Astronomical observations demonstrate conclusively that T-Tauri-type outbursts can be of sufficient magnitude to scour the terrestrial-planet region of our Solar System. The Hubble Space Telescope image at left shows just such an outburst of the binary XZ-Tauri, taken in 2000. The white crescent embedded in the plume marks the leading edge of that outburst five years before. In five years the leading edge of the plume progressed a distance equivalent to 130 times the distance from our Sun to Earth.

Much confusion has arisen from decades of making computational models based upon the erroneous assumptions that the mineral assemblage characteristic of ordinary chondrites formed in equilibrium in an atmosphere of solar composition at very low pressures, ca. 10^-4 bars, and that ordinary-chondrite-like matter comprises planetary interiors.

Herndon and Suess [3] have shown that ordinary chondrite formation necessitates, not an atmosphere of solar composition, but instead an atmosphere depleted in hydrogen by a factor of about 1000. Subsequently, Herndon [4] showed the impossibility of ordinary chondrite-matter being in equilibrium with a gas of solar composition, and showed as well the necessity of some oxygen depletion relative to solar matter. Moreover, the ordinary chondrites appear, not primary, but rather as a secondary mixture, leaving only two types of primary matter, the oxygen-rich carbonaceous chondrite-type matter and the oxygen-starved enstatite chondrite-type matter [2]. 

As early as 1940, scientists, including the renowned Harvard geophysicist Francis Birch, built geophysics upon the premise that the Earth is like an ordinary chondrite, one of the most common types of meteorites observed impacting Earth, while totally ignoring another, albeit less abundant type, called enstatite chondrites. As Herndon [5] discovered in 1980, if the Earth is indeed like a chondrite meteorite as widely believed for good reasons, Earth is like an enstatite chondrite, not an ordinary chondrite. Imagine melting a chondrite in a gravitational field. At elevated temperatures, the iron metal and iron sulfide components will alloy together, forming a dense liquid that will settle beneath the silicates like steel on a steel-hearth. The Earth is like a spherical steel-hearth with a fluid iron-alloy core surrounded by a silicate mantle.

The Earth’s core comprises about 32.5% of the planets mass. Only the enstatite chondrites (filled circles), not the ordinary chondrites (open circles), have the sufficiently high proportion of iron-alloy that is observed for the core of the Earth, as shown in at left. Moreover, components of the interior of the Earth, shown at right, can be identified with corresponding components of an enstatite chondrite meteorite: (1) The inner core being nickel silicide; (2) Earth-core precipitates CaS and MgS at the core-mantle boundary; (3) The lower mantle consisting of essentially FeO-free MgSiO₃; and, (4) The boundary between the upper and lower mantle being a compositional boundary with the matter below that boundary, the endo-Earth, being like an enstatite chondrite [5-8]. Those discoveries and insights led to a fundamentally different view of Earth formation, dynamics, energy production, and energy transport process [1, 9, 10].

In the 1940s and 1950s, the idea was generally discussed about planets “raining out” from inside of giant gaseous protoplanets with hydrogen gas pressures on the order of 10²-10³ bars [11-14]. But, in the early 1960s, scientists instead began thinking of primordial matter, not forming dense protoplanets, but rather spread out into a very low-density “solar nebula” with hydrogen gas pressures on the order of 10^-4 to 10^-5 bars. The idea of low-density planetary formation, often referred to as the “standard model”, envisioned that dust would condense at fairly low temperatures, and then would gather into progressively larger grains, and become rocks, then planetesimals, and ultimately planets [28, 29].

These two ideas about planetary formation embody fundamentally different condensation processes which are the underlying cause for the two unique primary types of chondritic matter. The immediate implication is that both processes were operant during the formation of the Solar System. The relative extent and region of each process can be ascertained to some certitude from thermodynamic considerations together with planetary data. Even within present limitations, a consistent picture emerges that is quite unlike the so-called “standard model of solar system formation” [1].

From thermodynamic considerations it is possible to make some generalizations related to the condensation process in an atmosphere of solar composition. At the foundation, there are two dominant considerations, one essentially independent of pressure and one a strong function of pressure, which together are responsible for formation of the two primary types of Solar System matter.

In an atmosphere of solar composition, oxygen fugacity is dominated by the gas-phase reaction H₂ + ½O₂ = H₂O which is a function of temperature, but is essentially independent of pressure over a wide range of pressures where ideal gas behavior is approached. Oxygen fugacity controls the condensate state of oxidation at a particular temperature. At high temperatures the state of oxidation is extremely reducing, while at low temperatures it is quite oxidizing. The state of oxidation of the condensate ultimately becomes fixed at the temperature at which reaction with the gas phase ceases and/or equilibrium is frozen-in by the separation of gases from the condensate.

Condensation of an element or compound is expected to occur when its partial pressure in the gas becomes greater than its vapor pressure. Generally, at high pressures in solar matter, condensation is expected to commence at high temperatures, while at low pressures, such as 10^-4 to 10^-5 bar, condensation is expected to progress at relatively low temperatures at a fairly oxidizing range of oxygen fugacity. At low temperatures, all of the major elements in the condensate may be expected to be oxidized because of the great abundance of oxygen in solar matter relative to the other major condensable elements [30]. Beyond these generalizations, in this low-pressure regime, precise theoretical predictions of specific condensate compounds may be limited by kinetic nucleation dynamics and by gas-grain temperature differences arising because of the different mechanisms by which gases and condensate lose heat.

Among the thousands of known chondrites, only a few, like the famous Orgueil carbonaceous chondrite, have a state of oxidation and mineral components with characteristics similar to those expected as a condensate from solar matter at low pressures. Essentially all of the major elements in these few chondrites are oxidized, including sulfur.

The idea of planetary formation from a diffuse solar nebula, with hydrogen pressures on the order of 10^-4 to 10^-5 bar, envisioned that dust would condense at fairly low temperatures, and then would gather into progressively larger grains, and become rocks, then planetesimals, and ultimately planets. In the main, that picturesque idea leads to the contradiction of the terrestrial planets having insufficiently massive cores, because the condensate would be far too oxidized for a high proportion of iron metal to exist. But as evidenced by Orgueil and similar meteorites, such low-temperature, low-pressure condensation did in fact occur, perhaps only in the evolution of matter of the outer regions of the Solar System or in interstellar space, and thus may contribute to terrestrial planet formation only as a component of late-addition planetary veneer.

On the basis of thermodynamic considerations, Eucken suggested in 1944 core-formation in the Earth as a volatility-controlled consequence of successive condensation from solar matter in the central region of a hot, gaseous protoplanet with molten iron metal first raining out at the center [11]. Except for a few investigations initiated in the 1950s and early 1960s [12, 13, 15, 16], that idea languished when interest was diverted to Cameron’s low-pressure solar nebula models [17].

On the basis of thermodynamic considerations, Herndon and Suess [18] showed at the high-temperatures for condensation at high-pressures, solar matter is sufficiently reducing, i.e., it has a sufficiently low oxygen fugacity, for the stability of some enstatite chondrite minerals as shown in the figure at left, where the phase boundary between iron gas and liquid iron metal is a function of temperature and pressure in an atmosphere of solar composition, calculated from thermodynamic data, but the oxygen fugacity values are independent of pressure However, formation of enstatite-chondrite-like condensate would necessitate thermodynamic equilibrium being frozen-in at near-formation temperatures. At present, there is no adequate published theoretical treatment of solar-matter condensation from near the triple-point. But from thermodynamic and metallurgical considerations, some generalizations can be made. At the high temperatures at which condensation is possible at high pressures, nearly everything reacts with everything else and nearly everything dissolves in everything else. At such pressures, molten iron, together with the elements that dissolve in it, is the most refractory condensate.

Chondrite elemental abundances are nearly identical to solar element abundances for the relatively non-volatile rock-forming elements. If Earth is like a chondrite element, as widely believed, then adding to Earth’s mass the corresponding proportion of gaseous elements and those elements which form highly volatile compounds, calculated from solar abundances, yields an estimate of the mass of protoplanetary-Earth being in the range 275-305m_E, not very different from the mass of Jupiter, 318m_E. The formation of early-phase close-in gas giants in our own planetary system is consistent with observations and implications of near-to-star giant gaseous planets in other planetary systems [19-21]. It is thus reasonable to expect that the giant planets possess interior rock-plus-alloy kernels of enstatite-chondritic-like matter as they each possess internally generated magnetic fields [22].

In the absence of evidence to the contrary, the observed enstatite-chondritic composition of the terrestrial planets, as indicated by their massive cores, permits the deduction that these planets formed by raining out from the central regions of hot, gaseous protoplanets [14]. With the possible exception of Mercury, the outer veneer of the terrestrial planets may contain other components derived from carbonaceous-chondrite-like matter and from ordinary-chondrite-like matter. Mars, for example, may have an extensive outer veneer, while for Earth, it is ≤18% by mass. Satellites may possess an internal kernel of enstatite-chondritic-matter. The particular importance of enstatite-chondritic-matter derives from the highly reduced state of oxidation during formation, which forced certain oxyphile elements, such as uranium, into the alloy portion, rather than into the silicate, resulting in the possibility of georeactor-like magnetic field generation in planets of our Solar System [22].

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Herndon, J. M., and Suess, H. C., Can enstatite meteorites form from a nebula of solar composition? Geochimica et Cosmochimica Acta, 40, 1976, 395-399.