Since I was a kid, the only thing I knew about the four jovian planets was that they are big, gaseous and cold. Yet, they are so much more interesting than it seems at first glance. In this post I’ll talk about some special characteristics of each of them. But why does our solar system have these gaseous planets in the first place?
The solar system formed from a giant interstellar nebula of gas about 4.5 billion of years ago. When the Sun formed during the collapse of the nebula, the clouds of gas began orbiting it. Soon, small planetesimals formed from accreted dust. Terrestrial planets formed in the inner solar systems, since the proximity to the sun allowed only rocks and metals to exist. The lower temperatures in the outer solar system allowed ice and hydrogen compounds to exist. Hydrogen and helium compounds were the primary components of the initial nebula, thus the jovian planets are significantly larger than their terrestrial counterparts.
Jupiter
Jupiter is the biggest and heaviest planet in the solar system. One of its most peculiar features is its heat emission. In fact, due to its internal heat generation it emits almost twice the amount of energy it receives from the Sun. At first glance, it seems that this is due to Jupiter’s large size, since it still retains a lot of heat from accretion and differentiation. However, it is not enough to account for the present rate of heat loss. The leading theory is that Jupiter is still contracting, which converts gravitational potential energy to thermal energy.
Saturn
The most striking features of Saturn are definitely the rings. We think of them as continuous sheets, however they are made out of numerous reflective ice particles. These range from small bits of dust to large boulders. However, I believe that the most interesting thing about the rings are the gaps and ripples.
The gaps happen because gravitational attraction causes particles to bunch up at certain orbital distances and to be pushed away at others. One of the sources of these gravitational tugs are gap moons. They are tiny moons located within the rings. They can create a gap in the rings by keeping a space clear of other particles. They also tug at the edges of the rings, creating beautiful ripples. It might seem that the ripples on each side move in opposite directions. This is because the particles nearer to Saturn orbit faster than the ones further away. This can be explained with Kepler’s second law.
Rings are not unique to Saturn, as all of the other jovian planets also have them. However, they are much darker and and the particles are sparser. Hence, it took a long time to discover them. The rings of Uranus were discovered in 1977, the ones on Jupiter in 1979 by the Voyager, and the ones on Neptune in 1989 by the Voyager 2.
Uranus
Many planets are slightly tilted on their axis. For example, Earth has a 23 degree tilt and Saturn a 27 degree one. However, Uranus is tilted by 98 degrees. It rotates around on its side, which creates the most extreme seasonal variations in the solar system. It is suspected that this tilt is due to a tremendous collision with another body during the planet’s formation.
Due to its crazy tilt, Uranus also rotates in the opposite direction to other planets. Uranus and Venus are the only planets that rotate clockwise around their axis. It is generally accepted that all the planets initially spun counterclockwise, but collisions with Earth sized objects changed the orbits of Uranus and Venus. Collisions with massive bodies were relatively common in the early solar system. The Earth’s moon formed as a result of a collision, which can be seen with the evidence of past volcanic activity on its surface.
Neptune
Neptune is the furthest planet from the Sun. Its gorgeous blue color is given by methane in the atmosphere. Yet, I think its biggest moon, Triton, is the most fascinating aspect of this planet.
Triton is has a surface temperature of −235.2 °C, which makes it even colder than Pluto. It is the only large moon in the solar system that orbits its planet in retrograde. Normally, satellites orbit their host planet in the same direction as the planet’s rotation. However, captured moons like Triton do not always obey this rule.
What’s especially interesting in this case is that Triton seems too large to have been captured. One hypothesis states that Triton was paired with another Kuiper belt object. When they passed close to Neptune, Triton lost energy and got captured, while the other object gained energy and flew away. It’s another example of conversation of energy being used to answer tricky astronomy questions.
Another fascinating thing about this moon is its geological activity. Triton is smaller than our Moon, but the surface contains evident of relatively recent volcanism. Other areas contain “cantaloupe terrain”, which is the result of tectonic activity. Pieces of ice of different densities rose and fell, shaping the terrain. Why would such a cold and small world have tectonic activity? The leading hypothesis is tidal heating, the same thing that drives volcanism on Jupiter’s moon Io. Due to Triton being captured, it can be inferred that it had a very elliptical orbit and quicker rotation. The tidal forces would’ve circularised its orbit and heated the interior, which is why the geological activity rapidly subsided. A fascinating moon in the far reaches of the solar system.
References
A. (2019, August 8). Ring ripples reveal how long a day lasts on Saturn. Science News. https://www.sciencenews.org/article/ring-ripples-reveal-how-long-day-lasts-saturn
Bennett, J. O., Donahue, M., Schneider, N., & Voit, M. (2020). The Cosmic Perspective. Pearson Education, Incorporated.
In Depth | Neptune –. (2020). NASA Solar System Exploration. https://solarsystem.nasa.gov/planets/neptune/in-depth/
In Depth | Triton –. (2020). NASA Solar System Exploration. https://solarsystem.nasa.gov/moons/neptune-moons/triton/in-depth/
Jupiter | Facts, Surface, Moons, Great Red Spot, & Rings. (2020). Encyclopedia Britannica. https://www.britannica.com/place/Jupiter-planet