The recipe for the distinctive colour of Jupiter’s Great Red Spot has long been a mystery, but now researchers have an idea of how it got so red. Start with an ammonia-rich cloud deck and add intense solar radiation for eons.

Ammonia forms the top layer of Jupiter’s atmosphere. Below that are clouds of ammonium hydrosulphide, a compound that breaks up at temperatures found on Earth but is stable on Jupiter. The lowest clouds are water. All three gases form white clouds, so what gives them a red hue?

at Northern Arizona University in Flagstaff suspects high-energy radiation from space splits molecules in the clouds, with the fragments combining to form complex compounds that look red to the eye.

Competing reds

He found that irradiating ammonium hydrosulphide with high-energy protons produces coloured granules. They look green when the sample is held at -113°C, slightly cooler than the cloud tops, and look reddish when cooled to -223°C, colder than the clouds.

When the granules are compared to the light spectrum coming from the Great Red Spot, Loeffler says some of the visible wavelengths match up to his low-irradiance experiments. He suspects the green in his samples may come from a sulphur radical produced in the lab.

at the Jet Propulsion Laboratory in California says a better mix of ingredients is ammonia and the hydrocarbon acetylene. Last year, he reported that irradiating this mix with ultraviolet light produced reddish material that more closely matched the red spot spectrum. And when blended with white clouds, it matches shades of red seen across Jupiter’s surface.

The missing piece

Sulphur is a much more abundant ingredient in Jupiter’s atmosphere than acetylene, but Carlson says the hydrocarbon is concentrated in the right place to redden the planet’s iconic spot. Acetylene forms when sunlight splits methane molecules in the upper atmosphere, then drifts down to the ammonia clouds where solar ultraviolet can trigger further reactions to produce the reddish compounds.

Carlson says the red spot is darker than the rest of the cloud deck because its ammonia clouds rise higher than those on the rest of the planet, so they encounter more acetylene molecules to make more of the red stuff.

The red material Carlson made “has optical properties that are an excellent match to the spectrum of the Great Red Spot,” says Larry Sromovsky of the University of Wisconsin-Madison. By adjusting particle size and concentration, their model could match the visible spectra of other reddish clouds on Jupiter, unlike Loeffler’s material.

The problem, Sromovsky says, is that for Jupiter’s Great Red Spot to be as red as it is, the gas giant’s atmosphere would have to produce far more of this reddish material than it possibly can. The missing piece could be reddish organic molecules called tholins, which are found on Titan and many other cold or icy bodies in the solar system, but have not been identified on any planet other than Pluto.

Read more: NASA spacecraft could plumb depths of Jupiter’s Great Red Spot

A pair of gnashers

Wrong teeth

They looked like loons but honked like geese, and are kin to a group of modern birds that includes ducks, geese and chickens. Meet the Vegaviidae, a newly named group of waterbirds that seemingly lived through the mass extinction that took out the dinosaurs.

Although the Vegaviidae are now extinct, they are the first bird group known to have survived the mass extinction, says at the in Buenos Aires.

At the end of the Cretaceous Period 66 million years ago, a mass extinction – probably caused by an asteroid impact – wiped out a swathe of species, including all non-avian dinosaurs. Birds survived the disaster, but which groups carried the flame has been unclear.

Bird fossils from the end-Cretaceous between 72 and 66 million years ago are few and fragmentary, says at the American Museum of Natural History in New York.

One of the few known species is Vegavis iaai, discovered on Vega Island off West Antarctica and . Vegavis was a fish-eating diving bird that resembled a modern loon. However, Agnolin says its skeleton shows that it was related to ducks and geese, and to land fowl such as chickens.

Last year, palaeontologists described a second Vegavis fossil that included a syrinx, the bird version of vocal cords. They concluded that the birds honked like geese.

Fossil relations

Agnolin and his colleagues have now compared that fossil with other bird fossils from the southern polar regions. They believe that Vegavis forms a group with three other bird species and some unnamed, mostly fragmentary fossils. They call the group Vegaviidae.

Two of the Vegaviidae lived during the late Cretaceous: Polarornis gregorii in Antarctica and Neogaeornis wetzeli in Chile. The third, Australornis lovei, lived in New Zealand soon after the asteroid impact – suggesting the group survived the mass extinction.

It seems that the Vegaviidae diversified before the mass extinction. Their survival supports the idea that birds from southern continents were vital in the evolution of modern birds.

It’s not clear why this group lived on. However, there are also signs that the bones of Vegavis and Polarornis grew rapidly, something that also happens in modern birds. Agnolin says that might have given them an edge over another group: the Enantiornithes, or “opposite birds”, which died out at the end of the Cretaceous.

“A circum-Antarctic clade of diving, primitive, duck-like birds makes sense,” says at the Natural History Museum of Los Angeles County in California. But he says the family tree proposed by Agnolin is unconvincing, because the analysis does not include enough species. This means the birds might not all belong in one group after all, and therefore that we may still not have identified a bird group that survived the extinction.

Agnolin is right to place Polarornis with Vegavis, says at the Senckenberg Research Institute in Frankfurt, Germany. He is, however, also not convinced that the other species have enough in common to be labelled close relatives.

Nevertheless, most of the known modern-looking birds from the late Cretaceous were aquatic, so Mayr says the ancestors of today’s birds may have been at least semi-aquatic.

Journal reference: The Science of Nature, DOI:

Dune on the moon

TITAN’S methane lakes. Icy Enceladus spouting geysers of hot water. Sponge-like Hyperion. Ravioli-shaped Pan and Atlas. Iapetus with its equatorial ridge battered by ancient craters. Close-ups of those iconic rings engirdling the gas-giant planet itself, and gigantic hurricanes around its poles.

The Cassini probe, launched in 1997, has orbited the Saturn system for 13 years. What it has revealed is astounding – and challenges our understanding of planets and their satellites everywhere, says planetary scientist of the Southwest Research Institute in Boulder, Colorado. “If everything comes from the same processes, should we get so much diversity?” he asks. “It seems like every moon has a part of the story to tell.”

Right up to this weekend’s “Grand Finale”, Cassini has been collecting more data and snapping fresh images with a series of low swoops over Saturn’s moons and through its rings. But what it has already seen constitutes, arguably, the richest haul of discoveries from any mission yet mounted to another planet. “It’s going to be tough to say goodbye to Cassini,” says Salmon.

The rings

After a six-and-a-half-year journey, Cassini entered orbit around Saturn on 1 July 2004 – and immediately encountered the planet’s signature feature, its rings. Pictures collected by the Voyager probes when they flew past Saturn in 1980 and 1981 suggested the planet was girdled by about 10,000 rings, each a cloud of particles tightly confined to a narrow orbit. “Now the number is in the millions,” says , principal investigator for Cassini’s ultraviolet imaging spectrograph.

The rings are also complex: Cassini’s images have revealed clumps, holes, gaps and other structures. Some wave-like features are due to gravitational interactions with the moons embedded in the rings, but the origin of others is unclear.

The rings probably formed initially when a large moon came too close to Saturn and was ripped apart by gravitational forces. Esposito thinks this was early in the solar system’s history, and that the rings have gradually spread since then, perhaps forming moons in the process. Others think the rings go through cycles: moons collide forming new rings that coalesce into new moons which eventually collide again, with the current rings as little as 100 million years old.

Old rings are expected to be much more massive than young ones. In its past final weeks, Cassini has been repeatedly passing between the rings and the planet, accurately measuring the rings’ mass to perhaps resolve the question.

Enceladus

Before Cassini, researchers had expected this icy, 500-kilometre-diameter moon to be frozen solid. But on an early fly-by in February 2005, the spacecraft’s magnetometer “sensed something unusual going on with its magnetic field”, says Cassini project scientist of the Jet Propulsion Laboratory (JPL) in Pasadena, California.

A later pass showed that the south pole was much warmer than expected, and was spouting geysers of salty water into space. Enceladus circles Saturn twice for every orbit of the larger moon Dione, inducing a gravitational interaction that melts ice inside both moons. The process squeezes Enceladus, ejecting jets of water from large fracture zones near its south pole (pictured below). Cassini measured the composition of these jets, detecting raw materials for life including salt, water, carbon dioxide, methane, other organic molecules and, most recently, hydrogen, an ideal energy source for life.

Silica found in the jets can be produced only in water close to boiling point, indicating that hydrothermal vents are also present in the subsurface ocean – making the icy moon a hot target in the search for life.

Hyperion

Trapped in a gravitational resonance with Titan, Hyperion (right) tumbles chaotically in orbit. Subject of an early fly-by in September 2005, its light, porous-looking surface resembles a battered sponge, but no one quite knows why. One possibility is that it is a fragment of a larger object shattered in a past collision. The dark zones look lower than the light-coloured ridges, perhaps because they absorbed more sunlight, causing ices below them to evaporate and the dark layer to sink down.

Iapetus

At first glance, an equatorial ridge girdling Iapetus looks like a moulding mark on a factory-fresh rubber ball. A Cassini fly-by in 2007 revealed that the ridge is as heavily cratered as the rest of the 1500-kilometre-diameter moon’s surface, so it must have formed long ago. Iapetus’s surface is also oddly two-toned, with a darker leading edge. This is caused by gravitational forces that lock the moon into position around Saturn, causing its front face to sweep up dust.

Titan

When Voyager 1 passed Titan in 1980, it couldn’t see the surface of Saturn’s largest moon: solar ultraviolet radiation drives reactions between nitrogen and methane molecules in its atmosphere that yield a thick, orange-brown gunk. The purpose of Cassini’s Huygens lander, built by the European Space Agency, was to find out what lay beneath. Voyager had discovered that the temperature and pressure on Titan’s surface would allow liquid methane. Huygens, released on 14 January 2005, was made to withstand a wet or dry landing.

Photos taken during the lander’s 150-minute descent showed networks of branching streams possibly carved by liquid methane. But the touchdown was hard, on a cobblestone-strewn flood plain near Titan’s equator like “something you might see in Death Valley”, says of Cornell University in Ithaca, New York. But at around -180°C it was much colder, with a surface covered in plastic shavings and foam beads.

Huygens transmitted data from the surface for 72 minutes until its battery failed. In the years since, Cassini has probed Titan’s atmosphere and mapped its surface on successive fly-bys, confirming the presence of liquid methane. In radar observations a few weeks apart it found evidence that methane showers had soaked the soil, then evaporated – the first proof of precipitation beyond Earth.

Titan’s landscape is eerily calm, with methane seas and lakes that are “fantastically flat”, says Hayes. They are more transparent than water lakes: a radar echo from one was reflected from its bottom, 160 metres down. Bright “magic islands”, which appear briefly in the dark lakes before disappearing, are thought to be nitrogen bubbling out of solution.

Perhaps oddest of all, Titan has two ocean levels. Beneath the hydrocarbon seas on the surface, under a shell of water ice, lies salty liquid water. This hidden ocean is, says Hayes, “the most accessible laboratory for prebiotic chemistry in the solar system” – a potential habitat for life.

Mimas

At 396 kilometres in diameter, Mimas is the smallest known rounded body in the solar system. Seen closest by Cassini in February 2010, it’s not completely round, however: one side is dominated by the 130-kilometre Herschel crater with walls 5 kilometres high. The giant pit makes Mimas, right, look eerily like the planet-destroying Death Star in the “Star Wars” movies. It is, however, extremely vulnerable: made principally of water ice, cracks on its opposite side show that a past impact came close to shattering it.

Hurricanes on Saturn

Hurricanes on Earth tend to go towards the poles, but those on Saturn, pictured right, are fixed there. They have central eyes and eye-wall clouds like terrestrial hurricanes, and spin in the same way, but at 4000 kilometres across, three of them side by side would span Earth’s diameter. Terrestrial hurricanes are powered by heat released from warm ocean surfaces. There’s nothing like that on Saturn, so what powers its storms remains a mystery.

Pan

Fat, round, ravioli-shaped Pan orbits in a gap in Saturn’s A-ring, the outermost of the large, bright rings. Its central core is icy, but ring particles accumulate on a strip around its circumference, fattening Pan out to a 35-kilometre diameter. Revealed in great detail in images taken in March 2017, this belt is cratered, with signs of a small landslide pulled downhill by the moon’s gravity. Atlas, another moon in the A-ring, is similar, but its skirt shows no craters and looks fluffier. The moons’ growth may be limited by a gravitational tug of war between them and Saturn: if ring particles pile too high on Pan’s equator, the planet’s gravity tugs them off again.

Why the grand finale?

Almost 20 years after leaving Earth, Cassini’s plutonium-powered generators are running out of fuel needed to adjust its course. Left to drift, it might collide with one of Saturn’s moons, perhaps contaminating an environment that might contain or provide the conditions for life. To avoid that possibility, it will be steered into the atmosphere of Saturn itself, burning up like a meteor as it becomes part of the planet whose environment it has spent so long exploring.

This article appeared in print under the headline “Whole new worlds”