“Something is going on on Enceladus – is active and we want to know,” said astrophysicist Laura Danly, curator at the Griffith Observatory in Los Angeles. NASA’s Cassini mission to Saturn observed surface fissures on Enceladus that are unique in our Solar System and are perpetually erupting with water ice from its global subsurface ocean, that appear as parallel, evenly spaced “stripes” that are some 130 kilometers long and 35 kilometers apart.
How Enceladus Got Its Stripes
2019 research led by Douglas Hemingway with the Carnegie Institute for Science revealed the physics governing the fissures through which ocean water erupts from the moon’s icy surface, giving its south pole an unusual “tiger stripe” appearance. Working with Max Rudolph of the University of California, Davis and Michael Manga of UC Berkeley, Hemingway used models to investigate the physical forces acting on Enceladus that allow the tiger stripe fissures to form and remain in place. Their findings are published by Nature Astronomy.
The image above is a NASA/JPL-Caltech artist’s rendition of Enceladus South Pole.
The team was particularly interested in understanding why the stripes are present only on the moon’s south pole but were also keen to figure out why the cracks are so evenly spaced.
The answer to the first question turns out to be a bit of a coin toss. The researchers revealed that the fissures that make up Enceladus’ tiger stripes could have formed on either pole, the south just happened to split open first.
Enceladus experiences internal heating due to the eccentricity of its orbit. It is sometimes a little closer to Saturn and sometimes a littler farther, which causes the moon to be slightly deformed—stretched and relaxed—as it responds to the giant planet’s gravity. It is this process that keeps the moon from freezing completely solid.
Key to the formation of the fissures is the fact that the moon’s poles experience the greatest effects of this gravitationally induced deformation, so the ice sheet is thinnest over them. During periods of gradual cooling on Enceladus, some of the moon’s subsurface ocean will freeze. Because water expands as it freezes, as the icy crust thickens from below, the pressure in the underlying ocean increases until the ice shell eventually splits open, creating a fissure. Because of their comparatively thin ice, the poles are the most susceptible to cracks.
The researchers believe the fissure named after the city of Baghdad was the first to form. (The stripes are named after places referred to in the stories of One Thousand and One Nights, which are also called Arabian Nights.) However, it didn’t just freeze back up again. It stayed open, allowing ocean water to spew from its crevasse that, in turn, caused three more parallel cracks to form.
“Our model explains the regular spacing of the cracks,” Rudolph said.
The additional splits formed from the weight of ice and snow building up along the edges of the Baghdad fissure as jets of water from the subsurface ocean froze and fell back down. This weight added a new form of pressure on the ice sheet.
“That caused the ice sheet to flex just enough to set off a parallel crack about 35 kilometers away,” Rudolph added.
That the fissures stay open and erupting is also due to the tidal effects of Saturn’s gravity. The moon’s deformation acts to keep the wound from healing—repeatedly widening and narrowing the cracks and flushing water in and out of them—preventing the ice from closing up again.
For a larger moon, its own gravity would be stronger and prevent the additional fractures from opening all the way. So, these stripes could only have formed on Enceladus.
“Since it is thanks to these fissures that we have been able to sample and study Enceladus’ subsurface ocean, which is beloved by astrobiologists, we thought it was important to understand the forces that formed and sustained them,” Hemingway said. “Our modeling of the physical effects experienced by the moon’s icy shell points to a potentially unique sequence of events and processes that could allow for these distinctive stripes to exist.”
NASA scientists now are preparing for a mission to another ice-covered ocean world with possible plumes: Jupiter’s moon Europa. Scheduled to launch in 2024, NASA’s Europa Clipper spacecraft will study the moon from its deep interior to its surface to determine whether it has ingredients that make it a viable home for life.
The composite Hubble images show a suspected plume of material erupting two years apart from the same location on Jupiter’s icy moon Europa. The images bolster evidence that the plumes are a real phenomenon, flaring up intermittently in the same region on the moon. Both plumes, photographed in ultraviolet light by NASA’s Hubble’s Space Telescope Imaging Spectrograph, were seen in silhouette as the moon passed in front of Jupiter.
The newly imaged plume, shown at right, rises about 62 miles (100 kilometers) above Europa’s frozen surface. The image was taken Feb. 22, 2016. The plume in the image at left, observed by Hubble on March 17, 2014, originates from the same location. It is estimated to be about 30 miles (50 kilometers) high. The snapshot of Europa, superimposed on the Hubble image, was assembled from data from NASA’s Galileo mission to Jupiter.
The plumes correspond to the location of an unusually warm spot on the moon’s icy crust, seen in the late 1990s by the Galileo spacecraft (see PIA21444). Researchers speculate that this might be circumstantial evidence for water venting from the moon’s subsurface. The material could be associated with the global ocean that is believed to be present beneath the frozen crust.
The Astrobiological Mystery
“The really fascinating aspect of the Enceladean oceans is how much mystery there is to them and our drive to wonder how astrobiologically-relevant it is,” wrote astrophysicist, Caitlin Ahrens. a NASA Postdoctoral Program Fellow at the Goddard Space Flight Center, in an email to The Daily Galaxy.
“We can compare the subsurface icy ocean to our own oceans here on Earth,” Ahrens explains, “but there may be an astrobiological mechanism that we may not be totally familiar with. The chemicals that vaporize out of those plumes have already shown us potential pH levels, hydrogen, and ammonia as building blocks and “fuel”, plus there may be even more awesome and complex chemistries that the plumes aren’t outputting that are churning in the subsurface ocean (or we just haven’t detected them yet!). The potential is there, but in what way and how we can explore it further – we’re just getting started. I’d say it’s pretty high up on the potential-life hotspot list.”
According to a June, 2021 Nature article, Life on Enceludus, Cassini Mission measurements suggest hydrothermal activity on Enceladus that could support methanogenesis. Bayesian analysis of models simulating an abiotic or biotic ocean indicates the latter is more probable so long as the evolution of living organisms from inorganic or inanimate substances –abiogenesis– is sufficiently likely to occur.
In an email to The Daily Galaxy Douglas Hemingway wrote: “I would rate Enceladus as the top hotspot in the search for life mainly because of the ongoing eruptions, which are making the ocean material so readily accessible.”
Source: Cascading parallel fractures on Enceladus, Nature Astronomy (2019).
Image credit: A view of Enceladus’ southern hemisphere in color (IR-green-UV). The “tiger stripe” fractures, the source of plumes venting gas and dust into space, are visible at the center. [NASA/JPL-Caltech/SSI/Lunar and Planetary Institute, Paul Schenk (LPI, Houston)]