SPECTROSCOPY 37
Data from ESPRESSO First Light
“Both techniques are non-destructive
and work both in daytime and night-time conditions without any sample collection or preparation. Raman spectroscopy provides a very high confidence level in detection of a chemical. However, Raman signals are very weak. Typically only one Raman photon is produced for every 10 million laser photons for excitation. Alternatively, bio-fluorescence signals are very strong by orders of magnitudes and can be used to search for life on a larger scale,” he adds.
‘Habitable zone’ Another interesting recent development is the use of the so-called ESPRESSO spectrograph on the European Southern Observatory (ESO) Very Large Telescope in Chile, chiefly for the detection and characterisation of ‘rocky’ extra-solar planets. As Gaspare Lo Curto, Instrument Scientist at ESO, explains, these rocky planets, orbiting stars other than the Sun, have high density, similar to Earth, and are thought to be the best candidates to
harbour extraterrestrial life. ESPRESSO has the potential to discover many such planets, several of them within the ‘habitable zone’ of their stars, defined as a region of space around a star where the direct irradiation from the star would allow water – thought to be one of the fundamental ingredients of life – to exist in liquid form on the planet’s surface. ESPRESSO detects extra-solar planets thanks to the reflex motion they induce on their star. Te instrument records the chromatic signature of the star, its spectrum, and measures the movement in wavelength space of this ‘signature’ due to the Doppler effect of the star wobbling around the system’s centre of mass. Tis technique is called ‘radial velocity,’ because it measures the velocity variations of the radial motion (along the line of sight) of a star with respect to us, the observers. According to Lo Curto, ESPRESSO
will be ‘by far the most precise radial velocity machine in the world, reaching a precision of 10cm/s over a period of 10
years or more.’ To compare, a man walks at a speed of about 100cm/s, therefore ESPRESSO will be capable of measuring ‘precisely the motion of a star which is wobbling ten times slower than a walking man at many billions of kilometre distance.’
“ESPRESSO will benefit enormously
from the much larger light collecting power of the VLT. Te light collected by the 8m diameter telescope is delivered by a complex ‘train’ of prisms, lenses and mirrors, about 60m away, to the location of the instrument,” says Lo Curto. From here, the light beam angle and position is stabilised before sending the light into fibre optics that deliver it inside the spectrograph, which then disperses the light into its chromatic components with a very high spectral resolution. Because extreme stability is required for the system to achieve its goal of 10cm/s precision in 10 years, the spectrograph itself is kept in a vacuum, and its temperature is kept constant within a few thousandth of a degree Kelvin.
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