Date: November 30, 2009

Title: What Galileo Discovered at Jupiter


Podcaster: Adam Fuller of Columbia Astronomy

Organization: Columbia University Astronomy:

Description: In December 1995, NASA’s unmanned spacecraft, Galileo, dropped a probe into Jupiter’s atmosphere and provided us with the first in situ observations of an outer planet’s atmosphere. Along the way Galileo discovered an asteroid with a moon, watched Jupiter swallow a comet, detected life on Earth, and provided some insight to Jupiter’s formation. In today’s podcast we’ll discuss its discoveries, the next mission to Jupiter, and what this means for our understanding of the formation of the solar system.

Bio: Adam Fuller is currently a graduate student at the University of Unknown. He graduated from Columbia University with a B.S. in Astrophysics in 2009. He also has a B.A. in Journalism from North Carolina (Go Heels!). His research interests include planetary science, meteorology, and astrobiology. Outside of school, he is an avid marathoner, a failed comedian, and a dedicated uncle to three proto-astrophysicists.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Joseph Brimacombe.


Hello everyone, and welcome to Columbia Mondays! My name is Adam Fuller, and I’m a recent graduate of the undergrad astrophysics program at Columbia University in the City of New York. I’m currently a first year grad student in the Planetary Sciences department at Johns Hopkins University.

In today’s 365 Days of Astronomy podcast we’re talking about the Galileo Mission to Jupiter, the unmanned spacecraft NASA sent to Jupiter over 14 years ago. It was an incredibly successful mission that watched Jupiter swallow a comet, discovered an asteroid with a moon, and detected life on Earth. More importantly, it provided us with the first in situ observations of an outer planet’s atmosphere. Today we’ll talk about the discoveries the Galileo spacecraft made and the new questions it made us ask–questions we hope to answer with the next mission to Jupiter.

As the largest planet in the Solar System, Jupiter is slightly wider than 11 Earths lined up in a row. It’s 318 times more massive than Earth and is made up of 85% hydrogen, 13% helium, and 2% everything else. There is no solid surface like Earth’s, and, in fact, we don’t even know if it has a solid core like every other planet in our Solar System.

Instead, descend into Jupiter one twentieth of its diameter (4,440 miles, 7150 km), and you’ll encounter atomic hydrogen compressed under such enormous pressures and temperatures that it behaves more like a gigantic metallic ocean, nothing like anything we’ve ever experienced here on Earth.

Galileo was launched from Earth on October 19, 1989 with three primary scientific objectives: (1) study the geological composition and structure of the Galilean moons Io, Europa, Ganymede, and Callisto, (2) characterize the plasma physics at work in the Jovian magnetosphere, and, most importantly, (3) deploy the attached entry probe into Jupiter’s atmosphere to study its composition, structure, and dynamics. The probe piggy-backed on the orbiter during liftoff and most of the spacecraft’s trip to Jupiter. It eventually detached in July 1995. Both orbiter and probe reached Jupiter on December 7, 1995, and communicated to each other during the probe’s harrowing descent into the atmosphere.

Galileo was originally planned to launch in 1982, but a series of setbacks, including the Challenger disaster in 1986, pushed it’s launch back to 1989. Because the planets weren’t in the same position in 1989 as they were in 1982, the spacecraft’s trajectory was redesigned to include a flyby of Venus in February 1990 and two Earth flybys in December 1990 and December 1992. These provided gravitational assists to help Galileo reach Jupiter. The first Earth flyby became an historic event when NASA engineers used Galileo’s onboard instruments to “detect” life on Earth. Carl Sagan published a paper in Nature about Galileo’s discoveries that discussed what is now called the Sagan criteria for life:

1. Galileo discovered the “vegetation red edge,” the dramatic decrease in the amount of reflected red light by Earth’s surface. The light on the red end of the visible spectrum detected by Galileo is absorbed by the chlorophyll in plants for use in photosynthesis. (Green light isn’t absorbed but rather scattered back, giving plants their green color.)

2. Galileo also detected spectral absorption features caused by the large amounts of oxygen in the atmosphere. Earth has the only oxygen-rich atmosphere in the Solar System because life–primarily photosynthesis by plants–replenishes the oxygen much faster than it combines with rocks on the Earth’s surface or is absorbed by the oceans.

3. Methane spectral absorption features were also detected. There wasn’t much, but since the Earth is so oxygen-rich, oxidization should rapidly transform all methane in the atmosphere into carbon dioxide and water. On Earth, however, bacteria is constantly replenishing the supply of methane.

4. The big give-away, however, was when Galileo detected modulated narrowband radio transmissions that looked nothing like natural radio sources like lightning or instabilities in Earth’s magnetic field. These were clear signs of “intelligent” life on Earth.

Considered individually, each of these discoveries does not guarantee that life is present on Earth. But all four together, Carl Sagan said, “are consistent with the hypothesis that widespread biological activity now exists…on Earth.”

After this historic discovery, Galileo went on to perform close observations of two asteroids, 951 Gaspara and 243 Ida, in October 1991 and August 1993, respectively.

Traveling within 1,400 miles (2,400 km) of Ida, it discovered Dactyl, the first moon orbiting an asteroid. A few months before Galileo’s encounter with Ida and Dactyl, astronomers discovered comet Shoemaker-Levy 9 in orbit around Jupiter. In July 1992 it passed very close to Jupiter and was ripped apart by the planet’s intense gravity. By May 1994 it was a string of 21 fragments, each ranging in size from a thousand feet up to almost two miles across, and they were all on a collision course with Jupiter. Astronomers determined the collisions would occur over a week in July 1994, just out of viewing range from Earth and Hubble. But not Galileo! When Shoemaker-Levy 9 began striking Jupiter’s southern hemisphere on July 16, 1994, Galileo was still 149,000,000 miles (239,800,000 km) away, but it had a clear view of the impacts and ensuing fireballs. The biggest impact was by fragment G on the morning of July 18, 1994, and its explosion had the equivalent energy of 6,000,000 megatons of TNT or 11,765 times all the energy ever released by global nuclear weapons testing. Galileo was uniquely positioned to observe this and most of the other collisions, and it helped supplement global ground-based observations. One year later, on July 13, 1995, the Galileo spacecraft finally deployed the atmospheric entry probe. Both were still roughly 50,000,000 miles (80,000,000 km) and five months from Jupiter. They traveled together the remainder of the way and arrived simultaneously at Jupiter on December 7, 1995. The probe plunged into the Jovian atmosphere and, an hour later, was crushed and melted by the incredible pressures and temperatures deep within Jupiter. The orbiter remained in orbit, visiting the Galilean moons Io, Europa, Ganymede, and Callisto. It took more measurements of Jupiter’s atmosphere and magnetosphere until September 21, 2003. By then several onboard instruments had been damaged beyond repair by radiation, so the orbiter was deorbited and sent to join the remains of the probe in Jupiter’s atmosphere. Originally a two year mission, the Galileo spacecraft was in service for 8 years and spent a total of 14 years in space. Over the entire course of the mission, Galileo traveled almost 3 billion miles (4,631,778,000 km) and completed 34 orbits of Jupiter.

So what did we learn from Galileo? There was no realistic way of targeting a specific atmospheric feature for the probe’s entry site, so it was by chance that it descended into a unique region called a “5-micron hot spot.” These hot spots are typically restricted to Jupiter’s equatorial region where they cover about 15% of the surface but only 1% of Jupiter’s entire surface. They’re regions of relatively clear sky where deeper thermal emissions can be detected, allowing us to peer farther into the atmosphere. Because they’re regions of less cloud cover, they’re very dry and aren’t typical for global conditions.

It was as if scientists had to figure out Earth’s entire atmosphere after shooting a softball from Venus to Earth during inferior conjunction, having it fall “straight down [into] a thermonuclear fireball” while still in Earth’s upper stratosphere, and then landing in Death Valley.

But we did learn quite a bit from Galileo’s measurements. Carbon, nitrogen, sulfur, and the noble gases argon, krypton, and xenon were all found to be enriched in Jupiter’s atmosphere by a factor of 3 relative to their solar abundance. That is, the ratio of those elements to hydrogen in Jupiter’s atmosphere was roughly 3 times what is measured for the Sun. The total amount of helium in Jupiter’s atmosphere was found to be 30% greater than what was determined from Voyager data. This implied that helium was settling into the deep interior but at a slower rate than previously thought.

These updated abundances are important because they help us understand how Jupiter formed and its current structure. But because of uncertainties in how hydrogen and helium behave in the crushing pressures and temperatures found deep in Jupiter’s interior, models of Jupiter’s core calculate it to have between 0 and 14 times Earth’s mass. In other words, Jupiter may not even have a solid core! It could be liquid metallic hydrogen all the way down, or there could be a solid core as massive as Uranus.

Not all elements were measured to have super-solar abundances. Neon was found to be only one tenth solar abundance. Scientists think this could be due to it being carried along with helium into the interior. More importantly, water was also found to be depleted; its primary constituent, oxygen, was measured to be three tenths solar abundance. However, the abundance was still increasing when the probe failed, and the location of the probe’s entry, the 5-micron hot spot, is thought to be a particularly dry region. Galileo’s measurements also conflict with the derived oxygen abundance calculated from ground-based measurements of the speed of the ripples in Jupiter’s atmosphere caused by the Shoemaker-Levy 9 comet collisions. These suggest oxygen could be enriched up to 10 times the solar abundance. This is important because in Jupiter’s atmosphere, oxygen should be found almost exclusively in water, so the oxygen abundance is also the water abundance.

Jupiter’s oxygen abundance is the most important question remaining from the Galileo mission. If oxygen is enriched similar to other heavy elements in Jupiter, then this implies Jupiter formed in a colder region and migrated inward to its present location during the formation of the solar system. If oxygen is super enriched relative to other heavy elements, upwards of 10 times solar abundance or more, then this implies it formed in a warmer region, perhaps much closer to its present location. But if Galileo’s measurements, which show oxygen depleted relative to solar, are global, then we need new planetary formation and atmospheric models. It is no exaggeration to say that “the history of the solar system is found in the formation of the planet Jupiter.” And that’s why NASA plans to launch the Juno Mission to Jupiter in August 2011.

The Juno spacecraft is a polar orbiter that should arrive at Jupiter in August 2016 after a gravity assist from Earth in October 2013. Its primary mission will last one year during which it will complete 33 eleven-day long orbits. Juno should answer many questions we have about Jupiter and help us form a much clearer picture of the solar system’s cosmogony. Juno’s mission will focus on the following:

1. Providing a more accurate determination of the abundance of heavy elements in the atmosphere. This can help us determine what sort of model most accurately describes Jupiter’s formation.

2. Mapping the gravitational and magnetic fields so that we can infer the interior structure of Jupiter. Does it have a core? How massive is the core?

3. Using microwave frequencies to peer into the atmosphere down to pressures greater than 100 bars. This will help us determine the distribution of condensibles like ammonia and water.

4. Exploring the magnetosphere around Jupiter’s polar regions to determine how it couples with the atmosphere.

Well, I hope you’ve enjoyed this latest Columbia Monday podcast about Galileo and Jupiter. For more information about the public events at Columbia Astronomy visit Our next and last Columbia Monday podcast will be “Life in Technicolor” by Maria Pereira on Monday, December 28th. I’m Adam Fuller. Happy birthday wishes go out to my nephew BG. Everyone else, have a great day and keep listening.

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