Crucial Scientific Issues in Earth Science Revealed Only by Mantle Drilling: Understanding the Current State of the Oceanic Plates of a Life-bearing Planet

In the 1950s, the aim of the original mantle drilling projects was to obtain oceanic mantle samples in order to address the unanswered question of what constitutes the Earth's mantle. However, in the 21st century, it is widely accepted that the uppermost mantle is mainly composed of peridotite. Now, the challenge of mantle drilling is to understand crucial unsolved issues of earth science. Today's Earth is different from other planets due to the existence of life and plate tectonics. It is emphasized that mantle drilling is the only way to obtain the oceanic crust from top to bottom and an active mantle sample from an oceanic plate. The crucial issues that can only be addressed by mantle drilling are: (1) limits of life in an oceanic plate and its controlling factors, and (2) formation process of an oceanic plate and its modification. Modification of an oceanic plate, especially the weakening of plate strength, is required for plate tectonics. These two issues are interrelated. Long seismic profiles of oceanic plates reveal the diversity of Moho seismic reflection regions: clear, unclear, diffuse and non-Moho regions. Faults and/or fracturing in oceanic plates and subsequent seawater flow can modify oceanic plates locally, probably causing the diversity of oceanic Moho, as well as the rheological behavior of oceanic plates. Fluid flows along faults/fractures also extend the biosphere of oceanic plates. The first drilling sample should be a reference to the oceanic crust and the uppermost mantle, and define the nature of the Moho at the site, as well as constrain reasons for the diversity of the Moho in other areas. Deep sampling, such as mantle drilling in an old oceanic plate, can penetrate the biosphere/non-biosphere boundary, which tells us about the controlling factors of the limit of life. This information may help us find extraterrestrial life. After mantle drilling is completed, the borehole is the only window from the ocean floor to the mantle. An in-situ mantle observatory in the mantle hole to monitor plate movement and fluid flow with biological activity within an oceanic plate is also suggested. Detecting geoneutrinos at the mantle site allows the amounts and distributions of radioactive elements from the Earth's mantle to be measured. These provide basic information on the Earth's heat sources and the evolutionary history of the mantle.