Propellers and wakes in Saturn's rings

This figure shows results from an Nbody simulation of a small patch in Saturn's rings; click figure to zoom in. Small dots represent meter sized ring particles, while the circle at the center is a 150m moonlet that is embedded in the ring. All bodies are travelling to the right as they orbit Saturn, but keep in mind that those nearer Saturn (which is far downwards in this figure) orbit faster, so ring particles in the lower x<0 half of this figure are drifting towards the right side of the moonlight, while those in the upper x>0 half are drifting left of the moonlet. This Nbody simulation was performed by Shugo Michikoshi and Eiichiro Kokubo, and their results are detailed in this preprint.

The upper figure shows what happens in a low mass ring having a surface density of 60 grams/cm^2. As particles drift past the moonlet they receive a kick due to the moonlet's gravity, which in turn opens a propeller-shaped gap in the ring. The Cassini spacecraft has in fact observed many such propellers orbiting in Saturn's A ring, like the one seen below; see the CICLOPS website for more details about this image. Curiously, the model predicts that the propeller should appear as a dark gap in the ring, while the Cassini image below shows that a propeller is bright. The meaning of this is unclear, but it may indicate that the propeller gap is also filled with sunlight-reflecting dust grains that are produced as ring particles collide near the moonlet.

The lower Nbody simulation (in the lower half of the top graphic) shows results for a high mass ring of surface density 400 grams/cm^2. In this case, the higher ring gravity cause the ring particles to condense into ropy or taffy-like structures that are known as wakes. These wakes dominate the ring's appearance and completely wash-out the propeller that the moonlet is trying to form. The fact that propellers are seen in Saturn's A ring, while none have been observed in Saturn's B ring, suggest that the A ring is a relatively low mass ring that allows moonlets to form propellers, while the B ring is massive and full of gravitating wakes that inhibit any such propellers.

Messenger's first image from Mercury orbit

The planet Mercury acquired its first artificial satellite on March 17, when the Messenger spacecraft went into orbit about the planet. Two weeks later, the spacecraft acquired its first image from Mercury orbit, with 75,000 more images to be acquired during the next year. See this press release for more details and a much larger image, or visit the Messenger website.

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.

Monster solar flare imaged by Solar Dynamics Observatory

This massive solar flare erupted from the Sun's surface on February 24, and was imaged by NASA's Solar Dynamics Observatory. Check the SDO website for more info, plus a very dramatic movie of this explosion in space.

Stardust sees impact site on comet Tempel 1

The stardust spacecraft flew by comet Tempel 1 last night, and acquired the close-up image of the surface that is seen on the left. Recall that this comet was first visited by the Deep Impact spacecraft in 2005, which shot that comet with a one-ton projectile; the high-res image on the left shows the impact site prior to the collision. One of Deep Impact's science goals was to observe the resulting crater, but the impact generated so much dust, the crater was never seen by Deep Impact. Fortunately, the Stardust spacecraft could maneuvered so that it would encounter comet Tempel 1 six years later. Stardust acquired the fuzzy low-res image on the right, which does indeed show the impact site, with a shallow 100m crater just inside the smaller circle. Note also that the impact obliterated the dark region that is seen at 10 o'clock along the yellow circle (left), in the pre-impact image. The Deep Impact crater has been observed at last!

ISS transits the Sun during solar eclipse

Thierry Legault captured this amazing picture of the International Space Station as it transits the Sun during a partial solar eclipse by the Moon on January 4.

HR 8799 hosts jumbo planetary system

Christian Marois (Herzberg Institute of Astrophysics) and colleagues discovered a fourth giant planet orbiting HR 8799. The new planet, 'e' in the above image, lies inwards of planets b,c,d. This is typical when using imaging to hunt for extra-solar planets, since it is the closer-in planets that are harder to find, due to obscuration by the bright central star. A simulated star was actually removed from this image; the fuzzy blob seen in the center is due to imperfections in this star-subtraction process. These planets lie about 15 to 70 AU away from the central star, and have masses of ~5 to 10 Jupiter masses. So this is a jumbo planetary system, since the orbits and masses are several times larger than that the Solar System's. See the abstract of their paper in Nature for more details.