I. Introduction
In recent years, human interest in the space beyond our solar system has increased drastically due to endeavors such as the Hubble Space Telescope, the International Space Observatory, and the launch of the Cassini probe. Much of that interest stems from the discovery of extrasolar planets, worlds that exist around stars other than our own, and ignited the quest to discover a possible Earth-like world. The planets yet found have been coined “hot Jupiters”, gas giants hovering close to white-hot stars. With that came the need for knowledge on how our own Jupiter formed and its composition. Discovery of extrasolar worlds similar to Jupiter can be facilitated if a chemical and isotopic “signature” can be determined for large gas giants, further progressing the search for another Earth among the stars.
II. Methods of Analysis
Gathering data on a celestial object always garners problems. On rare occasions, it is possible to have direct analysis done. In the case of Jupiter, this is accomplished with launched probes containing mass spectrometers. A total of 7 NASA missions have been to or by this planet. Most atmospheric measurements of Jupiter’s stratosphere and troposphere were done while in orbit or transit of the planet. These include Pioneers 10 and 11, Voyager, Ulysses, the Galileo Orbiter, and Cassini. In
addition to the Galileo Orbiter, the Galileo Probe actually entered the atmosphere of Jupiter to take direct data.
The bulk of data gathered on Jupiter is land-based. Telescopes equipped to track the movement of objects in the sky as night progresses maintain a constant stream of spectra images. Every element has a specific series of black lines, or absorption spectra, that appear in a visible light spectrum. The isotopes of a given element have the same patterns offset by only a few Angstroms. By analyzing these spectra, it’s possible to determine the elemental composition of a stellar object. Overall, direct measurement is the most accurate method for gathering chemical data relating to an atmosphere. A probe designed to penetrate Jupiter in its totality to the core will provide invaluable data on stellar and planetary formation as well as creating a fingerprint of large gas giants that can be used to identify planets in other systems
through spectral analysis.
III. Planetary Formation
Approximately 4.7 billion years ago, our solar system as we know it did not exist. In its place was a bright protostar surrounded by a swirling disk of gas, dust, and icy planetesimals. For the most part, the gaseous disk was composed of H, D, 3He, and small amounts of various heavier elements. At some point, this disk was disrupted, possibly from the passing shockwave of a supernova, starting a chain of collapse. Angular momentum caused particles within the cloud to bond together, growing ever bigger as collisions of these particles increased. The gradual cooling of the disk provided the opportunity for solid grains to form, much along the lines of Bowen’s Reaction series. While the cloud remained above 2000 K, the cloud stayed in a gaseous state. The sequence of formation is shown below:
Temperature (K) Elements Condensing Form of Condensation
2000 Gaseous nebula
1600 Al, Ti, Ca Oxides
1400 Fe, Ni Nickel-iron grains
1300 Si Silicates
300 C Carbonaceous grains
300-100 H, N Ice particles (water and ammonia)
Table 1: Condensation reactions for the protosolar nebula.
As gravitational collapse continued throughout the cloud, the increasing pressure on molecules from coalescence initiated chemical reactions, changing the overall elemental composition. Of significant notice is the fusion of deuterium atoms together. All of the deuterium in existence originated with the Big Bang, creating a set ratio of D/H. In the cloud, much of the deuterium fused together to create 3He. This will be discussed in greater detail later regarding its importance. The gas giant planets of the solar system, Jupiter, Saturn, Neptune, and Uranus, are presumed to be the first to have formed for a very simple reason. The asteroid belt between Mars and Jupiter is likely to be the remains of a not-fully formed solid planet torn apart by the gravitational forces of Jupiter. The creation of Jupiter is still a debated topic, but the most widely accepted theory is that vibrations within the
gaseous cloud allowed for gravity to pull more and more particles towards the protoplanet. A crucial balance between vibrational energy pushing outward, gravity pulling inwards, and temperature was reached to allow the planet to maintain stability.
IV. Present Composition
The exact differentiation of Jupiter is impossible to know. The vast pressures of the planet have prohibited any sort of in-depth examination of the interior beyond the outer few kilometers. Therefore, most of the ideas about the planetary center are derived from exterior study. The very heart of the planet is something of a mystery. There has yet to be a way that scientists may receive data about the core, either
directly or indirectly. So the ideas about Jupiter’s center are rather vague. Two plausible situations exist: (1) The core is composed of a ball of supercondensed ices, primarily water, ammonia, and methane, similar to the composition of Saturn, and (2) Jupiter’s interior is a ball of silicic rocks and iron. In either case, the minimum mass of the core would have to be approximate to three times Earth in order to have had the necessary gravity to capture and maintain the remaining layers.
Above this relatively tiny core lays a double layer of He and H. Approximately 80% of Jupiter’s radius out from the core, pressures within the atmosphere are in excess of 600 millibars (average pressure on Earth at sea level is 1013 millibars). It is at this estimated pressure, H and He fractionate out from one another and settle into two immiscible layers. At these pressures, for reasons not yet understood, hydrogen goes from a molecular fluid to an atomic metallic fluid. Immediately below it is a layer of 3He, formed by the dissolution of HD compounds.
The outermost part of the planet is the atmosphere. Jupiter’s atmosphere is stratified into three distinct layers, distinguished by pressure areas. The lowermost of those layers, the lower troposphere, is fairly homogeneous with clouds composed of H2O, NH4SH, or NH3. For certain, the condensation of ammonia is the primary component for the molecular haze in the layer. The upper troposphere is a bit more stratified,
primarily methane bands with a molecular haze. The outermost stratosphere is fully stratified methane bands. It is the stratosphere and upper troposphere that are of most relevance to this paper.
V. Isotopic Fractionation
The basic composition of Jupiter consists of H, C, N, O, He, and S. The primary stable isotopes of the first three are to be addressed here. While there are numerous other elements present in the Jovian atmospheres, the abundances are so low as to not contribute significantly. The relative abundances of these molecules are:
Molecule Abundance / H2
H2 1
HD 2×10-5
He 0.2
CH4 7×10-4
CH3D 3×10-7
Table 2: General abundances in the Jovian atmosphere
Hydrogen (1H and D)
Despite its light mass, hydrogen in both of its stable forms accounts for 50-70% of Jupiter’s composition by mass with no detection of 3H. In the protosolar cloud, the D/H ratio has been extrapolated to be 1.97 +/- 0.36×10-5. This is thought to be the
initial ratio for Jupiter as well. H and D are found in numerous molecules in the atmosphere, some of which are difficult to analyze even in situ due to a polarizing affect on the molecules. Because of this, the combination of spectral data and direct measurement has lead to a range of ratios for the fractionation of D/H.
Common hydrogen-rich molecules include H2, CH4, and NH3. Water measurements provide another uncertainty. O/H values have been difficult to determine in the deeper parts of the atmosphere. However, data from the Galileo spacecraft has given an upper limit of 0.35 times the solar value. More information about water will be discussed in the oxygen section. No data has yet been discovered to suggest that there is a preference between hydrogen and deuterium in any of the molecules.
Fractionation of D and H is accomplished by three main mechanisms: the breakdown rate of hydrogen-bearing compounds under UV irradiation, the diffusion rate for the molecules, and the exchange rate between hydrogen and deuterium in the molecules. This first fractionation method is seen particularly well in the lower troposphere with HD. Molecules containing D are dissociated or ionized, depending on the prevailing atmospheric conditions at the time. H atoms are freed to settle into the metallic liquid layer of Jupiter. The D has two possibilities for it: (1) The atoms are free to settle into their own layer somewhere between the metallic hydrogen and the core, or (2) Fusion of D atoms in the planetary interior to form 3He, which has been shown to be immiscible with H, forming a central layer around the core.
Solar values for hydrogen are accepted to be approximately 4.4 x 10-5. The difference between that value and the values stated for Jupiter can be explained through the previously mentioned fusion of deuterium to helium, lowering the overall hydrogen ratio. But the primary reason given for these variances is measurement error. Hydrogen concentration is often measured through molecules like singularly charged and
doubly charged NH3. In the single-charge species, there is a spectra overlay with water in the mass spectrometer readings during analysis which frequently results in a much higher than expected amount.
Carbon (12C and 13C)
Carbon was believed to have been incorporated into the Jovian atmosphere at formation through the accretion of planetesimals in the protosolar cloud, so it is reasonable to state that its abundance is equal or nearly equal to solar values. It exists in
numerous forms throughout the cloud layers with the most prevalent listed below.
Common C Molecules
CH4 CH3D
C2H2 C2H6
C2H4 C4H2
CH3
Table 5: Commonly occurring carbon-bearing compounds
The reported abundances of carbon cover a vast range, from less than the solar value to many times that value. The only widely accepted concentration is that carbon,
primarily in the form of CH4, constitutes less than 1% of the atmosphere. Though 13C does exist within the atmosphere, it is overwhelmingly dominated by 14C. To date, there has been no data found to show preference for one isotope over the other in any
molecular state. Previously, the carbon concentration was thought to be solar much
similar to hydrogen with little reason to doubt otherwise. It was only when the Galileo Probe began taking direct measurements that it was discovered Jupiter possesses a vast carbon enrichment, approximately three times the solar value. More interestingly was the discovery of what was responsible for a molecular “haze” that covers the entire planet, very similar to a smog cloud. Galileo found that the haze was a thin cloud of benzene, formed when the CH4 in the upper stratosphere is irradiated with UV rays from the sun. This was a startling discovery since it had been presumed that only the very simplest of molecules could exist on other planets.
Nitrogen (14N and 15N)
Nitrogen concentrations prove to be of great interest in “dating” the age of a stellar system by a ratio of its isotopes. 14N is only produced in stars, therefore the amount of 14N in a planet is solely constrained by the amount present in the protosolar cloud. ISO measurements show that the 14N/15N ratio in interstellar medium to be 435 +/- 50, so planets should exhibit a 14N depletion over time relative to the solar winds. For Jupiter, N is primarily found in the form of NH3 with a calculated 15N/14N ratio of 2.3 +/- 0.3 x 10-3. Though important as a component of the atmospheric clouds, NH3 comprises only an approximate 0.3% of the cloud composition. Its highly unlikely that nitrogen exists in any other form other than this fully
reduced one or in any layer above Jupiter’s troposphere. Ammonia in the atmosphere becomes increasingly destroyed at higher altitudes due to the influx of UV rays from the Sun therefore its likely any other form of nitrogen would reside below the ammonia clouds though this has yet to be proven. Similarly, no method for the isotopic fractionation of nitrogen has been discovered.
VI. Interpretations and Conclusions
The planetary composition of Jupiter is much more varied than previously imagined, shown by the in situ analysis preformed by Galileo. The ratio of the elements and isotopes in the atmosphere has created a thumbprint of sorts for identifying gas giant planets in other parts of the galaxy through spectrographic means. The presence of complex molecules such as benzene provides a basis for larger organic molecules to form, thus supplying the basis for life. Finding such a thing in our own stellar system gives hope for identifying the same characteristics in planets yet undiscovered.
