Exploring Jupiter’s Interior

Danielle Corriveau, Kylie Couture, Xinjie Liu, Mikayla Rychel

Figure 1. Image Credits: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill https://www.nasa.gov/image-feature/jpl/dark-and-stormy-jupiter.

As the biggest planet in our solar system, Jupiter has sparked the curious minds of people for centuries. Among the many mysteries surrounding Jupiter, its interior remains one of the most fascinating secrets that we seek to understand. Although there is no certainty, it is widely accepted that Jupiter has a core and we will explore theories about its composition that support this theory. The spacecraft, Juno, is currently orbiting Jupiter to delve deep in discovering the planet’s evolution and atmosphere, sending back information that continues to amaze peoples minds. Jupiter’s core and composition will be assessed to determine what really lies in Jupiter’s interior and for the possibility of the core being composed of dense materials instead of solids.

About Jupiter

Figure 2. A true-color simulated image of Jupiter pieced together by four images taken by NASA’s Cassini spacecraft on December 7, 2000. Image credit: NASA/JPL/University of Arizona https://www.jpl.nasa.gov/spaceimages/details.php?id=pia02873

Named after the king of the Roman gods, Jupiter formed about 4.5 billion years ago. Jupiter is the biggest planet in our solar system, with a radius of 43,440.7 miles, and is the fifth planet from the Sun. The distance between Jupiter and the Sun is 484 million miles, or 5.2 astronomical units (AU), therefore taking sunlight 43 minutes to reach its surface.1

Jupiter is predominantly composed of hydrogen and helium, making it a gas giant. Deep inside its atmosphere, pressure and temperatures increase, and compress the hydrogen into a liquid creating the largest ocean in the solar system. Jupiter likely has three distinct cloud layers that altogether span close to 44 miles (71 kilometers). The top layer is thought to be composed of ammonia ice, with the middle layer to be ammonium hydro-sulfide crystals, and the innermost layer to be water ice and vapor. With its fast rotation, it is believed that it drives electrical currents in this region and generates its magnetic field which influences the region of space called the Jovian magnetosphere. It reaches 600,000 to 2 million miles towards the Sun, and tapers almost 600 million miles behind Jupiter. This magnetic field rotates with the planet and collects any particles with an electric charge.1

 

 

Jupiter's Atmosphere
Jupiter’s atmosphere is made up predominantly of hydrogen gas and helium gas, much like the Sun. This was determined by the Galileo spacecraft in 1995, when a probe was dropped 100 km below the clouds of Jupiter to measure its composition.²
Jupiter’s Core Dynamo & Magnetic Field
It is believed that large planetary fields are created by a dynamo process. The dynamo effect is a magnetic field that is generated by electrically charged particles that continuously move in a planet’s core. Dynamos are found in thermal or composition convection in large fluid regions.³ On Earth, the dynamo is found within the liquid outer core. As Jupiter has a very large field, it is assumed that a large dynamo force would be needed, indicating the fluidity of its core.4

Figure 3. A visual representation of Jupiter’s large magnetosphere. Image credit: John Spencer http://www.boulder.swri.edu/~spencer/jupmag5na.jpg

 

Planet and Solar System Formation

The history of how our solar system has formed remains a challenge to explain as we are unable to study the process itself. The current understanding of the theory for the formation of planets is related to that of stars and the overall creation of the solar system.

Figure 4. Artist’s conception of a protoplanetary disk. Image credit: NASA/JPL-Caltech/T. Pyle
https://exoplanets.nasa.gov/news/229/these-arent-the-planets-youre-looking-for/

The prevailing theory is that a star and its planets are formed out of a collapsing interstellar cloud of dust and gas within a larger cloud called a nebula. As the material in the cloud collapses, gravity pulls the materials closer together. The center of the cloud is compressed and rises in temperature, causing the material to churn and flatten into circumstellar or protoplanetary disks (Figure 4).5

These flat rotating disks of dust and gas reaching up to tens to hundreds of AU, are the birthplace of planets.6 The disk continues to spin around the star, and the material within begins to stick together and grow, attracting more material as the disk becomes larger. At this point, the baby planets, or planetesimals, begin to form. The interior is composed of mostly rocky materials, while the exterior is composed of gas and ice. This allowed for the formation of smaller, rocky planetesimals close to the star to form, such as Mercury, Venus, Earth and Mars. Jupiter, Saturn, Uranus and Neptune are giants of ice and gas and were formed further away.5

Jupiter is believed to be the first planet to have formed in the solar system. The massive gas giant was supposedly formed one million years after the formation of the Sun. This was difficult to discover as we previously had no samples from anything beyond Jupiter’s asteroid belt. Thomas Kruijer, a researcher at Lawrence Livermore National Laboratory, stated that isotopes from meteorite samples had to be used and examined to determine Jupiter’s maximum age.7

It was theorized that after Jupiter formed, it had migrated closer and then farther away from the Sun. This path that Jupiter took is called the Grand Tack. According to this model, Jupiter was likely formed around 3.5 AU from the Sun, however, but the wild currents of gas and dust particles caused the planet to roam as close as 1.5 AU, the orbit that Mars is currently in.8 Saturn also followed this pattern before all of the dust particles between them were driven out and the two planets’ bound paths became inverted.8 The end result of this migration placed Jupiter where it currently resides, at 5.2 AU from the Sun.

 

The 'Grand Tack' Model
The “Grand Tack” is a model where Jupiter traversed towards the Sun until Saturn was formed, which caused Jupiter to return and settle into its current orbit.9
The Effects of Jupiter’s Migration on the Solar System
Jupiter’s potential migration not only affected Saturn but could have also affected the size of Mars, as the planet is low in mass and evacuated the Solar System’s innermost region.8, 9

Figure 5. A Solar Sytem model. Image credits: CC 2.0 https://www.flickr.com/photos/11304375@N07/2818891443

 

Figure 6. An artist’s interpretation of a a young sun-like star surrounded by a planet-forming disk of gas and dust. Image credit: NASA/JPL-Caltech
https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA12008

 

With current understandings, a critical component of planetary formation is the necessity of a dense core for the accumulation of materials and elements. The key to understanding what composes Jupiter’s core lies within understanding how the planet itself was formed. The theories on how Jupiter was formed is directly related to what lies at its core. However, much like what Jupiter’s core is actually made of, the planet’s detailed formation is also a mystery. The most popular theory on how gas giants were formed is the core-nucleated accretion theory, which involves gas being gravitationally accreted onto a sufficiently massive core.10 With the interior of the protoplanetary disk being exposed to more rocky conditions, which produced the terrestrial planets, and the exterior to ice and gas, it is probable that at the center of Jupiter lies a core composed of rock or ice.5

 

 

 

Theories on Jupiter’s Core

The most accepted theory about Jupiter’s core is that it is dense and composed of heavy elements. The core grew from a nearby collection of debris, water ice, and pieces of comets and asteroids. These materials fused together becoming what is called Planetesimals, and these large chunks of matter collided with one another to form Jupiter’s core. When the core was large enough, helium and hydrogen were attracted and continued to accumulate until Jupiter was fully formed.11

Data from the Galileo probe mass spectrometer that was dropped into Jupiter revealed that Jupiter is depleted in water and oxygen, but has carbon levels 1.7 times higher then the Sun. Using this information, it was questioned where all the water that had helped build Jupiter’s core was. It was proposed in 2004 that instead of water ice, Jupiter’s core was originally made of mainly tar, as tar is sticky and collects more rocks and is more durable compared to water ice. During the initial formation, the core grew large enough to accrete gas from the solar nebula; these gases were hydrogen and helium. The accretion caused energy to heat up Jupiter and caused the tar to react and create methane, the third most abundant gas found in the solar system.12

It was proposed in 2010 that Jupiter’s core composition has actually shrunk due to a collision with a protoplanetary and has a mixture of hydrogen, helium and heavy elements. With such a large body crashing into Jupiter’s core, there is a possibility that the collision could have lead to the core decaying and gases to rise up to Jupiter’s upper atmosphere layer.13

Another theory is that Jupiter has no core at all. After the Sun’s birth, it is theorized that a large cloud of gas and dust surrounded the Sun, and in this cloud contained the initial materials to form Jupiter. As the temperature cooled down, the cloud condensed, which lead to small particles such as gas and dust to accumulate and caused density discrepancy among different regions. This accumulation enhanced gravitational power until it became large enough to form Jupiter.11

 

What is Metallic Hydrogen?
Hydrogen is a diatomic molecule, otherwise known as molecular hydrogen or H2. When compressed, the molecules change into a solid before turning into a metal, all while maintaining its diatomic structure. After this process, it is called metallic molecular hydrogen. But if the molecules continue to be compressed, the hydrogen molecules will eventually dissociate, yielding metallic atomic hydrogen or H.16
Metallic Hydrogen in Relation to Jupiter’s Core
Jupiter is thought of to be mainly composed of hydrogen and helium, but the hydrogen gas turns into a liquid from the extreme pressures and high temperatures found deep in the planet’s atmosphere. With the increased pressure, electrons are forced off of the hydrogen atoms and causes the liquid to electrically conduct like metal. These electrical currents are what help generate Jupiter’s magnetic field.17

 

Figure 7. A model of Jupiter’s interior, composed of a rocky core and a layer of liquid metallic hydrogen. Image credit: Kelvinsong CC BY-SA 3.0 https://en.wikipedia.org/wiki/Jupiter#/media/File:Jupiter_diagram.svg

 

Juno’s Mission to Jupiter

Juno’s principle goal is to understand Jupiter’s origin and evolution. Its mission objective is to explore its atmosphere in order to measure composition, temperature, cloud motions and discover what percentage of Jupiter’s atmosphere is water, which would give further evidence towards which planet formation theory is correct. Juno will also map Jupiter’s magnetic and gravitational fields, study its interior to determine if there is a core, as well as its magnetosphere near the north and south poles.18

Figure 8. The Juno spacecraft and its science instruments. Image credit: NASA/JPL https://www.nasa.gov/mission_pages/juno/spacecraft/index.html

Juno was launched August 5th, 2011, from Cape Canaveral, Florida.19 But the spacecraft circuited our solar system for five years for regular check-ups and testing on its equipment before returning to Earth for a boost.20 When two objects in space fly near each other, they each feel a gravitational pull, but the smaller object will feel a bigger tug. When Juno reached Earth, a technique called the gravity assist, Juno took a small amount of the planets’ momentum to orbit the Sun, and used it to reach Jupiter. This flyby was essential to the mission’s success as during the time of launch, there was not a rocket powerful enough to send a spacecraft directly to Jupiter.21

Juno arrived in Jupiter’s orbit on July 4th, 2016. To enter Jupiter’s orbit, Juno needed to complete the Jupiter Orbit Insertion, where it fired its engines at exactly the right moment and direction for the right amount of time entirely on its own to safely enter into orbit. Juno relies on solar power, so it needs to stay exposed to sunlight in Jupiter’s orbit, an area which is called the polar orbit. This orbit will take Juno over Jupiter’s poles, in a north-south direction. It takes Juno 11 days to complete a revolution around Jupiter, so a route was designed for Juno to cover the entirety of Jupiter’s surface by the time the mission is complete.22

On Juno’s 11th orbit around Jupiter, it was discovered that the rotating zones and belts seen in the atmosphere can reach up to

1,900 miles (3,000 kilometers). Hydrogen is then conductive enough to be dragged into near-uniform rotation by the immensely powerful magnetic field. This data also held information about Jupiter’s interior structure and its composition. Regions of

Figure 9. An artist’s concept of the lightning distribution in Jupiter’s northern hemisphere, using a JunoCam image. Image credit: NASA/JPL-Caltech/SwRI/JunoCam https://www.jpl.nasa.gov/news/news.php?feature=7151

surprising magnetic field intensity were discovered on Jupiter, with the northern hemisphere having more complex magnetic fields then the southern hemisphere. Around halfway from the equator and the north pole is an area of intense and positive magnetic field, but it is bordered by areas that are negative and less intense. The magnetic field is different in the southern hemisphere, as it is consistently negative with increasing intensity.23

On Juno’s 12th orbit, its discoveries were sent back to NASA to reveal the origins of Jupiter’s mysterious lightning (Figure) and help to improve our understanding of Jupiter’s thermal composition. Using the Microwave Radiometer Instrument (MWR), Juno detected 377 lightning discharges from its first eight orbits. These discoveries showed that the lightning distribution was found at both poles, but most of the activity was at the north pole. Scientists believe that this is because heat is generated from the sunlight that heats up the equator and creates stability in the upper atmosphere as warm air is prevented from escaping. But the poles don’t have this stability, and this allows warm gases from the interior to rise, driving convection and thus creating lightning. Juno’s 13th science pass will be completed on July 16th, 2018.24

Juno has yet to come to a general consensus of what Jupiter’s core is made of, though it is still one of the primary reasons of this mission. With Juno’s mission extended until 2021, hopefully the answer to what Jupiter’s core is composed of will be discovered.25

 

 

Conclusion

As Jupiter was the first planet formed in our solar system, the planet could contain many answers to the mysteries of what our solar system and its planets are composed of. With many elements pertaining to Jupiter that remain unknown, there are ongoing efforts to seek explanations. When seeking to understand what Jupiter’s core could be composed of, it is important to look at how a gas giant is formed. Information such as where Jupiter may have originally been formed, its atmospheric and central composition, elements such as metallic hydrogen and Jupiter’s magnetosphere and the multitude of possible theories can help contribute to Jupiter’s interior. With Juno’s ongoing mission to orbit Jupiter until 2021, we can potentially learn what Jupiter’s interior contains and finally have an answer for one of the greatest mysteries of our solar system.

 

References

1 NASA, Jupiter: In Depth, https://solarsystem.nasa.gov/planets/jupiter/in-depth/ (Accessed 9 June 2018).

2 NASA, The Atmosphere of Jupiter, https://www.missionjuno.swri.edu/jupiter/atmosphere?show=hs_jupiter_atmosphere_story_what-is-jupiters-atmosphere-made-of. (Accessed 12 June 2018).

3 D.J. Stevenson, Earth and Planetary Science Letters 208, 1 (2003).

4 C.A. Jones and R. Holme, Geophysical Research Letters 44, 5355 (2017).

5  NASA, HubbleSite, Discovering Beyond: How do Planets Form? http://hubblesite.org/hubble_discoveries/discovering_planets_beyond/how-do-planets-form. (Accessed 2 June 2018).

6 J.P. Williams and L.A. Cieza, Annual Review of Astronomy and Astrophysics 49, 67 (2011).

7  J. Bennett, Popular Mechanics, 167 (2017).

8  NASA, Jupiter’s Youthful Travels Redefined Solar System, https://www.nasa.gov/topics/solarsystem/features/young-jupiter.html (Accessed 12 June 2018).

9 K. Batygin and G. Laughlin, Proceedings of the National Academy of Sciences of the United States of America (2015).

10 S. Ginzburg and R. Sari, Monthly Notices of the Royal Astronomical Society 469, 278 (2017).

11  NASA, What’s in Jupiter’s Core, http://www.missionjuno.swri.edu/origin?show=hs_origin_story_whats-in-jupiters-core (Accessed 10 June 2018).

12  T. Fitzpatrick, Space Scientist Proposes New Model for Jupiter’s Core,

https://source.wustl.edu/2004/12/space-scientist-proposes-new-model-for-jupiter-core/ (Accessed 10 June 2018).

13 S. L. Li, C. Agnor, and D. N. C. Lin, The Astrophysical Journal 720, 1161(2010).

14 H. F. Wilson, and B. Militzer. Physical Review Letters 108, (2012).

15 R. Sanders, Jupiter’s Rocky Core Bigger and Icier, Model Predicts, https://www.berkeley.edu/news/media/releases/2008/11/25_core.shtml (Accessed 14 June 2018).

16 Forbes, Is Metallic Hydrogen Possible, https://www.forbes.com/sites/quora/2017/10/30/is-metallic-hydrogen-possible/#4e9ad02e343d (Accessed 14 June 2018).

17 NASA. Jupiter: In Depth, https://solarsystem.nasa.gov/planets/jupiter/in-depth/ (Accessed 14 June 2018).

18 NASA, Mission, https://www.missionjuno.swri.edu/mission/ (Accessed 5 June 2018).

19 NASA, Launch, https://www.missionjuno.swri.edu/launch (Accessed 5 June 2018).

20 NASA, Deploy the Craft, https://www.missionjuno.swri.edu/deploy-the-craft (Accessed 5 June 2018).

21 NASA, Earth Flyby, https://www.missionjuno.swri.edu/earth-flyby. (Accessed 5 June 2018).

22 NASA, Orbit, https://www.missionjuno.swri.edu/orbit (Accessed 5 June 2018).

23  NASA, News: Juno Provides Infrared Tour of North Pole, https://www.missionjuno.swri.edu/news/juno-provides-infared-tour-of-north-pole (Accessed 5 June 2018).

24 NASA, News: Juno Solves 39 Year Old Mystery, https://www.missionjuno.swri.edu/news/juno-solves-39-year-old-mystery (Accessed 6 June 2018).

25 NASA, News: NASA Replans Juno’s Jupiter Mission, https://www.missionjuno.swri.edu/news/nasa-replans-junos-jupiter-mission (Accessed 14 June 2018).

Figure 10. An animation of the surface of Jupiter. Image credit: NASA/JPL/University of Arizona
http://photojournal.jpl.nasa.gov/catalog/PIA02863