02 - Full Moon
at 19:14 UT
05 - Taurid
(south) meteor shower peaks. Active
between 25 Sept and 25 Nov.
Associated with Comet 2P/Encke.
09 - Moon near
Mars (morning sky) at 14h UT. Mag. +0.3.
12 -Taurid (north) meteor
shower peaks. May produce the occasional
17 -Leonid meteor shower
peaks at 9h UT. Arises from debris ejected
Comet Tempel-Tuttle in 1533. Expect about 25
to 30 meteors per hour under
dark skies. Predictions of enhanced activity
between 21-22h UT on 17 Nov
(favours sky watchers in Asia).
21 -Alpha Monocerotid meteor
shower peaks at 15:25 UT. A usually minor
shower active 15-25 Nov. Radiant is near Procyon.
Predictions of enhanced
activity this year. Timing favours Far East
Asia, Australia and across the
Pacific to Alaska.
00 0 0
0 0 0//
Get the complete calendar version
Wednesday, June 29, 2005
Cassini Probe Spies Lake-Like Feature on Titan
You might have thought Saturn's moon Titan was a somewhat dead issue after the Cassini spacecraft did not find convincing evidence for methane seas that scientists had predicted would exist. But the smoggy moon is back in the news today as a new Cassini image reveals a dark feature that scientists speculate might be a lake. The feature is "remarkably lake-like," according to a NASA statement that noted the appearance of smooth, shore-like boundaries unlike any seen previously on Titan.
The feature lies in Titan's cloudiest region, which is presumably the most likely site of recent methane rainfall. This, coupled with the shore-like smoothness of the feature's perimeter makes it hard for scientists to resist speculation about what might be filling the lake, if it indeed is one.
Thirty-nine more Titan flybys are planned for Cassini's prime mission. In future flybys the science teams will search for opportunities to observe the lake feature again and to look for mirror-like reflections from smooth surfaces elsewhere on Titan. Such reflections would strongly support the presence of liquids.
A recent flyby of Saturn's hazy moon Titan by the Cassini spacecraft has revealed evidence of a possible volcano, which could be a source of methane in Titan's atmosphere.
Images taken in infrared light show a circular feature roughly 30 kilometers (19 miles) in diameter that does not resemble any features seen on Saturn's other icy moons. Scientists interpret the feature as an "ice volcano," a dome formed by upwelling icy plumes that release methane into Titan's atmosphere. The findings appear in the June 9 issue of Nature.
Titan, Saturn's largest moon, is the only known moon to have a significant atmosphere, composed primarily of nitrogen, with 2 to 3 percent methane. One goal of the Cassini mission is to find an explanation for what is replenishing and maintaining this atmosphere. This dense atmosphere makes the surface very difficult to study with visible-light cameras, but infrared instruments like the visual and infrared mapping spectrometer can peer through the haze. Infrared images provide information about both the composition and the shape of the area studied.
The highest resolution image obtained by the visual and infrared mapping spectrometer instrument covers an area 150 kilometers square (90 miles) that includes a bright circular feature about 30 kilometers (19 miles) in diameter, with two elongated wings extending westward. This structure resembles volcanoes on Earth and Venus, with overlapping layers of material from a series of flows.
In the center of the area, scientists clearly see a dark feature that resembles a caldera, a bowl-shaped structure formed above chambers of molten material. The material erupting from the volcano might be a methane-water ice mixture combined with other ices and hydrocarbons. Energy from an internal heat source may cause these materials to upwell and vaporize as they reach the surface. Future Titan flybys will help determine whether tidal forces can generate enough heat to drive the volcano, or whether some other energy source must be present. Black channels seen by the European Space Agency's Huygens probe, which piggybacked on Cassini and landed on Titan's surface in January 2005, could have been formed by erosion from liquid methane rains following the eruptions.
Scientists have considered other explanations. They say the feature cannot be a cloud because it does not appear to move and it is the wrong composition. Another alternative is that an accumulation of solid particles was transported by gas or liquid, similar to sand dunes on Earth. But the shape and wind patterns don't match those normally seen in sand dunes.
The data for these findings are from Cassini's first targeted flyby of Titan on Oct. 26, 2004, at a distance of 1,200 kilometers (750 miles) from the moon's surface.
The visual and infrared mapping spectrometer instrument can detect 352 wavelengths of light from 0.35 to 5.1 micrometers. It measures the intensities of individual wavelengths and uses the data to infer the composition and other properties of the object that emitted the light; each chemical has a unique spectral signature that can be identified.
Forty-five flybys of Titan are planned during Cassini's four-year prime mission. The next one is Aug. 22, 2005. Radar data of the same sites observed by the visual and infrared mapping spectrometer may provide additional information.
Oort cloud, is a postulated spherical cloud
of comets situated about 50,000 to 100,000 AU from
the Sun. This is approximately 1000 times the distance
from the Sun to Pluto or roughly one light year, almost
a quarter of the distance from the Sun to Proxima
Centauri, the star nearest the Sun. The Oort cloud
would have its inner disk at the ecliptic from the
Kuiper belt. Although no direct observations have
been made of such a cloud, it is believed to be the
source of most or all comets entering the inner solar
system (some short-period comets may come from the
Kuiper belt), based on observations of the orbits
of comets. Source: Wikipedia