Sunday, August 30, 2009

Circumstellar debris disk orbiting HD 32297

This is the circumstellar debris disk that orbits the star HD 32997, imaged with the Palomar 5m telescope by Dimitri Mawet and colleagues. The star lies at the cross, but its light has been blocked by a phase mask coronograph, which is a device that shifts the phase some of that starlight so that the star's light waves interfere with itself destructively, effectively making the very bright star dissapear from this image. This is very useful, since it also reveals the light from the much fainter circumstellar material.

The colored blobs indicate that there is a ring or perhaps a disk of dust in orbit about this star, with that disk/ring seen nearly edge on. The dust grains are visible because they are reflecting starlight, and the colors indicate the intensity of that reflected light. Of particular interest to me is the asymmetry seen in this disk, with one side being brighter than the other by ~50%.

These dusty disks usually have rather short lifetimes, since dust grains destroy each other when the collide with each other. Consequently, other unseen `planetesimals' are implicated here, since collisions by these asteroidal or cometary bodies are needed to continually resupply the disk with the dust seen here. And since comets or asteroids are evidently forming in this system, it seems plausible that larger planets might have formed here, too. Additional details are also available in the paper by Mawet et al.

Tuesday, August 18, 2009

Ingredients for prebiotic life found in comet

The Stardust spacecraft encountered comet Wild 2 on January 2, 2004, and collected samples of the comet's coma and tail. Those samples parachuted to Earth on January 15, 2006, and have been studied in labs ever since. From these samples, Jamie Elsila at NASA/Goddard reports the first ever detection of glycine in a comet. Glycine is a common amino acid, which when combined with others can build proteins, which are important building blocks in the chemistry of life. The detection of glycine in comet Wild 2 strengthens the argument that comet impacts on the early Earth provided the prebiotic chemistry that ultimately allowed life to begin on Earth. However, critics of this cometary-delivery theory would argue that comet impacts are far too fiery and energetic to deposit complex organic molecules on Earth whole and unscathed. Regardless, this interesting finding highlights the value of sample-return missions, which can provide valuable insight into composition of ancient and primitive bodies, like as comets and asteroids, that are the primordial building blocks of the planets. See this press release for more details.

Tuesday, August 11, 2009

Planetary-sized impacts around HD 172555?

Casey Lisse (JHU/APL) and colleagues recently used the Spitzer Space Telescope to collect infrared spectra of the dust that orbits the relatively young 12 million year-old star HD 172555. Their observations are described in this preprint. Their spectra shows that this star's circumsteller dust is, as expected, rich in silicate, which is the principle ingredient in circumstellar dust. What is surprising here is that this dust is a glassy silicate, like tektite or obsidian, which tends to form when rocky bodies collide at high speeds of ~10km/sec. These spectra also indicate the presence of ample amounts of SiO gas, which is vaporized rock. From these spectra, Lisse and colleagues infer that this system suffered a recent giant impact via the collision of two large ~1000km bodies (the size of Ceres, the largest asteroid in our Solar System). They estimate that this giant impact occurred within the past ~100 thousand years. There are two possible interpretations of these observations. (1) Collisions among ~1000km-sized protoplanets at HD 172555 indicate that this system is currently undergoing planet formation. This is an important step in the planet-formation process, and is necessary if one wishes to ultimately produce a system of ~10,000km-sized terrestrial planets. (2) Alternatively, giant impacts are instead destroying the protoplanets that orbit HD 172555, and that astronomers are witnessing the collisional destruction of a young planetary system. Which outcome is more likely is presently unclear. See this Spitzer page for more details, as well as the above artist's rendition of a giant impact.

Saturday, August 8, 2009

Small moonlet discovered in Saturn's B ring

The Cassini spacecraft spotted this tiny moonlet as it orbits within Saturn's vast and dense B ring. Again, the rings are observed very near equinox, so the Sun's illumination streams nearly along the ring plane, and small objects can cast long shadows here. From the 40km length of the moonlet's shadow, Cassini scientists can infer its diameter of 0.4km. The view here is of the outer part of the B ring. Saturn is far off to the left, and the dark region on the right is the Cassini Division, in which the Huygen's ringlet (grey ribbon) also resides. Check the CICLOPS website for more details and other great images of Saturn's ring/satellite system.

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.

Thursday, August 6, 2009

Topography at outer edge of Saturn's B ring?

This very interesting Cassini image shows a close-up of the outer edge of Saturn's B ring. Saturn is far to the left, so the ring's orbital motion here is either up or down (and my best guess says down). The Cassini Division is to the right, just beyond the B ring. The Huygen's ringlet is the prominent gray band that orbits 300km beyond the B ring's edge.

Recall that Saturn is almost at equinox (Tuesday August 11!), so the Sun is just above Saturn's equator. Consequently, sunlight is streaming almost parallel to the ring plane, which allows even very modest vertical structures in the ring to cast long shadows across the ring plane. Evidently, the B ring's outer edge has topography, since it cast shadows that are hundreds of kilometers long! Judging by how ragged the shadows are, this ring-edge seems to resemble a mountain range, which is quite a surprise since the rest of the ring-plane is extremely flat. Note also the bright diagonals, one of which is clearly casting a shadow. These streaks might be due to ring material moving radially, perhaps due to avalanches of ring-matter tumbling down the supposed mountainside? Such radial motion would then get dragged along a diagonal due to the ring's faster orbital speed in regions closer to Saturn. But at this stage, this is all just speculation...

This image was acquired on July 26, 2009, and can be found at the Cassini Equinox Mission's raw image archive. A followup comment will describe how to use this archive effectively to search for other interesting Cassini images.

Saturday, August 1, 2009

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.

Rover spots meteorite on Mars?

The Opportunity rover recently spotted this rock on Mars, whose texture resembles a meteorite. The rover is seen using its X-ray spectrometer, which should reveal its composition, and will hopefully determine whether this rock is indeed a meteorite. See this press release for more pictures and details.