Wednesday, June 24, 2009

Enceladus might have a saltwater ocean

Frank Postberg (Max Planck Institute, Germany) has a Nature letter on Cassini's detection of sodium salts in Saturn's E ring; see also this press release. This is a very interesting result, because it implies that Saturn's satellite Enceladus might have a liquid water ocean beneath its icy surface. Recall that in 2005, the Cassini spacecraft spotted geysers shooting tiny ice crystal from cracks in Enceladus' surface (pictured). Those ice grains go into orbit about Saturn and form that planet's tenuous E ring. During subsequent passages through the E ring, Cassini's dust detector was then used to determine the composition of those ice grains, and found them to contain salt at the 1% level. Because those ice grains originated inside Enceladus, Postberg and co-authors argue that these grain's high salinity is possible if, under Enceladus' ice, there is also a liquid ocean there that lies on top of a rocky core that is the source of the salt. Note that there is also an astrobiology angle here, too, since if Enceladus is warm enough to maintain a liquid water ocean, then there is also the possibility for ocean life there, too.

Friday, June 12, 2009

Shadows along the Keeler Gap

This Cassini image is looking towards the outer edge of Saturn's main A ring. The dark band near the ring's outer edge is the Keeler gap, which is maintained by the small 8km satellite Daphnis, which is the white speck there that casts a shadow across the ring plane. Daphnis' orbit is also inclined slightly relative to the ring plane, which carries it above/below the ring plane with each orbit about Saturn. Due to this up/down motion, Daphnis' gravity on the ring also pulls the nearby ring material at the gap's edge up/down by about 1 km. And because Saturn is near its equinox, the Sun's illumination here is almost horizontal across the ring plane, causing these km-high piles of ring particles to appear brightly lit on their sunward side, which also casts shadows across the ring plane. Check the Cassini/CICLOPS website for more details, or the recent paper in AJ by Weiss et al (subscription required).

Wednesday, June 10, 2009

Shadows cast by disk-embedded planets

A recent theory paper by Hannah Jang-Condell (U. Maryland) examines the shadows that might be cast by recently-formed planets as they orbit within the circumstellar disk in which they formed. Her numerical models show that the planet's gravity will 'depress' the disk there. If that disk were then viewed by an astronomer at optical wavelengths, then that depressed spot would resemble a dark pothole, since that depression is not illuminated by the central star (see Figure). The exception is at the pothole's far side, which would instead appear as an illuminated bright spot. Note that extra-solar planets are difficult to see via direct imaging. However this work suggests a new technique that might be used to discover unseen planets indirectly---by searching for these planet's darkened potholes and dimples that they create in a planet-forming disk.