Cronian

The Hexagon

CASSINI ISS · 2013 & 2017

Nothing in nature should hold six corners at this scale, and yet it does. A jet stream at Saturn's northern latitudes has locked itself into a hexagon roughly thirty thousand kilometres across, wide enough to line up two Earths side by side, and it has kept that geometry through every year humans have watched it. Voyager saw it in 1980. Cassini found it exactly where it had been left.

What changed was the colour. In 2013 the interior read deep blue, a polar atmosphere still clean after years of winter darkness. By 2017, with northern summer approaching, ultraviolet sunlight had brewed photochemical haze and the same region had gone soft yellow. Only the vortex at the dead centre stayed blue, its descending air apparently sweeping itself clear of smog.

The corners mark where a fast eastward wind, running near 320 kilometres per hour, sits between slower flows and buckles into a standing wave. The shape is what stability looks like on a planet with no continents, no coastlines, and nothing at all to interrupt the flow.

Hexagon width

~30,000 km (over two Earth diameters)

Jet-stream speed

~322 km/h (200 mph)

Centre latitude

78 degrees north

Saturn rotation

10 h 33 m

PIA21611 · Two circular views of a planet's north pole side by side. Both show a clear six-sided outline of cloud bands ringing a small tight spiral at the centre. The left view is dominated by cool blue-grey tones inside the hexagon, and the right view is filled with pale mustard-yellow haze, with only the central spiral still blue. · NASA/JPL-Caltech/Space Science Institute

Video: Animated time-lapse of Saturn north polar hexagon rotating with central polar vortex · NASA/JPL-Caltech/SSI/Hampton University

The Decagon

HUBBLE OPAL · SCIENCE ADVANCES · 2 SEPTEMBER 2026

Saturn's south pole has grown a ten-sided wave. Hubble's Outer Planet Atmospheres Legacy program confirmed it in a paper published on 2 September 2026: a decagon between about 58 and 63 degrees south, sitting inside a jet stream and reaching through several layers of atmosphere. It is the first large regular polygon seen in the southern hemisphere.

The first hint came from people with backyard telescopes. In 2024, researchers and the amateur astronomers Trevor Barry and Jean-Paul Oger noticed a faint wavy band in ground-based images shared through an observer archive. Hubble data showed the outline faintly in 2023 and sharply by 2025.

It drifts east at 2.5 metres per second, a brisk walk, inside a jet running at 116. Cassini saw nothing like it in thirteen years. Nobody knows why it formed now, or whether it will last as long as the hexagon has.

Latitude

about 58 to 63 degrees south

Shape

10-sided wave, still strengthening

Drift

2.5 m/s east

Jet at 60.5 degrees south

116 m/s

Vertex oscillation period

32 days

Paper

Sánchez-Lavega et al., Science Advances, DOI 10.1126/sciadv.aee4251

HEIC2612A · A polar view of Saturn's south pole showing concentric atmospheric bands with a faint ten-sided polygonal outline visible in the blue-tinted polar region, surrounded by warmer gold and cream zonal bands. · NASA, ESA, STScI, A. Sánchez-Lavega (UPV), A. Simon (NASA-GSFC), M. Wong (UC Berkeley), Processing: A. Pagan (STScI)

Polar Aurora

CASSINI VIMS · 4 MICRONS

Saturn's poles glow in a light no eye was built for. At four microns, six times the wavelength of visible light, the planet's warm interior bleeds red through the cloud deck and the aurora sits on top of it in cold blue, a ring drawn exactly where magnetic field lines funnel charged particles down into the upper atmosphere.

Cassini's infrared spectrometer found emission inside the polar cap as well, structure nobody had predicted and brighter than the models allowed. Saturn's aurorae are also louder than Earth's: the same particle streams that light them drive radio emission strong enough to be recorded as sound, pulsing roughly once per Saturn day.

This composite stitches together observations from 2006 and 2008, taken from different distances and viewing angles. It reads as a long exposure of a magnetic field at work, and it is a reminder that most of what a planet does happens in wavelengths we have to build instruments to notice.

Wavelength

4 microns (aurora), 5 microns (interior)

Instrument

Visual and Infrared Mapping Spectrometer

Radio pulse period

~10.7 h (Saturn kilometric radiation)

Observations combined

2006 and 2008

PIA11396 · A dark curved planetary limb seen from above the pole. A luminous electric-blue ring hangs over the polar region, with a second brighter blue patch inside it. Beneath, the planet glows in deep red and orange where heat escapes from below, with darker bands where thick cloud blocks it. · NASA/JPL/University of Arizona

Lightning

CASSINI RPWS · SED DATA

Saturn's atmosphere crackles with electrical storms powerful enough to be heard from orbit. Cassini's Radio and Plasma Wave Science instrument recorded Saturn Electrostatic Discharges, bursts of radio noise from lightning bolts deep in the atmosphere, throughout its thirteen-year mission. The most dramatic episodes came from the great storm systems that erupt in Saturn's mid-latitudes.

The 2010-2011 Great White Spot, a planet-encircling storm, produced lightning flashes ten thousand times more powerful than anything measured on Earth. The radio pops Cassini recorded during these storms, when shifted into the audible range, sound like a crackling fire heard through a broken speaker.

Unlike Earth, where lightning is common and spread across the globe, Saturn's storms are rare and enormous. They build for decades in the deep atmosphere before erupting, then run for months. The energy they release in a single event dwarfs anything in terrestrial meteorology.

Instrument

RPWS Saturn Electrostatic Discharge detector

Great White Spot

2010-2011, planet-encircling storm

Flash power

~10,000x terrestrial lightning

Storm recurrence

Roughly every 20-30 years

PIA14903 · Saturn viewed from above showing a bright white turbulent storm band wrapping around the planet's northern mid-latitudes, disrupting the normally smooth zonal banding. · NASA/JPL-Caltech/Space Science Institute

Audio: Saturn lightning recorded by Cassini RPWS · NASA/JPL-Caltech/University of Iowa

Video: Time-lapse sequence showing bright lightning strike flashes on Saturn night side under ringlight · NASA/JPL-Caltech/Space Science Institute

Where It Ended

FINAL SIGNAL · 15 SEPTEMBER 2017

The last picture Cassini ever made shows a place: a patch of Saturn's night side, lit only by ringlight, where the spacecraft would arrive a few hours later at more than a hundred thousand kilometres per hour. By the time it did, the region had rotated into daylight. Nobody saw the entry.

The choice was deliberate. Cassini had found a habitable ocean at Enceladus and standing lakes on Titan, and it was nearly out of propellant. Rather than risk an uncontrolled crash contaminating either moon, the team spent the spacecraft on the planet, turning the final plunge into an atmospheric sampling run no orbiter could otherwise perform.

It held its antenna on Earth and kept talking on the way down. The last signal left Saturn and arrived eighty-three minutes later, at 11:55:46 UTC on 15 September 2017, and then the carrier went flat. Cassini became, in a strictly literal sense, part of the planet it had spent thirteen years describing.

Atmospheric entry

15 September 2017, ~9.4 degrees north

Entry speed

~113,000 km/h (70,000 mph)

Final signal received

11:55:46 UTC, 83-minute light delay

Grand Finale orbits

22, beginning 26 April 2017

PIA21895 · A grainy monochrome frame showing a faintly lit curve of atmosphere against black. The illumination is dim and even, coming from ringlight rather than the sun, so cloud detail is barely resolved and the horizon fades away at both edges of the frame. · NASA/JPL-Caltech/Space Science Institute

Video: Cassini narrow-angle camera movie swooping from north pole southward during first ring gap dive · NASA/JPL-Caltech/Space Science Institute/Hampton University

Scale

SCALE · SATURN SYSTEM

Saturn is 120,536 kilometres across the equator, wide enough to line up nine Earths, and light enough to float: its mean density is below that of water. It is also visibly flattened by its own spin, a full twelve thousand kilometres narrower pole to pole than across the middle, because it completes a rotation in ten and a half hours.

The rings are the harder thing to hold in the head. They span about 282,000 kilometres and are typically about ten metres thick. Scale the main rings to the width of a football pitch and they would come out some thirty times thinner than a sheet of paper. Their entire mass, weighed during the Grand Finale, amounts to less than half of Mimas.

Then the moons, which refuse to be one kind of object. Titan is larger than Mercury and has weather. Enceladus, 504 kilometres across, would not span Arizona and holds a global ocean. Mimas is a cratered ball of ice that keeps a gap wider than Mercury open in the rings. All of it sits 1.4 billion kilometres out, in one percent of Earth's sunlight.

Saturn equatorial diameter

120,536 km (9.4 Earths)

Mean density

0.69 g/cm3, less than water

Main rings

~282,000 km across, ~10 m thick

Total ring mass

~1.5 x 10^19 kg, under half of Mimas

PIA17172 · A wide panorama of a dark planet and its full ring system lit from behind, rendered in muted blues and golds. The rings arc across the entire width of the image. In the lower right, between a faint outer ring and a broad blue one, sits a small pale blue point of light with a much fainter companion just beside it. · NASA/JPL-Caltech/Space Science Institute

The Sound of Saturn

CASSINI RPWS · SONIFICATION

Saturn is loud in a band nobody can hear. Its aurorae drive radio emission called Saturn kilometric radiation, and when the recordings are shifted down into the audible range the planet turns out to sound like a siren winding up and down, keyed to its own rotation. Cassini began picking it up in April 2002, still 374 million kilometres out.

The instrument had no microphone. It measured oscillating electric and magnetic fields with three ten-metre antennas, and converting frequency into pitch is the same trick a radio set performs. What comes through is a whole plasma environment: whistlers from lightning in Saturn's clouds, dust grains striking the antennas as pops, and the steady beat of the magnetosphere.

The beat turned out to be a problem. The radio period drifts, roughly 10.7 hours but never fixed, and it differs between the northern and southern hemispheres. A gas planet offers no surface to time, so its day had to be established another way. This is the closest thing Saturn has to a pulse, and it will not hold still.

Emission

Saturn kilometric radiation, auroral origin

Apparent period

~10.7 hours, and drifting

First detected

April 2002, at 374 million km

Instrument

RPWS, three 10 m electric antennas

PIA11396 · A polar view in false colour where a cold blue ring of auroral emission overlays a body glowing deep red from internal heat. The blue ring is uneven in brightness, broader on one side, with a separate bright blue region inside the ring nearer the pole. · NASA/JPL/University of Arizona

Audio: Saturn kilometric radiation, sonified from RPWS wideband data · NASA/JPL-Caltech/University of Iowa

The Cassini Division

CASSINI ISS · 28 JANUARY 2016

From Earth it looks like an absence: a dark lane cut between the bright B ring and the dimmer A ring, nearly as wide as the planet Mercury. Close up, the absence turns out to be populated. Thinned, with faint ringlets threading a gap almost five thousand kilometres across.

Mimas made it, from a quarter of a million kilometres away. Particles in the division orbit Saturn almost exactly twice for every single orbit Mimas completes, so the same small moon tugs at them from the same place over and over again. Given enough repetitions, a resonance becomes a canyon.

Giovanni Domenico Cassini saw the gap in 1675 through a refractor, with no way to know what held it open. Three centuries later the spacecraft carrying his name crossed four degrees above the ring plane, resolved the ringlets inside the shadow he had found, and measured the resonance responsible. Some observations take a very long time to close.

Width

~4,800 km (2,980 miles)

Radius from Saturn

117,600-122,200 km from centre

Cause

2:1 orbital resonance with Mimas

First recorded

1675, by G. D. Cassini

PIA18365 · Tightly packed horizontal bands of grey and cream running across the frame. A broad darker corridor separates the densely striped brighter region on one side from a smoother, dimmer region on the other. Within the dark corridor, several very fine pale lines are visible. · NASA/JPL-Caltech/Space Science Institute

Video: Concatenation of 301 Cassini images showing Mimas orbital resonance waves sculpting outer B ring edge · NASA/JPL/Space Science Institute

Spokes

CASSINI ISS · 19 MARCH 2009

They appear over a few hours, run radially across the B ring like smudges on glass, and then they are gone. Nothing else in the ring system behaves this way. Ring particles obey Kepler and orbit at different speeds depending on radius, so any radial feature should shear apart almost at once. The spokes form anyway, faster than the shearing can erase them.

The working explanation is electrostatic. Micron-sized dust, charged by sunlight and by electrons in Saturn's magnetosphere, lifts clear of the larger ring bodies and is carried by the magnetic field, which rotates with the planet rather than with the ring. The dust makes the spokes visible, and the field decides where they point.

They also keep a schedule. Spokes cluster around Saturn's equinoxes, when sunlight strikes the rings almost edge-on, and they fade for years in between. Voyager found them in 1980 and Cassini arrived in the wrong season entirely, with the rings tilted well into the sun. It waited, and caught them returning as the 2009 equinox approached.

Location

B ring only, 92,000-117,600 km radius

Typical length

up to ~20,000 km

Dust grain size

~1 micron

Rotation

with Saturn's magnetic field, ~10.7 h

PIA11452 · A steeply foreshortened sweep of ring material filling the frame diagonally. Several soft, pale wedge-shaped streaks lie across the banding at right angles to it, fanning slightly outward and blurring at their edges rather than ending sharply. · NASA/JPL/Space Science Institute
PIA11639 · A wide, dim view of the rings seen from a shallow angle, contrast pushed hard. Faint luminous streaks stand out against darker ring material like breath on cold glass. Two tiny bright specks sit near the rings, one inside the outer ring edge and one beyond it, with a star at upper right. · NASA/JPL/Space Science Institute

Video: Time-lapse animation showing dark radial dust spokes rotating across Saturn sunlit B ring · NASA/JPL/Space Science Institute

Ring-Plane Crossing

GRAND FINALE · 22 DIVES

Twenty-two times, Cassini threaded a gap nobody had ever flown. Three thousand kilometres above the cloud tops, three hundred kilometres inside the innermost visible ring, at roughly a hundred and twenty thousand kilometres per hour. The first pass was made with the dish antenna turned forward as a shield, because no one knew what was in there.

What was in there was mostly nothing, and a little dust. The plasma-wave instrument recorded the hits as a rush of pops and static, thousands of grains per crossing, none larger than smoke particles. After the first dive the team stopped using the antenna as armour and started pointing the cameras instead.

The images that came back resolved ring structure at a few hundred metres per pixel, including the Janus 2:1 spiral density wave, the only major density wave in the B ring. Two ring particles orbit for every orbit Janus completes, and the resonance stacks into visible crests: spiral-galaxy physics compressed into a few hundred kilometres.

Altitude at dive

~3,000 km above cloud tops

Clearance from D ring

~300 km

Relative speed

~122,000 km/h (76,000 mph)

Best ring resolution

~530 m per pixel

PIA21627 · A dense field of fine parallel ridges filling the whole frame, like the grain of cut wood or a fingerprint. The ridges crowd closer together toward one side of the image, with brighter crests separated by narrow darker troughs. · NASA/JPL-Caltech/Space Science Institute
PIA21628 · A narrow strip of ring resolved into dozens of thin lanes, tinted in subtle shifts of tan, grey and pale blue-green that mark differences in particle size and composition between neighbouring lanes. · NASA/JPL-Caltech/Space Science Institute

Audio: Ring-plane crossing: dust grains striking the spacecraft, recorded by RPWS · NASA/JPL-Caltech/University of Iowa

The F Ring and Its Shepherds

CASSINI ISS · 29 OCTOBER 2005

The F ring is the ring system's unstable edge: a narrow bright core a few dozen kilometres wide, wrapped in strands that braid, kink and knot, and visibly different from one orbit to the next. It is the only major ring that changes on human timescales.

Two small moons are responsible. Prometheus, 86 kilometres across, orbits just inside the ring, and Pandora, 81 kilometres, just outside. Prometheus is the aggressive one. Near the far point of its slightly eccentric orbit it reaches into the ring, draws material out along a dark channel, and leaves a streamer trailing behind it. The pattern repeats on a cycle of about 68 days.

Watching them together is watching accretion and disruption argue in real time. Material clumps, a shepherd passes, the clump smears and re-forms somewhere else. The F ring is the closest thing available to a live demonstration of how rings and small moons trade material back and forth, running continuously about 140,000 kilometres from Saturn's centre.

F ring radius

~140,200 km (2.32 Saturn radii)

Core width

~50 km

Prometheus / Pandora

86 km and 81 km across

Orbital periods

0.613 d and 0.629 d

PIA07653 · A thin bright thread of ring material runs across a black field. Two small irregular lumpy moons sit on either side of it, one slightly larger and closer to the thread, both lit from the same side and showing dim grey surfaces without clear surface detail. · NASA/JPL/Space Science Institute

In Saturn's Shadow

CASSINI ISS · 15 SEPTEMBER 2006

Cassini spent twelve hours inside Saturn's shadow with the sun completely blocked, which is the only way to take this picture. Backlit, the rings invert. Dense regions go dark because they hold light back, and the faint dusty ones light up from within. Structures nobody had catalogued appeared at once, including rings that were entirely new.

The mosaic runs 165 frames. Somewhere on the left, interior to the G ring and above the brighter main rings, there is a pale dot about a pixel across with a fainter dot beside it. That is Earth, and the Moon, seen from 1.5 billion kilometres away through the gap in another planet's ring system.

Nobody on Earth knew the photograph was being taken. The image is beautiful. It also answers a positional question: this is where we actually are, and this is how much of the frame we occupy once the camera is far enough away to include the whole system at once.

Date acquired

15 September 2006

Frames combined

165

Distance from Saturn

~2.2 million km

Earth in frame

about one pixel, interior to the G ring

PIA08329 · A dark planet silhouetted against black, encircled by rings that glow from behind. The tenuous outer rings shine brightest as broad luminous halos, while the normally brilliant main rings appear as a darker banded belt. A single faint speck sits in the empty space inside the outermost glowing ring. · NASA/JPL/Space Science Institute

Huygens

HUYGENS DISR · 14 JANUARY 2005

On 14 January 2005 a probe the size of a small car fell for two and a half hours through an orange sky and came to rest on a damp plain of water-ice pebbles, 1.2 billion kilometres from the people who built it. It remains the only landing ever performed in the outer solar system.

Huygens had ridden Cassini for seven years, separated on Christmas Day 2004, and coasted for three weeks with no way to be steered or corrected. Its descent imager returned more than 350 pictures: haze, then drainage channels cut into bright highlands, then a dark shoreline, then the ground itself, close enough to resolve two rounded cobbles about fifteen and four centimetres across.

The surface measured 94 kelvin and behaved like wet sand under the probe's weight. Huygens kept transmitting for another seventy minutes, until Cassini dropped below its horizon and the relay ended. What it left behind is a description of a landscape shaped by rain, rivers and beaches, running entirely on methane instead of water.

Landing site

10.3 S, 192.3 E, 14 January 2005

Descent duration

2 h 27 min

Surface temperature

93.7 K (-179 C)

Descent images returned

more than 350

PIA07232 · A low, level view across a flat orange-brown plain strewn with rounded pebble-shaped objects of varying size. The two largest sit just below centre. The ground is uniformly tinted amber, the horizon is not visible, and the light is flat and diffuse with almost no shadows. · ESA/NASA/JPL/University of Arizona
PIA07230 · A mosaic looking down on pale highland terrain veined with dark branching channels that run downhill toward a broad flat darker region, resembling an aerial photograph of river drainage meeting a coastline. Resolution varies visibly between the stitched tiles. · ESA/NASA/JPL/University of Arizona

Audio: Titan descent, from Huygens acoustic and radio measurements · ESA/HASI-PWA Team, processed by The Planetary Society

Video: Animated descent sequence from Huygens DISR camera breaking through haze to Titan landing site · ESA/NASA/JPL/University of Arizona

Kraken Mare

CASSINI ISS · 14 SEPTEMBER 2011

There is a sea on Titan larger than the Caspian, and it is made of methane and ethane at 94 kelvin. Kraken Mare spreads across the moon's high northern latitudes, dark and smooth enough that Cassini occasionally caught sunlight glinting off its surface, the way a lake announces itself from an aircraft window.

Radar mapped its coastlines as fjords, flooded valleys and river mouths. Where the beam could reach bottom, the shallows measured tens of metres. Toward the centre the signal simply did not come back, which means the basin is deeper than the instrument could sound. Estimates put the central depth beyond a hundred metres.

This is the only other place in the solar system with stable liquid standing on a surface, and it runs a full cycle: methane evaporates, forms cloud, rains, carves channels and pools. The chemistry is alien and the hydrology is deeply familiar, which is most of what makes Titan difficult to look away from.

Surface area

~400,000 sq km, larger than the Caspian Sea

Composition

liquid methane and ethane at ~94 K

Sounded depth

beyond 100 m at centre, radar found no floor

Location

centred near 68 degrees north

PIA14584 · A hazy pale orange globe with soft, low-contrast markings. Near the top, a distinctly darker irregular patch spreads across the northern latitudes with ragged inlets along its edges. The limb fades gradually into a bright detached haze layer rather than a hard edge. · NASA/JPL-Caltech/Space Science Institute

Xanadu Regio

CASSINI RADAR · 30 APRIL 2006

Xanadu was the first thing anyone noticed about Titan's surface: a continent-sized bright region, roughly the area of Australia, distinct enough to be picked out from Earth through the haze. For two decades it was a smudge with a borrowed name and no landscape attached to it. Cassini's radar turned it into terrain.

The terrain is startlingly Earth-like. Mountain chains rise in the interior. Rivers descend from them through hilly country and cross plains in branching networks. Circular basins read as degraded impact craters, softened by erosion, and the region as a whole appears to be old, high ground that has been rained on for a very long time.

Radar-bright means rough at centimetre scales, which here probably means fractured ice, boulders and unsorted debris instead of the smooth organic sand filling Titan's equatorial dune fields. Xanadu reads as the moon's exposed bedrock: high, old, and slowly being taken apart by methane weather.

Dimensions

~4,000 km east-west by 2,000 km north-south

Location

near the equator, centred ~15 S

Instrument

synthetic-aperture radar, T13 flyby

Titan diameter

5,150 km, larger than Mercury

PIA08552 · A long narrow greyscale radar strip. Bright speckled ridges and ranges run through the middle, cut by darker sinuous lines that branch like river systems. Two large soft-edged circular depressions interrupt the bright terrain, their rims incomplete and blurred. · NASA/JPL
PIA08429 · A radar close-up in grainy greyscale. Two roughly circular outlines stand out against bright rough ground, each with a darker smoother interior and a broken, partly erased rim, surrounded by fine bright speckle. · NASA/JPL

The Tiger Stripes

CASSINI ISS · 11 AUGUST 2008

Four parallel gashes cross the south pole of a moon five hundred kilometres wide, each about a hundred and thirty kilometres long, spaced with a regularity that looks engineered. They are named for cities in The Arabian Nights: Alexandria, Cairo, Baghdad, Damascus. They are the warmest places on Enceladus and the reason the moon matters at all.

Cassini flew this mosaic at 14.5 metres per pixel, close enough to pick out individual blocks along the fracture walls and to locate where the jets emerge. The fractures run roughly two kilometres wide and five hundred metres deep, and infrared measurements found their floors near 180 kelvin against a surface averaging 75.

Something is opening and closing them. Saturn's tides flex the ice shell on a 1.37-day orbit, the fractures work like valves, and around five gigawatts of heat escapes through them continuously. On a body this small that budget should be impossible, which was the first clue that there is liquid water underneath.

Fractures

four, each ~130 km long

Image scale

14.5 m per pixel

Fracture-floor temperature

up to ~180 K vs ~75 K surface

South polar heat output

~5 gigawatts

PIA11114 · A high-resolution greyscale mosaic of icy ground. Two long troughs cross the frame, one diagonally near the top and one horizontally below centre, each flanked by raised rubbly flanks and broken blocks. The surrounding plain is smooth and almost crater-free, creased by finer parallel grooves. · NASA/JPL/Space Science Institute

The Plumes

CASSINI ISS · BACKLIT PLUMES

Put the sun behind Enceladus and the moon stops being a sphere. It becomes a dark disc with a hundred separate columns of ice standing off its south pole, some rising hundreds of kilometres, thin enough to see stars through and bright enough to have been mistaken for a smear on the optics the first time anyone looked.

At least 101 geysers have been identified, nearly all of them sitting on the tiger stripes. They spray water vapour and ice grains fast enough to escape the moon entirely, which is where Saturn's diffuse E ring comes from. Cassini flew directly through the plume, tasting it repeatedly, and on 28 October 2015 dropped to 49 kilometres above the surface to sample the densest part.

The sample came back with water, salts, silica, carbon dioxide, methane, ammonia and molecular hydrogen. Hydrogen implies hot rock reacting with water on a seafloor. Taken together it describes a habitable environment under twenty kilometres of ice, on a moon generous enough to spray it into space where a passing spacecraft could catch it.

Identified geysers

at least 101

Closest plume pass

49 km, 28 October 2015

Plume contents

H2O, CO2, CH4, NH3, H2, salts, silica

Enceladus diameter

504 km

PIA11688 · A dark crescent of moon against black, with a fan of narrow luminous spikes rising from its lower limb. Individual jets are distinguishable near the surface before merging into a broad diffuse glow higher up. The nightside disc is faintly visible as a lighter grey circle. · NASA/JPL/Space Science Institute

Audio: Radio and plasma waves travelling between Saturn and Enceladus, recorded by RPWS · NASA/JPL-Caltech/University of Iowa

Video: Animated Cassini ISS imaging of icy cryovolcanic jets erupting from Enceladus south pole · NASA/JPL/Space Science Institute

The Global Ocean

GRAVITY + LIBRATION · 2014-15

The ocean was never seen. It was inferred twice, by two methods that had no particular business agreeing as closely as they did. The first was gravity: precise Doppler tracking during three close flybys showed more mass beneath the south pole than a uniform ball of ice should have, consistent with liquid water displacing frozen.

The second was a wobble. Cassini tracked surface features across seven years and measured Enceladus rocking very slightly as it orbits, a libration far too large for a body frozen through. The only way the crust rocks that freely is if it is not attached to the core, which means a global ocean between them rather than a regional sea.

Current estimates put the ice shell near twenty kilometres thick on average and considerably thinner over the south pole, with liquid water beneath it everywhere. A moon 504 kilometres across, cold enough to be geologically dead, is instead holding an ocean in contact with rock. That combination is why Enceladus is now a destination rather than a curiosity.

Ocean extent

global, beneath the entire ice shell

Ice shell thickness

~20 km average, thinner at the south pole

Evidence

gravity from 3 flybys, 7-year libration measurement

Announced

2014 (gravity), 2015 (global ocean)

PIA19656 · A cutaway illustration of a small moon with a wedge removed. Layers are shown from the outside in: a white icy crust, a deep blue shell of liquid water wrapping the whole body, and a dark grey rocky core at the centre. Thin white jets rise from cracks at the bottom of the sphere. · NASA/JPL-Caltech

Herschel

CASSINI ISS · 13 FEBRUARY 2010

Mimas is 396 kilometres across and carries a 130-kilometre crater on one side, roughly a third of its own diameter. The resemblance to a certain fictional battle station was noted immediately and has never been shaken off. The comparison is unfair mainly because Mimas is by some distance the more improbable object.

Herschel's walls stand about five kilometres high, its floor lies ten kilometres below the rim, and a central peak rises six kilometres from that floor. Whatever struck here came close to breaking the moon apart, and fractures on the far side may be shock damage from the same impact. Mimas survived by being small enough to absorb it and cold enough to keep the scar.

Cassini made its closest approach on 13 February 2010, passing within about 9,500 kilometres, and this mosaic came from that flyby at 180 metres per pixel. The same modest moon quietly maintains the Cassini Division tens of thousands of kilometres inward, holding open a gap nearly as wide as Mercury by resonance alone.

Mimas diameter

396 km

Herschel crater

130 km wide, 10 km deep, 5 km walls

Closest approach

13 February 2010, ~9,500 km

Image scale

180 m per pixel

PIA12568 · A small grey cratered sphere seen nearly face-on, dominated by one enormous circular crater right of centre with steep bright walls, a hummocky floor and a prominent central peak. The rest of the surface is saturated with smaller overlapping craters whose walls appear brighter than their floors. · NASA/JPL/Space Science Institute

The Equatorial Ridge

CASSINI ISS · 10 SEPTEMBER 2007

Iapetus has a seam. A chain of mountains runs along its equator for roughly 1,300 kilometres, up to twenty kilometres wide, with peaks reaching thirteen kilometres above the surrounding ground. It follows the equator so precisely that coincidence is not a serious option. Nothing else in the solar system looks like this.

Cassini flew past once, on 10 September 2007, and resolved the peaks at twenty-three metres per pixel from under four thousand kilometres out. Dark material drapes the slopes, and bright ice shows where an impact has punched through.

The explanations are all uncomfortable. Iapetus may have spun much faster when young, frozen into a flattened shape, and cracked along the equator as it slowed. Or it may have had a small ring of its own that decayed and came down in a line. Neither has been demonstrated. The ridge is still the most conspicuous unexplained landform Cassini found.

Ridge length

~1,300 km along the equator

Peak height

up to ~13 km

Image scale

23 m per pixel

Flyby

10 September 2007, the only close pass

PIA08372 · A close, low-angle view of jagged mountain peaks casting long shadows across very dark ground. The slopes are almost black, while a bright white patch of exposed ice sits near the centre where a crater has cut through the dark surface layer. · NASA/JPL/Space Science Institute

Two-Tone World

CASSINI ISS · IAPETUS MOSAIC

One hemisphere of Iapetus reflects about a twentieth of the light that hits it. The trailing one more than half. The boundary between them is a graded transition, and the dark face always points forward along the moon's orbit. Giovanni Cassini noticed the effect in 1671, by failing to see the moon on one side of Saturn.

The mechanism appears to be runaway thermal migration. Dark dust, most likely swept up from Saturn's distant outer moons, coats the leading face. Dark absorbs more sunlight, warms to about 129 kelvin, and water ice sublimates away, leaving more dark residue behind and darkening further. The vapour recondenses on the cold bright hemisphere and at the poles.

It is a feedback loop that needed the moon's own slow rotation to run: Iapetus turns once every 79 days, so each face bakes for nearly forty at a stretch. The result reads as two different worlds photographed together, and is one of the cleanest demonstrations available that a surface can paint itself.

Albedo contrast

~0.05 leading vs ~0.50 trailing

Iapetus diameter

1,470 km

Rotation

79.3 days, tidally locked

Dark-side temperature

~129 K, warm enough to sublimate ice

PIA11690 · A sphere split into two starkly different halves. The left portion is near-black with a faint brownish cast, the right is bright white-grey and heavily cratered. Between them the tone shifts gradually, with dark patches scattered on the bright side and bright crater walls on the dark side. · NASA/JPL/Space Science Institute
PIA08384 · A false-colour globe with a heavily cratered pale trailing hemisphere and an intensely reddish-brown region encroaching from one limb. A large shallow impact basin dominates the lower half, and the polar areas are the most densely cratered part of the frame. · NASA/JPL/Space Science Institute

The Spacecraft

CASSINI-HUYGENS · 1997-2017

It left Cape Canaveral on 15 October 1997 weighing five and a half tonnes, most of that propellant, carrying twelve instruments and a European probe bolted to its flank. It could not reach Saturn directly. It flew past Venus twice, then Earth, then Jupiter, borrowing speed for nearly seven years before it arrived.

Orbit insertion was a ninety-minute engine burn on 30 June 2004, executed after threading the gap between the F and G rings with no possibility of a second attempt. What followed was thirteen years and 294 orbits: 127 Titan flybys, 23 passes at Enceladus, 453,048 images and some 635 gigabytes of science data.

Cassini found geysers on Enceladus, seas on Titan, moons nobody knew were there, and a ring system lighter than expected and possibly far younger than the planet it circles. It travelled 7.9 billion kilometres. When the propellant ran low, the team chose to destroy it rather than risk contaminating either of the moons it had made interesting.

Launch

15 October 1997, Titan IVB/Centaur

Saturn orbit insertion

30 June / 1 July 2004

Mission at Saturn

13 years, 294 orbits

Data returned

453,048 images, ~635 GB

PIA03883 · An illustration of a large spacecraft with a white dish antenna and a long boom, seen against Saturn and its rings. A pale exhaust plume trails from the engine at the base. The rings cut across the background at a shallow angle and the planet is lit from one side. · NASA/JPL

Pioneer 11

PIONEER 11 · 1 SEPTEMBER 1979

Nobody had been to Saturn, so nobody knew whether the rings would shred a spacecraft. Pioneer 11 crossed the ring plane at 114,000 kilometres an hour with no way to find out first. It came within 21,000 kilometres of the cloud tops, passed within about 4,000 kilometres of a small moon nobody had catalogued, and found a new ring.

The route was chosen for the spacecraft behind it. Voyager 2 had to take the same outer path to be swung on toward Uranus and Neptune, and Pioneer 11 was the test that it could be done.

It kept going and last made contact in 1995. It carries a plaque that gives our address.

Closest approach

1 September 1979

Distance from cloud tops

about 21,000 km

Ring-plane speed

about 114,000 km/h

Discovered

The F ring and two moons

Last contact

1995

P-21280 · Saturn and its rings photographed by Pioneer 11 during its 1979 flyby · NASA / Ames Research Center

Voyager 1

VOYAGER 1 · 12 NOVEMBER 1980

Voyager 1 could have gone to Pluto. That option was built into its flight path, with an arrival around 1986. The alternative was a close pass of Titan, to see what the orange haze was hiding. Mission planners chose Titan.

The Titan flyby bent the spacecraft up out of the plane of the planets, and no further planet was ever going to be on its route. Once it had its data, Voyager 2 was cleared to go on to Uranus and Neptune. The cameras saw only haze. The other instruments found a thick nitrogen atmosphere.

It is now in interstellar space, the most distant human-made object.

Closest approach

12 November 1980

Distance from Titan

about 6,500 km

Option given up

A Pluto flyby, about 1986

Discovered

Titan's atmosphere

Now

Interstellar space

PIA01483 · Saturn seen as a crescent from Voyager 1 departing the system in November 1980 · NASA / JPL

Voyager 2

VOYAGER 2 · 25 AUGUST 1981

Nine months after its twin, Voyager 2 passed Saturn at about 101,000 kilometres and skipped a close look at Titan. That kept its trajectory open for two more planets.

It reached Uranus in 1986 and Neptune in 1989. It is still the only spacecraft to have visited either.

Its route depended on Voyager 1. Only after Voyager 1 had its data at Titan was Voyager 2 cleared to go on. Between them the pair covered both choices: one looked closely, one kept moving.

Closest approach

25 August 1981

Distance from Saturn

about 101,000 km

Went on to

Uranus (1986) and Neptune (1989)

Now

Interstellar space

PIA01364 · Saturn's ring system photographed in detail by Voyager 2 · NASA / JPL