Young Astronomers Blog, Volume 28, Number 1.
Two of the most interesting moons in the solar system, other than Titan, are Enceladus and Europa. Enceladus is one of the smaller spherical moons of Saturn. Europa is one of the Galilean moons of Jupiter. Both moons are covered with water ice and astronomers believe they both have a global underground ocean of liquid water. Despite the cold of the outer solar system, these oceans remain liquid because of the tidal effects from Jupiter and Saturn. This is important because where there is water, there is life (at least on Earth) and astronomers think these two moons might have the right ingredients for life.
Astronomers estimate that Europa’s ocean might hold twice as much water as the oceans of the Earth. Using images from the Hubble telescope and the Galileo spacecraft, evidence of water/ice plumes rising from Europa have been detected. In November 2019, a NASA team announced the discovery of water vapor above Europa. NASA is currently preparing the Europa Clipper, a mission to explore Europa in the 2020s.
Enceladus is one of the most reflective objects in the solar system. In 2005, the Cassini spacecraft arrived at Saturn. When Cassini turned its cameras on Enceladus, they revealed four long canyons/fissures at Enceladus’ southern pole. These canyons, called “tiger stripes”, were found to have jets of water ice erupting into space. Much of the material from these eruptions falls back onto Enceladus preserving its reflective surface. Some of the material feeds Saturn’s E ring. Measurements by Cassini revealed that the jets include salts and organic materials, possibly ingredients to support life.
In December 2019, Doug Hemingway, Max Rudolph, and Michael Manga published an article in Nature Astronomy explaining how the “tiger stripes” formed. They concluded that fissures such as these could have formed at either pole where the ice is thinnest and tidal effects are the greatest. One called Baghdad formed first, relieving pressure from the underground ocean and preventing fissures from forming at the north pole. Baghdad was followed by the other three, Cairo, Damascus and Alexandria, which formed parallel to the first. They also concluded that stronger gravity on larger moons would have prevented the formation of similar fissures. This explains why they are only observed on Enceladus, which is a relatively small moon.
Sources and Further Reading
“Europa” NASA Solar System Exploration (accessed December 9, 2019). https://solarsystem.nasa.gov/moons/jupiter-moons/europa/overview/
“Europa Clipper” NASA (accessed December 9, 2019). https://europa.nasa.gov/
Lonnie Shekhtman “NASA Scientists Confirm Water Vapor on Europa” NASA. November 18, 2019. https://www.nasa.gov/feature/goddard/2019/nasa-scientists-confirm-water-vapor-on-europa
“Enceladus” NASA Solar System Exploration (accessed December 9, 2019). https://solarsystem.nasa.gov/moons/saturn-moons/enceladus/in-depth/
“Enceladus: Ocean Moon” NASA Solar System Exploration (accessed December 9, 2019). https://solarsystem.nasa.gov/missions/cassini/science/enceladus/
Monica Young “How Saturn’s Moon Got Its Stripes” Sky & Telescope. December 9, 2019. https://www.skyandtelescope.com/astronomy-news/how-moon-enceladus-got-its-stripes/
Erika K. Carlson “How the icy moon Enceladus got ‘tiger stripes’ at its south pole” astronomy.com December 9, 2019. http://www.astronomy.com/news/2019/12/how-the-icy-moon-enceladus-got-tiger-stripes-at-its-south-pole
Carnegie Institution for Science “How Enceladus got its stripes” phys.org. December 9, 2019. https://phys.org/news/2019-12-enceladus-stripes.html
Technical Reading
Paganini et al. “A measurement of water vapour amid a largely quiescent environment on Europa”, Nature Astronomy (2019). https://doi.org/10.1038/s41550-019-0933-6. https://www.nature.com/articles/s41550-019-0933-6#citeas
Douglas J. Hemingway et al. “Cascading parallel fractures on Enceladus”, Nature Astronomy (2019). DOI: 10.1038/s41550-019-0958-x, https://www.nature.com/articles/s41550-019-0958-x