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The Solar System Through Chandra's Eyes
April 5, 2004
Chandra's speciality is probing the super-hot regions of the universe where matter has been heated to millions of degrees Celsius by shock waves from exploding stars and galaxies, or by the crush of gravity around black holes.
But Chandra has also shown that the relatively peaceful realms of space such as our solar system sometimes shine in X-ray light.
Planets, satellites and comets typically have temperatures well below 1000 degrees, but they still can produce X-rays in a number of ways, most of which involve the Sun directly or indirectly. Although the X-ray power is weak, ranging from a few megawatts to a few gigawatts for Jupiter, it provides information difficult to come by with other telescopes.
Very close to home, Chandra has detected evidence of X-rays from Earth's geocorona (extended outer atmosphere) through which Chandra moves. The geocoronal X-rays are caused by collisions between hydrogen atoms in the geocorona with carbon, oxygen and neon ions that are streaming away from the Sun in the solar wind. During the collisions, the solar ions capture electrons from the hydrogen atoms, then kick out X-rays as the captured electrons drop to lower energy states.
This process, called charge exchange because an electron is exchanged between the neutral atoms in the atmosphere and the ions in the solar wind, operates throughout the solar system. It is especially important for comets, which have extended atmospheres. By observing X-rays from comets, it is possible to study the elements present in the solar wind, the structure of the comet's atmosphere, and cometary rotation. In the future it may be possible to detect X-radiation from collections of hundreds of comets around stars other than the Sun. Young stars would be the most promising candidates because they have vigorous stellar winds.
Chandra has been used to prospect for elements on the Moon. Oxygen, magnesium, aluminum and silicon were detected over a large area of the lunar surface. The lunar X-rays are caused by fluorescence due to the impact of solar X-rays on the surface of the Moon. Fluorescent X-rays give a direct measurement of elements present, independent of assumptions about the type of mineral or other complications. When longer observations of the Moon are made with Chandra, they should help to determine if the Moon was formed by a giant impact of a planetoid with the Earth about 4.5 billion years ago, or by some other process.
Planetary atmospheres can also exhibit fluorescence of solar X-radiation. Chandra detected fluorescent radiation from oxygen and other atoms in the Venusian atmosphere between 120 and 140 kilometers (75 to 90 miles) above the surface of the planet. In contrast, the optical light from Venus is caused by the reflection of sunlight from clouds 50 to 70 kilometers above the surface. Future X-ray images will enable scientists to examine regions of the Venusian atmosphere that are difficult to investigate otherwise.
Fluorescent X-rays from oxygen atoms in the Martian atmosphere probe similar heights. A huge Martian dust storm was in progress when the Chandra observations were made. Since the intensity of the X-rays did not change when the dust storm rotated out of view, astronomers were able to conclude that the dust storm did not affect Mars's upper atmosphere. They also found evidence that Mars is still losing its atmosphere to deep space.
A faint halo of X-rays was detected some 7,000 kilometers above the surface of Mars. These X-rays are presumably due to the solar wind charge-exchange process operating in the tenuous extreme upper atmosphere of Mars.
Jupiter has an environment capable of producing X-rays in a different manner because of its substantial magnetic field. X-rays are produced when high-energy particles from the Sun get trapped in its magnetic field and accelerated toward the polar regions where they collide with atoms in Jupiter's atmosphere. Chandra's image of Jupiter shows strong concentrations of X-rays near the north and south magnetic poles.
Io, Europa and the Io Plasma Torus
|Io and Europa
Reference: Elsner et al.
The Astrophysical Journal, 572:1077-1082, 2002 June 20
Weaker X-ray signals have been detected from two of Jupiter's moons, Io and Europa, and from the Io Plasma Torus, a doughnut-shaped ring of energetic particles that circles Jupiter.
Gases such as sulfur dioxide are produced by Io's volcanos, escape from Io and become trapped in an orbit around Jupiter, where they are accelerated to high energies. Collisions between the particles within the torus, and with the surfaces of Io and Europa can account for the observed X-rays.
Like Jupiter, Saturn has a strong magnetic field so it was expected that Saturn would also show a concentration of X-rays toward the poles. However, Chandra's observation revealed instead an increased X-ray brightness in the equatorial region. Furthermore, Saturn's X-ray spectrum, or the distribution of its X-rays according to energy, was found to be similar to that of X-rays from the Sun. This indicates that Saturn's X-radiation is due to the reflection of solar X-rays by Saturn's atmosphere. The same process may be responsible for the weak equatorial X-radiation observed from Jupiter. Further observations should help clarify whether Saturn's magnetic polar regions ever flare up in X-rays, as do Jupiter's.
Astronomers have used the lack of X-rays from Saturn's largest moon, Titan, to draw some interesting conclusions. On January 5, 2003, Titan - the only moon in the solar system with a thick atmosphere - crossed in front of the Crab Nebula, a bright, extended X-ray source. Titan's transit enabled Chandra to image the one-arcsecond-diameter X-ray shadow cast on Chandra by the moon. This tiny shadow corresponds to the size of a dime as viewed from two and a half miles. The diameter of Titan's shadow was found to be larger than the known diameter of its solid surface. This difference in diameters yields a measurement of about 550 miles (880 kilometers) for the height of the X-ray absorbing region of Titan's atmosphere.
The extent of Titan's upper atmosphere is consistent with, or slightly (10-15%) larger, than that implied by Voyager I observations made at radio, infrared, and ultraviolet wavelengths in 1980. Saturn was about 5% closer to the Sun in 2003, so increased solar heating of Titan may have caused its atmosphere to expand. This may have important implications for the upcoming Cassini-Huygens mission. The Cassini-Huygens spacecraft will reach Saturn in July of 2004 to begin a four-year tour of Saturn, its rings and its moons that will include 44 close flybys of Titan. If Titan's atmosphere has really expanded, the trajectory may have to be changed.
This article was based on previous Chandra press releases and an article by Scott Wolk in the recent Chandra newsletter: http://cxc.harvard.edu/newsletters/news_11/solar.html
which contains references to the original scientific papers.