Jupiter’s Arrested Development

The banded behemoth Jupiter was born when the rest of our Solar System formed approximately 4.56 billion years ago, as the relentless force of gravity pulled whirling, swirling gas and dust together into a gigantic ball, that ultimately grew to become this enormous King of Planets. The newborn Jupiter gobbled up most of the mass that had been left over after our Sun’s fiery birth, and it ended up with more than double the mass of the combined material of all the other bodies in our Sun’s family. Indeed, Jupiter contains the same ingredients as a star–but it did not manage to attain sufficient mass to light its nuclear-fusing stellar fires. In August 2018. astronomers from the University of Bern and Zurich and of ETH Zurich (Switzerland) published their research showing how Jupiter was born, and the data that they collected from meteorites indicates that the growth of this behemoth of a planet had been delayed for two million years. The researchers propose that collisions with kilometer-sized blocks generated high energy, which meant that during this phase hardly any accretion of gas could occur and the planet could only grow very slowly.

With a radius of 43,440.7 miles, Jupiter is 11 times wider than Earth. Imagine Jupiter as a basketball, and Earth as a nickel. The process that gives birth to giant planets like Jupiter has long been a hotly debated topic, and for several decades there as been disagreement among planetary scientists about how such mysterious planetary births might occur. Now, the team of Swiss scientists have explained this puzzle about how Jupiter formed, and have also presented their new measurements. The research results are published in the August 27, 2018 issue of the journal Nature Astronomy under the title: The formation of Jupiter by hybrid pebble-planetesimal accretion.

“We could show that Jupiter grew in different, distinct phases,” noted Dr. Julia Venturini in an August 2018 University of Bern Press Release. Dr. Venturini is a postdoc at the University of Zurich. “Especially interesting is that it is not the same kind of bodies that bring mass and energy,” added Dr. Yann Alibert in the same Press Release. Dr. Alibert is Science Officer of Planet5 and lead author of the new paper.

Giant planet formation is believed to start with a planetary embryo quickly accreting very small, mere centimeter-sized pebbles. Then, rapidly, the baby planet builds a core during its first million years of existence. The two million years that follow this initial stage are dominated by the slower accretion of larger, kilometer-sized building blocks called planetesimals–and these large rocks serve as the seeds from which a baby giant planet can grow. The planetesimals blast into the growing toddler planet with great energy, and release large amounts of heat. “During the first stage the pebbles brought the mass. In the second phase, the planetesimals also added a bit of mass, but what is more important, they brought energy,” Dr. Alibert continued to explain. After the passage of three million years, Jupiter had grown into a body sporting 50 Earth masses. At this point, the third formation phase began, which was dominated by the runaway accretion of gas. This ultimately resulted in today’s gas-giant planet Jupiter, that now is more than 300 Earth masses.

In The Distant Kingdom Of Jupiter

Jupiter is named for the king of the ancient Roman gods (Greek Zeus), and it is circled by an entourage of 79 known moons. Planetary scientists are particularly interested in the quartet of large Galilean moons: Io, Europa, Ganymede, and Callisto, that were discovered by the great Italian astronomer Galileo Galilei in 1610, and were named in his honor.

Jupiter’s gaseous cloud tops, composed of ammonia and water, are a complicated, beautiful, and intricate strange sea of swirls and stripes that are actually very windy and frigid. These beautiful clouds float in an atmosphere composed of hydrogen and helium. Jupiter’s famous Great Red Spot is really a giant hurricane-like vortex storm that is larger than Earth. This crimson storm has roared for hundreds of years.

Jupiter also has a system composed of several rings. Unlike the famous rings of Saturn, however, Jupiter’s rings are dim and dusty, and not made up of the ice particles that jitter-bug around in Saturn’s gossamer rings.

From an average distance of 484 million miles, Jupiter is situated 5.2 astronomical units (AU) from our Sun. One AU is equal to the mean distance between our planet and the Sun, which is approximately 93,000,000 miles. From this distance, in our Solar System’s outer domain, it takes the light streaming out from our Sun about 43 minutes to make the journey from our Star to Jupiter.

Jupiter has the shortest day of any planet in our Solar System. A single day on Jupiter lasts only about 10 hours, which represents the time it takes for Jupiter to rotate or spin around once on its axis. Jupiter travels one complete orbit around our Star in about 12 Earth years–or 4.333 Earth days.

The equator of our Solar System’s banded giant is tilted, with respect to its orbital path around our Sun, by only 3 degrees. This means that Jupiter spins in a nearly upright position, and it does not experience seasons that are as extreme as the other major planets of our Sun’s family.

Our Solar System formed more than 4.5 billion years ago when a relatively small, very dense blob–embedded within the folds of a molecular cloud— collapsed under the merciless pull of its own gravity. These dark, frigid, enormous, and lovely molecular clouds haunt our Milky Way Galaxy in large numbers, and they are the strange cradles where baby stars are born. Billowing, undulating, and veiled in blackness, these clouds are composed mostly of gas with a smaller amount of very fine smoke-like dust. As the dense, relatively small blob undergoes this relentless gravitational collapse, most of its material gathers at the center and ignites as the result of the process of nuclear fusion–and a star is born. What is left of the blob swirls and whirls around the newborn protostar, and evolves into what is called a protoplanetary accretion disk. The rotating disk of gas and dust does a mesmerizing dance around the baby star. This type of disk orbited our newborn Sun, and the very tiny particles of naturally “sticky” dust within it, collided and “glued” themselves to one another to create bigger things. Eventually, a vast population of planetesimals formed from this dusty rubble. The planetesimals then went on to collide and merge together to form the eight major planets of our Solar System.

When Jupiter was born, it might have become a star. However, it never made it. The energy hurled out by the infalling material made Jupiter’s interior grow searing-hot, and the larger it grew, the hotter it became. Finally, when the material gobbled up from the surrounding, turbulent disk was used up, Jupiter may have sported the impressive mass of more than 10 times what it now has. It is also likely that Jupiter had a central temperature of a broiling 50,000 Kelvin, and a bright luminosity that was about 1% as great as that of our Sun.

However, if Jupiter had been born somewhat heavier, it would have grown ever hotter, and hotter, and hotter, as it shrunk in size–until its nuclear-fusing furnace caught fire, and it became a star. If this had occurred, our Sun would have had a binary companion star, and we would not be here. Most of the stars inhabiting our Galaxy exist in systems that contain two or more sibling stars.

Nevertheless, the planet Jupiter is like a star in its composition. Like our own Sun, it is composed mostly of hydrogen and helium, and deep in its mysterious and alien atmosphere, pressures and temperatures increase. This increase compresses the hydrogen gas into a liquid. This gives Jupiter the distinction of possessing the largest ocean in our entire Solar System–an ocean made up of liquid hydrogen instead of water. Astronomers think that, at depths approximately halfway to the Jovian center, the pressure grows so great that electrons are squeezed off from the hydrogen atoms, thus turning the liquid into an electrically conducting substance like metal. Jupiter’s rapid rotation is believed to drive electrical currents in this strange and alien region, thus generating the planet’s strong magnetic field. However, it is still unknown if, deeper down, Jupiter contains a central solid core–or if, instead, it harbors a searing-hot, thick, and dense soup. Planetary scientists think that Jupiter’s core could be up to 90,032 degrees Fahreheit at these depths, and be made mostly of iron and silicate minerals akin to quartz.

Gas-giants like Jupiter do not have a true surface like our Earth and other solid planets. Jupiter, like others of its kind, is primarily a big ball of swirling gases and liquids. A spacecraft could not land on Jupiter, but it couldn’t safely fly through the gases of this banded behemoth either. The extreme and highly destructive pressures and temperatures deep within the planet would fatally crush, melt, and vaporize any spacecraft dispatched to fly into this very alien giant world.

Jupiter’s appearance is an intricate tapestry woven of strange spots and clown-colored bands. This gaseous world is thought to contain a trio of cloud layers in its “skies” that, when taken together, span approximately 44 miles. The uppermost cloud is thought to be made up of ammonia ice, while the middle cloud is composed of ammonium sulfide crystals. The innermost Jovian cloud layer is possibly made up of water ice and vapor.

The vivid clown-like hues that compose Jupiter’s thick bands are thought to be plumes of sulfur and phosphorus-containing gases that are rising up from the planet’s much warmer interior.

Because Jupiter has no solid surface to slow them down, its whirling spots can linger for many years. This colorful, bizarre world is savaged by rushing, rampaging winds, some reaching impressive speeds of up to 335 miles per hour at the equator. The famous Jovian Great Red Spot is a whirling, swirling oval composed of clouds, and it has been observed for more than three centuries. However, more recently, a trio of smaller ovals were observed to merge–and then create–what is now known as the Little Red Spot. This smaller crimson storm is about 50% the size of it larger red-hued sibling. Planetary scientists do not as yet know if these oval spots and planet-circling clown-colored bands are shallow or reach deeply into the mysterious Jovian interior.

The Jovian environment is probably inhospitable to life as we know it. The pressures, temperatures, and materials that are found on this strange world are probably too extreme and volatile to create a comfortable environment for delicate life-forms.

In contrast, some of Jupiter’s moons could possibly be comfortable small worlds where life could form and flourish. Indeed, Jupiter’s icy, cracked-eggshell Galilean moon, Europa, is just such a promising small world. There are signs that a vast global ocean sloshes around beneath Europa’s shattered icy crustal shell, where aquatic forms of life could possibly swim comfortably and flourish.

Arrested Development

The new model explaining Jupiter’s primordial birth matches meteorite data that were presented by a team of astronomers in 2017. Initially, Dr. Venturini and Dr. Alibert were puzzled when they learned of the 2017 findings. Measurements of the compositions of meteorites revealed that in the very ancient Solar System the solar nebula was separated into two distinct domains during a span of two million years. From this, it could be concluded that Jupiter played the important role of a barrier when it increased in size from about 20 to 50 Earth masses. During this time interval, the evolving giant planet probably perturbed the dust disk, forming an over-density that captured the pebbles beyond its orbit. It is for this reason that the material from the outer regions was unable to combine with the material of the inner regions, until the toddler Jupiter finally attained enough mass to perturb and send rocks flying towards the inner regions of our young Solar System.

“How could it have taken two million years for Jupiter to grow from 20 to 50 Earth masses? That seemed much too long. That was the triggering question that motivated our study,” commented Dr. Venturini in the August 28, 2018 University of Bern Press Release.

A discussion using email began among NCCR Planet5 researchers of the Universities of Bern and Zurich and ETH Zurich. The following week experts in astrophysics, cosmochemistry, and hydrodynamics arranged a meeting in Bern, Switzerland. “In a couple of hours we knew what we had to calculate for our study. This was only possible within the framework of the NCCR, which links scientists from various fields,” Dr. Alibert commented in the same Press Release.

The calculations conducted by the Swiss researchers showed that the time the young Jupiter spent in the mass range of 15 to 50 Earth masses was actually considerably longer than previously thought. During this stage of the toddler Jupiter’s development the collisions with kilometer-sized rocks provided sufficient energy to heat up the young planet’s gaseous atmosphere, and also prevented rapid cooling, contraction, and additional accretion of gas.

According to the research, the formation of Jupiter occurred in three stages:

Stage 1: Up to the first one million years. At this point the baby Jupiter grows by accretion of pebbles. Large primordial planetesimals possess high collision velocities that result in destructive collisions. These collisions produce small, second-generation planetesimals.

-Stage 2: 1-3 million years. The energy that is churned out as the result of the accretion of small planetesimals prevents rapid gas accretion, and prevents the young Jupiter from rapidly growing in size.

-Stage 3: Beyond 3 million years: Jupiter is now massive enough to accrete large quantities of gas.

“Pebbles are important in the first stages to build a core quickly, but the heat provided by planetesimals is crucial to delay gas accretion so that it matches the timescale given by the meteorite data,” the astrophysicists summarize. They are also convinced that their results provides new information that can be used for solving the long-standing mysteries of the formation of the duo of outermost giant planets, Uranus and Neptune–as well as exoplanets that are in this same mass regime.



Source by Judith E Braffman-Miller

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