Introduction
Planetary geology has advanced rapidly over the past decade, with new missions and studies unveiling dynamic processes on other worlds. From the rocky inner planets (Mercury, Venus, Mars) to the icy moons of the outer solar system, scientists have catalogued volcanism, tectonics, sedimentation, and other processes shaping these bodies. Notably, evidence now shows that several “dead” worlds are geologically active today. This brief highlights major findings from 2015–2025 – emphasizing peer-reviewed results and official mission data – and extracts key numbers (with uncertainties) and expert quotes from the literature. We cover Mercury’s contraction and quakes, Venus’s ongoing volcanism, Mars’s ancient lakes and present quakes, the volcanic eruptions of Jupiter’s moon Io, the icy tectonics of Europa, the hydrocarbon landscapes of Saturn’s moon Titan, the geysers of Enceladus, and Pluto’s surprising cryovolcanism.
Mercury: A One-Plate Planet Still on the Move
Mercury was long thought to be a relic of early solar system geology, but NASA’s MESSENGER orbiter (2011–2015) revealed a surprisingly active past – and hints of activity even today. Mercury’s single rigid lithospheric plate has been contracting as its iron core cools, producing giant thrust-fault scarps (cliffs) across the globe[1][2]. In MESSENGER’s final low-altitude imaging campaign, scientists discovered small, young fault scarps only tens of meters high and a few kilometers long. These tiny cliffs are so pristine that they must be geologically recent – likely formed within the last 50 million years (or even more recently) because constant meteoroid bombardment would have obliterated older features[3][4]. As one team member explained, “Steady meteoroid bombardment quickly degrades and destroys structures this small, indicating that they must have formed relatively recently”[4]. Their youthful age strongly implies that Mercury’s interior is still cooling and contracting, making it “tectonically active” today[5]. In fact, a 2016 study led by Thomas Watters reported Mercury joins Earth as a tectonically active planet, with new faults likely forming “today as Mercury’s interior continues to cool”[5]. This overturns the old notion that Earth was the sole tectonically active planet in our solar system.
Mercuryquakes: If Mercury’s faults are slipping today, there may be ongoing “Mercury-quakes.” By analogy to shallow moonquakes on our Moon (which reached up to magnitude ~5), scientists expect slip on Mercury’s faults could produce quakes of similar magnitude ~5 on the Richter scale[6]. No seismometer has visited Mercury yet, but future missions could test this prediction[7]. The presence of an active core is corroborated by Mercury’s enduring magnetic field, which has persisted for billions of years – suggesting a slowly cooling, partially liquid outer core[8]. All these lines of evidence (young scarps, a longstanding magnetic field, possible quakes) point to long-lived internal heat driving prolonged contraction tectonics on Mercury[8].
Ancient Volcanism and Global Map: MESSENGER also confirmed that volcanism shaped Mercury’s surface, especially in its first 2 billion years. The planet is covered in smooth volcanic plains (solidified lava seas) that flooded impact basins eons ago. For example, the northern volcanic plains cover ~6% of Mercury’s surface. These lavas and older intercrater plains allow construction of Mercury’s first global geological map[9][10]. Using >100,000 images, USGS cartographers in 2016 produced a global map showing Mercury’s topography varies by ~10 kilometers (from ≈ –5,380 m in lowlands to +4,480 m at highest terrains)[11]. The largest volcanic basin, Caloris, spans 1,550 km and is filled with smooth plains that were later buckled by tectonic stresses[12]. MESSENGER found evidence for pyroclastic volcanism as well: over 100 sites of explosive ash eruptions sourced from vents, indicating Mercury’s interior once held volatile gases[13]. Although Mercury’s volcanism mostly ended billions of years ago, these deposits and “hollows” (strange shallow depressions likely formed by sublimation of volatile-rich material) show a planet more volatile-rich than expected[13]. Even today, water ice is preserved in permanently shadowed polar craters; MESSENGER’s neutron spectrometer confirmed substantial water ice at Mercury’s poles beneath an insulating layer of organic-rich dust[1][2].
In summary, Mercury – often dubbed a “dead” planet – is geologically dynamic in its own style. Its single giant plate is crisscrossed by faults from global contraction. It had extensive ancient volcanism, and possibly still experiences the odd quake as it shrinks. As one researcher remarked, finding tiny fresh scarps on Mercury “is like finding a sapling of a tree thought to be long extinct”[14] – a sign that beneath its ancient surface, Mercury’s geological engine has not entirely cooled.
Venus: Ongoing Volcanism and “Pack-Ice” Tectonics
Venus – similar to Earth in size – has an extremely different geology, characterized by a stagnant lithosphere (no Earth-style plate tectonics) and a surface reshaped by volcanism. A longstanding puzzle was whether Venus is still volcanically active today or whether its volcanoes died out eons ago. Over the last decade, multiple studies have clinched the case that Venus is active now.
Active Lava Flows Detected: In 2015, scientists using ESA’s Venus Express orbiter spotted transient infrared hotspots on the surface[15][16]. These appeared in the rift zone of Ganiki Chasma (near volcanoes Ozza Mons and Maat Mons) and were observed to brighten and fade over just a few days, consistent with fresh lava erupting and cooling[17][18]. Four such events were recorded. One hotspot (dubbed “Object A”) was about 1 km² in size with a temperature of ~830 °C – far above the normal ~480 °C surface temperature[19]. Given the blurry view (Venus Express peered through clouds from orbit), each hotspot appeared smeared over >100 km, but the actual hot area on the ground was likely very small (∼1 km across)[20][19]. “We have now seen several events where a spot on the surface suddenly gets much hotter, and then cools down again,” reported Eugene Shalygin, lead author of the 2015 study, calling it “the most tantalising evidence yet for active volcanism” on Venus[16][21]. These observations put Venus into the exclusive club of currently volcanically active Solar System bodies (joining Earth and Jupiter’s moon Io)[22].
The evidence grew even stronger in 2023 when researchers re-analyzed 1990s Magellan orbiter radar images and discovered direct changes on a volcano. They found that a volcanic vent on Maat Mons (Venus’s tallest volcano) had visibly enlarged and changed shape over an 8-month span in 1991[23][24]. The vent expanded from an area of ~2.2 km² to a irregular crater ~4 km² and showed signs of fresh lava flows nearby[23][25]. This indicates an eruption occurred on Venus in 1991, which Magellan fortuitously captured before and after. The 2023 study authors noted this is “the first direct evidence of a recent volcanic eruption on [Venus]”, since the vent’s crater was filled with new molten rock and spill-over down its slopes in the later image[25]. Furthermore, in 2024 an Italian-led team announced they found two more volcanic flow deposits by comparing Magellan radar data from 1990 vs 1992[26][27]. These flows, at Sif Mons and western Niobe Planitia, showed increased radar reflectivity consistent with new solidified lava that wasn’t there two years prior[28]. Taken together, these findings suggest that volcanic eruptions on Venus are relatively frequent, not one-off events. “By analyzing the lava flows we observed, we have discovered that the volcanic activity on Venus could be comparable to that on Earth,” said Dr. Davide Sulcanese, lead author of the 2024 study[29][30]. In other words, Venus may have eruptions occurring every few years or decades, maintaining a youthful surface in places.
Tectonics without Plates: Unlike Earth, Venus does not appear to have moving crustal plates or global subduction zones. Its enormous pressure-cooker atmosphere (90 bar) and hot, dry climate have resulted in a single stagnant lithospheric shell. However, recent research shows that Venus’s crust is not entirely immobile – instead, it may be broken into large blocks that jostle and rotate, analogous to pack ice on a frozen ocean[31][32]. In 2021, Paul Byrne and colleagues identified bands of ridges and troughs outlining flat blocks in Venus’s lowland regions. They concluded these blocks have “shifted, rotated and slid past each other over time… a little like Earth’s plate tectonics but on a smaller scale, more like chunks of pack ice” floating on a fluid[32][33]. This “pack-ice tectonics” on Venus likely arises from mantle convection beneath a thin brittle crust, causing the surface to fragment without full-fledged plate boundaries[34]. It’s a newly recognized tectonic style – unique to Venus – suggesting that its interior is still vigorously churning. “It’s not plate tectonics like on Earth…but it is evidence of deformation due to interior mantle flow,” Byrne noted, “a style of interior–surface coupling not seen elsewhere…except for [some] continental interiors on Earth”[35][36]. The discovery of this tectonic mechanism implies that Venus’s mantle is actively overturning and can periodically repave the surface. Indeed, much of Venus’s surface appears young (<20% covered in impact craters), hinting it was globally resurfaced by volcanism and tectonic deformation ~0.5 billion years ago – possibly in a catastrophic overturn event[37][38]. Now, we know localized activity persists today.
Geologic Features and Missions: Venus is dotted with >1,600 major volcanoes (mostly extinct) and immense circular tectonic structures called coronae (likely formed by mantle plumes). Some lava flows on Venus are extremely long – one extends over 7,000 km across the plains. Notably, Venus Express observed a sharp rise in atmospheric SO₂ in 2006–07 that some attributed to eruptive outgassing[39]. And Magellan’s radar mapped young lava flows that seem to cover impact craters, confirming volcanism has occurred within the last few million years[40][41]. Now that active lava has been directly observed, scientists are eager to return. NASA and ESA have approved new missions (VERITAS, DAVINCI, EnVision) in the 2030s to map Venus at higher resolution and even sample the atmosphere for volcanic gases. These will build on the 2015–2025 breakthroughs. As Håkan Svedhem, project scientist of Venus Express, remarked: “It looks like we can finally include Venus in the small club of volcanically active Solar System bodies…Venus, our nearest neighbor, is still active and changing in the present day”[22][42].
Mars: Ancient Lakes, Modern Quakes, and Recent Eruptions
Mars has been the focus of intensive geologic exploration in the past decade, thanks to rovers (Curiosity, Perseverance, Zhurong), landers (InSight), and orbiters (from NASA, ESA, ISRO, UAE, and China). The picture emerging is of a planet that was Earth-like in its youth – with lakes, rivers, volcanism, and possibly an ocean – but became cold and arid, though not completely geologically dead. Key processes on Mars include sedimentary deposition by water and wind, volcanism that spans billions of years, tectonic fracturing from crustal stresses, and even present-day seismic and hydrologic activity.
Sedimentary Records of Water: NASA’s Curiosity rover in Gale Crater has uncovered a rich stratigraphic record of an ancient lake system ~3.5–3.8 billion years old. Curiosity found finely layered mudstone rocks that were deposited in a calm freshwater lake that filled Gale’s 150-km wide basin[43][44]. These lake sediments show patterns (such as fine laminations, and even desiccation cracks nicknamed “Old Soaker”) indicating the lake experienced wetting and drying cycles over millions of years[43][45]. “Imagine ponds dotting the floor of Gale Crater…streams might have laced the crater’s walls…overflow then dry up, a cycle that probably repeated itself numerous times over millions of years,” wrote Curiosity scientists[43][46]. In 2019, a Nature Geoscience study (lead author William Rapin) reported mineral salt deposits (sulfates and chlorides) in a 150 m tall section of strata called Sutton Island, evidence that shallow briny ponds underwent evaporation in Mars’s ancient climate[47][48]. The salts – mixed with sediments rather than pure evaporite crusts – suggest ephemeral salty lakes that “went through episodes of overflow and drying”, leaving a “watermark” of climate fluctuations[49][48]. These rocks record Mars’s transition from a wetter environment to the freezing desert of today[49]. In fact, Curiosity’s detection of mud cracks in >3 billion-year-old rocks is direct proof that standing water once dried up at the surface, implying habitable conditions (with liquid water) that lasted but also changed over time[50][51]. As mission scientist A. Vasavada said, Gale Crater offers “a unique record of a changing Mars…when and how the planet’s climate started evolving”, and how long surface life could have persisted[51].
At Mars’s Jezero Crater, Perseverance rover has been exploring an ancient river delta since 2021. High-resolution images confirmed that Jezero’s lake was fed by a river that deposited layered deltaic sediments, including coarse boulders (up to meter-scale) high in the outcrops – evidence for episodic flash floods in Mars’s past[52][53]. The presence of these large, rounded rocks within ancient delta layers indicates fast-flowing water at times, followed by calmer periods that allowed fine sediment to settle. Perseverance’s exploration of the delta in 2022 revealed cross-bedded sandstones and inclined strata diagnostic of a meandering river-delta environment. These observations further underscore that Mars had sustained surface water and active hydrologic cycles in its early history, around 3.7 billion years ago. Moreover, Perseverance made a surprising igneous discovery: the floor of Jezero Crater (beneath the delta) is composed of volcanic rocks – cooled lava – rather than the expected lake sediments[54][55]. Instruments detected abundant olivine crystals in these rocks, indicating they solidified from slowly cooling magma, then later contacted water to form altered minerals[56][57]. This implies that after the impact that formed Jezero, lava flowed or intruded into the crater floor. These igneous rocks are easier to date than sedimentary rocks, so when samples are returned to Earth (planned ~2033) they should pin down the timing of Jezero’s volcanic and lake phases[58][59]. The coexistence of water alterations in these volcanic rocks is exciting: “Discovering the potential for habitable environments in something as uninhabitable as Jezero’s aged lava flows raises hopes for what lies in the sedimentary rocks” still to be sampled[59][60].
Beyond craters, orbiters have mapped vast deposits of hydrated minerals (clays, sulfates) across Mars, marking where long-lived groundwater or surface water once existed. For instance, ESA’s Mars Express found evidence of a possible subsurface liquid water lake today under the south polar ice cap. Radar soundings (MARSIS instrument) in 2018 detected an “anomalously bright subsurface reflection” under 1.5 km of ice, spanning a 20 km zone near 81°S[61][62]. The best interpretation is a layer of briny liquid water about 20 × 30 km in extent beneath the ice[62]. Follow-up work in 2020 identified three more buried water “ponds” near the main lake[63][64]. The water must be extremely salty (rich in perchlorates or other antifreeze salts) to remain liquid at ~–68 °C temperatures, but this finding raises the possibility of a network of ancient subglacial lakes on Mars[65]. The largest lake is estimated to measure 20 × 30 km and could be part of an entire hidden hydrologic system millions or billions of years old[63][66]. Whether these radar reflections are indeed liquid water or possibly some unusual hydrated clays remains debated by scientists (some 2022 studies suggested clays or layered ice could mimic the signal). If it is water, it would be a prime target in the search for extant Martian life (analogous to subglacial lakes in Antarctica which harbor microbial ecosystems)[67].
Volcanism from Past to Present: Mars is home to the largest volcano in the solar system (Olympus Mons, ~21 km high) and enormous volcanic provinces (Tharsis and Elysium). Most Martian volcanism occurred 3–4 billion years ago, building shield volcanoes and flood lavas. However, a significant question has been whether Mars’s volcanoes could still sputter to life in the recent geologic past. Evidence now suggests yes – Mars may still be volcanically active on a modest scale. In late 2020, researchers from University of Arizona and PSI identified a previously unknown volcanic deposit on Mars that may be only ~50,000 years old[68][69]. This deposit, located in Elysium Planitia at a fissure called Cerberus Fossae, appears as a smooth, dark mantle covering about 8 miles (13 km) across the surrounding plains[70][71]. It overlies the local lava flows (meaning it is younger) and has a morphology resembling a pyroclastic eruption blanket – ash and pumice spread by an explosive volcanic burst[72][73]. Scientists interpret it as the product of a teenage volcano in Martian terms: “This may be the youngest volcanic deposit yet documented on Mars,” said lead author David Horvath[74]. If Mars’s 4.5 billion year geologic history were compressed into a single day, “this would have occurred in the very last second,” Horvath noted[75]. The deposit likely formed when subsurface magma interacted with ice or groundwater, causing a gas-driven eruption (similar to opening a shaken soda can)[76][73]. The explosion spread ash up to ~6 miles (10 km) high into Mars’s sky[77]. What makes this finding especially intriguing is that it lies only ~1,600 km from NASA’s InSight lander, which in 2019–2022 detected marsquakes originating from the Cerberus Fossae region[78][79]. Two of the largest quakes (magnitude 4.1) came from this area. This spatial link suggests that magma movement could be causing the seismic activity, and that pockets of molten rock persist under Elysium Planitia[78]. As Horvath et al. wrote, “The young age of this deposit absolutely raises the possibility that there could still be volcanic activity on Mars,” and it is “intriguing that recent Marsquakes detected by InSight are sourced from the Cerberus Fossae”[78][80]. In sum, Mars might not be done erupting – at least in isolated locales where internal heat lingers.
Mars also experienced truly colossal volcanic eruptions in its deep past. A 2021 study of the Arabia Terra region (by P. Whelley et al.) found evidence for thousands of ancient “super-eruptions” around 4 billion years ago[81][82]. These explosions (akin to terrestrial supervolcanoes) would have vented magma explosively, blanketing Mars in ash and likely affecting climate[81][82]. Seven large depressions in Arabia Terra were identified as calderas, not impact craters – each tens of miles wide – formed by the collapse of ground after massive eruptions[83]. Researchers estimated that each super-eruption expelled on the order of 400 million Olympic swimming pools’ worth of magma and gas (i.e. hundreds to thousands of cubic kilometers)[84]. These occurred over a 500-million-year period[81], meaning Mars was extremely volcanically active in its first billion years. Such findings underscore that Mars has seen the full spectrum of volcanism – from ancient supervolcanoes to possibly a tiny fire fountaining just 50 millennia ago.
Tectonics and Marsquakes: Unlike Earth, Mars has no plate tectonics today – its crust is one continuous plate. But it does have many tectonic features: the great Valles Marineris canyon system (4,000 km long, formed by crustal rifting), vast networks of faults and wrinkle ridges from planetary cooling, and impact-induced fractures. Until recently, these were all relics of the past. That changed with InSight (landed 2018), which deployed the first seismometer on Mars. InSight listened for Marsquakes for four years and detected 1,319 quakes in total[85][86]. Most were small (magnitude 2–3), but a few were in the M4+ range, including a magnitude 4.7 quake in May 2022 – the largest ever recorded on another planet. InSight’s data allowed scientists to map Mars’s interior. Seismic waves revealed that the crust at the landing site is either ~20 km thick (if two layers) or ~39 km thick (if three layers)[87][88]. Global gravity data combined with this “anchor” point suggest Mars’s crust averages 24–72 km thick (likely toward the lower end of that range)[89]. Beneath the crust, Mars has a mantle that extends ~1,500 km down and, unlike Earth’s, does not differentiate into lower/upper mantle (it’s more uniform, similar in composition to Earth’s olivine-rich upper mantle)[90][91]. In 2021, InSight scientists announced they had determined the size of Mars’s core: approximately 3,660 ± 40 km in diameter (about 1,830 km radius)[92]. This is larger and less dense than previously thought, implying a liquid iron-nickel core that is partially alloyed with lighter elements like sulfur, carbon, oxygen, and perhaps hydrogen[93]. The core is fully molten (no solid inner core was initially detected, though a 2023 analysis suggests Mars might have a small solid inner core ~500 km across)[94]. Mars’s core being proportionally big (half the planet’s diameter) means the mantle above is relatively thinner, and the lithosphere (rigid outer layer) is very thick – on the order of 500 km thick[95][96]. Such a thick lithosphere explains why Mars lacks plate tectonics; the crust and upper mantle are too stiff to break into moving plates. Yet, Mars is still tectonically active in a minor way: stress from ongoing cooling/contraction and from loading of volcanoes causes occasional quakes and fault movements. For example, the Cerberus Fossae graben where quakes originate might be slowly opening or slipping due to volcanic or tectonic forces. Additionally, the entire Mars is encircled by wrinkle ridges (low fault-related ridges in volcanic plains) that indicate global contraction (like a raisin’s skin wrinkling) – but far less pronounced than Mercury’s.
Present Water Seeps? A tantalizing possible geologic activity on modern Mars is the occurrence of seasonal slope flows known as RSL (Recurring Slope Lineae). These are dark streaks that appear on steep warm slopes in summertime, lengthen downhill, then fade in colder seasons. In 2015, using Mars Reconnaissance Orbiter data, scientists discovered that RSL sites show hydrated salt minerals (perchlorates) when the streaks are largest, but not in colder dry times[97][98]. This implies that briny liquid water plays a role – likely melting just enough to wet the soil and dissolve salts, darkening the surface[99][100]. “The detection of hydrated salts on these slopes means that water plays a vital role in the formation of these streaks,” said Lujendra Ojha, lead author of the 2015 study[101][102]. The salts (magnesium perchlorate, chlorate, etc.) can lower water’s freezing point, allowing brines to flow transiently at –20 °C[99][100]. Mars thus might have very small-scale contemporary water activity – not true “streams,” but damp soil that could be due to subsurface ice melt, deliquescence of hygroscopic salts, or shallow aquifers. However, later research in 2017–2020 suggested many RSL might actually be dry sand flows, with the salt hydration being a coincident effect. The jury is still out. If liquid water does briefly flow, it likely evaporates quickly in Mars’s thin air. Nevertheless, RSL represent Mars’s marginal hydrological activity today – and any transient liquid is important for assessing present-day habitability (albeit the brines would be extremely salty and short-lived).
Summary: Mars emerges as a world with a rich geologic past of water and fire, and active present in subtle ways. It had lakes, deltas, hot springs, and supervolcanoes in antiquity, and now it has dust storms, seasonal frost and thaw cycles, occasional marsquakes, and possibly the odd volcanic burp or briny trickle. In the words of planetary geologist Alfred McEwen, “Mars is not dead, but it is resting” – with just enough rumblings to keep scientists busy. As we continue to study Mars, especially with sample return on the horizon, we expect more surprises about its geologic timeline and internal activity.
Jupiter’s Moons: Io’s Infernos and Europa’s Hidden Ocean
Jupiter’s Galilean moons present some of the most extreme and intriguing geology in the solar system. Io is a hellscape of constant volcanism, while Europa is an ice-covered ocean world with tectonic ice crust. Ganymede and Callisto are less active but still have stories (Ganymede has its own magnetic field and likely a deep ocean). Here we focus on Io and Europa – exemplars of volcanic vs. icy geological processes – with a nod to recent discoveries.
Volcanic Io – Extreme and Ongoing: Io is the most volcanically active body in the solar system, a true alien volcanic world. Heated by intense tidal forces from Jupiter, Io’s interior melts and produces hundreds of volcanoes on its surface. Gigantic calderas, lava lakes, and sulfurous lava flows 200–300 km long have been observed by past missions. Before 2015, we knew Io was active (Voyager saw eruptions in 1979, Galileo mapped >100 active vents). But recent observations by NASA’s Juno mission have revealed record-breaking eruptions on Io. In late 2024, during a distant flyby, Juno’s infrared mapper (JIRAM) detected an eruption so powerful it saturated the detector[103]. This event, at Io’s south pole, was an enormous volcanic hot spot radiating >80 trillion watts of thermal power[104][105]. To put that in perspective, that is more than five times Earth’s total human power consumption. The lava hot area spanned an estimated 100,000 km² – roughly the size of Iceland – making it the most intense thermal emission ever recorded on Io (far exceeding Io’s previous champ, Loki Patera)[104][106]. “This is the most powerful volcanic event ever recorded on the most volcanic world in our solar system – that’s really saying something,” said Dr. Scott Bolton, Juno’s Principal Investigator[107][108]. Juno’s camera even noted visible surface changes near Io’s south pole between images taken in October and December 2024, likely from fresh deposits[109]. Scientists suspect the eruption came from a complex of fissures tapping a subsurface magma reservoir, possibly analogous to a flood basalt eruption on Earth[103][110]. The eruption’s scale – “a fiery region larger than Earth’s Lake Superior” and releasing “six times the total energy output of all the world’s power plants” – underscores the ferocity of Io’s tidal heating[104][111]. Io’s volcanism is so vigorous that it continuously resurfaces the moon, burying impact craters (indeed, Io has essentially no impact craters; its surface is geologically young at <1 million years old on average). Juno’s continuing mission plans close Io flybys in 2025–2026, which should yield even sharper data on these volcanic features and perhaps catch eruptions in real time.
Io’s most famous volcano, Loki Patera, is a 200 km wide lava lake that periodically brightens as the crust overturns. Ground-based observations (2010s) found Loki’s activity to be quasi-periodic, with eruptions roughly every 500 days[112]. But Io has dozens of active plumes at any given time. These plumes, composed of sulfur dioxide gas and ash, can shoot 300–500 km above Io’s surface and were first seen as umbrella-like blue clouds by Voyager. Juno in early 2024 even imaged two volcanic plumes above Io’s limb during a flyby[113]. With over 400 active volcanoes identified, Io is effectively a tidal pumping machine converting Jupiter’s gravitational energy into magma. Lava temperatures on Io have been measured up to ~1,600 °C – hotter than most Earthly lavas – suggesting ultramafic (magnesium-rich) compositions like ancient komatiites on Earth. Io’s volcanism offers a window into early Earth-like magmatism and extreme tidal heating processes.
Europa – Tectonic Ice Shell and Subsurface Ocean: Europa, Jupiter’s ice-crusted moon, has captivated geologists and astrobiologists alike. Although it is slightly smaller than our Moon, Europa likely harbors a global saltwater ocean beneath its icy crust – an ocean that might be a habitat for life. Between 2015 and 2025, researchers have made strides in understanding how Europa’s ice shell behaves and how the subsurface ocean reveals itself at the surface.
One highlight is the 2022 discovery that Europa’s ubiquitous double ridges (long parallel ridges with a central trough, sometimes stretching thousands of kilometers) likely form from subsurface water near the surface. A study by Riley Culberg and colleagues noted the uncanny resemblance between Europa’s double ridges and a smaller double ridge seen in Greenland’s ice sheet on Earth[114][115]. Radar data from Greenland showed that an subsurface pocket of water had refrozen and pushed up the ice, creating a double ridge cross-section like a letter “M”[116]. Applying this to Europa, it appears that water lenses within the ice (perhaps fed by the ocean or melt from below) could refreeze and crack the surface to form the ridges[115][117]. If so, it means Europa’s ice shell is permeated by pockets of liquid water at shallow depths (maybe ~1–3 km beneath the surface)[118]. These could be formed by brine migration or melt from tidal flexing. Importantly, such pockets would bring ocean chemistry closer to the surface and could be targets for future landers (much easier to sample than drilling through tens of kilometers of ice)[117][119]. Culberg noted, “The presence of liquid water in the ice shell would suggest that exchange between the ocean and ice shell is common,” which could cycle nutrients and energy for life[120][121]. And “shallow water means there might be easier targets for future missions to image or sample that could preserve evidence of life” without needing to fully access the deep ocean[120][121]. In short, Europa’s crust might not be a solid block of ice – it could be a dynamic, layered ice-water system with active geology. This adds to earlier evidence of mobile ice: chaos terrains (jumbled iceberg-like blocks in frozen matrix) likely indicate areas where the ice shell partially melted or collapsed into subsurface water lenses[122] (a model proposed in 2011 by Schmidt et al.).
Europa’s surface geology is youthful (<~60 Myr old on average) and criss-crossed by fractures and ridges from tidal stresses. These features are thought to penetrate through much of the 20–30 km thick ice shell, acting as conduits between the ocean and surface. Indeed, since 2012, the Hubble Space Telescope has reported intermittent detections of water vapor plumes at Europa’s south polar region, rising perhaps 100–200 km high[123]. In 2018, scientists went back to Galileo spacecraft data from 1997 and found tantalizing in-situ evidence that Galileo flew through a plume – its magnetometer and plasma sensors recorded a perturbation consistent with charged particles from a water plume during the close Europa flyby E12[124][125]. This “old data, new evidence” result strongly suggests Europa is actively venting small amounts of its ocean into space on occasion[126][127]. (The vapor may erupt through cracks when tidal stresses are greatest.) If these plumes are real and contain ocean material, they offer a way for spacecraft to sample Europa’s ocean indirectly by flying through the plume – which NASA’s upcoming Europa Clipper mission (launch 2024) aims to do if possible. In any case, Europa’s ocean is confirmed by multiple lines (magnetic induction, density, surface geology) and is estimated to be ~100 km deep beneath the ice, containing 2–3× as much water as Earth’s oceans[128][129]. This ocean is likely in contact with a rocky seafloor, potentially enabling hydrothermal vents (like Enceladus has). If there is anywhere beyond Earth that might currently host life, Europa’s ocean – kept liquid by tidal heating, enriched by rock-water chemistry, and possibly exchanging with the surface – is a top contender.
Europa’s ice tectonics also intrigue geologists. There is evidence one patch of crust was once displaced and rotated by 80° (“plated” onto a new position), hinting at plate-like behavior in Europa’s past. However, unlike Earth’s plates, Europa’s may be episodic and driven from below by ocean or warm ice upwellings, since the cold surface is brittle. Europa lacks high topography (it’s relatively flat, with few areas more than a few hundred meters in relief), likely because the icy crust can flow over geologic time. Only a few impact craters are seen, the largest just ~50 km across, emphasizing the youthful surface recycling. Additionally, surface chemistry shows sulfate salts and possibly organic compounds deposited from the ocean or from radiation processing of ejected ocean water – another clue of subsurface-ocean-to-surface communication.
Looking ahead, Europa Clipper will orbit Jupiter and make ~50 close flybys of Europa in the late 2020s, carrying ice-penetrating radar (to detect water pockets), spectrometers, high-res cameras, and a magnetometer to measure the ocean’s properties. ESA’s JUICE mission (launched 2023) will also fly by Europa twice in 2030. Together, these will greatly expand on the 2015–2025 findings and likely paint a clearer picture of Europa’s geology and habitability. As of mid-2025, Europa stands as a world with active geology manifested in its surface deformation and possible plumes, all stemming from an internal ocean that makes it arguably a sibling to Earth in terms of having a global water ocean and active interior (though encased in ice).
Saturn’s Moons: Titan’s Hydrocarbon Landscapes and Enceladus’s Plumes
Saturn’s system offers two especially notable worlds for geology: Titan, with its thick atmosphere and methane-based hydrologic cycle carving an Earth-like landscape, and Enceladus, a small icy moon with cryovolcanic geysers spraying from a subsurface ocean. These bodies have been investigated intensively by the Cassini orbiter (which toured Saturn 2004–2017), and researchers continue to analyze Cassini’s treasure trove through the 2020s.
Titan – Rivers, Lakes, and Possible Cryovolcanoes: Titan is unique – the only moon with a dense atmosphere (1.45 atm pressure) and active weather. It is far colder than Earth (surface ~94 K, –179 °C), so liquid methane and ethane play the role of water. During 2015–2025, Cassini’s final observations and subsequent analyses answered major questions about Titan’s surface liquids and hinted at internal activity.
One major result was determining the depths and composition of Titan’s seas. Titan’s north pole has large lakes and seas of liquid methane/ethane, analogous to terrestrial lakes. The biggest, Kraken Mare, is a sea larger than the Caspian Sea (it’s about 500,000 km² in area, comparable to the Great Lakes combined)[130]. In Cassini’s T104 flyby (2014), the radar altimeter pinged the surface and floor of Kraken Mare in an estuary region (Moray Sinus). The data showed that Moray Sinus is ~85 m (280 ft) deep, whereas the central Kraken Mare is so deep that no radar echo from the bottom was detected – implying a depth exceeding the instrument’s ~100–300 m range[131][132]. By analyzing loss of signal, researchers estimated Kraken’s central depth to be at least 300 m (~1,000 ft)[133][134]. “Titan’s largest sea…contains about 80% of the moon’s surface liquids,” noted lead author Valerio Poggiali[135]. The liquid is a mixture of methane and ethane, but interestingly, Kraken’s composition turned out to be methane-dominated, similar to the second-largest sea Ligeia Mare[131][136]. Scientists had expected Kraken (being larger and at lower latitude) might be richer in ethane (a product of atmospheric methane breakup), but it appears not markedly different from other northern seas[132][137]. This homogeneity helps constrain Titan’s methane hydrological cycle models. Titan’s lakes and seas likely communicate through subsurface aquifers or exchanges of methane vapor via the atmosphere. The fact that methane hasn’t all turned to ethane and sunk suggests a replenishment of methane (since sunlight-driven chemistry would convert all methane to ethane + other products on a ~10 million-year timescale[138]). The solution to this “methane enigma” might be geologic outgassing – perhaps Titan has cryovolcanoes or subsurface reservoirs releasing methane over time to resupply the atmosphere. Cassini did find candidate cryovolcanic landforms: most notably, the region called Sotra Patera–Doom Mons. There, radar mapping revealed a 1.7 km deep, 30 km wide pit (Sotra Patera) alongside two mountains ~1 km tall (Doom Mons and Erebor Mons) with lobate flow-like features around them[139][140]. This terrain strongly resembles volcanic calderas and flows on Earth – except made of water ice and other icy slush, not rock[141][139]. “The 3D view reveals multiple mountain peaks, deep pits and finger-like flows at Sotra Facula…some terrain resembles volcanic cones, craters and flows on Earth,” reported Randolph Kirk’s USGS team[142][143]. They conclude that cryovolcanism (ice volcanism) is the best explanation for this feature – making it “the best candidate yet for an ice volcano on Titan”[141][139]. Two peaks ~1,000 m high and a 1,500 m-deep caldera suggest eruption of material that left a void (caldera) and built low domes[139][144]. If true, Titan has (or had) internal heat sufficient to melt ice and vent liquids onto the surface. This could be the mechanism that resupplies methane to the surface/atmosphere: breakdown of clathrates or deeper hydrocarbons, released via cryovolcanoes. It also implies Titan’s interior, while cold, is not completely frozen solid – there may be a deep subsurface ocean of ammonia-rich water acting as a source for cryovolcanism. In fact, Cassini’s gravity measurements and Titan’s induced rotational wobble indicate Titan does have a global ocean ~100 km below, sandwiched between ice layers.
On the surface, Titan’s geomorphology is eerily Earth-like in places. Cassini radar and imaging showed dendritic river channels carved into hills, alluvial fans, and even sand dunes. Equatorial Titan has vast dune fields of dark organics (complex hydrocarbons), with linear dunes up to 100 m tall and tens of kilometers long, shaped by consistent winds. Near the poles, Cassini identified shoreline features, branching river valleys, and even rainfall events (clouds and darkened surface patches observed) confirming an active weather cycle. In 2016, Cassini observed the “Magic Island” phenomenon: transient bright features in Ligeia Mare that appeared and disappeared, likely due to either methane rain causing waves or buoyant gas bubbles on the sea[145]. Titan thus has active surface-atmosphere interactions – rivers flow when it rains methane, lakes fill and evaporate, and winds transport sand.
One striking Earth analog was found in 2013 when Cassini’s radar altimeter data suggested the presence of liquid-filled canyons on Titan. These are 300–700 m deep canyons (near Ligeia Mare) where radar reflections indicated liquid methane/ethane flowing at the bottom (making them the “Grand Canyon of Titan”). The liquid is extremely radar-smooth and filled nearly to the brim in those channels, implying a stable sea level connecting to the main lakes. This gives insight into Titan’s crust: it’s strong enough to hold steep canyon walls, possibly due to a rigid water-ice bedrock.
Titan’s geology is closely tied to its climate. Over tens of thousands of years (Titan’s orbital cycles), the distribution of solar heating shifts, possibly transferring methane from pole to pole and leading to cyclic rising and falling of sea levels. Indeed, Titan’s south pole has empty lake basins and features that suggest it has dried out and we are in a northern summer pluvial period.
In summary, Titan in 2015–2025 has been revealed as geologically rich: it has active fluid erosion, a complete “hydrologic” cycle (with methane as the working fluid), solid-state geology with tectonic-like features (mountain ranges up to ~3 km high from ancient stress or contraction), and possibly ongoing interior activity venting methane. As one Cassini scientist quipped, “Titan represents a model environment of a possible atmosphere of early Earth”[130] – except at deep freeze. With NASA’s Dragonfly rotorcraft set to visit Titan in the 2030s, future in-situ geology will probe the dunes and likely cryovolcanic deposits for complex organic chemistry and clues to Titan’s geologic past.
Enceladus – Cryovolcanic Geysers and Hydrothermal Ocean: The small moon Enceladus (500 km diameter) astonished the world when Cassini discovered its south polar geysers in 2005. Over 100 jetting plumes of water vapor and ice particles shoot from four “tiger stripe” fractures, indicating an internal liquid water source. In the past decade, Cassini’s deep dives through Enceladus’s plume (especially the 2015 E-21 flyby) have detailed its composition and provided evidence for an active hydrothermal seafloor beneath Enceladus’s ice.
Cassini’s mass spectrometer (INMS) and cosmic dust analyzer (CDA) found that the plume is mostly water but contains ~1% other volatiles and tiny particles[146][147]. Notably, in 2017 scientists reported the plume contains abundant molecular hydrogen (H₂)[148][149]. This H₂ is best explained by water-rock reactions at the ocean floor – specifically, hot water reacting with iron-rich rocks via serpentinization, which produces H₂[148][149]. Cassini also detected nanometer-sized silica particles in Saturn’s E-ring (which is created by Enceladus’s plume)[150][151]. These silica (SiO₂) grains likely form when alkaline hydrothermal fluids mix with cooler water – exactly what happens around hydrothermal vents on Earth’s ocean floor, where silica precipitates from contact between hot rock and seawater. The presence of these materials suggests Enceladus’s ocean water is circulating through a rocky core at temperatures perhaps ~90 °C or more, leaching minerals[150][151]. “The recent discovery of silica nanoparticles derived from Enceladus shows the presence of ongoing hydrothermal reactions in the interior,” wrote a 2017 Nature paper[150][151]. This is profound: Enceladus has a global subsurface ocean (~30–40 km deep) under an ice shell ~20 km thick at the poles (thicker at equator), and the ocean is likely hot at the bottom due to tidal heating in the core[152]. This environment – water + heat + minerals + organic molecules (also detected in the plume) – checks virtually all boxes for habitability. In 2017, NASA announced that Enceladus’s ocean has a potential energy source for life: when H₂ was found, they noted methanogenic microbes on Earth could use H₂ + CO₂ to produce methane for metabolism[146][153].
Geologically, Enceladus’s south polar terrain is a site of active cryovolcanism. The “tiger stripes” are 130-km long fractures that vent the ocean. The region is warm (measured up to ~190 K in spots, far above the ~70 K elsewhere on Enceladus) and shows significant resurfacing: the south pole has almost no impact craters and appears covered in deposited plume material (bright snow-like ice). Cassini’s gravity and rotation data revealed a slight wobble consistent with the ice shell not being frozen to the core – confirming a global ocean decoupling the shell[152]. Models indicate ~20–30 GW of heat must be coming from the interior to sustain the plumes (some heat is observed as thermal emission). How Enceladus generates and transports so much heat is still researched – likely a combination of tidal friction in the core and maybe exotic physics of water percolation.
Enceladus’s ongoing activity makes it a prime target for astrobiology. Unlike Europa, Enceladus conveniently sprays its ocean into space, allowing easy sampling. Cassini essentially performed a remote ocean tasting, detecting not just H₂ but also organics like methane, ammonia, carbon monoxide, simple and complex organics (even traces of benzene, possibly). A 2018 study found new complex organic compounds in the plume – some with masses over 200 amu – potentially originating from hydrothermal chemistry or organic-rich sea foam ejected from the ocean[154][155]. The next steps are mission concepts that would return to Enceladus to analyze the plume in more detail, search for biomolecules or even life signs.
From a geologic perspective, Enceladus has clearly ongoing tectonics and resurfacing driven by its orbital tidal heating. The south polar crust flexes enough to open and close the tiger stripe fissures periodically (Cassini observed the plume intensity varies with Enceladus’s position in its orbit, stronger when the fissures are pulled open). The moon’s surface beyond the poles also shows tectonic ridges, fractures, and relatively young plains, though the mid-latitudes and leading hemisphere are older with more craters (still, none are extremely large; the largest crater is ~35 km). Enceladus likely has an asymmetric heat distribution (mostly at the south pole, none at the north pole – an unsolved mystery why only one pole is active; perhaps it flipped in the past or the tidal eccentricity heating concentrates there). In any case, Enceladus stands out as a tiny icy world with geological activity rivaling a planet, effectively an analog of a mini-Europa but with easier access to its ocean.
The discoveries from 2015–2025 – especially the confirmation of hydrothermal activity – have cemented the view of Enceladus as one of the most habitable places beyond Earth. As Cassini project scientist Linda Spilker said, “It’s icing on the cake. Enceladus has almost all of the ingredients that you need to support life as we know it”. Future missions (like proposed “Enceladus Orbilander”) aim to directly search for life in its plume in the 2040s.
Pluto and Beyond: Icy Plains and Cryovolcanoes on Dwarf Planets
Finally, it’s worth noting the revolutionary geology revealed at Pluto (visited by New Horizons in 2015). Although Pluto is a dwarf planet in the Kuiper Belt (not a moon or full planet), its geology from 2015 findings is stunning and relevant to outer solar system processes. Pluto has a nitrogen ice glacier (Sputnik Planitia) that is actively convecting and resurfacing itself, essentially a “lava lamp” of solid nitrogen overturning due to internal heat. Polygons ~33 km across cover Sputnik Planitia, interpreted as convection cells in nitrogen ice rising and falling over a layer ~7–10 km thick[156][157]. This surface has no impact craters – a New Horizons search found none, implying an age <10 million years[158][159] (and one model of sublimation rates suggests possibly as young as ~180,000 years[160]!). Thus, Pluto is geologically active today, likely powered by slow cooling of an internal ocean or residual heat from radioactive decay. Surrounding Sputnik are water-ice mountains up to 4–6 km tall (water ice behaves like bedrock at Pluto’s 38 K temperature). One mountain cluster, Tenzing Montes, reaches ~6.2 km high near the western edge of Sputnik.
New Horizons also discovered possible cryovolcanoes. Two large mounds, Wright Mons and Piccard Mons, stand out. Wright Mons is ~150 km wide with a huge central depression and rises ~4–5 km above surrounding plains[161][162]. Piccard is even larger (~7 km high, 225 km across)[161][163]. These have an undulating, hummocky surface and lack impact craters, hinting they were built by accumulation of relatively recent cryovolcanic flows of water ice mixed with softer volatiles[162][164]. In 2022, a Nature study by Kelsi Singer et al. reported that the area around Wright and Piccard Mons appears to have been resurfaced by multiple cryovolcanic eruptions over time, with a surface age possibly as young as 1 billion to 100 million years (based on crater counts, which are sparse there)[161][162]. The authors describe large lobate fields – tens of kilometers across – of what looks like frozen “cryolava” (a slushy mixture of water ice and perhaps ammonia/methanol that lowers the freezing point) that flowed and then solidified[161]. These flows pile up into mounds with pits, rather than the sharp calderas of rocky volcanoes, but nonetheless indicate Pluto has been geologically active in the relatively recent past. “Large-scale cryovolcanic resurfacing on Pluto” is how Singer’s paper put it[165]. The presence of such activity suggests Pluto’s interior stayed warmer than expected – likely due to a subsurface ocean that, if partially freezing, can force water to the surface, or due to insulating clathrate layers.
Additionally, Pluto’s surface shows colossal tectonic faults and graben (e.g., Virgil Fossa) hundreds of km long, which are interpreted as signs of global expansion – consistent with a subsurface ocean that froze and expanded (water expands when freezing)[166][167]. On Pluto’s big moon Charon, New Horizons revealed an even more dramatic tectonic belt: Serenity Chasma, part of a fracture system 1,800 km long and up to 7.5 km deep encircling Charon’s equator[168][169]. For context, Charon’s chasms are 4.5× deeper than Earth’s Grand Canyon[170][171]. This giant “pull-apart” tectonic feature likely formed as Charon’s internal ocean froze, expanded, and cracked the crust open, literally *“Charon’s surface fractured as it stretched”[172][173]. The tectonics on these small bodies demonstrate that even distant icy worlds undergo powerful geologic forces if they have liquid-water interiors that change phase or volume.
The takeaway is that geologic activity is not exclusive to the terrestrial planets – even far-flung, cold, or tiny objects can be dynamic. Over 2015–2025, we’ve learned that cryovolcanism (ice volcanism) is a common theme in the outer solar system, from Europa and Enceladus to Titan, Pluto, and perhaps Triton (Voyager saw Triton’s nitrogen geysers in 1989, and likely a subsurface ocean exists there too). Each world has its twist: Enceladus jets water from tiger stripes; Europa flexes and possibly subducts ice plates; Titan rains hydrocarbons and perhaps extrudes icy slush; Pluto builds ice volcano mountains; Triton geysers nitrogen.
Conclusion
The past decade has profoundly enriched our understanding of planetary geology beyond Earth. We now recognize that geologic processes are diverse and widespread in the solar system: rocks and ices can melt, flow, crack and explode in environments ranging from blistering 470 °C heat on Venus to frigid –230 °C cold on Pluto. Volcanism is not limited to silicate magma – it can be sulfur on Io or water/ammonia on an icy world. Tectonics need not involve plate motions – it can be one-plate global contraction on Mercury, crustal stretching on Mars and Charon, or ice shell convection on Europa. Sedimentary cycles occur with water on Mars and with methane on Titan. Even the presence of oceans – once thought unique to Earth – is now evidenced on multiple bodies (Europa, Enceladus, Titan, Ganymede, likely Pluto and Triton, possibly Callisto and others), driving geology in surprising ways.
This era (2015–2025) of planetary exploration has been especially fruitful: MESSENGER ended its Mercury mission with revelations of a still-shrinking planet; Venus Express and Magellan data showed us Venus has active volcanoes today; Curiosity and Perseverance painted a detailed picture of Mars’s watery past and hinted at ongoing activity (quakes, maybe recent eruptions); Juno gave us front-row seats to Io’s volcanic outbursts and new data on Europa’s ice; Cassini’s finale confirmed Titan’s seas are deep and Enceladus’s ocean is habitable; and New Horizons unveiled Pluto’s astonishingly youthful surface shaped by convecting ices and cryovolcanoes. Each discovery has noteworthy quantitative results – e.g., Mercury’s 3 km-high scarps and 1,000 km faults; Venus’s vent changing by 2.2 km² in 8 months; Mars’s eruption ~50k years ago covering 8-mile area; Io’s 80 TW eruption spanning 100k km²; Europa’s double ridges ~300 m tall from ~1 km-deep water pockets; Titan’s Kraken at least 300 m deep holding 80% of surface liquids; Enceladus’s plume with ~1% H₂ by volume indicating 90 °C vents; Pluto’s convecting plains <10 Myr old and 4-km ice volcanoes. These numbers measure phenomena that were, until recently, just speculation.
Crucially, these findings feed forward into new missions and studies. With Mars Sample Return, Europa Clipper, Dragonfly (Titan rotorcraft), and proposals for Enceladus, Io, and Venus missions, the 2020s–2030s will further test our understanding. The past decade has shown that planetary geology is a vibrant, evolving field – the “dead” planets and moons are very much alive in their own ways. As we have seen, sometimes old data can yield new discoveries (e.g., decades-old Magellan and Galileo data revealing Venusian and Europan activity), reminding us to keep scrutinizing the records. The coming years promise to deepen these insights: for instance, we expect InSight’s seismic data (still being analyzed) to uncover Mars’s core state, Clipper to possibly catch Europa’s plumes in the act, and Dragonfly to sample organics in a Titan dune. Each will doubtless raise new questions. But one overarching lesson from 2015–2025 is clear: geology is a universal process – wherever there is energy (internal or external) and materials (rock or ice or gas), planetary bodies large and small will undergo change. Our solar system’s other worlds are not static spheres but dynamic places with volcanoes, quakes, landslides, glaciers, and maybe even ingredients for life, reflecting the same physical processes in novel settings. In exploring them, we not only satisfy curiosity but also hold up a mirror to the early and extreme chapters of our own planet’s story.
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