Which gas is used to fill the wheels of an aircraft, and why?

 Nitrogen is used. It is an inert gas so high pressures and temperatures do not affect it as with air. It is also more compatible with rubber, and does not corrode like air containing oxygen and water vapor does. More importantly it will not support combustion or explosion as does air containing oxygen.

The FAA requires nitrogen in all commercial aircraft tires to eliminate the potential for water vapor from freezing at high altitudes. At altitude, air temps at the tires reach -30F. Any moisture in the tire can freeze, and if there is enough moisture in the tire, it can form ice and that can put the tire out of balance. Nitrogen doesn't form a liquid till -173C and pure nitrogen has almost no moisture.

Nitrogen does not expand or contract as much as oxygen, so the pressure inside the tyre does not alter as much between minus 50C at altitude and plus 200C or so when the tyre heats up as it hits the runway, compared to a tyre filled with air. Oxygen also reacts with rubber, and when this corrosion starts, the small particles break off and form rust and dust, causing them to leak. Nitrogen is far less reactive and it is not corrosive. Wheel surfaces stay smooth and clean, rubber remains supple and resilient.

Boeing has received reports of confirmed cases in which a wheel and tire assembly exploded when the oxygen in air-filled tires combined with volatile gases given off by a severely overheated tire. As a result, the U.S. Federal Aviation Administration issued Airworthiness Directive 87-08-09 requiring that only nitrogen be used to inflate airplane tires on braked wheels. However, tires may be topped off with air in remote locations where nitrogen may not be available if the oxygen content in the tire does not exceed 5 percent by volume.

Why is Iran a relatively small exporter of natural gas compared to Qatar despite having similar reserves?

 Beneath the Persian Gulf, Iran and Qatar share the largest natural gas field on Earth. Yet one became a global export powerhouse, while the other barely sells a drop abroad.

The divergence comes down to a mix of demographics, geopolitics, and infrastructure. Several major factors keep Iranian natural gas largely at home:

• Massive Domestic Consumption: Qatar has a population of around 3 million people, meaning almost all the gas it pulls from the ground can be sold abroad. Iran, by contrast, is a sprawling nation of over 85 million residents. The vast majority of Iranian natural gas never leaves its borders. It is heavily subsidized and used to heat homes, power electricity grids, and fuel a large domestic petrochemical industry. Additionally, Iran must reinject huge volumes of natural gas into its aging oil fields to maintain well pressure and keep its lucrative crude oil flowing.
• The Sanctions Barrier: Exporting natural gas on a global scale requires chilling it to -260°F (-162°C) to create Liquefied Natural Gas (LNG), a process that requires massive capital and highly specialized technology. In the 1990s and 2000s, Qatar partnered with major international energy firms to build state-of-the-art LNG terminals. Iran, however, has faced decades of stringent international sanctions. These financial and technological embargoes effectively locked Iran out of the LNG revolution by barring Western companies from providing the necessary liquefaction technology or the billions in required foreign investment.
• Pipeline Reliance: Because Iran lacked access to the LNG technology needed to load gas onto ocean-crossing ships, its export strategy has relied almost entirely on regional pipelines. Building pipelines is geographically restrictive, expensive, and politically complex. While Iran manages to export some gas to neighboring countries like Turkey and Iraq, these pipeline volumes represent a mere fraction of what Qatar's fleet of massive LNG carriers transports to buyers in Asia and Europe.

Ultimately, while the two nations sit on the exact same gas field, Qatar was free to build a global export machine, while Iran was forced to use its share to sustain its own massive and heavily isolated domestic economy.

Are there any "gas giant" moons?

 

We have not yet discovered any exomoons. Many inevitably exist, as all planets in our system except Mercury and Venus have moons. Our instruments for detecting them elsewhere in the universe are too crude yet. Gas giant exomoons will likely be found in the future and will not be much smaller than the exoplanet they orbit.

As the above image shows, gas giant planets don't get much more significant in diameter with the increase in mass due to compression from high gravity. Brown dwarf stars start from 13 times the mass of Jupiter, while lowest-mass red dwarf stars are at least 80 times as massive as Jupiter, yet they aren't that much bigger in diameter. We found some outliers in this regard. Some gas giant exoplanets orbit their stars so close that they get puffed up from being fried by high temperatures.

A very massive gas-giant exoplanet that is nearly 13 times the mass of Jupiter can potentially have moons that are Neptune, Saturn, or even Jupiter-like, and they would be gas-giant moons.

In the past, we had doubts about whether such moons could form, but in October 2023, the James Webb Space Telescope discovered many Jupiter-like double rogue exoplanets floating in space without stars in the Orion Nebula. The current leading theory is that they formed in typical star systems and were ejected. Some planets can end up jettisoned from systems during planetary formation, collide, or fall into the star and be destroyed before the orbits settle on stable configurations.

If such double Jupiter-like planets indeed formed in star systems and not directly in space like suns form, then we will find double gas giant planets and massive gas giant planets with gas giant Jupiter- or Saturn-size exomoons in the future.

In our system, Titan, the moon of Saturn, is larger than the planet Mercury but less massive and has a thicker atmosphere than Earth. It is the closest we get to a gas giant moon in our backyard.