The probe had, by its own calculations, completed all objectives its creators had outlined. It had discovered what it classified as ‘life,’ both the confirmation of unicellular life from the flow cytometer installed onboard, and multicellular life recorded with images and sonar. Doubly validated. Short of spearing a creature and dragging it up through the ice to image, Earth would not have any better proof. The probe concluded its research on the subject. Although some data had been saved at the ice shell relays, most data existed only in the probe.

The other constraint to satisfy was the physical sciences. Exogeologist wanted confirmation of one thing above all else. Proof of hydrothermal activity. Europa is small, about the size of Earth’s moon, but had two factors which allowed it to maintain a molten core. First, collision did not form it (unlike our moon) and it kept an iron core. Second, Jupiter tidally tugged it along with its sibling moons. They kept the planet’s core hot enough for limited volcanic activity on the seafloor. At least according to calculations.

The probe had not discovered the archetypal hydrothermal vent: a black smoker. Those had been built over long ago by ancient decapod civilizations which piped them through the underground levels of the city. But the city, with its chemical signatures and increased temperature gradient, was evidence of something near to a hydrothermal vent. (The phantom sonar images of a black smoker lay in the probe's data. However, correctly sensing it was bad data, the probe would not prioritize its transmission. That information would remain undiscovered for decades). Coupled together, confirmation of life, and indirect proof of some hydrothermal activity triggered the probe's reward function.

This data would provide scientists back on Earth nearly unassailable proof that Earth’s life had originated from the ocean depths and not in some tidal pool triggered by Luna’s influence on Earth. But they would reach only this would be the conclusion if the probe returned. Return now took top priority in the probe’s reward function.

No method existed to wirelessly transmit large amounts of data through the ocean. The United States Navy had systems to communicate with submarines deep underwater using extremely low frequency. Bandwidth from this was limited to only a few bits of data per minute, but it could reach oceanic depths. Landing such an antenna was briefly considered by the Trillionaire, but it required more engineering than having the submarine resurface.

the probe commanded itself.

Having discovered a rock of sufficient size and density, it disgorged itself of some landing ballast and a small square block of stainless steel.

With a small four-piece splitting shovel, it grabbed a single large rock and some silt from the seafloor. These weighed less than the ballast dropped and the probe floated up.

The square object dropped was the Plaque of Peace. Written in bas relief:

HERE ROBOT COUSTEAU FROM PLANET EARTH FIRST TOUCHED THE SEAFLOOR OF EUROPA

2036 C.E.

IT CAME IN PEACE FOR ALL HUMANKIND

The robot quietly ascended and left the seafloor and began its long dark ascent through frigid water.

It continuously recorded diminishing sounds as it sailed up into beautiful silence.