by Marcus Chown

from issue 2214 of New Scientist magazine

27 November 1999, page 20
from NewScientist Website

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 thought.

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 tryptophan and 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 ones.


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.







Inner circles

by Jeff Hecht

from issue 2146 of New Scientist magazine

08 August 1998, page 11
from NewScientist Website

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 carbohydrates.

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, California.

"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 Mars or 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
from TheGuardian Website

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 galaxies apart.

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 Aldering.

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 astronomers observe!

This has been described by Nobel laureate Steven Weinberg as "the worst failure of an order-of-magnitude estimate in the history of science".


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," says Aldering.

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 question.


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.