Enlarge (credit: NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstädt/Thomas Thomopoulos )

Ever since the Voyager mission sent home images of Jupiter's moon Io spewing material into space, we've gradually built up a clearer picture of Io's volcanic activity. It slowly became clear that Io, which is a bit smaller than Mercury, is the most volcanically active body in the Solar System, with all that activity driven by the gravitational strain caused by Jupiter and its three other giant moons. There is so much volcanism that its surface has been completely remodeled, with no signs of impact craters.

A few more details about its violence came to light this week, with new images being released of the moon's features, including an island in a lake of lava, taken by the Juno orbiter. At the same time, imaging done using an Earth-based telescope has provided some indications that this volcanism has been reshaping Io from almost the moment it formed.

Fiery, glassy lakes

The Juno orbiter's mission is primarily focused on studying Jupiter, including the dynamics of its storms and its internal composition. But many of its orbital passes have taken it right past Io, and this week, the Jet Propulsion Laboratory released some of the best images from these flybys. They include a shot of Loki Patera , a lake of lava that has an island within it. Also featured: the impossibly sheer slopes of Io's Steeple Mountain.

Enlarge (credit: NASA/JPL-Caltech/University of Arizona )

Mars has a history of liquid water on its surface, including lakes like the one that used to occupy Jezero Crater , which have long since dried up. Ancient water that carried debris—and melted water ice that presently does the same—were also thought to be the only thing driving the formation of gullies spread throughout the Martian landscape. That view may now change thanks to new results that suggest dry ice can also shape the landscape.

It’s sublime

Previously, scientists were convinced that only liquid water shaped gullies on Mars because that’s what happens on Earth. What was not taken into account was sublimation , or the direct transition of a substance from a solid to a gaseous state. Sublimation is how CO ₂ ice disappears ( sometimes water ice experiences this, too).

Frozen carbon dioxide is everywhere on Mars, including in its gullies. When CO ₂ ice sublimates on one of these gullies, the resulting gas can push debris further down the slope and continue to shape it.

Enlarge / Trying out the robot on a glacier. (credit: NASA/JPL-Caltech )

Icy ocean worlds like Europa or Enceladus are some of the most promising locations for finding extra-terrestrial life in the Solar System because they host liquid water. But to determine if there is something lurking in their alien oceans, we need to get past ice cover that can be dozens of kilometers thick. Any robots we send through the ice would have to do most of the job on their own because communication with these moons takes as much as 155 minutes.

Researchers working on NASA/JPL’s technology development project called Exobiology Extant Life Surveyor (EELS) might have a solution to both those problems. It involves using an AI-guided space snake robot. And they actually built one.

Geysers on Enceladus

The most popular idea to get through the ice sheet on Enceladus or Europa so far has been thermal drilling, a technique used for researching glaciers on Earth. It involves a hot drill that simply melts its way through the ice. “Lots of people work on different thermal drilling approaches, but they all have a challenge of sediment accumulation, which impacts the amount of energy needed to make significant progress through the ice sheet,” says Matthew Glinder, the hardware lead of the EELS project.

Enlarge / Artist's conceptions of what the surfaces of two dwarf planets might look like. (credit: SWRI )

Active geology—and the large-scale chemistry it can drive—requires significant amounts of heat. Dwarf planets near the far edges of the Solar System, like Pluto and other Kuiper belt objects, formed from frigid, icy materials and have generally never transited close enough to the sun to warm up considerably. Any heat left over from their formation was likely long since lost to space.

Yet Pluto turned out to be a world rich in geological features, some of which implied ongoing resurfacing of the dwarf planet's surface. Last week, researchers reported that the same might be true for other dwarf planets in the Kuiper Belt. Indications come thanks to the capabilities of the Webb telescope, which was able to resolve differences in the hydrogen isotopes found on the chemicals that populate the surface of Eris and Makemake.

Cold and distant

Kuiper Belt objects are natives of the distant Solar System, forming far enough from the warmth of the Sun that many materials that are gasses in the inner planets—things like nitrogen, methane, and carbon dioxide—are solid ices. Many of these bodies formed far enough from the gravitational influence of the eight major planets that they have never made a trip into the warmer inner Solar System. In addition, because there was much less material that far from the Sun, most of the bodies are quite small.

Enlarge / Ridges and basins in the Eridania Basin on Mars. (credit: NASA/JPL-Caltech/University of Arizona )

Early in Earth's history, the heat left over from the collision that formed the Moon left its surface an ocean of magma. As it cooled, its crust was frequently shattered by massive impacts that dwarfed the one that did in the dinosaurs. Somewhere in between that and the onset of plate tectonics, it's thought that a distinct process caused parts of the crust to sink, while volcanism brought material to the surface that would later form the continental crust.

While we can model this period, we can't really search for evidence to back our models, since any of this early crust has been eroded or transformed by the plate tectonics that eventually ensued. However, a team of researchers is suggesting that there might be a way to see what this process looked like, and it doesn't involve a time machine. Instead, it involves studying the surface of Mars.

Lots of volcanoes

On Mars, plate tectonics never got going. So, while some areas of the planet have been transformed in a way that keeps us from studying the earliest periods of Mars' history—looking at you, Olympus Mons —scientific consensus is that nearly half of the planet's surface is over 3.6 billion years old. This provides the opportunity to study processes that occurred in the first half-billion or so years after Mars formed.

Enlarge / That is actually a moon. (credit: NASA/JPL/Space Science Institute )

The once-exclusive club of Solar System objects that host oceans is getting increasingly crowded. On Wednesday, Nature released a paper providing evidence that Saturn's moon Mimas has a subsurface ocean beneath its heavily cratered crust. The evidence for this ocean comes in the form of orbital oddities that are seemingly impossible to explain by anything other than the presence of an ocean.

Solid looks

Of Saturn's seven major moons, Mimas orbits closest to the planet, taking less than a day to complete an orbit. It's also the smallest of the major moons, with a diameter of just under 400 kilometers (about 250 miles). Despite its diminutive size, Mimas hosts the second-largest crater on any moon in the Solar System. The Herschel Crater dominates the surface of the moon, giving it an appearance that evokes the Death Star.

Even outside of Herschel, the moon's surface is heavily pocked by craters, suggesting it has been static for most of the moon's history. That's in sharp contrast to moons like Europa and Enceladus, where the subsurface oceans allow the constant remodeling of their surfaces, leaving them with much sparser crater histories. So Mimas seemed like a very poor candidate for hosting an ocean.

Enlarge / Each panel shows the modeled effects of early Earth’s bombardment. Circles show the regions affected by each impact, with diameters corresponding to the final size of craters for impactors smaller than 100 kilometers in diameter. For larger impactors, the circle size corresponds to size of the region buried by impact-generated melt. Color coding indicates the timing of the impacts. The smallest impactors considered in this model have a diameter of 15 kilometers. (credit: Simone Marchi, Southwest Research Institute)

When it comes to space rocks slamming into Earth, two stand out. There’s the one that killed the dinosaurs 65 million years ago (goodbye T-rex, hello mammals!) and the one that formed Earth’s Moon . The asteroid that hurtled into the Yucatan peninsula and decimated the dinosaurs was a mere 10 kilometers in diameter. The impactor that formed the Moon, on the other hand, may have been about the size of Mars. But between the gigantic lunar-forming impact and the comparatively diminutive harbinger of dinosaurian death, Earth was certainly battered by other bodies.

At the 2023 Fall Meeting of the American Geophysical Union, scientists discussed what they’ve found when it comes to just how our planet has been shaped by asteroids that impacted the early Earth, causing everything from voluminous melts that covered swaths of the surface to ancient tsunamis that tore across the globe .

Modeling melt

When the Moon-forming impactor smashed into Earth, much of the world became a sea of melted rock called a magma ocean ( if it wasn’t already melted ). After this point, Earth had no more major additions of mass, said Simone Marchi , a planetary scientist at the Southwest Research Institute who creates computer models of the early Solar System and its planetary bodies, including Earth. “But you still have this debris flying about,” he said. This later phase of accretion may have lacked another lunar-scale impact, but likely featured large incoming asteroids. Predictions of the size and frequency distributions of this space flotsam indicate “that there has to be a substantial number of objects larger than, say, 1,000 kilometers in diameter,” Marchi said.

Enlarge / Image of a planet-forming disk, with gaps in between higher-density areas. (credit: ALMA(ESO/NAOJ/NRAO); C. Brogan, B. Saxton )

Where do planets come from? The entire process can get complicated. Planetary embryos sometimes run into obstacles to growth that leave them as asteroids or naked planetary cores. But at least one question about planetary formation has finally been answered—how they get their water.

For decades, planetary formation theories kept suggesting that planets receive water from ice-covered fragments of rock that form in the frigid outer reaches of protoplanetary disks, where light and heat from the emerging system’s star lacks the intensity to melt the ice. As friction from the gas and dust of the disk moves these pebbles inward toward the star, they bring water and other ices to planets after crossing the snow line, where things warm up enough that the ice sublimates and releases huge amounts of water vapor. This was all hypothesized until now.

NASA’s James Webb Telescope has now observed groundbreaking evidence of these ideas as it imaged four young protoplanetary disks.The telescope used its Medium-Resolution Spectrometer (MRS) of Webb’s Mid-Infrared Instrument (MIRI) to gather this data, because it is especially sensitive to water vapor. Webb found that in two of these disks, massive amounts of cold water vapor appeared past the snow line, confirming that ice sublimating from frozen pebbles can indeed deliver water to planets like ours.

Enlarge (credit: USGS )

With Europa and Enceladus getting most of the attention for their subsurface oceans and potential to host life, other frozen worlds have been left in the shadows—but the mysterious Jovian moon Ganymede is now making headlines.

While Ganymede hasn’t yet been observed spewing plumes of water vapor like Saturn’s moon Enceladus, Jupiter’s largest moon is most likely hiding an enormous saltwater ocean. Hubble observations suggest that the ocean—thought to sit under 150 km (95 miles) of ice—could be up to 100 km (60 miles) deep. That’s ten times deeper than the ocean on Earth.

Ganymede is having a moment because NASA’s Juno mission observed salts and organic compounds on its surface, possibly from an ocean that lies beneath its crust of ice. While Juno’s observations can't provide decisive evidence that this moon has an ocean that makes Earth look like a kiddie pool, the Juno findings are the strongest evidence yet of salts and other chemicals making it to the exterior of Ganymede.