Last week, NASA made a close flyby with the Cassini spacecraft of Saturn’s fascinating moon Enceladus. This moon is a geologically active world with active water and ice geysers on its surface, and a planet-wide liquid ocean beneath its icy crust. The flyby was part of a series of three planned for this month, which itself is part of a longer series of “lasts” for Cassini. After the close moon flybys of Enceladus and other Saturn moons this year, the spacecraft will depart Saturn’s equatorial plane – where moon flybys occur most frequently – to begin a year-long setup of the mission’s daring final year.
And now here are the first of the “last” close Cassini images of Enceladus. This moon is 312 miles (503 km) wide.
Visible here is mostly the northern hemisphere, with a wide variety of terrain from impact craters to very youthful tectonic terrain. I have rotated the images so north is towards the top, and have contrast-enhanced some images, plus included some nightside images.
This pass of the Saturn moon Enceladus was known as the Enceladus E20 pass.
The crescent views show the nightside of Enceladus lit by sublight reflected off Saturn, also known as Saturnshine.
This time, the Cassini spacecraft obtained by far the sharpest imagery of the north polar region of Enceladus ever obtained.
This part of Enceladus has been seen before. Way back on August 26, 1981, the Voyager 2 spacecraft saw the far north of Enceladus. But this is by far the closest pass of the north polar region. Voyager 2 saw it from 71,400 miles (115,000 km) away. This time, the north polar region of Enceladus was in places seen from as close as 1,142 miles (1,839 km) with higher resolution cameras.
During most of the Cassini mission to date, the southern hemispheres of Saturn and its major moons were tilted towards the sun (all of Saturn’s major moons apart from Iapetus orbit Saturn in more or less in the plane of Saturn’s rings around Saturn’s equator).
On August 11, 2009, the Saturn system underwent an equinox, brining northern spring / southern autumn to this world.
Since then, as Saturn slowly orbits the sun, the sun has been shining further north onto Saturn and the moons and, over the last six years, it is now approaching northern summer (Saturn orbits the sun once every 29.5 years). The Saturn system seasons last approximately 7.4 years. Saturn and the main moons are tilted by 26.7 degrees in respect to the orbit around the sun. That is in contrast to Earth’s tilt of about 23.4 degrees.
The north polar area including the actual north pole has fractures running through. The craters appear largely softened as after the impacts that formed them, the ice crust of Enceladus has warmed and softened before freezing solid again.
The southern hemisphere and the south pole of Enceladus are known to be active with geysers emanating from the area close to the south pole through huge linear faults. The geysers are still active, although it is the south polar night on Enceladus.
Last week’s flyby is a prelude to that planned for late October, when Cassini will come “dizzyingly close” to Enceladus, according to NASA. It’ll pass only 30 miles (49 kilometers) above the moon’s south polar region, making its deepest-ever dive through the moon’s plume of icy spray, over the night side.
Enceladus is one of a handful of known geologically active worlds, with activity in the form of jets and geysers in the south polar region (not visible here). It is thought to be due to tidal heating between interactions due to the mid-sized moon Dione and Saturn tidally heating Enceladus.
Some ice crystals fall back as a kind of snow and frost and some escape from Enceladus and end up forming the E Ring in orbit around Saturn.
By the way, Enceladus orbits Saturn at an average distance of 147,800 miles (238,000 km) once every 1 day, 8 hours and 53 minutes. Enceladus is kronesynchronous, meaning it keeps the same face turned towards Saturn permanently.
Enceladus is also quite dense, 60% denser than water ice, roughly 50-50 rock and ice. It has an average surface temperature of minus 324 Fahrenheit (minus 198 Celsius).
For its grand finale, Cassini will repeatedly dive through the space between Saturn and its rings. Linda Spilker, Cassini project scientist at JPL, said:
We’ll continue observing Enceladus and its remarkable activity for the remainder of our precious time at Saturn. But these three encounters will be our last chance to see this fascinating world up close for many years to come.
Bottom line: Images from last week’s close flyby of Saturn’s moon Enceladus on October 14, 2015. The flyby is one of three planned for this month. These three will be the final close flybys of Enceladus with the Cassini spacecraft before it ends its mission.
Andrew R. Brown, an avid follower of the space program, writes frequently about space topics for EarthSky. Over several years, he has also suggested observations that were carried out by imaging teams of some space missions. He has lives in Ashford, Kent, United Kingdom and works for local government, Kent County Council.