3:2 mean motion resonance at approximately from the Sun. This changed the direction of migration, causing them to migrate away from the Sun and out of the inner system to their current locations. All of this happened over a period of 3–6 million years, with the final migration of Jupiter occurring over several hundred thousand years. Jupiter's departure from the inner solar system eventually allowed the inner planets—including Earth—to form from the rubble. There are several problems with the grand tack hypothesis. The resulting formation timescales of terrestrial planets appear to be inconsistent with the measured elemental composition. It is likely that Jupiter would have settled into an orbit much closer to the Sun if it had migrated through the solar nebula. Some competing models of Solar System formation predict the formation of Jupiter with orbital properties that are close to those of the present day planet. Other models predict Jupiter forming at distances much farther out, such as . Based on Jupiter's composition, researchers have made the case for an initial formation outside the molecular nitrogen (N2) snowline, which is estimated at from the Sun, and possibly even outside the argon snowline, which may be as far as . Having formed at one of these extreme distances, Jupiter would then have migrated inwards to its current location. This inward migration would have occurred over a roughly 700,000-year period, during an epoch approximately 2–3 million years after the planet began to form. In this model, Saturn, Uranus and Neptune would have formed even further out than Jupiter, and Saturn would also have migrated inwards. Physical characteristics Jupiter is a gas giant, being primarily composed of gas and liquid rather than solid matter. It is the largest planet in the Solar System, with a diameter of at its equator. The average density of Jupiter, 1.326 g/cm3, is about the same as simple syrup (syrup USP), and is lower than those of the four terrestrial planets. Composition Jupiter's upper atmosphere is about 90% hydrogen and 10% helium by volume. Since helium atoms are more massive than hydrogen molecules, Jupiter's atmosphere is approximately 24% helium by mass. The atmosphere also contains trace amounts of methane, water vapour, ammonia, and silicon-based compounds. There are also fractional amounts of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia. Through infrared and ultraviolet measurements, trace