Hey Bill Nye! Why Don't Gas Giants Have Gas Moons?

A very small person asks a very big question: why aren't the moons of gaseous planets also made of gas?

Aria: Hi Bill Nye. My name is Aria. My question is where do all moons come from and why do gas giants not have gas moons?

Bill Nye: Wow. That is a great question Aria. Wow. First of all I was alive when people figured out or satisfied themselves as to where our moon came from. And it's generally agreed that our moon was created when the earth was hit with another pretty big thing, another asteroid. And the impact was very hard and the energy of the smashing was converted to heat and both the earth and the moon were hot and molten and they both cooled off and here we are with this other separate thing in orbit around us. Now why do gas giants not have gas moons is a great question and the answer is almost certainly because of gravity.

So when you have these little things that spun off of, let's say Jupiter or Saturn, they were too small to sustain themselves as just balls of gas. The gas shrunk down, gravity pulled it down and formed a solid thing. Whereas on Jupiter and Saturn there's so much spin, so much heat created that these are held in the gaseous state or what we call gaseous state by the energy that's coming out of them, where as the smaller ones cooled off so much that they turned solid. It is just a fantastic question, Aria. And astrophysicists or planetary scientists speculate about this all the time, but what a fantastic thing that you made this observation. The moons of Jupiter, the big ones Io, Callisto, Europa and Ganymede those big moons are cooled off. They're not gaseous like Jupiter, what they orbit. That is fantastic. Wow. And maybe more will be learned about this.

And I gotta tell you, Aria, it's very reasonable that in your lifetime we will send the right spacecraft to Europa and the moon of Saturn, Enceladus and Titan, and look for signs of life. Instead of being a gas giant, in this one example, Europa is the moon of Jupiter that has twice as much seawater as the earth and that water is under a layer of ice. And I mention it because it's evidence of how things cool off when they're in space and they're small – or relatively small. The ice forms because the heat of creation, things smash together and that heat is radiated into space so there's a crust of ice, but the water underneath is kept liquid by the gravitational exercising of its orbit around Jupiter. It's amazing. What an observation. Aria, you're making my day. I hope you will be involved in missions to Europa and you will look for signs of life below the ice. I mean who knows if you have an ocean for four and a half billion years things are going to happen. There could be living things there. There could be Europanians out there, fish people that you'll interact with. Keep us posted. Thank you.

It took a very small person to ask a big question, one that planetary scientists pondered for a long time. There are four gas giants in our solar system – Jupiter, Saturn, Uranus and Neptune – but why are their moons not made of gas? They’re solid, unlike the planets they orbit.


As always, Bill Nye is on standby to explain the workings of the world and universe around us. The widely accepted theory on how Earth’s moon was created is known as the Giant Impact Hypothesisis. It states that a very young proto-Earth was struck by an asteroid, and in that collision both Earth and the debris that chipped off it became molten. They both cooled off, and now we have a moon; nicely spherical (although not perfectly) and solid, in orbit around us.

So why don’t gas giants have gas moons? The simple answer: gravity. When Jupiter or Saturn have undergone similar collisions or events that resulted in a fragment being spun off into their orbits, those new moons were too small to sustain themselves in a gaseous state. Gravity pulled the gas down until it formed a solid sphere. A gas giant, on the other hand, stays gaseous because of its size; there is so much heat generated from its spin that it cannot cool down into a solid state. Jupiter, for example, has 67 known moons and even the bigger ones like Io, Callisto, Europa and Ganymede, have cooled off – they're non-gaseous, unlike Jupiter itself.

Much can be learned after we know the state of moons; we now know that Europa has twice as much seawater as Earth, but that the water is under a crust of ice. One day, we may find life in those oceans – they’ve been around for 4.5 billion years, so it’s likely Europanians may be living there, says Nye.

Bill Nye's most recent book is Unstoppable: Harnessing Science to Change the World.

A map of America’s most famous – and infamous

The 'People Map of the United States' zooms in on America's obsession with celebrity

Image: The Pudding
Strange Maps
  • Replace city names with those of their most famous residents
  • And you get a peculiar map of America's obsession with celebrity
  • If you seek fame, become an actor, musician or athlete rather than a politician, entrepreneur or scientist

Chicagoland is Obamaland

Image: The Pudding

Chicagoland's celebrity constellation: dominated by Barack, but with plenty of room for the Belushis, Brandos and Capones of this world.

Seen from among the satellites, this map of the United States is populated by a remarkably diverse bunch of athletes, entertainers, entrepreneurs and other persons of repute (and disrepute).

The multitalented Dwayne Johnson, boxing legend Muhammad Ali and Apple co-founder Steve Jobs dominate the West Coast. Right down the middle, we find actors Chris Pratt and Jason Momoa, singer Elvis Presley and basketball player Shaquille O'Neal. The East Coast crew include wrestler John Cena, whistle-blower Edward Snowden, mass murderer Ted Bundy… and Dwayne Johnson, again.

The Rock pops up in both Hayward, CA and Southwest Ranches, FL, but he's not the only one to appear twice on the map. Wild West legend Wyatt Earp makes an appearance in both Deadwood, SD and Dodge City, KS.

How is that? This 'People's Map of the United States' replaces the names of cities with those of "their most Wikipedia'ed resident: people born in, lived in, or connected to a place."

‘Cincinnati, Birthplace of Charles Manson'

Image: The Pudding

Keys to the city, or lock 'em up and throw away the key? A city's most famous sons and daughters of a city aren't always the most favoured ones.

That definition allows people to appear in more than one locality. Dwayne Johnson was born in Hayward, has one of his houses in Southwest Ranches, and is famous enough to be the 'most Wikipedia'ed resident' for both localities.

Wyatt Earp was born in Monmouth, IL, but his reputation is closely associated with both Deadwood and Dodge City – although he's most famous for the Gunfight at the O.K. Corral, which took place in Tombstone, AZ. And yes, if you zoom in on that town in southern Arizona, there's Mr Earp again.

The data for this map was collected via the Wikipedia API (application programming interface) from the English-language Wikipedia for the period from July 2015 to May 2019.

The thousands of 'Notable People' sections in Wikipedia entries for cities and other places in the U.S. were scrubbed for the person with the most pageviews. No distinction was made between places of birth, residence or death. As the developers note, "people can 'be from' multiple places".

Pageviews are an impartial indicator of interest – it doesn't matter whether your claim to fame is horrific or honorific. As a result, this map provides a non-judgmental overview of America's obsession with celebrity.

Royals and (other) mortals

Image: The Pudding

There's also a UK version of the People Map – filled with last names like Neeson, Sheeran, Darwin and Churchill – and a few first names of monarchs.

Celebrity, it is often argued, is our age's version of the Greek pantheon, populated by dozens of major gods and thousands of minor ones, each an example of behaviours to emulate or avoid. This constellation of stars, famous and infamous, is more than a map of names. It's a window into America's soul.

But don't let that put you off. Zooming in on the map is entertaining enough: celebrities floating around in the ether are suddenly tied down to a pedestrian level, and to real geography. And it's fun to see the famous and the infamous rub shoulders, as it were.

Barack Obama owns Chicago, but the suburbs to the west of the city are dotted with a panoply of personalities, ranging from the criminal (Al Capone, Cicero) and the musical (John Prine, Maywood) to figures literary (Jonathan Franzen, Western Springs) and painterly (Ivan Albright, Warrenville), actorial (Harrison Ford, Park Ridge) and political (Eugene V. Debs, Elmhurst).

Freaks and angels

Image: Dorothy

The People Map of the U.S. was inspired by the U.S.A. Song Map, substituting song titles for place names.

It would be interesting to compare 'the most Wikipedia'ed' sons and daughters of America's cities with the ones advertised at the city limits. When you're entering Aberdeen, WA, a sign invites you to 'come as you are', in homage to its most famous son, Kurt Cobain. It's a safe bet that Indian Hill, OH will make sure you know Neil Armstrong, first man on the moon, was one of theirs. But it's highly unlikely that Cincinnati, a bit further south, will make any noise about Charles Manson, local boy done bad.

Inevitably, the map also reveals some bitterly ironic neighbours, such as Ishi, the last of the Yahi tribe, captured near Oroville, CA. He died in 1916 as "the last wild Indian in North America". The most 'pageviewed' resident of nearby Colusa, CA is Byron de la Beckwith, Jr., the white supremacist convicted for the murder of Civil Rights activist Medgar Evers.

As a sampling of America's interests, this map teaches that those aiming for fame would do better to become actors, musicians or athletes rather than politicians, entrepreneurs or scientists. But also that celebrity is not limited to the big city lights of LA or New York. Even in deepest Dakota or flattest Kansas, the footlights of fame will find you. Whether that's good or bad? The pageviews don't judge...

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Thumbs up? Map shows Europe’s hitchhiking landscape

Average waiting time for hitchhikers in Ireland: Less than 30 minutes. In southern Spain: More than 90 minutes.

Image: Abel Suyok
Strange Maps
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  • However, as this map shows, thumbing a ride still occupies a thriving niche – if at great geographic variance.
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Technology may soon grant us immortality, in a sense. Here's how.

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