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Probe Data

The Probe Data is here!

Table of contents
Introduction--A Quick Overview of the Probe Mission
What conditions on Jupiter are like
Observing Jupiter's Atmosphere....Indirectly

Introduction--A Quick Overview of the Probe Mission

Named for the chief of the Roman gods, Jupiter contains more mass than all the other planets combined. It is more than 300 times the mass of Earth. So far, understanding of this huge gas planet is limited to a two -dimensional view (pictures of the planet's cloud tops) of a three -dimensional process (Jupiter's weather). To explain the incredible coloring, swirling shapes and high-speed motion of its cloud tops, scientists must explore far below the visible surface. They must know the structure and composition of the atmosphere, nature and location (altitude) of the deeper cloud layers, and forces which drive the winds.

The chance to answer many of these questions was presented on December 7, 1995 when the Galileo Probe--like a flaming meteor--made history's first entry into the atmosphere of an outer planet. The Probe plunged into Jupiter's brilliantly-colored, swirling cloudtops at 106,000 mph, and then descended through turbulence, violent winds and three major cloud layers into the hot, dense atmosphere below. During the probe's descent, it measured the composition of Jupiter's atmosphere, the winds, temperatures, and clouds. The results of the probe mission are giving us clues to better understand Jupiter's history and the origin of the solar system.

During its entry, the gasses just in front of the Probe were brighter than the Sun and more than twice as hot as the sun's surface (28,000 degrees Fahrenheit). Wrenching forces slowed the probe from 106,000 mph to 100 mph in just four minutes. Once it finished its blazing entry the Probe, by comparison, operated in a far less severe environment. Jupiter's atmosphere is primarily hydrogen and helium. For most of its descent through Jupiter's three main cloud layers, the probe was immersed in gases at or below room temperature. Eventually, the Probe passed below these clouds, where rising pressures and temperatures caused it to lose radio contact with the orbiter. Ultimately the probe was destroyed by the high temperatures.

As the Probe passed through the cloud layers, its main computer received information from each science instrument. This science data was transmitted to the orbiter, which stored the data and then later relayed it to Earth. The Probe descent mission lasted 61.4 minutes, and then the radio signal was lost.

Probe Data in Your Classroom

Galileo Probe scientists have been waiting since the 1970's for data from their atmospheric explorer, the first "in-situ" (inside) exploration of Jupiter's atmosphere. Questions they hope to answer include "How do Jupiter's winds behave below the visible cloud tops?", "How does the temperature change as we get deeper into Jupiter's atmosphere?", and "Where are the clouds and what are they made out of?"

Online From Jupiter 97 participants will be able to see some of that same data. Through special arrangements with scientists who have experiments on the Probe, we'll be sharing some of the actual Probe data. This data will be posted in mid-February, 1997.

What conditions on Jupiter are like

Before we start talking about what the Probe did, we need to understand something about Jupiter's atmospheric conditions, and why scientists are interested in studying it. We'll start out by looking at Jupiter's composition and interior structure.

The composition of Jupiter's atmosphere is quite different from Earth's--Jupiter is composed mostly of hydrogen and helium, the same elements that make up most stars. In fact, we expect that Jupiter has basically the same composition as the Sun. Like the Sun, Jupiter has its own heat source, but Jupiter's heat comes from heat left over from the formation of the planet 4.5 billion years ago, and heat produced today due to the slight contraction of the planet under its own gravity. This means that Jupiter's composition might be very much like the original solar nebula from which it--and the solar system--formed, so investigating Jupiter's atmosphere is a way for us to investigate the early solar system.

In its present state Jupiter emits about twice as much heat energy as it receives from the Sun. The loss of this heat -- residual energy left over from the planet's formation --- means that Jupiter is slowly but steadily cooling and losing energy. Temperatures in Jupiter's core, which were about 50,000 degrees Celsius (C) in the planet's hot, early phase, are now about 30,000 degrees C -- 100 times hotter than any terrestrial surface, but 500 times cooler than the temperature at the center of the Sun. Temperatures on Jupiter are now thought to range from 30,000 degrees C at the core to minus 120 degrees at the top of the cloud banks.

Of course, Jupiter's atmosphere is made up of other elements besides hydrogen and helium. There are traces of compounds such as ammonia, methane, sulfur, water, and various hydrocarbons. It's expected that Jupiter will have amounts of the these compounds in roughly the same relative proportions as the Sun, too. However, there may be differences in how much of any compound is found in Jupiter compared to the Sun. These differences may give important clues into how Jupiter was formed and the nature of processes in the inaccessible interior. The Galileo Probe directly sampled the atmosphere to determine its composition, an extremely important science objective.

We do not have any direct information on the interior structure of Jupiter, but there are theoretical models of the interior; these must match up to known measurements of Jupiter's mass, volume, and slightly non-spherical shape caused by its rapid spin (a day on Jupiter lasts 9 hours and 48 minutes, but Jupiter's radius is roughly 71, 500 km, over 11 times that of Earth, so rotational speed at the equator is quite high!).

We expect that at the center of Jupiter is a rocky core that may be about 30 times the mass of Earth, made from approximately the same rocky materials that the Earth is made from. Surrounding the rocky core and extending to about half the radius of Jupiter, we expect there is a region of what is termed "liquid metallic hydrogen." This form of hydrogen only occurs under extreme high pressure and temperature, like that deep in the interior of Jupiter. As you might expect from its name, liquid metallic hydrogen is an electrically conducting liquid form of hydrogen. Since it is electrically conducting, fluid motions in this region almost certainly produce Jupiter's strong magnetic field, about 10 times stronger than the magnetic field of the Earth. This situation is similar to the way in which the magnetic field of the Earth is created by fluid motions in its liquid iron core, which surrounds the inner solid iron core at the center of the Earth. We know about the Earth's interior from earthquake studies, but we do not have such information about Jupiter.

Outward beyond the liquid metallic hydrogen region, there probably exists a slurry of liquid hydrogen and helium mixed with gaseous hydrogen and helium. Farther out from the center, hydrogen and helium exist as gas, not liquid. This is the region called the atmosphere, and it is the upper part of the atmosphere that was sampled by the Galileo Probe. Although only the upper atmosphere was sampled, the information obtained helped us understand even the deep interior, because we obtained basic information on how temperature is related to pressure, a relation that is expected to be true even in the interior regions. Additionally, the chemical composition of the atmosphere will give clues about Jupiter's interior.

Observing Jupiter's Atmosphere....Indirectly

Since the Galileo Probe was the first spacecraft to actually go into Jupiter's atmosphere, you might have wondered how scientists already knew something about Jupiter's composition. Amazingly, it's possible to use long-distance observations (both from spacecraft and from telescopes here on Earth) to gather some information about Jupiter's atmosphere. For example:

Spectral Observations

One of the most powerful ways to examine a planet's atmosphere is to use an instrument known as a spectrometer. If you've ever looked at a prism, you already have some understanding of how a spectrometer works: light comes in one side, and is refracted, or bent, within the prism. Since different wavelengths of light (or, in the case of visible light, different colors) are bent by different amounts, the light that comes out of the prism is "separated" into its different wavelengths.

What makes this especially useful is that the atoms and molecules in the atmosphere may absorb certain wavelengths of light. When a scientist takes the light from a planetary atmosphere and feeds it into a spectrometer, the result isn't a pretty rainbow--it's a rainbow with gaps, with black lines showing where some of the light has been "absorbed". Once these "absorption lines" are identified (meaning, figuring out what atoms or molecules cause the absorption), scientists are able to tell what's in the atmosphere. Of course, this only gives us a good idea about what's in the uppermost layers of the atmosphere.

a schematic drawing of occulation geometry


We've also been able to determine the temperature and pressure in the outermost 80 kilometers of the Jovian atmosphere. Imagine that we have a spacecraft that is flying by Jupiter (see the figure below). When the spacecraft is on the far side of the planet, we can't see the spacecraft at all (the spacecraft at the bottom of the figure). Any radio signals that the craft is sending out are blocked by the planet itself, or by the planet's dense atmosphere. At other times, the spacecraft has a clear line of sight to Earth; the radio signal doesn't pass through the planet's atmosphere. However, when the spacecraft's line of sight passes through the thinner, outer layers of the planet's atmosphere, the atmosphere acts like a prism, and the spacecraft's signal is refracted. Since the amount of refraction depends on the composition of the atmosphere scientists can get more clues into what is in Jupiter's atmosphere.

Probe Data

The Probe Data is in from the Principal Investigators involved with the Galileo Mission.

This data can be used by students to make spreadsheets and develop hypotheses based on the interpretations of the data.

Data Set - Posted 2/6/97




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