Chemical & Engineering News,
December 4, 1995

Copyright © 1995 by the American Chemical Society.

Galileo And Its Atmospheric Probe Rendezvous With Jupiter

Elizabeth K. Wilson, C&EN West Coast News Bureau

After six years in space, spacecraft
nears its destination to begin study
of Jupiter and its mysterious atmosphere

Galileo Galilei, the 17th-century scientist branded as a heretic by the Catholic Church for his Copernican beliefs, waited a long time for exoneration. Although the Italian astronomer and physicist pioneered the use of astronomical telescopes, discovered Jupiter's moons, and formulated the basic law of falling bodies, the church didn't repeal its ruling until 1992.

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Artist's rendition depicts Galileo's atmospheric probe-its parachute deployed and heat shiled released-beginning its descent into Jupiter's atmosphere.

The modern-day Galileo, a 2-1/2-ton spacecraft arriving at Jupiter on Dec. 7 after six years in space, is hardly in danger of ecclesiastical condemnation, but like its namesake, it promises to turn science on its ear.

Within Galileo's grasp are answers to some of the most chemically provocative questions in planetary science: What substances create Jupiter's colors? How much water does the planet contain? What lies underneath the water clouds? What are the structures and compositions of Jupiter's many moons? And, most important, what does Jupiter's makeup tell us about the formation of the solar system?

Engineers and scientists who have literally spent their careers on this mission are collectively holding their breath. "I've been in a state of high alert for months," says Alvin Seiff, one of the pioneers of robotic space exploration at the National Aeronautics & Space Administration's Ames Research Center in Mountain View, Calif. "Somehow, I don't think it can get much more intense."

Launched on Oct. 18, 1989, from the space shuttle Atlantis, Galileo was sent on a complicated routine of planetary flybys designed to sling it toward Jupiter. During its journey, it has flown by two asteroids, Gaspra and Ida, past Earth twice and Venus once, and has observed the impact of comet Shoemaker-Levy 9 on Jupiter.

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Despite several misfortunes, including its high-gain antenna's failure to unfold and the recent malfunction of its tape deck, the craft is "believed to be in excellent health," Project Manager William J. O'Neil told a Jet Propulsion Laboratory (JPL) press conference last month in Pasadena, Calif. These glitches shouldn't affect the atmospheric probe data.

At around 2:15 PM PST Dec. 7, Galileo's atmospheric probe, which was released from the main spacecraft July 13, is slated to begin a high-speed plunge into Jupiter's clouds at 6.5°N. Less than an hour later, mission control at JPL should receive the first signals from the Galileo orbiter to indicate that the probe survived entry. Meanwhile, the spacecraft will be maneuvering into orbit around the giant planet, swooping by for a closeup study of Jupiter's volcanic moon Io in the process.

The probe's life in Jupiter's atmosphere will be short - it will transmit data up to the orbiter for a mere 75 minutes. About 10 hours later, at a temperature of about 2,000 K, the probe and its six instruments will have melted and vaporized.

Jupiter is a chemist's dream. It is a huge planet blanketed in layers of ammonia, ammonium hydrosulfide (NH4HS), water, and unknown gases. Violent convection currents, caused by temperature gradients generated by the still-lingering heat of Jupiter's formation, stir thick clouds into a psychedelic mass of swirling, colored storms. In some ways, Jupiter has been viewed as a chemical Rosetta stone, its composition thought to have the same ratio of elements as the primordial solar system. Elemental abundances on Jupiter are expected to say volumes about the evolutionary processes that formed the planet.

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On July 13, Galileo- launched from Kennedy Space Center in Florida in 1989 - released its atmospheric probe (schematic bottom image) that will descend into Jupiter's atmosphere on December 7th.

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The primary scientific objective of the mission is to understand the history of the solar system, says Richard E. Young, probe scientist at Ames Research Center.

As the Galileo probe plunges toward the planet, its instruments will measure the ratio of hydrogen to helium; atmospheric composition, temperature, and pressure; the transfer of heat and radiation from one atmospheric level to the other; the size and density of particulates; and even lightning.

At a relative speed of 106,000 mph, the probe will hit the atmosphere at a horizontal angle of 8.3°. The angle must be precise: only 1.5° deeper and the probe could burn up instantly; 1.5° shallower, it could "skip" off the atmosphere like a stone.

The probe will the slow to 1,000 mph within two minutes, experiencing a deceleration equivalent to 250 to 300 times Earth's gravity, Young says. "This is the most difficult atmospheric entry ever attempted."

After about 75 minutes, the probe will have descended about 150 km into the jovian atmosphere where the pressure could reach between 20 and 30 bars. The distance is only a minute portion of the giant planet's radius of more than 71,000 km.

"Imagine Jupiter is a tomato," Young says. "We're only going to pierce the skin of the tomato."

Because Galileo's tape recorder may be less reliable than before, engineers have cut back on some of the recording redundancies from the probe that would have served as extra data sets. "But it's still very likely we'll get all the data from the probe," says Marcie Smith, probe manager at Ames.

Scientists believe the probe will first encounter clouds of ammonia ice crystals about 1 &mgr;m in size or smaller. They also are fairly sure there is some water on Jupiter, just because of the cosmic abundance of oxygen, and because of the planet's quantities of hydrogen. (Toward the center of Jupiter, under enormous pressure and high temperature, the hydrogen becomes metallic, which may be the source of the planet's intense magnetic field.)

The giant planets, which also include Saturn, Uranus, and Neptune, are thought to have formed from "leftover gas that didn't quite get sucked into the sun," says Andrew P. Ingersoll, planetary science professor at California Institute of Technology.

To Sidebar: Like other spacecraft, Galileo has weathered major glitches

Earth, and indeed all the planets, presumably had similar compositions at the time of their formation. But whereas Earth's relatively weak gravity couldn't hang onto lighter elements like hydrogen and helium, the gargantuan Jupiter retained most of its original substance.

Thus, "you have a very hydrogen-rich atmosphere. Instead of getting things like nitrogen, you have ammonia; and instead of CO2, it's methane," Ingersoll says. If this theory is correct, Jupiter's composition should nearly match the makeup of the sun and the primordial solar system.

However, this model does have problems, scientists say. Spectroscopic studies suggest that on Jupiter, carbon and nitrogen are richer relative to the sun by a factor of two. One explanation, Ingersoll says, is that at some point during formation, half of the hydrogen and helium were lost.

But another monkey wrench has been thrown into the works as a result of the collision of the comet Shoemaker-Levy 9 with Jupiter. Scientists originally thought Jupiter might be very dry: Spectral data from the Voyager missions indicated that water was scarce, perhaps existing in a proportion less than that of the sun. (On the sun, only water's constituent oxygen and hydrogen exist, whereas on Jupiter, temperatures are cool enough to permit water molecules to form.)

In July 1994, impacts from Shoemaker-Levy 9 generated visible spreading ripples - known as gravitational waves - in Jupiter's atmosphere that propagated outward, like stones dropped in a lake. The speed of the ripples suggest the waves rode through a water cloud underneath the upper atmosphere.

"I was led to the conclusion that the amount of water was 10 times what we'd expect if Jupiter were just a cooled-down piece of the sun," Ingersoll says.

If water is indeed as dense as some think, deep inside Jupiter's atmosphere it should be perpetually raining. "There's nothing for it to fall on, but it would just be continuous rain," Ingersoll says. But what does the extra water do to the Rosetta stone picture? The answer may be a third process that added water to the planet, such as the introduction of a number of icy bodies - like comets.

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Hubble Space Telescope image made October 5, 1995, indicates the Galileo atmospheric probe entry site, 6.5 degrees North of Jupiter's equator.

The first-ever delve by a probe into this mysterious atmosphere may help clear up some of the confusion.

For example, "we sort of expect we might be seeing sulfur in the clouds, because sulfur was detected in the comet impacts," says Glenn Orton, coinvestigator for the probe's net flux radiometer and nephelometer at JPL. "But there's still a raging debate about whether that's part of the comet or Jupiter."

Scientists also are keenly interested in the atmospheric structure of Jupiter - the way pressure and temperature vary with depth.

It's not clearly understood how heat is transported from the planet's interior to the atmosphere. Jupiter emits about twice as much energy as it absorbs from the sun, with the excess heat coming from the primordial heat of gravitational collapse. This heat would cause moist convection, the hot atmosphere cooling and condensing as it moves upward. In addition, Jupiter's striking band structure is caused by winds that rush east-west across the planet's equator at speeds of up to 250 mph, reversing direction with each band.

The net flux radiometer will measure how much light and heat are transferred from one level to another in the atmosphere. The atmospheric structure experiment on the probe will measure pressure, temperature, and accelerations taken near the probe's center of gravity as it descends.

"If you understand the structure, you understand a great deal about the atmosphere of Jupiter," says Ames's Seiff, principal investigator for the atmospheric structure experiment.

The probe's nephelometer (from the Greek word nephele, meaning cloud) will characterize Jupiter's clouds by measuring the scattering of a narrow beam of laser light by atmospheric particles. "Using scattering theory we can get a fair idea of what's doing the scattering," says Boris Ragent, nephelometer principal investigator at Ames.

"This is so much better than anything we've done before, it's fantastic. We're answering zero-order questions, like 'What's there?'" Ragent says.

The probe also contains a neutral mass spectrometer, designed to measure the chemical constituents of Jupiter's atmosphere within a range of 2 to 150 daltons. The instrument will sample gas over a pressure range of 150 millibars to 20 bars as it descends into the atmosphere "depending on how long the probe survives," says Paul R. Mahaffy, a member of the instrument development team at NASA's Goddard Space Flight Center, Greenbelt, Md.

To Sidebar: On route to Jupiter, Galileo toured planets, asteroids

"We expect to detect all sorts of [chemical] species that haven't been detected remotely because they don't produce signals of sufficient intensity," Mahaffy says.

The lightning and radio emission instrument on the probe will look for radio bursts and optical flashes. Voyager sent back images of lightning 100 to 1,000 times more intense than anything on Earth. Lightning may also be evidence of water - scientists can't think of a mechanism on Jupiter that produces lightning without it, Young says.

The probe entry site was originally chosen for its most nearly "typical" features, but clearly, more global information is needed. "We can't define an entire planet by a single survey, any more than we could if we released one meteorologic balloon," Seiff says.

Because only the slower, low-gain antenna can be used to transmit data, engineers have cut back on the number of images the orbiter was originally slated to record, including the probe entry site.

So at the time the probe enters the atmosphere, ground-based telescopes will be scanning Jupiter in visible through thermal IR wavelengths at observatories around the world, from Kitt Peak in Arizona to the Pic du Midi in the French Pyrenees, says Orton.

After the probe's dramatic encounter with Jupiter, the majority of the spacecraft's original objectives will be accomplished as it orbits the planet 11 times over the next 23 months.

The Galileo orbiter's instruments will continue to record and measure magnetic fields, energetic particles, plasmas, plasma waves, and dust. Additionally, two radio science experiments will take place and a heavy-ion counter will measure the charged-particle environment of the spacecraft. In particular, the near-infrared mapping spectrometer, which will study the surface/atmospheric composition and conduct thermal mapping of the planet, and the ultraviolet spectrometer, which detects atmospheric gases and aerosols, should illuminate the moons that surround Jupiter.

The 16 known satellites of Jupiter resemble its own solar system. Galileo will focus on the four largest, planet-sized moons: Io, Callisto, Ganymede, and Europa.

While the probe is making its precipitous descent, the orbiter will initially swing past Io, whose orbit lies within Jupiter's intense radiation belts. This will be the only time Galileo visits Io because of the risk of radiation damage to its delicate instruments.

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Color composite taken by Voyager
captures the dynamics of the
jovian atmosphere.
Galileo will study Jupiter's
four largest moons:
Io (top center),
Ganymede (upper left),
Europa (center),
and Callisto (lower left).

Io's curious volcanic composition was first revealed by Voyager images, which showed active volcanoes ejecting sulfur dioxide.

The UV spectrometer experiment includes an extreme UV instrument that will be particularly useful for detecting high-energy particles that circulate in Io's torus. The torus is formed from a cloud of ions that surrounds Io. As Io orbits Jupiter, the cloud takes on a doughnut shape that circles the planet.

"The fascinating thing about Jupiter is that its moons are inside the magnetosphere, while on Earth our moon is a long way away compared to the size of the magnetic field," says Charles Hord of the University of Colorado, Boulder, who is principal investigator for Galileo's UV spectrometer.

Io's torus is thought to be formed when sulfur dioxide is spewed out from the moon's surface, breaking into oxygen and sulfur. As photons from the sun or electrons in Jupiter's magnetosphere ionize an atom of sulfur or oxygen, the "magnetic field grabs hold of that ion. It must go with the lines of force, so it is immediately accelerated to temperatures as hot as the solar corona, 30,000 to 40,000 K," Hord explains. "That is really hot."

Galileo will fly by Ganymede four times, Callisto three times, and Europa three times. The UV instrument will look for gases, perhaps water vapor, escaping from Ganymede and Callisto.

It may also detect things like atomic oxygen, or the hydroxyl radical, which radiates in the UV region, Hord says. "And actually, if there's enough oxygen, it could form a little bit of ozone," he says, of which there has been some evidence from Earth-based observations.

Io is probably devoid of water, while Europa appears to be in a state of transition, Hord says. "It looks almost like there's been running water on the surface; there are no craters. Europa is still icy, but it's losing it; it's changing."

The near-IR mapping spectrometer will be taking measurements at wavelengths between 0.7 and 0.52 &mgr;m. "It's a very diagnostic region, where there are lots of vibrational transitions where molecules can absorb," says Robert Carlson, principal investigator for the near-IR mapping spectrometer at JPL.

"Chemical maps" will be created by remote sensing of surface compositions of Jupiter's satellites. "We know there's water, and probably iron-bearing silicates," Carlson says. "Who knows what we'll find."

On Europa, some observers think dark linear features are cracks in ice with brownish material, possibly frozen hydrocarbons, welling up from an ocean below, Carlson says.

Atmospheric probe scientist Young (left) and atmospheric probe manager Smith, both of Ames Research Center, pose in front of a full-scale model of the probe at the Jet Propulsion Laboratory

A major task of the instrument will be to focus on the 5-&mgr;m region, looking into the deep atmosphere of Jupiter for water variation. Other chemicals, such as phosphine (PH3) and germane (GeH4), produced deeper in the atmosphere act as tracers for convection. "We expect to see a lot of enhancement of phosphine and germane in the polar regions" where heat flux may be related to enhanced diffusion, Carlson says.

The first results from the probe will arrive on Earth within a week. Then, over the next two years, images, spectra, and other measurements will be sent to Earth in regular packages, delivered to waiting, eager scientists.

Ingersoll fondly remembers the success of the Voyager mission as he anticipates Galileo. "I'm hoping that going into Jupiter - the probe, plus getting really close to the satellites, studying the atmosphere - is going to yield just as many surprises," Ingersoll says. He sums up what keeps scientists in the risky field of space exploration: "I really got hooked on discovery."

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