Number 805, December 13, 2006
by Phil Schewe, Ben Stein, and Davide Castelvecchi
X-Ray Rainbow
In 1670 Isaac Newton demonstrated the composite
nature of sunlight when he sent a carefully collimated sunbeam
through a prism, which spread out the light into a rainbow of
colors; by sending a beam of single color through a second prism
(with no further spreading) Newton showed that the color was not
being imposed by the prism but was intrinsic to the light itself.
Now physicists using the Advanced Photon Source at Argonne National
Lab, in Illinois, have spread out a beam of X-rays (which are, after all, just a
more energetic version of visible light) into a rainbow of colors.
Trying to reflect X-rays from a surface is difficult because X-ray
wavelengths are some 10,000 times shorter than those for visible
light. Glancing reflection of only a few tenths of a degree is
normally possible, and even then the beam of X-rays will suffer very
little wavelength-dependent spreading. However, another phenomenon,
Bragg diffraction, allows for scattering of X-rays from a crystal
through large angles; in this case the incoming X-rays scatter not
merely from a top layer of atoms in the crystal but from numerous
atomic planes. Furthermore, if the atomic planes are not parallel
to the crystal surface the diffracted X-ray beam will be spread out
prismatically into a range of component wavelengths (or colors).
In
the Argonne experiment an incoming beam of 9-kiloelectronvolt X-ray photons with
angular spread of only 1 micro-radian (two-tenths of an arcsecond)
was backwards scattered and spread out into an X-ray rainbow with an
angular dispersion of 230 micro-radians (see figure at
Physics News Graphics).
Argonne physicist Yuri
Shvyd'ko (shvydko@aps.anl.gov, 630-252-2901) says that his rainbow
is not just a novelty but will have many practical applications in
X-ray optics. These include compression of X-ray pulses in time and
the development of X-ray monochomators (which fashion X-ray beams of
pure wavelength, or color) and much higher-resolution X-ray
spectrometers.
Shvyd'ko et al., Physical Review Letters, 8 December
2006
Contact Yuri Shvyd'ko
Argonne National Laboratory
shvydko@aps.anl.gov
Tel: 630-252-2901
Best Evidence Yet for Recent Water on Mars
Photographs returned by
the Mars Global Surveyor spacecraft have revealed new bright
deposits in Martian gullies that suggest water has flowed on the
surface of the planet sometime in the last seven years. The images,
taken in 2004 and 2005, show what appear to be deposits of minerals
left by bursts of water running down the sides of two gullies,
according to Michael Malin, the chief scientist for the spacecraft's
camera system. The light-colored deposits were not there in pictures
taken in 1999.
While previous evidence has shown water ice and
water vapor exist below the surface of Mars, this is the "strongest
evidence to date that water still flows occasionally on the surface
of Mars," Malin said at a NASA press conference on Dec. 6. Cold
temperatures on the planet, coupled with a thin atmosphere, do not
allow water to persist on the surface of Mars. Researchers think
water could remain liquid long enough, after breaking through from
an underground source, to carry debris downhill before freezing or
evaporating.
These new findings heighten the intrigue surrounding the potential
for life on Mars; given water and a steady source of heat, bacteria
can grow in extremely harsh environments. The shapes of the
deposits are consistent with what one would expect to see if the
materials were carried by water, NASA scientists said. The pictures
show the liquid easily flowed around small obstacles on the way down
a hill, eventually branching out like fingers at the end.
With the
flow patterns traveling down the slope for 500 to 600 yards, project
scientist Kenneth Edgett estimates the volume of each liquid burst
was equivalent to "five to 10 swimming pools of water." The light
tone of the deposits could be from frost being continuously
replenished by ice from within the body of the deposit. Or the light
color could be a salty crust, which would be a sign of water's
effects on concentrating the salts. The deposits are unlikely to be
caused by dry dust sliding down the slope because dust moved by
rover tracks, dust devils and fresh craters on Mars is typically
darker than surrounding areas.
Enlarge text
Shrink text
Print
Digg this
Del.icio.us
Furl