Eliana Willis

NOORDWIJK, THE NETHERLANDS — Life might arise in the darkest of places: the moon of a planet wandering the galaxy without a star.

The gravitational tug-of-war between a moon and its planet can keep certain satellites toasty enough for liquid water to exist there — a condition widely considered crucial for life. Now computer simulations suggest that, given the right orbit and atmosphere, some moons orbiting rogue planets can stay warm for over a billion years, astrophysicist Giulia Roccetti reported March 23 at the PLANET-ESLAB 2023 Symposium. She and her colleagues also report their findings March 20 in the International Journal of Astrobiology.

“There might be many places in the universe where habitable conditions can be present,” says Roccetti, of the European Southern Observatory in Garching, Germany. But life presumably also needs long-term stability. “What we are looking for is places where these habitable conditions can be sustained for hundreds of millions, or billions, of years.”

Habitability and stability don’t necessarily need to come from a nearby sun. Astronomers have spotted about 100 starless planets, some possibly formed from gas and dust clouds the way stars form, others probably ejected from their home solar systems (SN: 7/24/17). Computer simulations suggest that there may be as many of these free-floating planets as there are stars in the galaxy.

Such orphaned planets might also have moons — and in 2021, researchers calculated that these moons need not be cold and barren places.

Unless a moon’s orbit is a perfect circle, the gravitational pull of its planet continually deforms it. Resulting friction inside the moon generates heat. In our own solar system, this process plays out on moons such as Saturn’s Enceladus and Jupiter’s Europa (SN: 11/6/17; SN: 8/6/20). A sufficiently thick, heat-trapping atmosphere, likely one dominated by carbon dioxide, might then keep the surface warm enough for water to remain liquid. That water could come from chemical reactions with the carbon dioxide and hydrogen in the atmosphere, initiated by the impact of high-speed charged particles from space.

But such a moon won’t stay warm forever. The same gravitational forces that heat it up also mold its orbit into a circle. Gradually, the ebb and flow of gravity felt by the moon deforms it less and less, and the supply of frictional heat dwindles.

In the new study, Roccetti and her colleagues ran 8,000 computer simulations of a sunlike star with three Jupiter-sized planets. These simulations showed that planets that are ejected from their solar system will often sail off into space with their moons in tow.

The team then ran simulations of those moons, assumed to be the size of Earth, whizzing around their planets along the orbit they ended up with during the ejection. The goal was to see if gravitational heating occurred and if it lasted long enough for life to potentially originate there. Earth may have become habitable within a few hundred million years, although the earliest evidence of living organisms here date to about 1 billion years after the planet formed (SN: 1/26/18).

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Because an atmosphere is crucial to heat retention, the team did their calculations with three alternatives. For moons with an atmosphere the same pressure as Earth’s, the period of potential habitability lasted at most about 50 million years, the team found. But it can last nearly 300 million years if the atmospheric pressure is 10 times that of Earth, and for about 1.6 billion years at pressures 10 times greater still. That amount of pressure may sound extreme, but it’s close to conditions on the similarly sized Venus.

Warmth and water might not be enough to let living organisms appear, though. Moons of free-floating planets “will not be the most favorable places for life to arise,” says astrophysicist Alex Teachey, of the Academia Sinica Institute of Astronomy & Astrophysics in Taipei, Taiwan.

“I think stars, due to their incredible power output and their longevity, are going to be far better sources of energy for life,” says Teachey, who studies the moons of exoplanets. “A big open question … is whether you can even start life in a place like Europa or Enceladus, even if the conditions are right to sustain life, because you don’t have, for example, solar radiation that can help along the process of mutation for evolution.”

But Roccetti — although not an astrobiologist herself — thinks moons of orphan planets have a few  important advantages. They will have some, but not too much, water, which many astrobiologists think is a better starting point for life than, say, an ocean world. And not having a star nearby means there are no solar flares, which in many cases will destroy the atmosphere of an otherwise promising planet.

“There are many environments in our universe which are very different from what we have here on Earth,” she says, “and it is important to investigate all of them.” More

THE WOODLANDS, TEXAS — A young, ultrabright Jupiter may have desiccated its now hellish moon Io. The planet’s bygone brilliance could have also vaporized water on Europa and Ganymede, planetary scientist Carver Bierson reported March 17 at the Lunar and Planetary Science Conference. If true, the findings could help researchers narrow the search for icy exomoons by eliminating unlikely orbits.

Jupiter is among the brightest specks in our night sky. But past studies have indicated that during its infancy, Jupiter was far more luminous. “About 10 thousand times more luminous,” said Bierson, of Arizona State University in Tempe.

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That radiance would have been inescapable for the giant planet’s moons, the largest of which are volcanic Io, ice-shelled Europa, aurora-cowled Ganymede and crater-laden Callisto (SN: 12/22/22, SN: 4/19/22, SN: 3/12/15). The constitutions of these four bodies obey a trend: The more distant the moon from Jupiter, the more ice-rich its body is.

Bierson and his colleagues hypothesized this pattern was a legacy of Jupiter’s past radiance. The team used computers to simulate how an infant Jupiter may have warmed its moons, starting with Io, the closest of the four. During its first few million years, Io’s surface temperature may have exceeded 26° Celsius under Jupiter’s glow, Bierson said. “That’s Earthlike temperatures.”

Any ice present on Io at that time, roughly 4.5 billion years ago, probably would have melted into an ocean. That water would have progressively evaporated into an atmosphere. And that atmosphere, hardly restrained by the moon’s weak gravity, would have readily escaped into space. In just a few million years, Io could have lost as much water as Ganymede may hold today, which may be more than 25 times the amount in Earth’s oceans.

A coruscant Jupiter probably didn’t remove significant amounts of ice from Europa or Ganymede, the researchers found, unless Jupiter was brighter than simulated or the moons orbited closer than they do today.

The findings suggest that icy exomoons probably don’t orbit all that close to massive planets. More

Venus has active volcanism. A new analysis of decades-old images reveals the first definitive sign of a volcano erupting on the hellish planet next door.

NASA’s Magellan spacecraft observed the volcano Maat Mons twice between 1990 and 1992. Sometime in the 243 Earth days between each observation, the volcanic vent appears to have morphed from a 2.2-square-kilometer circle to a 4-square-kilometer blob. That change indicates that an eruption had occurred, researchers report online March 15 in Science and at the Lunar and Planetary Science Conference in The Woodlands, Texas.

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“This world is not quiet, not quiescent, not dead,” says planetary scientist Paul Byrne of Washington University in St. Louis who was not involved in the new work.

Venus is about the same size and mass as Earth so it should have a similar amount of internal heat. And that heat must escape somehow. Scientists have long thought that Venus should be volcanically active. “We’ve just never had something we can point to. And now we do,” Byrne says. He’s also confident that volcanoes on Venus can still erupt now.

“There’s no way you have a planet that big that was doing something 30 years ago and stopped,” he says. “It’s definitely still active today.”

Planetary scientist Robert Herrick spotted the change after painstakingly poring through images of the Venusian regions considered most likely to be volcanically active. “This was a needle-in-a-haystack search with no guarantee that the needle exists,” says Herrick, of the University of Alaska Fairbanks.

Several features in these Magellan radar images look like they’ve changed between the first observation (top) and the second (bottom). But most of those differences occurred because the spacecraft was looking in opposite directions, giving different shading and illumination to the surface. Scientists were able to show that one crater’s apparent differences were due to those imaging differences (Unchanged Vent). Another one (Expanded Vent) was due to real changes on Venus’ surface — probably a volcanic eruption.R.R. Herrick and S. Hensley/Science 2023

Much circumstantial evidence for eruptions on Venus has been reported over the decades (SN: 10/22/10; SN: 6/19/15; SN: 10/18/16). But it has been difficult to tell whether any particular change was due to real geology on the ground, or just a mirage. Many reported differences have turned out just to be due to Magellan’s differing viewing angles over successive orbits around Venus.

“Fundamentally, looking at these images is very hard,” says radar scientist Scott Hensley of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “It’s not like people have not looked [for active volcanism]. People have been looking over the years.”

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Still, the vent’s change in the images alone was not enough to convince Hensley and Herrick that they were seeing evidence for active volcanism. So, Hensley ran more than 100 computer simulations of what Maat Mons would have looked like to Magellan under different imaging conditions. “None of them ever looked like [the 4-square-kilometer blob] on the second cycle,” Hensley says. The change must be real, he concluded.

The volcano’s change in shape suggests that it probably didn’t explosively explode like Washington’s Mount St. Helens did in 1980, Byrne says (SN: 11/1/16). Instead, the eruption was probably more like the long, slow lava drainage from Hawaii’s Kilauea volcano in 2018, only bigger, he says (SN: 1/29/19).

The finding gives scientists an idea of what to expect — and some new ideas for research — when upcoming missions return to Venus (SN: 6/2/21). In the late 2020s or early 2030s, NASA plans to launch VERITAS, a satellite that will map the whole planet from space, and EnVision, which will take high-resolution satellite images of targeted regions.

“The cool part is it means that Venus is volcanically active now. In these upcoming missions, we are going to see things happening,” Herrick said in his March 15 talk. “We already had plans to try and look for new things and changes with time in both of those missions … we now know that that’s a valuable thing to do.”

This work is awe-inspiring, said planetary scientist Darby Dyar of Mount Holyoke College in South Hadley, Ma., who was not involved in the new work. “Everybody in this room should be salivating over the features we’re going to see” in images from future missions. More

Saturn’s moon Enceladus is shrouded in a thick layer of snow. In some places, the downy stuff is 700 meters deep, new research suggests.

“It’s like Buffalo, but worse,” says planetary scientist Emily Martin, referring to the famously snowy city in New York. The snow depth suggests that Enceladus’ dramatic plume may have been more active in the past, Martin and colleagues report in the Mar. 1 Icarus.

Planetary scientists have been fascinated by Enceladus’ geysers, made up of water vapor and other ingredients, since the Cassini spacecraft spotted them in 2005 (SN: 12/16/22). The spray probably comes from a salty ocean beneath an icy shell.

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Some of that water goes to form one of Saturn’s rings (SN: 5/2/06). But most of it falls back onto the moon’s surface as snow, Martin says. Understanding the properties of that snow — its thickness and how dense and compact it is — could help reveal Enceladus’ history, and lay groundwork for future missions to this moon.

“If you’re going to land a robot there, you need to understand what it’s going to be landing into,” says Martin, of the National Air and Space Museum in Washington, D.C.

To figure out how thick Enceladus’ snow is, Martin and colleagues looked to Earth — specifically, Iceland. The island country hosts geological features called pit chains, which are lines of pockmarks in the ground formed when loose rubble such as rocks, ice or snow drains into a crack underneath (SN: 10/23/18). Similar features show up all over the solar system, including Enceladus.

Pit chain craters in Iceland, like those shown here, helped planetary scientist Emily Martin and colleagues verify that they could measure the depth of craters on Enceladus. Martin took this image during a field excursion.E. Martin

Previous work suggested a way to use geometry and the angle at which sunlight hits the surface to measure the depth of the pits. That measurement can then reveal the depth of the material the pits sit in. A few weeks of fieldwork in Iceland in 2017 and 2018 convinced Martin and her colleagues that the same technique would work on Enceladus.

Using images from Cassini, Martin and colleagues found that the snow’s thickness varies across Enceladus’ surface. It is hundreds of meters deep in most places and 700 meters deep at its thickest.

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It’s hard to imagine how all that snow got there, though, Martin says. If the plume’s spray was always what it is today, it would take 4.5 billion years — the entire age of the solar system — to deposit that much snow on the surface. Even then, the snow would have to be especially fluffy.

It seems unlikely that the plume switched on the moment the moon formed and never changed, Martin says. And even if it did, later layers of snow would have compressed the earlier ones, compacting the whole layer and making it much less deep than it is today.

“It makes me think we don’t have 4.5 billion years to do this,” Martin says. Instead, the plume might have been much more active in the past. “We need to do it in a much shorter timeframe. You need to crank up the volume on the plume.”

The technique was clever, says planetary scientist Shannon MacKenzie of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. Without rovers or astronauts on the ground, there’s no way to scoop up the snow and see how far down it goes. “Instead, the authors are very cleverly using geology to be their rovers, to be their shovels.”

MacKenzie was not involved in the new work, but she led a mission concept study for an orbiter and lander that could one day visit Enceladus. One of the major questions in that study was where a lander could safely touch down. “Key to those discussions was, what do we expect the surface to be?” she says. The new paper could help “identify the places that are too fluffy to land in.” More