Journey To A Distant Nebula

A Nebula is a diffuse mass of interstellar dust or gas or both, visible as luminous patches or areas of darkness depending on the way the mass absorbs or reflects incident radiation. Originally, the word "nebula" referred to almost any extended astronomical object (other than planets and comets). The etymological root of "nebula" means "cloud". We sometimes use the word "nebula" to refer to galaxies, various types of star clusters and various kinds of interstellar dust/gas clouds. More strictly speaking, the word "nebula" should be reserved for gas and dust clouds and not for groups of stars.

Mercury's Soft Center

By tracking a subtle wobbling of the planet Mercury as it spins about its axis, researchers using a trio of ground-based telescopes have found strong evidence that the planet has a molten core.

Careful measurements of Mercury's spin--to an accuracy of one in 100,000--reveal that the planet's interior is decoupled from its exterior, providing strong evidence of a molten core.

Astronomer Jean-Luc Margot of Cornell University, Stan Peale of the University of California, Santa Barbara, Ray Jurgens and Martin Slade of the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and Igor Holin of the Space Research Institute in Russia report their findings in the cover story of the May 4, 2007, journal Science.

While most models for the formation of Mercury suggest the planet has an iron-rich core, many predict that the core is solid after billions of years of cooling. Others predict that small amounts of sulfur and other trace elements mixed with the iron have lowered the core's freezing point, keeping the planet's outer core from completely solidifying over that time.

While peering into the deepest interior of a planet--even our own--is difficult even with on-site technology, the ground-based telescope data collected by Margot and his colleagues provided evidence that strongly supports the latter scenario, suggesting the core is at least partially molten and may contain at least small amounts of sulfur.

The researchers used National Science Foundation's (NSF) Arecibo Observatory in Puerto Rico, part of NSF's National Astronomy and Ionosphere Center, NSF's Robert C. Byrd Green Bank Telescope in W. Va., and the NASA/JPL 70-meter antenna at Goldstone, Calif., to beam radar signals to the planet and then carefully analyze the echoes that returned.

To obtain their measurements, the astronomers compared the properties of the return signal as it struck the distributed telescope locations on Earth's surface. The amplitude of the wobbling was twice what the researchers expected for a solid planet, but on par with an object that has a solid exterior and liquid core.