by Marcus Chown
from issue 2214 of New Scientist
27 November 1999, page 20
The dominance of left-handed molecules among the building blocks of
life might not be a trick of the light. Researchers in Belgium have
found that the structure of two of the amino acids that make up
proteins cannot be explained by a rare form of ultraviolet light in
the interstellar cloud where Earth was born, as some astronomers had
Amino acids come in two mirror-image forms, but life opts almost
exclusively for the so-called left-handed form. Last year,
astronomers showed that the
Orion Molecular Cloud - adjoining
the Orion Nebula - contained circularly polarized light that
preferentially destroyed right-handed amino acids (New
Scientist, 8 August 1998, p 11).
Now two Belgian scientists have cast doubt on this theory. In a
paper to be published in the journal Space Science Reviews,
biochemist Corinne Cerf and astrophysicist Alain Jorissen
of the Free University of Brussels say that the cosmic
mechanism for destroying right-handed amino acids might not work for
tryptophan and proline, two of the twenty amino acids
commonly used by living organisms.
The electric field of circularly polarized light rotates one of two
ways around its direction of motion. Each of the right and
left-handed forms of the amino acid molecules absorbs one type of
light better than the other. If only one type is present, that light
can break down the mirror form that absorbs it more efficiently.
Astronomers suggested that this might
have happened in the interstellar cloud from which the Earth
congealed, having found only one type of circularly polarized
light in the Orion Nebula.
But Cerf and Jorissen found two exceptions to the theory when they
looked at how amino acids absorb ultraviolet light.
"The data show that
proline do not behave like other amino acids," says Jorissen.
As a result, both their mirror forms
might survive in the presence of just one type of light. The
researchers think this may happen because of differences in the
structure of the side chains of these molecules.
According to Stephen Mason of King's College London,
Cerf and Jorissen are correct that circularly
polarized light cannot select for one form of tryptophan and proline.
But he argues that although some wavelengths preferentially destroy
right-handed amino acids, other wavelengths destroy left-handed
As the light from stars covers a wide
range of wavelengths, Mason says that overall effect on the
handedness of amino acids should be zero.
by Jeff Hecht
from issue 2146 of New Scientist
08 August 1998, page 11
POLARIZED light from space could solve a mystery about the origin of
life. According to an international team of astronomers, a rare form
of ultraviolet light from the Orion Nebula might account for the
chemical structure of the building blocks of proteins and
Life on Earth uses amino acids and sugars in only one of two
possible mirror-image forms—amino acids are left-handed and sugars
right-handed. The choice was vital, because proteins can fold
consistently into their complex shapes only if made entirely of one
form or another, but not both. Yet most inorganic chemical reactions
form equal numbers of left and right-handed molecules. So where did
this bias come from?
Reports last year of excess left-handed amino acids in the meteorite
that fell near Murchison, Australia, in 1969 suggested the choice
originated in outer space, but did not reveal how the excess came
about (In Brief, 20 September, p 26). Now James Hough, dean
of natural sciences at the University of Hertfordshire, says that
the answer may be a strong dose of circularly polarized light, which
could have produced an excess of particular kinds of amino acids in
the dust that formed the Solar System.
In circularly polarized light, the direction of polarization rotates
continuously. Ultraviolet wavelengths of such light can force
chemical reactions to make molecules of mostly one-handedness when
they would otherwise have churned out half of one and half of the
other. Right-handed light destroys right-handed molecules, leaving
an excess of left-handed ones.
Circular polarization is rare and hard to spot, so Hough and his
colleagues built a special instrument for the Anglo-Australian
Telescope near Coonabarabran in New South Wales. With it, they found
that as much as 17 per cent of the light reflected from gas clouds
where stars are forming in Orion was circularly polarized. Although
they could only observe in the infrared, they say in Science (vol
281, p 672) that ultraviolet light—obscured by the clouds—should
have the same polarization.
If the Solar System formed in a similar environment, Hough says, one
would expect a 5 to 10 per cent excess in the handedness of
molecules, the same range found in the Murchison meteorite. If that
excess persisted on the young Earth, it could have tipped the scales
in selecting building blocks for early life.
All those "ifs" bother Jeff Bada
of the Scripps Institution of Oceanography in La Jolla,
"It's just a series of maybe steps,"
he says. "To me that makes the whole thing a big maybe."
But Hough says what's appealing about
his theory is that in the regions of Orion much larger than our
Solar System, the polarization was all right-handed, which could
account for the left-handed bias of amino acids on earth. Sugars, on
the other hand, are too unstable to have arrived directly from
space, says Hough. Their right bias may be the byproduct of the
early biological synthesis of amino acids.
Bada and Hough agree that life on
Europa could test the theory.
"If it has [right-handed] amino
acids," Bada says, "it throws the whole thing out."
A Snap In The Dark
A New Probe With The Largest
Camera Ever Launched Into Space Could Throw Light On The Universe's
Latest Mystery - Dark Energy
by Marcus Chown
November 21, 2002
It is invisible, it pervades space and it is unaccountably speeding
up the expansion of the universe. The "dark energy" was discovered
four years ago and nobody has the slightest idea what it is, despite
the fact it is controlling the fate of the universe.
Now astronomers are proposing a space probe that could nail the
cosmology's stickiest problem once and for all. It's called the
Supernova acceleration probe (Snap)
and it will boast the largest camera put in space - 30 times bigger
than the camera on Nasa's Hubble space telescope.
Its task? To make detailed measurements of more than 2,000 exploding
stars, or "supernovae", as they detonate in far away galaxies.
"It is one of the most important
scientific projects of the coming decade," says cosmologist
Max Tegmark, of the University of Pennsylvania.
The dark energy burst on to the
scientific scene in 1998, when astrophysicists in the US discovered
that distant "Type Ia" supernovae - a class believed to explode with
a standard luminosity - were fainter than they ought to be, taking
into account their distance from Earth. Evidently, the universe's
expansion had speeded up since the stars exploded, pushing them
further away than expected and making them appear fainter.
It was a bombshell.
The universe's constituent galaxies are flying apart and the sole
force affecting them ought to be their mutual gravitational pull. It
should be braking the expansion, not speeding it up. The only thing
that could be accelerating things is space itself. Contrary to
expectations, it could not be empty. It must contain something
unknown to science - a dark energy - exerting a kind of cosmic
repulsion on the universe, countering gravity and driving the
Snap, which has just been confirmed by the US Department of
Energy as one of its highest scientific priorities, will probe
that cosmic repulsion.
The aim, during its 32-month "primary
mission", will be to measure the brightness and detailed properties
of the Type Ia supernovae, and measure their distances, or "red
shifts", with the aid of an on-board spectrograph.
"We want to see how the dark energy
changes with distance, or 'look-back time'," says one of Snap's
originators, Greg Aldering, of Lawrence Berkeley
Laboratory in California.
One possibility is that the dark energy
is associated with the "cosmological constant", a repulsive force of
empty space proposed by Einstein (which he later dubbed his biggest
mistake). Its central characteristic is that the energy concentrated
in a given volume of space is constant in the universe's history.
Another possibility is "quintessence", a
type of dark energy that can vary over space and time.
"By seeing how the dark energy
changes with 'look-back time', we hope to be able to distinguish
between different models of what the dark energy is," says
The embarrassing fact remains that
physicists are at sea when it comes to understanding the dark
energy. Their best theory - quantum mechanics - predicts an energy
for empty space that is 1 followed by 123 zeroes bigger than what
This has been described by Nobel laureate Steven Weinberg as
"the worst failure of an order-of-magnitude estimate in the history
The Snap image system will consist of a
2-metre telescope with an unusually wide 1.5 field of view, and a
camera with a whopping 600 million picture elements, or "pixels".
The camera will contain two types of detectors - charged-coupled
detectors (CCDs) for optical light and
mercury-cadmium-telluride (HgCdTe) detectors for infrared
light, which is characteristic of distant supernovae whose light has
been "red shifted" by the expansion of the universe.
By comparison, NASA's Hubble space telescope has a 65,000 pixel
HgCdTe camera and a 2 million pixel CCD camera.
"Even Hubble's planned successor -
the Webb Telescope - will have far fewer pixels in its camera,"
He admits that building the camera will
be a challenge.
"Commercial aerospace hasn't flown a
camera as large as Snap's, which is an indication of how
difficult we are going to find it."
One of the challenges of such a large
number of pixels - especially in space - is keeping them cool in the
face of the sun's heat and waste heat from the camera electronics.
Snap will use tiny custom integrated circuitry - known as
application specific integrated circuits - to perform the read-out
of each chip in the camera. These require little power and generate
little waste heat.
The estimated cost of the probe is a few hundred million dollars, a
fraction of the price of Hubble.
With a diameter of 2.5m and length of
6m, and weighing in at 1.6 tonnes, it will be launched in a 3-day
highly elliptical orbit.
"We aim to have the closest point of
its orbit at roughly the same longitude as Berkeley, so that
each orbit we could download the huge amount of data using an
11m radio dish at the Berkeley Space Science Lab," says Aldering.
One technical challenge will be to keep
the camera in the shade at -133 C - the optimum temperature for the
electronics - while the telescope is at room temperature, 20° C. It
is unusual to have a space probe dedicated to a single scientific
Nevertheless, Aldering stresses
that Snap's camera will produce unprecedented views, of use to the
wider astronomical community.
"It will reveal objects far more
distant than even the famous Hubble Deep Field, while
imaging an area of the sky 6,000 times bigger," he says.