This coming Wednesday, Oct. 28, 2015, the Cassini spacecraft will make a historic close flyby (dubbed “E21”) of Saturn’s tiny icy moon Enceladus, not only passing very close to the surface, but also making the deepest dive yet through the water vapor geysers which erupt from the south pole. These plumes are connected to a global ocean of salty water deep below the surface ice, which may be a habitable environment for some form of life.
This incredible upcoming event was the subject of a NASA news teleconference yesterday, which outlined what will happen during the encounter and what Cassini could find.
On Wednesday, Cassini will make its closest approach to Enceladus at 8:22 a.m. PDT (11:22 a.m. EDT) Wednesday at an altitude of only 30 miles (49 kilometers) above the moon’s south polar region, speeding past the moon at about 19,000 mph. Cassini has flown through the plumes before, but this will be the deepest dive yet by the spacecraft. The plumes erupt from large fractures in the surface at the south pole called “Tiger Stripes” by the Cassini team, originating from the global ocean below.
Cassini has previously found evidence for hydrothermal activity on the ocean floor of Enceladus, and scientists are hopeful this flyby will provide more details on just how much activity is actually occurring. The spacecraft’s ion and neutral mass spectrometer (INMS) instrument will search for molecular hydrogen, which will help answer this question.
“Confirmation of molecular hydrogen in the plume would be an independent line of evidence that hydrothermal activity is taking place in the Enceladus ocean, on the seafloor,” said Hunter Waite, INMS team lead at Southwest Research Institute in San Antonio. “The amount of hydrogen would reveal how much hydrothermal activity is going on.”
Of particular interest to the Cassini scientists is the composition of the plumes themselves. Sampling of the plumes during previous flybys has already detected water vapor, ice particles, sodium, potassium, methane, carbon monoxide, carbon dioxide, and nitrogen, as well as both simple and complex organics. Since this flyby will be deeper into the plumes, scientists hope for an even more detailed analysis of heavier, more complex molecules, including organics. The CDA instrument can detect up to 10,000 particles per second in the plume, and is also expected to reveal how much material the plume is actually spraying into space. The first initial analysis results might be available within a couple of weeks after the flyby, with more detailed findings in the following weeks and months.
According to Sascha Kempf, a CDA team co-investigator at the University of Colorado at Boulder: “There’s really no room for ambiguity. The data will either match what our models are telling us about the rate at which the plume is producing material, or our concept of how the plume works needs additional thought.”
The flyby may also help settle the debate as to whether the geysers are individual jets, or curtain eruptions, or both.
As outlined on the NASA website, here are seven key facts about this flyby:
- Enceladus is an icy moon of Saturn. Early in its mission, Cassini discovered Enceladus has remarkable geologic activity, including a towering plume of ice, water vapor and organic molecules spraying from its south polar region. Cassini later determined the moon has a global ocean and likely hydrothermal activity, meaning it could have the ingredients needed to support simple life.
- The flyby will be Cassini’s deepest-ever dive through the Enceladus plume, which is thought to come from the ocean below. The spacecraft has flown closer to the surface of Enceladus before, but never this low directly through the active plume.
- The flyby is not intended to detect life, but it will provide powerful new insights about how habitable the ocean environment is within Enceladus.
- Cassini scientists are hopeful the flyby will provide insights about how much hydrothermal activity—that is, chemistry involving rock and hot water—is occurring within Enceladus. This activity could have important implications for the potential habitability of the ocean for simple forms of life. The critical measurement for these questions is the detection of molecular hydrogen by the spacecraft.
- Scientists also expect to better understand the chemistry of the plume as a result of the flyby. The low altitude of the encounter is, in part, intended to afford Cassini greater sensitivity to heavier, more massive molecules, including organics, than the spacecraft has observed during previous, higher-altitude passes through the plume.
- The flyby will help solve the mystery of whether the plume is composed of column-like, individual jets, or sinuous, icy curtain eruptions—or a combination of both. The answer would make clearer how material is getting to the surface from the ocean below.
- Researchers are not sure how much icy material the plumes are actually spraying into space. The amount of activity has major implications for how long Enceladus might have been active.
See the video below for an animation of the flyby:
Cassini will also be able to acquire new images of Enceladus’ surface during the flyby; the views of the south polar terrain will be among the highest-resolution yet, lit by light reflected from Saturn (“Saturnshine”) since the south pole is in darkness right now. Those closest images will however be blurry due to the speed the spacecraft is moving at, but post-flyby processing is expected to remove most of the blurriness. As noted during the teleconference, there should also be some cool images of Enceladus and its geysers with Saturn in the background. The first images from the flyby are expected to be released probably by Friday, Oct. 30.
“Cassini truly has been a discovery machine for more than a decade,” said Curt Niebur, Cassini program scientist at NASA Headquarters in Washington. “This incredible plunge through the Enceladus plume is an amazing opportunity for NASA and its international partners on the Cassini mission to ask, ‘Can an icy ocean world host the ingredients for life?'”
Enceladus has two distinct hemispheres, heavily cratered in the north, but smoother and younger in the south. It is the southern hemisphere where the fractures and geysers are prominent, which have reshaped the surface in the area, unlike in the north, where the terrain is older and largely unchanged for billions of years.
The final close flyby of Enceladus will be on Dec. 19, 2015, at a distance of 3,106 miles (4,999 kilometers), which will focus on determining how much heat is emanating from there moon’s interior. The most recent of these last three flybys of Enceladus was on Oct. 14, 2015, at a distance of 1,142 miles (1,839 kilometers), which produced some spectacular new images of the north polar region. There is also an online toolkit for all three of the final flybys. The closest ever flyby of Enceladus was in 2008, when Cassini passed only 16 miles (25 kilometers) above the surface. The spacecraft flew through the plumes during that flyby also, but at a higher altitude than it will Wednesday.
Cassini isn’t designed to find living organisms themselves, but the data obtained Wednesday should help scientists to better understand the potential habitability of the subsurface ocean. Just the fact that we even can sample the spray from an alien ocean is of course exciting enough in itself; not too long ago it wasn’t even known that such watery environments existed elsewhere in the Solar System, including Jupiter’s moons Europa and Ganymede and possibly others as well.
“We’ve been following a trail of clues on Enceladus for 10 years now,” said Bonnie Buratti, a Cassini science team member and icy moons expert at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “The amount of activity on and beneath this moon’s surface has been a huge surprise to us. We’re still trying to figure out what its history has been, and how it came to be this way.”
A video of the news teleconference is below:
A subsequent mission, such as the Enceladus Life Finder (ELF) being contemplated, will be necessary to determine whether or not there actually are microbes or other organisms in the Enceladean ocean. As Cassini imaging team leader Carolyn Porco also noted recently in a podcast, if there are microbes in the ocean, then they could be ejected into space through the plumes, and since Saturn’s outer E ring is composed largely of material from the plumes, then that ring itself could even have microbes in it, or at least their remnants – an amazing thought.
“We’ll continue observing Enceladus and its remarkable activity for the remainder of our precious time at Saturn,” said Linda Spilker, Cassini project scientist at JPL. “But these three encounters will be our last chance to see this fascinating world up close for many years to come.”
Cassini has been orbiting Saturn since 2004, with the mission scheduled to end in 2017. The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. JPL manages the mission for NASA’s Science Mission Directorate in Washington.
Continue to follow AmericaSpace for more updates as they become available. More information about the Cassini mission is also available here.