In this blog post, we’ll look at what LIBS is and why Curiosity uses it to analyse Martian soil.
What is LIBS?
LIBS stands for Laser-Induced Breakdown Spectroscopy. Put simply, it is a method of identifying the constituent elements by firing a strong laser pulse at a sample to instantly turn it into a plasma state, and then analysing the spectrum of light emitted as the plasma cools.
The process is as follows. When a high-power laser reaches the surface of a material, the material absorbs the energy and is instantly heated to a very high temperature, turning into plasma. In the plasma state, some particles become ionised and enter an unstable state; as this state stabilises, light of specific wavelengths is emitted. As each particle has its own unique wavelength, measuring the intensity of each wavelength with a spectrometer reveals which elements are present, and by comparing the brightness (intensity) at the same wavelength, the approximate composition ratio can also be estimated.
Why is LIBS suitable for Mars exploration?
The news that Curiosity had landed on Mars to investigate the composition of soil and rocks attracted a great deal of interest. However, this raises a question: how can a spacecraft or rover accurately analyse Martian soil directly on site? The answer to this question is LIBS.
Firstly, the sample requirement is extremely small. It is known that LIBS can perform analysis using a minuscule amount—equivalent to one hundred-millionth of a drop of water—with a single laser pulse. This is an advantage that makes it suitable even for rare meteorites or cultural artefacts that must not be damaged.
Secondly, analysis can be carried out non-destructively or with minimal damage. Although the surface is momentarily heated by the laser, there is no need for destructive processing—such as collecting a sample and dissolving it in a solvent—as is required with traditional chemical reagent-based analysis. It can be applied directly to samples where preserving the original form is crucial.
Thirdly, it allows for layer-by-layer analysis. By repeatedly firing the laser at the same point, surface dust and by-products are removed, enabling the continuous acquisition of spectra from progressively deeper layers. This makes it possible to identify compositional differences between the surface and deeper layers, thereby detecting stratigraphic structures or buried materials.
Fourthly, it is relatively less susceptible to external contamination. As LIBS measures light without physical contact with the sample, there are fewer sources of contamination during handling, and impurities adhering to the surface can be removed through repeated irradiation, allowing for a more accurate determination of the actual internal composition.
Finally, it is suitable for the harsh environment of Mars. Mars has a much thinner atmosphere than Earth (approximately 1/200th of Earth’s) and experiences extreme temperature variations between day and night. As LIBS utilises laser and plasma-emitted light, it is not significantly affected by changes in atmospheric pressure or temperature; spectral analysis is possible provided the emitted light can be accurately captured.
In fact, Curiosity transmitted spectral data obtained via LIBS on Mars back to Earth, and signals from various elements were observed in those spectra. Furthermore, by noting that the higher the signal intensity at a specific wavelength, the higher the concentration of that element, it was possible to estimate the composition of Martian soil.
Technological Advancements and Future Prospects
In the past, LIBS was considered solely as laboratory equipment due to the size and power requirements of high-power laser systems. However, with the miniaturisation of laser engineering and optical components, the development of equipment capable of on-site analysis is now becoming a reality.
This trend towards miniaturisation is the key technological factor that has enabled LIBS equipment to be installed on exploration rovers such as Curiosity. On-site optical analysis, which was difficult to imagine just a few years ago, is now possible, and as a result, the application of LIBS is becoming increasingly widespread in various fields, including not only space exploration but also disaster sites on Earth, cultural heritage conservation and on-site mineral exploration.
Rather than completely replacing traditional chemical analysis methods, laser-induced breakdown spectroscopy (LIBS) is establishing itself as an analytical tool that is ‘available anytime, anywhere’, not confined to the laboratory. We hope that, spurred on by the success of Curiosity’s mission, LIBS technology will take another significant leap forward.
