First X-ray view of Venus!
29 November 2001X-ray emission has been detected from far-flung galaxies and exotic stellar objects in the Universe. But it is also observed from celestial objects much closer to home - even within our Solar System.
For instance, comets visiting our Solar System also radiate in X-rays - a mystery which scientists using data from an XMM-Newton observation of Comet McNaught-Hartley hope to fully explain shortly.
In the meantime, the same European-led team has turned its sights to one of Earth's immediate neighbours and have obtained the first ever X-ray pictures of Venus. The results of this original study have been presented this week at the "New Visions of the X-ray Universe in the XMM-Newton and Chandra era" symposium at ESA's Space Technology and Research Centre (ESTEC) at Noordwijk in the Netherlands.
It was logical for Konrad Dennerl, of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany to attempt an X-ray observation of Venus. "The discovery of unexpectedly bright X-ray emission from comets has led to increased interest in X-ray studies of Solar System objects. The planet with its rich dense atmosphere containing lots of carbon dioxide, the absence of a strong magnetic field, and its proximity to the Sun represents a close planetary analogue to a comet."
The orbit of Venus is such that its angular separation from the Sun, as seen from the Earth, never exceeds 48 degrees. XMM-Newton could not be used for such an observation because its design dictates that it should never point closer than about 70 degrees from the Sun. But NASAs Chandra X-ray observatory, with a lower 45-degree limit, could fulfil the task.
Venus was also a challenging target because with its high optical surface brightness (second brightest after the Sun and brighter than our Moon), the planet's optical light could degrade the eventual X-ray measurements.
The observation was carried out in two sessions: on 10 January 2001, spectra were obtained with the observatorys LETG/ACIS-S imaging spectrometer. Three days later, an image was obtained with the ACIS-I imager. A total of 6.5 hours of X-ray data was gathered. Whilst the space observatory looked at the planet, Konrad Dennerl used his own 4 inch Newtonian telescope to take an optical image.
It should be stressed that Venus does not generate X-rays by itself. There is no invisible powerhouse on the planet! The X-ray emission which has been observed is the result of fluorescent scattering of the X-rays from the Sun entering the planetary atmosphere.
The X-ray image clearly detects Venus as a half-lit crescent, with considerable brightening on the sunward limb. It is very similar to the optical view from Garching, although the limb is brighter in X-rays than in the optical view. Optical brightness is stronger inside the crescent.
The spectrum corresponds well with what could be expected from the fluorescent scattering of these solar X-rays. The radiation is observed at discrete energy levels, mainly from ionised oxygen, carbon and nitrogen.
Continuing their investigation, Konrad Dennerl and his colleagues performed detailed simulations, taking into account all the parameters of the planet's atmosphere at different altitudes. The simulations agree well with the real X-ray data, whilst the optical view displays a different brightness distribution. It is seen that limb brightening depends very closely on the density and the chemical composition of the Venusian atmosphere. Measurement of this brightening thus provides direct information about the atmospheric structure.
"The intensity of individual fluorescence lines for different elements can change markedly with altitude," says Konrad Dennerl. "For example, the deeper in the atmosphere the X-ray emission occurs, the more absorbing layers are above. Solar X-rays do not reach below 100 km."
Whilst there is practically no variation in the optical flux from Venus on time scales of hours, the X-ray flux displayed pronounced variability from minute to minute a phenomenon yet to be fully explained.
One thing is for sure: this pioneering view of Venus demonstrates that by studying X-ray fluorescence, the new era X-ray space observatories can remotely contribute to planetary science and delve into areas perhaps impossible to investigate otherwise.
"Discovery of X-rays from Venus with Chandra" by K. Dennerl, V. Burwitz and J. Englhauser (Max Planck Institute for Extraterrestrial Physics, Garching), C. Lisse (University of Maryland), and S. Wolk (Harvard Smithsonian Centre For Astrophysics, Cambridge, Massachusetts) is to appear in Astronomy and Astrophysics.
For more information please contact:
Dr. Konrad Dennerl
Max Planck Institute for Extraterrestrial Physics, Garching
Tel: +49 89 30000 3862
Dr. Fred Jansen
XMM-Newton project scientist
Tel: +31 71 565 4426