In situ recording of Mars soundscape.

Before the Perseverance rover landing, the acoustic environment of Mars was unknown. Models predicted that: (1) atmospheric turbulence changes at centimetre scales or smaller at the point where molecular viscosity converts kinetic energy into heat1, (2) the speed of sound varies at the surface with frequency2,3 and (3) high-frequency waves are strongly attenuated with distance in CO2 (refs. 2-4). However, theoretical models were uncertain because of a lack of experimental data at low pressure and the difficulty to characterize turbulence or attenuation in a closed environment. Here, using Perseverance microphone recordings, we present the first characterization of the acoustic environment on Mars and pressure fluctuations in the audible range and beyond, from 20 Hz to 50 kHz. We find that atmospheric sounds extend measurements of pressure variations down to 1,000 times smaller scales than ever observed before, showing a dissipative regime extending over five orders of magnitude in energy. Using point sources of sound (Ingenuity rotorcraft, laser-induced sparks), we highlight two distinct values for the speed of sound that are about 10 m s-1 apart below and above 240 Hz, a unique characteristic of low-pressure CO2-dominated atmosphere. We also provide the acoustic attenuation with distance above 2 kHz, allowing us to explain the large contribution of the CO2 vibrational relaxation in the audible range. These results establish a ground truth for the modelling of acoustic processes, which is critical for studies in atmospheres such as those of Mars and Venus.

Remote pulsed Raman spectroscopy of inorganic and organic materials to a radial distance of 100 meters.

A portable pulsed remote Raman spectroscopy system has been fabricated and tested to 100 m radial distance. The remote Raman system is based on a directly coupled f/2.2 spectrograph with a small (125 mm diameter) telescope and a frequency-doubled Nd:YAG pulsed laser (20 Hz, 532 nm, 25 mJ/pulse) used as the excitation source in a co-axial geometry. The performance of the Raman system is demonstrated by measuring the gated Raman spectra of calcite, sodium phosphate, acetone, and naphthalene. Raman spectra of these materials were recorded with the 532 nm pulsed laser excitation and accumulating the spectra with 600 laser shots (30 s integration time) at 100 m with good signal-to-background ratio. The remote pulsed Raman system can be used for remotely identifying both inorganic and organic materials during daytime or nighttime. The system will be useful for terrestrial applications such as monitoring environmental pollution and for detecting minerals and organic materials such as polycyclic aromatic hydrocarbons (PAHs) on planetary surfaces such as Mars.

Observations in collinear femtosecond-nanosecond dual-pulse laser-induced breakdown spectroscopy.

In the work reported herein, we have combined a short-lived femtosecond laser-induced plasma (LIP) and a longer-lived nanosecond LIP in a collinear pulse configuration to examine the source(s) of atomic emission and signal-to-noise enhancement in dual-pulse laser-induced breakdown spectroscopy (LIBS). Initial studies indicate that the primary source of dual-pulse LIBS enhancement in the collinear configuration may in large part be a matter of pulse focus; focusing on the sample surface, for example, yields atomic emission enhancements whose lifetime correlates reasonably well with the femtosecond LIP emissive lifetime, suggesting that plasma-plasma coupling may play an important role at that pulse focus. At a second "optimal" focal position above the sample surface, alternatively, atomic emission and signal-to-noise enhancements correlate quite well with the nitrogen and oxygen atomic emission reductions previously seen following use of a femtosecond air spark and a nanosecond ablative pulse in the orthogonal dual-pulse configuration, suggesting that pressure or number density reductions due to femtosecond LIP formation in air may be significant at that pulse focus.

LIBS using dual- and ultra-short laser pulses.

Pre-ablation dual-pulse LIBS enhancement data for copper, brass and steel using ns laser excitation are reported. Although large enhancements are observed for all samples, the magnitude of the enhancement is matrix dependent. Whereas all of the dual-pulse studies used ns laser excitation we see interesting effects when using ps and fs laser excitation for single-pulse LIBS. LIBS spectra of copper using 1.3 ps and 140 fs laser pulses show much lower background signals compared to ns pulse excitation. Also, the atomic emission decays much more rapidly with time. Because of relatively low backgrounds when using ps and fs pulses, non-gated detection of LIBS is shown to be very effective. The plasma dissipates quickly enough using ps and fs laser pulses, that high pulse rates, up to 1,000 Hz, are effective for increasing the LIBS signal, for a given measurement time. Finally, a simple near-collinear dual-pulse fiber-optic LIBS probe is shown to be useful for enhanced LIBS measurements.

Fluorescent particle image velocimetry: application to flow measurement in refractive index-matched porous media.

This paper presents results in which particle image velocimetry (PIV) is used in conjunction with refractive index matching to measure fluid flow velocities within complex, multiphase systems. This application required the adaptation of PIV for use with fluorescent, rather than scattering, seed particles; we refer to the technique as fluorescent PIV (FPIV). We applied index-matched FPIV to the measurement of low flow velocities (tens of microns per second) at high spatial resolution (tens of microns) in a porous medium. We produced clear images of flowing particles in heterogeneous porous media and obtained reliable velocity vectors by a point-by-point interrogation of these images. We also found evidence of the intrapore mixing of porous media flow.

Comparison of some fiber optic configurations for measurement of luminescence and Raman scattering.

Measurements are made for a number of dual fiber optic configurations to determine their relative sensitivity using bare fibers and graded-refractive-index lenses. An analysis of the background fiber emission for a typical silica-on-silica fiber (Diaguide, 200-microm core) is presented, and the origin (core or cladding) for several prominent Raman peaks is determined. Also, a forward-scattering fiber geometry is introduced, and the dependence of sensitivity on the type of optical termination and fiber separation is determined.

Wavelength selection for fiber optic Raman spectroscopy. Part 1.

Blue wavelength excitation is usually preferred for analytical applications of Raman spectroscopy because of the lambda(-4) dependency of the Raman signal intensity on excitation wavelength. However, for remote Raman measurements using long optical fibers, the transmission spectrum of the fiber should be considered in determining the optimal excitation wavelength. In this note, a quick, approximate approach is developed to determine the optimal excitation wavelength for Raman spectroscopy over optical fibers.

In situ monitoring of ocean chlorophyll via laser-induced fluorescence backscattering through an optical fiber.

The first seagoing test of a prototype laser/fiber-optic system for in situ detection of ocean chlorophyll fluorescence is described. Radiation at 488 nm originating from a shipboard argon laser was transmitted through 20 of 200-microm core optical fiber to the distal tip mounted on the microstructure profiler, the Rapid Sampling Vertical Profiler. The backscattered fluorescence emission signal was collected through the same fiber and processed on board ship. A series of measurements indicated that (1) successful isolation of shipinduced vibrations could be achieved using our optical bench framework to maintain optical alignments; (2) ambient chlorophyll concentrations could be detected in situ; (3) a Raman scattering signal from water could also be detected and should provide an internal standard against which chlorophyll fluorescence may be calibrated.