Hot and Cold; Near and Far. Two Planets Reveal New Surface Details
New details have been released regarding the surface of Mercury, the planet closest to the sun as well as that of Pluto, the planet furthest from the sun.
Researchers at the Observatoire de Paris used ESA’s infrared space telescope (ISO) to measure variations in the thermal flux of the Pluto/Charon system. The data collected show that the temperature of Pluto’s surface is not uniform – with the coldest portions at -235°C and the warmest as high as -210°C. Comparing previous data on Pluto’s surface brightness in visible light to new data regarding its brightness in the infrared (i.e.,. where it is warmer or cooler) astronomers showed that there is a rough correlation between lower visible light brightness (darker material) and greater infrared brightness (higher temperature). This “anti-correlation is not perfect thus suggesting that the surface of Pluto may be porous.
Pluto’s surface is composed mostly of Nitrogen along with some Methane and Carbon Monoxide. Pluto’s stature as a true “planet” has been in dispute in recent years. It is now thought to be a rather large example of a class of icy bodies common in the extreme outer regions of the solar system. Neptune’s giant moon Triton, visited by the Voyager 2 probe in 1989, is thought to resemble Pluto in many ways. Although Pluto has yet to be visited by a spacecraft, the Pluto-Kuiper Express mission is scheduled to fly past Pluto and Charon in 2010.
Meanwhile, much closer to home, astronomers from Boston University used an Earth-based telescope to obtain the first detailed images of a Mercury’s surface taken on Earth. Images taken with the Mt. Wilson Observatory show similar features to those found on the side of Mercury which was imaged at close range by the Mariner 10 Probe in 1974 and 1975. These new images cover a portion of Mercury’s surface not observed by Mariner 10. Since telescope observations of Mercury require pointing a telescope close to the sun near the horizon, optimal viewing conditions are often hampered by atmospheric turbulence and glare. The team used a process of combining multiple images, and a little luck to obtain these images.
Due to its close proximity to the Sun, Mercury’s surface is heated, in some places, to over 400°C. None the less, radar observations of its polar regions suggest that water ice may be trapped within permanently shaded craters. Mercury’s cratered surface may resemble that of the Earth’s moon, but its interior is much more like that of Earth. Indeed, its core is much larger, in proportion to its diameter, than is Earth’s. Some astronomers speculate that Mercury is the remnant of a larger planet that was inflicted with a large impact early in its history which removed its outer mantle, leaving the metallic core behind. Mercury was last visited by Mariner 10 a quarter century ago. The next visit will now be in 2009 when the Messenger Mission visits Mercury.
The earliest astronomical observations – those using only the unaided human eye – showed the closest object in the sky, the Earth’s moon, to have a face rich with surface features. When the first telescope was fixed upon the moon by Galileo, he found a bewildering (and somewhat unexpected) array of geological features. They soon found themselves named after oceans and famous terrestrial mountain ranges.
The Hubble Space Telescope has been used in the past to observe Pluto and its moon Charon. However, this is not possible in the case of Mercury or Venus. Observation protocols prohibit such observations for fear that mis-callibration might result in too much light from the sun hitting sensitive instruments.
Soon thereafter, Galileo, and those who followed, looked at the planets (and even the sun) to see if they too had surface markings. The superior planets – i.e.,. those who orbit the sun further out than Earth, while distant, could be viewed with relative ease. By virtue of celestial mechanics, they always presented a full illuminated face to the Earth bound observer – especially when they were at their closest approach to Earth. They can be viewed against a moonless, sunless pitch black sky, as well as when they are directly overhead, minimizing the amount of Earth’s distorting atmosphere their light has to pass through.
However, the inferior planets, Mercury and Venus, again owing to celestial mechanics, present, at most, a thin crescent when they are closest to us. To get a good view of a partial or fully illuminated hemisphere of Mercury or Venus, you have to wait until the planets are opposite the sun – hence smaller in apparent size. They are also closer to the sun’s disk thus making them harder to observe since this require observations just before dawn or just after sunset, with the greatest amount of atmosphere between planet and observer.
None the less, early astronomers reported surface markings for both Mercury and Venus. In the case of Venus there is some vague correlation with its characteristic sideways “V” shaped cloud features. With Mercury, however, there was virtually no correlation between what people thought they saw then and what our space probes have shown us recently. This did not stop astronomers from making maps of Mercury’s surface however. One of the more detailed was made by Giovanni Schiaparelli (the same fellow who drew the infamous “canals” on Mars) in the 1880’s.
Related Links
° ISO measures temperature variations of the surface of Pluto, ESA news story
° New Ground Based Photos of Mercury’s Unseen Surface Obtained By BU Astronomers, Boston University press release
° “A digital high-definition imaging system for spectral studies of extended planetary atmospheres. Initial results in white light showing features on the hemisphere of Mercury unimaged by Mariner 10”, Baumgartner et al, [Abstract], The Astronomical Journal, 119:2458-2464, 2000 May.
° ISO, Infrared Space Observatory, ESA
Background Information
° Pluto, Welcome to the Planets, NASA JPL
° The Surface of Pluto, 7 March 1996, Space Telescope Science Institute
° Pluto-Kuiper Express, NASA JPL
° Mercury, Welcome to the Planets, NASA JPL
° Mercury, Preliminary Draft, Terrestrial Planets Science Working Group, Brown University
° MESSENGER mission to Mercury, Johns Hopkins University
