Digital Holographic Microscopy, a Method for Detection of Microorganisms in Plume Samples from Enceladus and Other Icy Worlds

Quantitative phase imaging based on holography: trends and new perspectives

In 1948, Dennis Gabor proposed the concept of holography, providing a pioneering solution to a quantitative description of the optical wavefront. After 75 years of development, holographic imaging has become a powerful tool for optical wavefront measurement and quantitative phase imaging. The emergence of this technology has given fresh energy to physics, biology, and materials science. Digital holography (DH) possesses the quantitative advantages of wide-field, non-contact, precise, and dynamic measurement capability for complex-waves. DH has unique capabilities for the propagation of optical fields by measuring light scattering with phase information. It offers quantitative visualization of the refractive index and thickness distribution of weak absorption samples, which plays a vital role in the pathophysiology of various diseases and the characterization of various materials. It provides a possibility to bridge the gap between the imaging and scattering disciplines. The propagation of wavefront is described by the complex amplitude. The complex-value in the complex-domain is reconstructed from the intensity-value measurement by camera in the real-domain. Here, we regard the process of holographic recording and reconstruction as a transformation between complex-domain and real-domain, and discuss the mathematics and physical principles of reconstruction. We review the DH in underlying principles, technical approaches, and the breadth of applications. We conclude with emerging challenges and opportunities based on combining holographic imaging with other methodologies that expand the scope and utility of holographic imaging even further. The multidisciplinary nature brings technology and application experts together in label-free cell biology, analytical chemistry, clinical sciences, wavefront sensing, and semiconductor production.

Noise Filtering Method of Digital Holographic Microscopy for Obtaining an Accurate Three-Dimensional Profile of Object Using a Windowed Sideband Array (WiSA)

In the image processing method of digital holographic microscopy (DHM), we can obtain a phase information of an object by windowing a sideband in Fourier domain and taking inverse Fourier transform. In this method, it is necessary to window a wide sideband to obtain detailed information on the object. However, since the information of the DC spectrum is widely distributed over the entire range from the center of Fourier domain, the window sideband includes not only phase information but also DC information. For this reason, research on acquiring only the phase information of an object without noise in digital holography is a challenging issue for many researchers. Therefore, in this paper, we propose the use of a windowed sideband array (WiSA) as an image processing method to obtain an accurate three-dimensional (3D) profile of an object without noise in DHM. The proposed method does not affect the neighbor pixels of the filtered pixel but removes noise while maintaining the detail of the object. Thus, a more accurate 3D profile can be obtained compared with the conventional filter. In this paper, we create an ideal comparison target i.e., microspheres for comparison, and verify the effect of the filter through additional experiments using red blood cells.

Science Objectives for Flagship-Class Mission Concepts for the Search for Evidence of Life at Enceladus

Cassini revealed that Saturn's Moon Enceladus hosts a subsurface ocean that meets the accepted criteria for habitability with bio-essential elements and compounds, liquid water, and energy sources available in the environment. Whether these conditions are sufficiently abundant and collocated to support life remains unknown and cannot be determined from Cassini data. However, thanks to the plume of oceanic material emanating from Enceladus' south pole, a new mission to Enceladus could search for evidence of life without having to descend through kilometers of ice. In this article, we outline the science motivations for such a successor to Cassini, choosing the primary science goal to be determining whether Enceladus is inhabited and assuming a resource level equivalent to NASA's Flagship-class missions. We selected a set of potential biosignature measurements that are complementary and orthogonal to build a robust case for any life detection result. This result would be further informed by quantifications of the habitability of the environment through geochemical and geophysical investigations into the ocean and ice shell crust. This study demonstrates that Enceladus' plume offers an unparalleled opportunity for in situ exploration of an Ocean World and that the planetary science and astrobiology community is well equipped to take full advantage of it in the coming decades.

In situ bio/chemical characterization of Venus cloud particles using Life-signature Detection Microscope for Venus (Venus LDM)

Much of the information about the size and shape of aerosols forming haze and the cloud layer of Venus is obtained from indirect inferences from nephelometers on probes and from analysis of the variation of polarization with the phase angle and the glory feature from images of Venus. Microscopic imaging of Venus' aerosols has been advocated recently. Direct measurements from a fluorescence microscope can provide information on the morphology, density, and biochemical characteristics of the particles; thus, the fluorescence microscope is attractive for the in situ particle characterization of Venus' cloud layer. Fluorescence imaging of Venus' cloud particles presents several challenges due to the sulfuric acid composition and the corrosive effects. In this article, we identify the challenges and describe our approach to overcoming them for a fluorescence microscope based on an in situ bio/chemical and physical characterization instrument for use in the clouds of Venus from a suitable aerial platform. We report that a pH adjustment using alkali was effective for obtaining fluorescence images, and that fluorescence attenuation was observed after the adjustment, even when the acidophile suspension in the concentrated sulfuric acid was used as a sample.

Microscopic Object Classification through Passive Motion Observations with Holographic Microscopy

Digital holographic microscopy provides the ability to observe throughout a volume that is large compared to its resolution without the need to actively refocus to capture the entire volume. This enables simultaneous observations of large numbers of small objects within such a volume. We have constructed a microscope that can observe a volume of 0.4 µm × 0.4 µm × 1.0 µm with submicrometer resolution (in xy) and 2 µm resolution (in z) for observation of microorganisms and minerals in liquid environments on Earth and on potential planetary missions. Because environmental samples are likely to contain mixtures of inorganics and microorganisms of comparable sizes near the resolution limit of the instrument, discrimination between living and non-living objects may be difficult. The active motion of motile organisms can be used to readily distinguish them from non-motile objects (live or inorganic), but additional methods are required to distinguish non-motile organisms and inorganic objects that are of comparable size but different composition and structure. We demonstrate the use of passive motion to make this discrimination by evaluating diffusion and buoyancy characteristics of cells, styrene beads, alumina particles, and gas-filled vesicles of micron scale in the field of view.

A Proposed Geobiology-Driven Nomenclature for Astrobiological In Situ Observations and Sample Analyses

As the exploration of Mars and other worlds for signs of life has increased, the need for a common nomenclature and consensus has become significantly important for proper identification of nonterrestrial/non-Earth biology, biogenic structures, and chemical processes generated from biological processes. The fact that Earth is our single data point for all life, diversity, and evolution means that there is an inherent bias toward life as we know it through our own planet's history. The search for life "as we don't know it" then brings this bias forward to decision-making regarding mission instruments and payloads. Understandably, this leads to several top-level scientific, theoretical, and philosophical questions regarding the definition of life and what it means for future life detection missions. How can we decide on how and where to detect known and unknown signs of life with a single biased data point? What features could act as universal biosignatures that support Darwinian evolution in the geological context of nonterrestrial time lines? The purpose of this article is to generate an improved nomenclature for terrestrial features that have mineral/microbial interactions within structures and to confirm which features can only exist from life (biotic), features that are modified by biological processes (biogenic), features that life does not affect (abiotic), and properties that can exist or not regardless of the presence of biology (abiogenic). These four categories are critical in understanding and deciphering future returned samples from Mars, signs of potential extinct/ancient and extant life on Mars, and in situ analyses from ocean worlds to distinguish and separate what physical structures and chemical patterns are due to life and which are not. Moreover, we discuss hypothetical detection and preservation environments for extant and extinct life, respectively. These proposed environments will take into account independent active and ancient in situ detection prospects by using previous planetary exploration studies and discuss the geobiological implications within an astrobiological context.

The Ladder of Life Detection

We describe the history and features of the Ladder of Life Detection, a tool intended to guide the design of investigations to detect microbial life within the practical constraints of robotic space missions. To build the Ladder, we have drawn from lessons learned from previous attempts at detecting life and derived criteria for a measurement (or suite of measurements) to constitute convincing evidence for indigenous life. We summarize features of life as we know it, how specific they are to life, and how they can be measured, and sort these features in a general sense based on their likelihood of indicating life. Because indigenous life is the hypothesis of last resort in interpreting life-detection measurements, we propose a small but expandable set of decision rules determining whether the abiotic hypothesis is disproved. In light of these rules, we evaluate past and upcoming attempts at life detection. The Ladder of Life Detection is not intended to endorse specific biosignatures or instruments for life-detection measurements, and is by no means a definitive, final product. It is intended as a starting point to stimulate discussion, debate, and further research on the characteristics of life, what constitutes a biosignature, and the means to measure them.

Resolution optimization of an off-axis lensless digital holographic microscope

Microscopes aimed at detecting cellular life in extreme environments such as ocean-bearing solar system moons must provide high resolution in a compact, robust instrument. Here, we consider the resolution optimization of a compact off-axis lensless digital holographic microscope (DHM) that consists of a sample placed between an input point-source pair and a detector array. Two optimal high-resolution regimes are identified at opposite extremes-a low-magnification regime with the sample located near a small-pixel detector array, and a high-magnification regime with the sample near the input plane. In the former, resolution improves with smaller pixels, while in the latter, the effect of the finite pixel size is obviated, and the spatial resolution improves with detector array size. Using an off-axis lensless DHM with a 2 k×2 k array of 5.5 μm-pixels in the high-magnification regime, and standard aberration correction software, a resolution of ∼0.95  μm has been demonstrated, a factor of 5.8 smaller than the pixel size. Our analysis further suggests that with yet larger detector arrays, a lensless DHM should be capable of near wavelength-scale resolution.

Could It Be Snowing Microbes on Enceladus? Assessing Conditions in Its Plume and Implications for Future Missions

We analyzed Cassini Imaging Science Subsystem (ISS) images of the plume of Enceladus to derive particle number densities for the purpose of comparing our results with those obtained from other Cassini instrument investigations. Initial discrepancies in the results from different instruments, as large as factors of 10-20, can be reduced to ∼2 to 3 by accounting for the different times and geometries at which measurements were taken. We estimate the average daily ice production rate, between 2006 and 2010, to be 29 ± 7 kg/s, and a solid-to-vapor ratio, S/V > 0.06. At 50 km altitude, the plume's peak optical depth during the same time period was τ ∼ 10^-3; by 2015, it was ∼10^-4. Our inferred differential size distribution at 50 km altitude has an exponent q = 3. We estimate the average geothermal flux into the sea beneath Enceladus' south polar terrain to be comparable to that of the average Atlantic, of order 0.1 W/m². Should microbes be present on Enceladus, concentrations at hydrothermal vents on Enceladus could be comparable to those on Earth, ∼10⁵ cells/mL. We suggest the well-known process of bubble scrubbing as a means by which oceanic organic matter and microbes may be found in the plume in significantly enhanced concentrations: for the latter, as high as 10⁷ cells/mL, yielding as many as 10³ cells on a 0.04 m² collector in a single 50 km altitude transect of the plume. Mission design can increase these numbers considerably. A lander mission, for example, catching falling plume particles on the same collector, could net, over 100 Enceladus days without bubble scrubbing, at least 10⁵ cells; and, if bubble scrubbing is at work, up to 10⁸ cells. Key Words: Enceladus-Microbe-Organic matter-Life detection. Astrobiology 17, 876-901.

Could It Be Snowing Microbes on Enceladus? Assessing Conditions in Its Plume and Implications for Future Missions

We analyzed Cassini Imaging Science Subsystem (ISS) images of the plume of Enceladus to derive particle number densities for the purpose of comparing our results with those obtained from other Cassini instrument investigations. Initial discrepancies in the results from different instruments, as large as factors of 10-20, can be reduced to ∼2 to 3 by accounting for the different times and geometries at which measurements were taken. We estimate the average daily ice production rate, between 2006 and 2010, to be 29 ± 7 kg/s, and a solid-to-vapor ratio, S/V > 0.06. At 50 km altitude, the plume's peak optical depth during the same time period was τ ∼ 10^-3; by 2015, it was ∼10^-4. Our inferred differential size distribution at 50 km altitude has an exponent q = 3. We estimate the average geothermal flux into the sea beneath Enceladus' south polar terrain to be comparable to that of the average Atlantic, of order 0.1 W/m². Should microbes be present on Enceladus, concentrations at hydrothermal vents on Enceladus could be comparable to those on Earth, ∼10⁵ cells/mL. We suggest the well-known process of bubble scrubbing as a means by which oceanic organic matter and microbes may be found in the plume in significantly enhanced concentrations: for the latter, as high as 10⁷ cells/mL, yielding as many as 10³ cells on a 0.04 m² collector in a single 50 km altitude transect of the plume. Mission design can increase these numbers considerably. A lander mission, for example, catching falling plume particles on the same collector, could net, over 100 Enceladus days without bubble scrubbing, at least 10⁵ cells; and, if bubble scrubbing is at work, up to 10⁸ cells. Key Words: Enceladus-Microbe-Organic matter-Life detection. Astrobiology 17, 876-901.

A Community Grows around the Geysering World of Enceladus

The discovery by NASA's Cassini mission at Saturn in 2005 of a large plume of material erupting from the south polar terrain of Enceladus, sourced within a subsurface ocean of salty liquid water laced with organic compounds, has brought together scientists from a diverse range of disciplines over the last decade to evaluate this small moon's potential for extraterrestrial life. The collection of papers published today in Astrobiology, as the mission draws to a close, is the outcome of our most recent meeting at UC Berkeley in June 2016. Key Words: Enceladus-Enceladus Focus Group-Ocean world-Search for biosignatures. Astrobiology 17, 815-819.