from Apollo 16's mapping camera shows the eastern edge of the familiar near side (top) and the strange and heavily cratered far side of the Moon. Surprisingly, the rough and battered surface of the far side looks very different from the near side which is covered with smooth dark lunar maria. The likely explanation is that the far side crust is thicker, making it harder for molten material from the interior to flow to the surface and form the smooth maria.
In October of 1846, William Lassell was observing the newly discovered planet Neptune. He was attempting to confirm his observation, made just the previous week, that Neptune had a ring. But this time he discovered that Neptune had a satellite as well. Lassell soon proved that the ring was a product of his new telescope's distortion, but the satellite Triton remained. The above picture of Triton was taken in 1989 by the only spacecraft ever to pass Triton: Voyager 2. Voyager 2 found fascinating terrain, a thin atmosphere, and even evidence for ice volcanoes on this world of peculiar orbit and spin. Ironically, Voyager 2 also confirmed the existence of complete thin rings around Neptune - but these would have been quite invisible to Lassell!
This dramatic image features a dark red Moon during a total lunar eclipse -- celestial shadow play enjoyed by many denizens of planet Earth last Saturday. Recorded near Wildon, Austria, the picture is a composite of two exposures; a relatively short exposure to feature the lunar surface and a longer exposure to capture background stars in the constellation Leo. Completely immersed in Earth's cone-shaped shadow during the total eclipse phase, the lunar surface is still illuminated by sunlight, reddened and refracted into the dark shadow region by a dusty atmosphere. As a result, familiar details of the Moon's nearside are easy to pick out, including the smooth lunar mare and the large ray crater Tycho. In this telescopic view, the background stars are faint and most would be invisible to the naked eye.
The Spirit rover attacked Mars again in 2005 September. What might look, above, like a military attack, though, was once again just a scientific one - Spirit was instructed to closely inspect some interesting rocks near the summit of Husband Hill. Spirit's Panoramic Camera captured the rover's Instrument Deployment Device above as moved to get a closer look at an outcrop of rocks named Hillary. The Spirit rover, and its twin rover Opportunity, have now been exploring the red planet for over three years. Both Spirit and Opportunity have found evidence that parts of Mars were once wet.
Spewed from a volcano, a complex plume rises over 300 kilometers above the horizon of Jupiter's moon Io in this image from cameras onboard the New Horizons spacecraft. The volcano, Tvashtar, is marked by the bright glow (about 1 o'clock) at the moon's edge, beyond the terminator or night/day shadow line. The shadow of Io cuts across the plume itself. Also capturing stunning details on the dayside surface, the high resolution image was recorded when the spacecraft was 2.3 million kilometers from Io. Later it was combined with lower resolution color data by astro-imager Sean Walker to produce this sharp portrait of the solar system's most active moon. Outward bound at almost 23 kilometers per second, the New Horizons spacecraft should cross the orbit of Saturn in June next year, and is ultimately destined to encounter Pluto in 2015.
Swooping below Saturn, the Cassini spacecraft spied several strange wonders. Visible in the distance are some of the many complex rings that orbit the Solar System's second largest planet. In the foreground looms the gigantic world itself, covered with white dots that are clouds high in Saturn's thick atmosphere. Saturn's atmosphere is so thick that only clouds are visible. At the very South Pole of Saturn lies a huge vortex that is a hurricane-like storm showing no sign of dissipating. The robotic Cassini spacecraft took the above image in January from about one million kilometers out, resolving details about 50 kilometers across.
Asteroid Itokawa. The unusual asteroid has been visited recently by the Japanese spacecraft Hayabusa that has been documenting its unusual structure and mysterious lack of craters. Recent analyses of the border regions between smooth and rugged sections of Itokawa indicate that jostling of the asteroid might be creating segregation between large and small rocks near the surface, like the Brazil nut effect. In late 2005, Hayabusa actually touched down on one of the smooth patches, dubbed the MUSES Sea, and collected soil samples that are to be returned to Earth for analysis. Hayabusa will start its three-year long return trip to Earth this month. Computer simulations show that 500-meter asteroid Itokawa may impact the Earth within the next few million years.
Small Worlds: Ceres and Vesta: Ceres and Vesta are, respectively, only around 950 kilometers and 530 kilometers in diameter - about the size of Texas and Arizona. But they are two of the largest of over 100,000 minor bodies orbiting in the main asteroid belt between Mars and Jupiter. These remarkably detailed Hubble Space Telescope images show brightness and color variations across the surface of the two small worlds. The variations could represent large scale surface features or areas of different compositon. The Hubble image data will help astronomers plan for a visit by the asteroid-hopping Dawn spacecraft, scheduled for launch on July 7 and intended to orbit first Vesta and then Ceres after a four year interplanetary cruise. Though Shakespeare might not have been impressed, nomenclature introduced by the International Astronomical Union in 2006 classifies nearly spherical Ceres as a dwarf planet.
