Hubble finds new moon orbiting Pluto

Mark Showalter (SETI Institute) just discovered a fourth moon orbiting Pluto, designated P4 for now. It is quite small, about 15 miles across, and its orbit resides between satellites Nix and Hydra. This discovery was quite accidental, because Showalter was observing the Pluto system to look for signs of any planetary rings there; see the Hubble website for more details. Since Pluto seems ripe with satellites, we might expect the New Horizons spacecraft, which flies by Pluto in 2015, to find even more tiny satellites.

Rare elongated crater on Mars

ESA's Mars Express imaged this elongated crater on Mars. Note that most craters are circular, even when the impactor strikes the planet at a shallow angle. However a train of interplanetary debris can leave an elongated scar, which might account for the crater seen here. But accounting for the origin of that hypothetical debris train can be problematic---perhaps this is debris from a comet or asteroid that was tidally disrupted by Mars? Or perhaps this debris is from a tidally disrupted satellite that had spiraled inwards and onto the planet due to the martian tide. Although this might seem farfetched, this in fact will be the ultimate fact of the Martian satellite Phobos, which will eventually impact Mars in tens of million years, due to its slow orbital decay that is driven by the martian tidal forces. See the Mars Express website for more details.

Daphnis maintains the Keeler Gap

The small Saturnian satellite Daphnis is responsble for maintaining the narrow Keeler gap, which lies near the outer edge of Saturn's main A ring. Daphnis' gravitational pull on the nearer ring particles also sculpts the gap's edges, resulting in wakes that are downstream of the satellite. This Cassini image is available at the CICLOPS website.

Close up view of Prometheus

On 27 January 2010, the Cassini spacecraft acquired this close-up view of the small 60km Saturnian satellite Prometheus. Shadows cast by its irregular surface suggests that this satellite might have the `flying saucer' shape that is also exhibited by Saturn's other small satellites Pan and Atlas (as detailed in Porco et al 2007). Note that this peculiar shape can result when a satellite forms while orbiting inside a dense planetary ring (such as perhaps Pan, which does orbit in Saturn's main A ring) while also under the influence of Saturn's strong gravitational tide. This in turn suggests that Prometheus (as well as its cohorts Atlas and Pandora) might actually have formed at the outer edge of the A ring and then migrated to its current position just beyond the A ring's outer edge. This kind of radial migration is in fact expected to be driven by the strong gravitational torques that the A ring exerts on the nearby satellites. Check the CICLOPS website for more details.

Iapetus' dichotomy explained

Iapetus is Saturn's icy yin-yang satellite. This Cassini image shows that the side of this satellite that leads in its orbit about Saturn is 10 times darker than its polar regions. This darkening was long thought to be due to contamination from Saturn's even more distant satellite Phoebe. Meteorites crashing into Phoebe will launch small debris that goes into orbit about Saturn, resulting in a vast dust ring that also contaminates Iapetus. This dust ring was only recently discovered by the Spitzer Space Telescope. Some of that dark dust gets deposited at Iapetus' leading face, which is then warmed by sunlight. Recent work by John Spencer and Tilmann Denk show that this can cause surface ice there to evaporate at the equator. That water vapor can migrate towards and then freeze out at the poles, brightening the satellite there, and giving it its yin-yang appearance. Additional images can be found at Cassini's CICLOPS website.

New radar image of rare triple asteroid system

A new triple asteroid system was discovered by Marina Brozovic and Lance Benner (JPL) via radar imaging; see this press release for details. Shown here are two small ~50m satellites orbiting a larger 700m asteroid named 1994 CC. Although other triple asteroid systems are known, most are in the main asteroid belt that lies between Mars and Jupiter. However, this is only the second triple asteroid system to have been detected among the near-Earth asteroid population.

Vertical ripples in Daphnis' edge-waves

The small speck casting a shadow here is Daphnis, which is an 8km satellite that orbits within the narrow Keeler gap that lies near the outer edge of Saturn's rings. Saturn is also very near equinox, so the sunlight that is illuminating this scene travels nearly parallel to the ring-plane. Consequently, this small satellite casts a rather prominant shadow across the ring. Daphnis also has a small inclination, so its motion carries it a bit above and then below the ring plane during each orbit of Saturn. Daphnis' gravity then tugs the nearby ring material up and down, too, which results in the vertical ripples that are seen at the Keeler gap's inner and outer edges. Note that these bright ripples also cast shadows as well. This image was acquired by the Cassini spacecraft on June 8, 2009, and many more such images can also be found at the CICLOPS website.