Light has been used in the geosciences in a number of applications and scientific discoveries for thousands of years. Using sunlight to illuminate a distant object, then recording the direction to the object from the position of the observer was the standard procedure of geodesy for centuries when generating maps or defining the layout of larger structures and buildings. In astronomy, natural light from the Sun or stars is observed with telescopes, possibly connected to spectrometers that split the received light into its different wavelengths. The originally used light detector, the human eye, was replaced during the past century by photographic films and during the past decades by electronic detectors such as CCDs (charge coupled devices), which convert the incoming light into electrical signals. These signals are then converted into numbers and stored in computer storage devices for image processing and analysis. The light colours range from ultraviolet to infrared, corresponding to a wavelength range between 100 nm (nanometer) and 10 μm (micrometer). For comparison, the human eye is sensitive to wavelengths roughly between 400 nm (violet) and 700 nm (red), with a maximum sensitivity at 555 nm (green).
Recording the variation of a star’s light intensity allows the detection of planets circling the star. Nearly all exoplanets have been detected and their properties discovered using these light curves. As our Sun is much closer, we can record two-dimensional images of our star, where the locally varying intensity is associated with sunspots, a good indicator for the varying activity of the Sun. Adding a spectrometer between the optics and the detector gives additional information about the different materials present in the star or the Sun. These so-called emission spectra not only indicate the presence or absence of different materials, but also their relative amount and the temperature of the light source. As the exact wavelengths of the different emission lines are known from laboratory measurements on Earth, the displacement of these lines in star spectra is a direct measure for the relative velocity between the Earth and the star. Displacement of the lines increases as the relative motion becomes faster.
Placing a spectrometer on board a planet-orbiting satellite the vertical structure of the atmosphere can be determined by recording the spectrum of a star. This is accomplished by recording spectra while the line-of-sight to that star traverses more and more through the upper layers of the planet’s atmosphere until the star has vanished behind the horizon. The change in the spectrum is caused by absorption of gas molecules in the atmosphere and therefore allows determining the atmosphere’s composition at different altitudes. For example, the SPICAM instrument on board the Mars Express spacecraft determines the vertical structure of the Mars atmosphere. Looking downward towards the planet’s surface, a spectrometer–camera combination not only records nice pictures of the surface but also retrieves the material composition of the surface and the composition of the atmosphere (which sunlight had to travel through to reach the surface) using the spectrum of the light reflected from the surface. Another instrument on board Mars Express, called OMEGA, as well as many optical instruments on Earth-orbiting satellites, works in a similar way. In all these cases the characteristic spectrum of the light source – the sunlight’s intensity variation with wavelength – is modified by the observed medium, thereby giving information about the scattering or absorbing material.
During the past fifty years a new field of observations using light was developed. The LASER (light amplification by stimulated emission of radiation) can be used for many applications. Using the very high energy density of a LASER beam, material can be evaporated and then investigated with spectroscopic means or mass spectrometers. As the LASER wavelength can be very precisely selected by various means, it can be used for selective excitation of molecules, which then emit light in a characteristic way allowing the observation of their detailed structure and composition (Laser-induced fluorescence). In particular, recently developed miniature LASER diodes allow the application of this technique in planetary research. This has introduced a new type of investigations in areas where only a limited amount of power is available as in planetary landing vehicles studying the surface material of Mars. A combination of these techniques is Laser-induced breakdown spectroscopy for heating and exciting target material that is then characterised by their spectroscopic signatures. They are useful for analysing any type of material (solid, liquid or gas). An example is the ChemCam system on board the Mars Curiosity rover.
LIDARs in the geosciences
Transmitting short LASER pulses and recording any echoes transforms a LASER into a LIDAR (light detection and ranging). LIDAR offers high space and time resolution measurements. In the LIDAR, the light pulses are sent through the atmosphere. The returned light scattered by various air molecules does not only give information about the type of molecules in the atmosphere and their location. Meteorological LIDARs are capable of monitoring a wide range of atmospheric parameters: the height, layering and densities of clouds, cloud particle properties (extinction coefficient, backscatter coefficient, depolarization), temperature, pressure, wind, humidity, trace gas concentration (ozone, methane, nitrous oxide, etc.).
Using only the range information of a reflected LIDAR signal very precise altitude profiles can be measured. These so-called LASER altimeters on board a plane or spacecraft are used for obtaining high-resolution maps with applications in archaeology, geography, geology, geomorphology, seismology, forestry and other areas. LASER altimeters are crucial devices in the automatic guidance systems for planetary landers. A special LASER altimeter application is the precise monitoring of the change of distance between Moon and Earth by aiming the beam at the reflectors placed on the Moon surface by the Apollo mission. As the precision depends on the wavelength, changes in distance can be detected with micrometer precision. The wavelength of a LIDAR is varied to suit the target: from about 10 micrometers to the UV (approximately 250 nm).
Spectroscopy as environment monitor
Any gas coming out of a chimney, a leak in a manufacturing plant or a car’s exhaust pipe can be identified by the way it absorbs light at different wavelengths. Mounting a light source and a detector at a location of interest and measuring the absorption spectra caused by the gas between them provides an effective way of detecting dangerous concentrations of pollutants. Automatic spectrum analysis can generate an alarm in case of problems. Two low-cost instrument types are used: A fixed broad-band light source like a light emitting diode is combined with a spectrometer, or a tunable diode LASER is scanned across the wavelength range of interest and a simple photo diode measures the variation of the light intensity caused by the gas. If the location to be monitored is at a larger distance like above a high chimney, a LIDAR can be used for the same purpose. These systems are used for routine air quality monitoring in urban areas, are easily networked and provide real-time information for traffic control authorities charged with smog control.
As LASER light sources can be very precisely controlled in wavelength and intensity with a time resolution in the picosecond range (1 000 000 000 000 picoseconds are 1 second), LASER diodes are nowadays used as standard means for rapid communication links. The rapid internet data links are based on glass fiber cables through which LASER light is sent, modulated with the information to be sent to the other end. While this is the standard way of using the glass fiber networks there is also another possibility: The laser light inside the thin glass cable is reflected at the inside of the cable and a small portion reflected backward to a receiver (Raman scattering). The details depend on the property of the fiber and its interaction with the environment. Small temperature variations at one location will cause a characteristic echo as does an increase in humidity or a change in pressure onto the cable. If one sends very short LASER pulses through the cable and monitors the echoes one can use the cable to monitor water reservoir dams or large constructions and buildings and receive an alarm in the event of a water leak or structural problem in the dam. The information not only reveals what is happening but also pinpoints the problem spot.
Though the list of possible applications could be extended much more, one trivial but important aspect should be added: The usage of light-illuminated solar cells as energy source makes many automatic geoscience instrument applications on Earth or in space possible where otherwise the maintenance would be prohibitive in costs and manpower or even impossible.
Walter Schmidt is a research manager at the Finnish Meteorological Institute in Helsinki since 1988. His main areas of responsibility are the development and operation of space instruments for planetary and solar system research. He completed his doctoral thesis in 1980 at the University of Freiburg, Germany, researching chemical reactions investigated with laser-induced fluorescence methods. From 2011 to 2015 he was President of the Division on Geosciences Instrumentation and Data Systems (GI) of the European Geosciences Union (EGU), and an associated editor of the EGU open access journal Geosciences Instrumentation and Data Systems.