The deep ocean continues to be our local frontier. We have charted the Moon, Mars, and even Venus in more detail than we have our own ocean floor. For example, whereas the seafloor is mapped to a resolution of about 5 kilometers globally [Sandwell et al., 2014] and about 25% is now mapped to roughly 100-meter resolution, the Moon’s entire surface was recently mapped to roughly 2-meter resolution [Fortezzo et al., 2020]. Apart from the shortage of detailed bathymetry, much remains unknown about the composition, structure, dynamics, biology, and other aspects of the seafloor.

Characterizing the ocean floor requires overcoming myriad engineering and technical challenges. The average depth to the seafloor is 3,682 meters, for example: No light penetrates, hydrostatic pressure reaches roughly 370 times atmospheric pressure, and temperatures drop to about 4°C [e.g., Ramirez-Llodra et al., 2010; Liu et al., 2021, 2022]. The lack of light complicates navigation and visual observations for both crewed and remotely operated vehicles (ROVs), and the pressure and cold can be punishing for sensitive instruments.

In 2023, scientists—including us—tested cutting-edge underwater exploration technology designed to overcome such challenges. From 16 May to 13 June, the team traveled to Kingman Reef and Palmyra Atoll, about halfway between the Hawaiian Islands and American Samoa, an isolated and little-explored area. There, we mounted the In-situ Vent Analysis Divebot for Exobiology Research, or InVADER, onto an ROV and sent it down deep to collect first-of-their-kind observations. This instrument can quickly analyze the composition of seawater as well as of seafloor rocks, sediments, and organisms, gathering data as fast as the ROV moves and allowing for efficient surveys of large areas.

The InVADER project embraces the concept of bringing the lab to sites of interest instead of taking snapshots of the field by collecting samples to be analyzed in the laboratory. In this way, the technology offers a novel way to explore and characterize the ocean floor. It also may represent a prototype for systems that could one day explore the oceans of icy moons such as Enceladus and Europa, which scientists have identified as sites that may potentially harbor life.

Laser Spectroscopy Under the Sea

InVADER comprises an integrated imaging and spectroscopy payload that includes Raman and laser-induced native fluorescence (LINF) technologies. Raman and LINF measurements are made by directing light from a dual-laser system that emits light at green and ultraviolet (UV) wavelengths at a target of interest. Raman spectroscopy involves detecting changes in the energy of the light that is scattered back from the target, and LINF involves detecting fluorescence emitted by the target as electrons excited by the laser return to their ground state.

These techniques are commonly used to identify minerals and molecules in a variety of applications, including in geochemistry and medicine. But they have yet to be used robustly for exploring the deep ocean, where they could enable in situ real-time measurements of minerals, organics, and dissolved ions on the seafloor and in the water column. The spring 2023 deployment of InVADER was the first time that standoff Raman (spectroscopy at a distance) was used for underwater exploration at such depths.

Prior underwater work included deploying a Raman probe that emitted and received light near target surfaces [e.g., Zhang et al., 2017]. This approach allows detection of relatively strong signals, whereas typical standoff Raman measurements must detect signals that get weaker with distance from the target. However, standoff Raman enables scientists to make measurements remotely and to explore environments and samples that are out of reach of the ROV carrying the instrumentation, an incredibly powerful feature.

Carrying out standoff Raman and fluorescence measurements in the deep sea is no easy task. As the laser light travels through the water column, it is partially absorbed before it reaches the sample. It also encounters particulate debris in the water, which can scatter the light and produce fluorescent signals that impede measurements. Furthermore, movement of the ROV platform during measurements can lead to signal degradation. In situ standoff measurements thus have lower signal-to-noise ratios and higher background fluorescence.

Before sending InVADER—the product of a multi-institution partnership—into the deep sea, we tested its performance and viability for field deployment using a laboratory setup at Impossible Sensing, a small start-up based in St. Louis that focuses on developing remote sensing technology for extreme environments. The setup allowed us to experiment with firing InVADER to target a series of mineral and organic standards in air and in different water compositions at distances of up to 7 meters.

Performing the testing involved overcoming a few unconventional challenges. For example, when InVADER arrived at Impossible Sensing’s headquarters—at the time located in an aging former church building—and was first turned on, a neighborhood-wide power outage occurred. (It’s unclear whether this outage was a coincidence or perhaps resulted from InVADER’s substantial electrical demands.) Thankfully, the outage caused no damage, although it forced adjustments in the laboratory setup to avoid repeat occurrences. A tilt in the building’s floor also proved challenging, and it took a while to account for the tilt and align InVADER’s laser correctly.

With these issues worked out, subsequent testing of InVADER in and out of the water necessitated changing the optical focusing distance and optimizing InVADER’s signal-to-noise ratio for different conditions. All told, it took a few months to achieve readiness for deployment in the field.

Bringing the Lab to the Field

In May 2023, we set out from Hawaii on cruise NA149 of the E/V Nautilus. We deployed the InVADER laser divebot on six of 16 dives by the ROV Hercules, with deployment depths ranging from 1,087 to 3,111 meters [Wagner et al., 2024]. Prior to each dive, we mapped the seafloor with multibeam sonar to locate target areas for our study.

We chose the top of an unnamed guyot—a flat-topped seamount—1,226 meters below the surface as the target of the first dive (Figure 1). To verify InVADER’s performance as it descended, we collected Raman and LINF spectra of seawater every 50 meters down to a depth of 300 meters and then every 100 meters below that.

Some of the first spectra collected showed a strong water signal and indicated the presence of marine snow (organic carbon descending through the water column) as well as sulfate. These results confirmed that the Raman and fluorescence lasers were functioning as expected underwater. Subsequent spectra showed decreasing organic carbon with increasing depth, again in agreement with expectations.

During the remaining five dives, we achieved a series of engineering and scientific accomplishments important for validating InVADER’s flexibility and usefulness for underwater exploration. For example, when the original calibration target for the green and UV lasers proved insufficient under the cold temperatures and extended durations of the deployments, we developed a new calibration target on the fly using high-density polyethylene that worked under the harsh deployment conditions. Similarly, we made engineering adjustments to correct for electronics issues resulting from the low temperatures. We also tested the focusing distance of the lasers from 3 to 10 meters and found 4 meters to be the best standoff distance for collecting optimal spectra.

Among the notable scientific observations made during the cruise, we detected enhanced fluorescence from seawater with increasing depth on InVADER’s third dive, a trend that may be attributable to increasing salinity. These measurements were particularly striking and initially unexpected. Fluorescence was suggested previously as a way to measure salinity in situ, but that approach has not been demonstrated [Simis et al., 2012; Stirchak et al., 2019]. Follow-up measurements in the laboratory will allow us to further resolve and understand our observations of fluorescence enhancement.

Following comprehensive depth profiling of the water column, the team turned its attention to exploring the ocean floor up the slope of another local guyot (Figure 2). Features detected by InVADER’s instruments appeared to be consistent with microcrystalline quartz and, intermittently, pigments and organic signals from sponges and detrital material. Collection of these observations is historic in that it marks the first time a standoff spectrometer was used in situ in a rover-type exploration at the bottom of Earth’s ocean. Furthermore, the ability to perform these measurements while in motion holds large promise for future endeavors in economic mineralogy surveys and baseline environmental characterizations.

Future Explorations on Earth and Other Ocean Worlds

The 2023 deployment of InVADER showcased the ability of this technology to expedite retrieval and interpretation of information on mineralogy, water column chemistry, and organic materials in the ocean. Broader use of the technology in the future has the potential to produce large data sets of standoff observations in the field and to enable long-term monitoring of biogeochemical changes in challenging deep-sea environments. It could also inform and guide sample collection efforts and robustly document settings where samples are retrieved.

Apart from its technological and scientific innovations, a major highlight of the InVADER program is that it leverages and aligns with efforts of multiple U.S. federal agencies to map and explore the ocean, increase ocean literacy, and characterize the ocean seafloor. It is also an example of an effective collaboration among research institutions and the private sector that supports small businesses.

The recent InVADER deployment further highlights the potential science return from using such a payload in planetary exploration campaigns on other ocean worlds, such as Enceladus or Europa. These places are thought to have oceans of water below their icy crusts that could harbor the chemical building blocks of life, or even life itself, and are high on scientists’ lists of desired destinations for future robotic exploration. InVADER-like technology could help determine exactly what minerals, molecules, and other materials are present in these extraterrestrial oceans. To be sure, further development would be needed to adapt the instrumentation for space travel and the extreme conditions of other worlds. But InVADER’s success so far suggests it could be a promising starting point for this development.

What’s next for InVADER? Within the next few years, the platform will explore black smoker hydrothermal vents. These tantalizing targets support a wide diversity of organisms and are often regarded as oases of life in the deep sea. They are also dynamic structures that are sensitive to geologic activity (e.g., underwater volcanic eruptions), and they may have been important sites for the origins of prebiotic chemistry on early Earth.

Planned developments to continue advancing InVADER include taking it deeper into the ocean to test its performance under increasingly extreme conditions and incorporating colocated laser induced breakdown spectroscopy (LIBS) with its Raman and fluorescence capabilities. The addition of LIBS will allow InVADER to measure simultaneously the mineralogical and elemental makeup of target materials, including rocks and organic compounds, providing richer and more complete pictures of the materials’ compositions.

With these ongoing efforts, we anticipate that InVADER will help reveal additional insights about Earth’s own local frontier and, one day, about frontiers beyond our planet.

Acknowledgments

The InVADER project is funded by NASA Planetary Science and Technology from Analog Research (PSTAR) grant 80NSSC18K1651. NOAA’s Ocean Exploration Cooperative Institute (OECI) and the Bureau of Ocean Energy Management’s Marine Minerals Program provided additional funds to develop and deploy the technology. The Ocean Exploration Trust’s NA149 expedition was funded by NOAA Ocean Exploration via OECI. Parts of this work were additionally supported by NASA Habitable Worlds grant 80NSSC20K0228 to L.M.B. and R.E.P. and carried out at the Jet Propulsion Laboratory (JPL), California Institute of Technology. This endeavor is accomplished with collaboration between several institutions and partnerships working closely together over a number of years. We thank all of the scientists and engineers of Impossible Sensing LLC, NASA JPL, SETI Institute, the University of Washington’s Applied Physics Laboratory, NOAA Ocean Exploration via the University of Southern Mississippi and the Ocean Exploration Institute, the Bureau of Ocean Energy Management’s Marine Minerals Program, the University of Southern California, the State University of New York at Stony Brook, the University of Southampton, the University of Hawaii, the Lunar and Planetary Institute, the Oak Crest Institute of Science, Citrus College, Honeybee Robotics, the Geological Survey of Belgium, and the crew of the Ocean Exploration Trust’s E/V Nautilus. The authors also want to acknowledge and honor the contributions to the InVADER mission of Jan Amend, professor of Earth and biological sciences at the University of Southern California, who unexpectedly passed away earlier this year. He was an exceptionally valuable member of the InVADER team, and his death was an incredibly large loss to the marine biology, oceanography, and geobiology communities both in the United States and across the globe. We send our condolences to Jan Amend’s family, close ones, and all those who had the honor of working with him.

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Author Information

Anastasia G. Yanchilina (ayanchil@caltech.edu), Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena; Laura E. Rodriguez, Lunar and Planetary Institute, Houston; Roy Price, Stony Brook University, Stony Brook, N.Y.; Laura M. Barge, NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena; and Pablo Sobron, Impossible Sensing LLC, St. Louis

Citation: Yanchilina, A. G., L. E. Rodriguez, R. Price, L. M. Barge, and P. Sobron (2024), Sensing remote realms of the deep ocean on Earth—and beyond, Eos, 105, https://doi.org/10.1029/2024EO240375. Published on 29 August 2024.

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