Short answers, pulled from the story.
Europa was discovered by Galileo Galilei on the 8th of January 1610, and independently by Simon Marius around the same time. On the 7th of January, Galileo observed Io and Europa together but could not separate them; the following night he saw them as distinct bodies for the first time.
The scientific consensus holds that Europa has a liquid saltwater ocean beneath its icy crust. The ocean may be about 100 kilometers deep, giving it a total volume between two and three times the volume of all of Earth's oceans combined. Heat from tidal flexing driven by Jupiter's gravity keeps the ocean from freezing.
Europa's subsurface ocean, in contact with a rocky seafloor, creates conditions that could support life similar to organisms found near hydrothermal vents in Earth's deep oceans. A 2010 model by Richard Greenberg of the University of Arizona proposed that irradiated surface ice could supply enough oxygen and peroxide to oxygenate Europa's ocean to Earth-like levels within just 12 million years.
The dark streaks, called lineae, are fractures in Europa's icy crust where warm ice welled up from below as the surface spread apart, a process similar to seafloor spreading on Earth. Spectrographic evidence suggests the reddish-brown coating on these features may include magnesium sulfate salts and sodium chloride deposited by water that emerged from the interior ocean.
NASA's Europa Clipper launched on the 14th of October 2024 aboard a Falcon Heavy rocket. It will orbit Jupiter and conduct 45 low-altitude flybys of Europa, using instruments including an ice-penetrating radar, a short-wave infrared spectrometer, and an ion-and-neutral-mass spectrometer to investigate the moon's habitability.
The Hubble Space Telescope detected what appeared to be water vapor plumes near Europa's south pole in 2012, with some interpretations placing them at heights up to 200 kilometers. However, a 2026 paper found no evidence of localized water vapor on Europa, raising the possibility that earlier detections resulted from inaccurate positioning of Europa's disk on telescope images.