Number 262, March 14, 1996 by Phillip F. Schewe and Ben Stein
THE SURFACE OF PLUTO HAS BEEN IMAGED for the first time. The Hubble
Space Telescope has snapped a series of high- resolution pictures throughout
Pluto's 6.4-day rotation period. The photo sequence reveals that Pluto
possesses more visible large-scale features than any planet except for
Earth. The features include a variety of dark and bright spots and a dark
stripe across the frosty north pole. Pluto had not previously been imaged
clearly before, even with the bigger Earth-based telescopes, because its
angular size on the sky is only a tenth of an arcsecond across. All of
this comes at a time when some astronomers want to take away Pluto's status
as a planet. (NASA press release, 7 March 1996.)
THE FIRST X-RAY HOLOGRAM WITH ATOMIC RESOLUTION has been made by scientists
at the Research Institute for Solid State Physics in Budapest, Hungary.
They sent a beam of 17-keV x rays into a perovskite crystal (SrTiO3). Some
of the x rays strike strontium atoms, where they eject inner-core electrons.
Filling this vacancy results in the emission of 14-keV "fluorescence"
x rays. Part of this x-ray wave scatters from other atoms, while part emerges
unscattered. The interference of the two waves can be monitored in a solid-state
detector at many angles. The ensuing hologram provides a direct three-dimensional
image of the strontium atoms in the crystal. In effect, the use of such
an atomic localized source of x rays is less ambiguous in determining the
internal structure of the solid than are conventional x-ray diffraction
techniques. (Miklos Tegze and Gyula Faigel, Nature, 7 March 1996.)
A NEW FORM OF ICE has been predicted to form at high pressures. Ice
has more solid forms than any other simple substance, with 10 known crystalline
structures. Using molecular dynamics simulations, a German-French-Italian
team has now predicted that "Ice XI" forms between 3 and 4 Megabars
of pressure at room temperature. The oxygen atoms arrange themselves into
a distorted hexagonal close-packed (hcp) lattice, a densely-packed structure
in which atoms essentially occupy the corners of equilateral triangles.
Interestingly, Ice XI is an insulator up to and beyond 7 Mbars, the kinds
of pressures at which ice exists in Jupiter. Numerous physicists have proposed
that ice may become metallic at high pressures, but the simulations suggest
that Jovian ice may not necessarily be metallic. The pressures that would
be required to make Ice XI can be experimentally achieved in diamond anvil
cells, the authors point out. (M. Benoit et al, upcoming paper in Phys.
Rev. Lett.)
IN THE LATE HEAVY BOMBARDMENT (LHB) EPOCH , a span of about 200 million
years some 4 billion years ago, the Moon sustained many large impacts.
Some astronomers believe that the projectiles responsible may have pestered
Mercury, Venus, Earth, and Mars as well. Others assert that the LHB phenomenon
was unique to the Earth-Moon system or that it did not happen at all, at
least not so suddenly. Now, a group of scientists at the University of
Manchester (UK) has dated a rock found here on Earth but which is believed
to have been a meteorite originating at Mars. The 4-billion-year age of
the object, determined by isotope dating, is much older than previously
studied Martian meteorites. The antiquity of the rock, say the researchers,
provides evidence for a widespread LHB effect. (R.D. Ash et al, Nature,
7 March 1996.)
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