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Planetary spectroscopy advances

20th June 2018


ESPRESSO instrument achieves first light with all four Unit Telescopes
Data from ESPRESSO First Light

In recent years, planetary spectroscopy has emerged as a useful tool to help scientists in the search for signs of extraterrestrial life. So, what types of technologies are being used? And what innovations can we expect in the coming years?

Remote Raman
 

One interesting recent example is a technology used by NASA and the University of Hawaii at Mānoa (UHM), which have jointly developed a remote Raman spectroscopy instrument to aid the search for extraterrestrial life. As Anupam Misra, researcher at UHM, explains, the system chiefly consists of two components – a nano-second pulsed laser for excitation and a time gated detector, such as intensified CCD. Since Raman signals are spontaneous, he points out that the laser excitation produces Raman photons, which ‘have a short lifetime and last for the same duration as the laser pulse width.’ A gated detector is also able to detect the Raman spectra in daylight by synchronising the timing of the arrival of photons with the opening and closing of the detector.

“Raman photons are different in energy from the laser photons. The Raman systems are designed to block all the laser photons and measure only the photons that are different from the laser wavelengths. Short gating of the system also effectively reduces the contribution from daylight and mineral phosphorescence background. The remote Raman system can record high quality Raman spectra of various targets in daylight,” says Misra.

In Misra’s view, Raman spectroscopy provides a very high level of confidence
in identifying molecules because the Raman spectrum of a molecule is ‘unique to the molecule and is normally used as a fingerprint method for chemical identification.’

“Raman spectroscopy has a number of advantages over passive spectroscopy, chief among them being the sharpness of spectral features. This allows much less ambiguous detection of specific species, especially in the presence of mineral mixtures,” he says.

Misra points out that Raman spectra of samples contain a ‘wealth of molecular fingerprint information’ that can be used to identify water containing minerals, biomarkers, biominerals, minerals and numerous chemical compounds. The technique also has the ability to detect both organic and inorganic components of a biogeological system, giving Raman a ‘large advantage in the detection of life.’

The new system will be used for the first time in the MARS2020 mission to look for evidence of life on Mars under a ‘SuperCam’ instrument. Looking ahead, Misra expects that the remote detection of extraterrestrial biological materials will be feasible using two spectroscopy techniques – remote Raman and remote bio-fluorescence spectroscopies, both of which he says ‘provide bio-detection at a range of several tens of metres with fast integration time of a few seconds.’

“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. The 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. This 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. The 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.

“ESPRESSO deploys three major technological advances with respect to its predecessors: a Laser Frequency Comb for optimum wavelength calibration - Nobel prize winning technology in 2005 - as well as a novel system to fold the beam inside the spectrograph to diminish the instrument size and with this its complexity, and two 9cm x 9cm CCD, among the largest monolithic CCDs in the world,” adds Lo Curto.

Simulation facility
 

Elsewhere, a new simulation facility at the Planetary Spectroscopy Laboratory (PSL) of the German Aerospace Centre (DLR) has recently been launched. It will be used mainly for the collection of compositional data for the surface of Venus – particularly for taking Venus-analogue emissivity measurements from 0.7 to 1.5 µm over the whole Venus surface temperature range.

The PSL is located in a temperature-controlled room and uses two Bruker Vertex 80V spectrometers located on an optical table equipped with external chambers for emissivity measurement. The instrument used for the new Venus facility is a recently upgraded Vertex 80V optimised for the near to far-infrared spectral range. It has also been equipped with an InGaAS (Indium-Gallium-Arsenid) detector, which has a very high sensitivity exactly in the wavelength range needed for Venus.

As Dr Jörn Helbert, Planetary physicist at the DLR’s Institute for Planetary Research, explains, up until recently it was commonly accepted in the scientific community that compositional data for the surface of Venus could only be obtained by landed missions because the planet’s permanent cloud cover ‘prohibits observation of the surface with traditional imaging techniques over most of the visible spectral range.’

However, landed missions carry a higher risk and the lifetime of a lander is severely limited by the harsh conditions of 95 bar pressure and 450°C surface temperature. Fortuitously, Helbert points out that Venus’ CO2 atmosphere is actually partially transparent in small spectral windows near 1 µm – and these windows have been used to obtain limited spectra of  Venus’ surface by ground observers, during a flyby of the Galileo mission at Jupiter, and from the VMC and VIRTIS instruments on the ESA VenusExpress spacecraft. In particular, the latter observations, which have been analysed at DLR, have revealed compositional variations correlated with geological features.

“These observations challenge the notion that landed missions are needed to obtain mineralogical information. However, any interpretation in terms of mineralogy of VNIR spectroscopy data from orbiters requires spectral libraries acquired under conditions matching those on the surfaces being studied. The PSL took up this challenge, building on nearly a decade of experience in high-temperature emission spectroscopy in the mid-infrared,” says Helbert.

“We are now at a point where the planetary spectroscopy community has fully embraced the need for laboratory studies under conditions relevant for the target bodies. Especially temperature, but also vacuum conditions have a strong influence on spectral characteristics and we will only get meaningful answers if we take this into account in the laboratory. Therefore we will see more analogue simulations chambers being setup for specific target bodies,” he adds.





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