There Could be as Many Water Worlds as Earths in the Milky Way

Artist’s impression of the strange landscape of a water world. Credit: Pilar Montañés

On July 12th, 2022, NASA released the first images acquired by the James Webb Space Telescope, which were taken during its first six months of operation. Among its many scientific objectives, Webb will search for smaller, rocky planets that orbit closer to their suns – especially dimmer M-type (red dwarf) stars, the most common in the Universe. This will help astronomers complete the census of exoplanets and gain a better understanding of the types of worlds that exist out there. In particular, astronomers are curious about how many terrestrial planets in our galaxy are actually “water worlds.”

These are rocky planets that are larger than Earth but have a lower density, which suggests that volatiles like water make up a significant amount (up to half) of their mass-fraction. According to a recent study by researchers from the University of Chicago and the Instituto de Astrofísica de Canarias (IAC), water worlds may be just as common as “Earth-like” rocky planets. These findings bolster the case for exoplanets that are similar to icy moons in the Solar System (like Europa) and could have significant implications for future exoplanet studies and the search for life in our Universe.

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Europa’s Nightside Glows in the Dark

This illustration of Jupiter's moon Europa shows how the icy surface may glow on its nightside, the side facing away from the Sun. Variations in the glow and the color of the glow itself could reveal information about the composition of ice on Europa's surface. Credit: NASA/JPL-Caltech

In a few years, NASA will be sending a spacecraft to explore Jupiter’s icy moon Europa. Known as the Europa Clipper mission, this orbiter will examine the surface more closely to search for plume activity and evidence of biosignatures. Such a find could answer the burning question of whether or not there is life within this moon, which is something scientists have speculated about since the 1970s.

In anticipation of this mission, scientists continue to anticipate what it will find once it gets there. For instance, scientists from NASA’s Jet Propulsion Laboratory recently conducted a study that showed how Europa might glow in the dark. This could be the result of Europa constantly being pummeled with high-energy radiation from Jupiter’s magnetic field, the study of which could tell scientists more about the composition of Europa’s ice.

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Icy Hot: Europa’s Frozen Crust Could Be Warmer Than We Thought

NASA is looking for a new Planetary Protection Officer to protect Earth and the other bodies of the Solar System from harmful contamination. Credit: NASA/JPL-Caltech/SETI Institute.

All the worlds may be ours except Europa but that only makes the ice-covered moon of Jupiter all the more intriguing. Beneath Europa’s thin crust of ice lies a tantalizing global ocean of liquid water somewhere in the neighborhood of 100 kilometers deep—which adds up to more liquid water than is on the entire surface of the Earth. Liquid water plus a heat source(s) to keep it liquid plus the organic compounds necessary for life and…well, you know where the thought process naturally goes from there.

And now it turns out Europa may have even more of a heat source than we thought. Yes, a big component of Europa’s water-liquefying warmth comes from tidal stresses enacted by the massive gravity of Jupiter as well as from the other large Galilean moons. But exactly how much heat is created within the moon’s icy crust as it flexes has so far only been loosely estimated. Now, researchers from Brown University in Providence, RI and Columbia University in New York City have modeled how friction creates heat within ice under stress, and the results were surprising.

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Cassini’s Close Flyby of Enceladus Yields Surprising, Perplexing Imagery

Craters near Enceladus' north pole region appear to be 'melting' into each other. Image taken by Cassini spacecraft on October 14, 2015. Credit: NASA/JPL-Caltech/Space Science Institute

If you thought Saturn’s moon Enceladus couldn’t get any more bizzare — with its magnificent plumes, crazy tiger-stripe-like fissures and global subsurface salty ocean — think again. New images of this moon’s northern region just in from the Cassini spacecraft show surprising and perplexing features: a tortured surface where craters look like they are melting, and fractures that cut straight across the landscape.

“We’ve been puzzling over Enceladus’ south pole for so long, time to be puzzled by the north pole!” tweeted NASA engineer Sarah Milkovich, who formerly worked on the Cassini mission.

While the Cassini mission has been at the Saturn system since 2004 and flown by this moon several times, this is the spacecraft’s first close-up look at the north polar region of Enceladus. On October 14, 2015 the spacecraft passed at an altitude of just 1,839 kilometers (1,142 miles) above the moon’s surface.

See more imagery below:

Craters and a possible straight fracture line mar the surface of Enceladus in this raw image from the Cassini spacecraft taken on October 14, 2015. Credit: NASA/JPL-Caltech/Space Science Institute.
Craters and a possible straight fracture line mar the surface of Enceladus in this raw image from the Cassini spacecraft taken on October 14, 2015. Credit: NASA/JPL-Caltech/Space Science Institute.

The reason Cassini hasn’t been able to see the northern terrain of Enceladus previously is that it was concealed by the darkness of winter. It’s now summer in the high northern latitudes, and scientists have been anxious to take a look at this previously unseen region. Gauging by the posts of “Wow!” and “Enceladus what are you doing??” by scientists on social media, the Cassini team is as excited and perplexed by these images as the rest of us.

“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. “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.”

Craters and fractures dot the landscape of the northern region of Enceladus in this raw image from the Cassini spacecraft taken on October 14, 2015.  Credit: NASA/JPL-Caltech/Space Science Institute.
Craters and fractures dot the landscape of the northern region of Enceladus in this raw image from the Cassini spacecraft taken on October 14, 2015. Credit: NASA/JPL-Caltech/Space Science Institute.

While these raw images just arrived this morning, already image editing enthusiasts have dived into the data to create composite and color images. Here are two from UT writer Jason Major and image contributor Kevin Gill:

A beautiful view of the night side of a crescent Enceladus, lovingly lit by Saturnshine. This was captured by the Cassini spacecraft during a close pass on Oct. 14, 2015. The 6.5-mile-wide Bahman cater is visible near the center. Credit: NASA/JPL-Caltech/Space Science Institute, image editing by Jason Major.
A beautiful view of the night side of a crescent Enceladus, lovingly lit by Saturnshine. This was captured by the Cassini spacecraft during a close pass on Oct. 14, 2015. The 6.5-mile-wide Bahman cater is visible near the center. Credit: NASA/JPL-Caltech/Space Science Institute, image editing by Jason Major.

Saturn's icy moon Enceladus on October 14th, 2015 during Cassini's latest encounter. Assembled from uncalibrated images using infrared, green, and ultraviolet light. Image Credit: NASA/JPL-CalTech/ISS/Kevin M. Gill
Saturn’s icy moon Enceladus on October 14th, 2015 during Cassini’s latest encounter. Assembled from uncalibrated images using infrared, green, and ultraviolet light. Image Credit: NASA/JPL-CalTech/ISS/Kevin M. Gill

In an email, Cassini imaging team leader Carolyn Porco explained the flyby: “Our cameras were active during most of this encounter, allowing the imaging team and other remote-sensing instrument teams to observe the Saturn-opposing side of Enceladus on the inbound leg of the encounter, and a narrow, sunlit crescent outbound.”

From previous imagery and study of this moon, it has been suggested that the fractured and wrinkled terrain on Enceladus could be the scars of a shift in the moon’s spin rate. The moon has likely undergone multiple episodes of geologic activity spanning a considerable portion of its lifetime.

A complex region of craters and fractures near the north polar region on Saturn's  moon Enceladus. Image from Cassini spacecraft taken on October 14, 2015. Credit: NASA/JPL-Caltech/Space Science Institute
A complex region of craters and fractures near the north polar region on Saturn’s moon Enceladus. Image from Cassini spacecraft taken on October 14, 2015. Credit: NASA/JPL-Caltech/Space Science Institute

While these images are incredible, get ready for even more. An even closer flyby of Enceladus is scheduled for Wednesday, Oct. 28, during which Cassini will come dizzyingly close to the icy moon, passing just 49 kilometers (30 miles) above the moon’s south polar region. NASA says that during this encounter, Cassini will make its deepest-ever dive through the moon’s plume of icy spray, collecting images and valuable data about what’s going on beneath the frozen surface. Cassini scientists are hopeful data from that flyby will provide evidence of how much hydrothermal activity is occurring in the moon’s ocean, and how the amount of activity impacts the habitability of Enceladus’ ocean.

Then another flyby — Cassini’s final scheduled close flyby of Enceladus — on Dec. 19 will examine how much heat is coming from the moon’s interior from an altitude of 4,999 kilometers (3,106 miles).

Enceladus hovers over Saturn's rings in this raw image from the Cassini spacecraft taken on October 14, 2015.  Credit: NASA/JPL-Caltech/Space Science Institute.
Enceladus hovers over Saturn’s rings in this raw image from the Cassini spacecraft taken on October 14, 2015. Credit: NASA/JPL-Caltech/Space Science Institute.

An interesting side note is that the Cassini mission launched 18 years ago today (October 15, 1997).

Again stay tuned for more, and you can see all of Cassini’s raw image here, and find out more details of the upcoming flybys at this CICLOPS page.