Matt Williams is a space journalist and science communicator for Universe Today and Interesting Engineering. He's also a science fiction author, podcaster (Stories from Space), and Taekwon-Do instructor who lives on Vancouver Island with his wife and family.
Uranus as seen through the automated eyes of Voyager 2 in 1986. (Credit: NASA/JPL).
The gas (and ice) giant known as Uranus is a fascinating place. The seventh planet from out Sun, Uranus is the third-largest in terms of size, the fourth-largest in terms of mass, and one of the least dense objects in our Solar System. And interestingly enough, it is the only planet in the Solar System that takes it name from Greek (rather than Roman) mythology.
But these basic facts really only begin to scratch the surface. When you get right down to it, Uranus is chock full of interesting and surprising details – from its many moons, to its ring system, and the composition of its aqua atmosphere. Here are just ten things about this gas/ice giant, and we guarantee that at least one of them will surprise you.
This Galielo image shows Jupiter's moon Ganymede in enhanced colour. The JWST aimed its instruments at our Solar System's largest moon to study its surface. Credit: NASA
Many people think that the answer to ‘what is the largest moon in the Solar System’ is our Moon. It is not. Our Moon is the fifth largest natural satellite. Ganymede, a moon of Jupiter, is the largest in this solar system. At 5,268 km at the equator, it is larger than Mercury, the dwarf planet Pluto, and three times larger than the Moon orbiting Earth. According to information from NASA,if Ganymede were to break free of Jupiter’s gravitational pull it would be classified as a planet.
Ganymede has an iron-metallic core that generates a magnetic field. The core is surrounded by a mantle of rock, which is, in turn, covered by a thick shell of ice and rock. The outer shell is up to 800 km thick. On top of the outer shell is a thin layer of material(accreted?) that is a mixture of ice and rock. In images taken by the Hubble Space Telescope in 1996, astronomers discovered a tenuous atmosphere of oxygen. The atmosphere would not support known life forms, but its very existence is interesting to science.
Through spacecraft observation, there are several facts known about the surface of Ganymede. Its surface shows two types of terrain. Forty percent of the surface is covered with highly cratered dark regions, while the remainder is covered by light grooved terrain. The light grooving forms intricate patterns across surface of the moon, some of which are thousands of km long. Sulcus, a term that means a groove or burrow, is often used to describe the grooving. This portion of the terrain was most likely formed by tensional faulting or the release of water from beneath the surface. Ridges as high as 700 m have been observed. The dark regions are heavily cratered, old and rough, and are believed to be the original crust of the moon.
Ganymede was discovered by Galileo on January 7, 1610. He made the discovery, along with three other Jovian moons. It marked the first time a moon was discovered orbiting a planet other than Earth. The discovery contributed to the acceptance of the heliocentric viewpoint over the geocentric that held sway prior to that.
Now you know the answer to ‘what is the largest moon in the Solar System’ and a few interesting facts about Ganymede. Jupiter has 63 moons, so there are plenty more facts for you to discover.
A true-color image of Jupiter taken by the Cassini spacecraft. The Galilean moon Europa casts a shadow on the planet's cloud tops. Credit: NASA/JPL/University of Arizona
Jupiter, which takes its name from the father of the gods in ancient Roman mythology, is the largest planet in our Solar System. It also has the most moon’s of any solar planet – with 50 accounted for and another 17 awaiting confirmation. It has the most intense surface activity, with storms up to 600 km/h occurring in certain areas, and a persistent anticyclonic storm that is even larger than planet Earth.
And when it comes to temperature, Jupiter maintains this reputation for extremity, ranging from extreme cold to extreme hot. But since the planet has no surface to speak of, being a gas giant, it’s temperature cannot be accurately measured in one place – and varies greatly between its upper atmosphere and core.
Currently, scientists do not have exact numbers for the what temperatures are like within the planet, and measuring closer to the interior is difficult, given the extreme pressure of the planet’s atmosphere. However, scientists have obtained readings on what the temperature is at the upper edge of the cloud cover: approximately -145 degrees C.
Because of this extremely cold temperature, the atmosphere at this level is composed primarily of ammonia crystals and possibly ammonium hydrosulfide – another crystallized solid that can only exist where conditions are cold enough.
However, if one were to descend a little deeper into the atmosphere, the pressure would increases to a point where it is ten times what it is here on Earth. At this altitude, the temperature is thought to increase to a comfortable 21 °C, the equivalent to what we call “room temperature” here on Earth.
Descend further and the hydrogen in the atmosphere becomes hot enough to turn into a liquid and the temperature is thought to be over 9,700 C. Meanwhile, at the core of the planet, which is believed to be composed of rock and even metallic hydrogen, the temperature may reach as high as 35,700°C – hotter than even the surface of the Sun.
Interestingly enough, it may be this very temperature differential that leads to the intense storms that have been observed on Jupiter. Here on Earth, storms are generated by cool air mixing with warm air. Scientists believe the same holds true on Jupiter.
A close-up of Jupiter’s great red spot, an anticyclonic storm that is larger than Earth. Credit: NASA/JPL-Caltech/ Space Science Institute
One difference is that the jet streams that drive storms and winds on Earth are caused by the Sun heating the atmosphere. On Jupiter it seems that the jet streams are driven by the planets’ own heat, which are the result of its intense atmospheric pressure and gravity.
During its orbit around the planet, the Galileo spacecraft observed winds in excess of 600 kph using a probe it deployed into the upper atmosphere. However, even at a distance, Jupiter’s massive storms can be seen to be humungous in nature, with some having been observed to grow to more than 2000 km in diameter in a single day.
And by far, the greatest of Jupiter’s storms is known as the Great Red Spot, a persistent anticyclonic storm that has been raging for hundreds of years. At 24–40,000 km in diameter and 12–14,000 km in height, it is the largest storm in our Solar System. In fact, it is so big that Earth could fit inside it four to seven times over.
Given its size, internal heat, pressure, and the prevalence of hydrogen in its composition, there are some who wonder if Jupiter could collapse under its own mass and trigger a fusion reaction, becoming a second star in our Solar System. There are a few reasons why this has not happened, much to the chagrin of science fiction fans everywhere!
This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of liquid metallic hydrogen. Credit: Kelvinsong/Wikimedia Commons
For starters, despite its mass, gravity and the intense heat it is believed to generate near its core, Jupiter is not nearly massive or hot enough to trigger a nuclear reaction. In terms of the former, Jupiter would have to multiply its current mass by a factor of 80 in order to become massive enough to ignite a fusion reaction.
With that amount of mass, Jupiter would experience what is known as gravitational compression (i.e. it would collapse in on itself) and become hot enough to fuse hydrogen into helium. That is not going to happen any time soon since, outside of the Sun, there isn’t even that much available mass in our Solar System.
Of course, others have expressed concern about the planet being “ignited” by a meteorite or a probe crashing into it – as the Galileo probe was back in 2003. Here too, the right conditions simply don’t exist (mercifully) for Jupiter to become a massive fireball.
While hydrogen is combustible, Jupiter’s atmosphere could not be set aflame without sufficient oxygen for it to burn in. Since no oxygen exists in the atmosphere, there is no chance of igniting the hydrogen, accidentally or otherwise, and turning the planet into a tiny star.
Scientists are striving to better understand the temperature of Jupiter in hopes that they will eventually be able to understand the planet itself. The Galileo probe helped and data from New Horizons went even further. NASA and other space agencies are planning future missions that should bring new data to light.
Our beautiful, precious, life-supporting Earth as seen on July 6, 2015 from a distance of one million miles by a NASA scientific camera aboard the Deep Space Climate Observatory spacecraft. Credits: NASA
For those people who have had the privilege of jet-setting or traveling the globe, its pretty obvious that the world is a pretty big place. When you consider how long it took for human beings to settle every corner of it (~85,000 years, give or take a decade) and how long it took us to explored and map it all out, terms like “small world” cease to have any meaning.
But to complicate matters a little, the diameter of Earth – i.e. how big it is from one end to the other – varies depending on where you are measuring from. Since the Earth is not a perfect sphere, it has a different diameter when measured around the equator than it does when measured from the poles. So what is the Earth’s diameter, measured one way and then the other?
Oblate Spheroid:
Thanks to improvements made in the field of astronomy by the 17th and 18th centuries – as well as geodesy, a branch of mathematics dealing with the measurement of the Earth – scientists have learned that the Earth is not a perfect sphere. In truth, it is what is known as an “oblate spheroid”, which is a sphere that experiences flattening at the poles.
Data from the Earth2014 global relief model, with distances in distance from the geocentre denoted by color. Credit: Geodesy2000
According to the 2004 Working Group of the International Earth Rotation and Reference Systems Service (IERS), Earth experiences a flattening of 0.0033528 at the poles. This flattening is due to Earth’s rotational velocity – a rapid 1,674.4 km/h (1,040.4 mph) – which causes the planet to bulge at the equator.
Equatorial vs Polar Diameter:
Because of this, the diameter of the Earth at the equator is about 43 kilometers (27 mi) larger than the pole-to-pole diameter. As a result, the latest measurements indicate that the Earth has an equatorial diameter of 12,756 km (7926 mi), and a polar diameter of 12713.6 km (7899.86 mi).
In short, objects located along the equator are about 21 km further away from the center of the Earth (geocenter) than objects located at the poles. Naturally, there are some deviations in the local topography where objects located away from the equator are closer or father away from the center of the Earth than others in the same region.
The most notable exceptions are the Mariana Trench – the deepest place on Earth, at 10,911 m (35,797 ft) below local sea level – and Mt. Everest, which is 8,848 meters (29,029 ft) above local sea level. However, these two geological features represent a very minor variation when compared to Earth’s overall shape – 0.17% and 0.14% respectively.
Meanwhile, the highest point on Earth is Mt. Chiborazo. The peak of this mountain reaches an attitude of 6,263.47 meters (20,549.54 ft) above sea level. But because it is located just 1° and 28 minutes south of the equator (at the highest point of the planet’s bulge), it receives a natural boost of about 21 km.
Mean Diameter:
Because of the discrepancy between Earth’s polar and equatorial diameter, astronomers and scientists often employ averages. This is what is known as its “mean diameter”, which in Earth’s case is the sum of its polar and equatorial diameters, which is then divided in half. From this, we get a mean diameter of 12,742 km (7917.5 mi).
The difference in Earth’s diameter has often been important when it comes to planning space launches, the orbits of satellites, and when circumnavigating the globe. Given that it takes less time to pass over the Arctic or Antarctica than it does to swing around the equator, sometimes this is the preferred path.
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