Tuesday, June 4, 2013

Jet Streams on Uranus and Neptune

Eight from the Sun to see in orbit.
But no one mentions Neptune for that odyssey.
What is it about it.
So vast it is...
Sixty Earths could sit within its grasp.
- Lawrence S. Pertillar, “Neptune”, stanza 3 line 1-5.

Figure 1: An illustration of Neptune’s interior.

In the Solar System, the known planets can be classified into three categories - terrestrial planets (Mercury, Venus, Earth and Mars), gas giant planets (Jupiter and Saturn) and ice giant planets (Uranus and Neptune). Compared to the other planets in the Solar System, very little is known about Uranus and Neptune because the only spacecraft to have ever visited them was NASA’s Voyager 2 which flew by Uranus in 1986 and Neptune in 1989. The bulk composition of an ice giant planet is very different from a gas giant planet such as Jupiter or Saturn. An ice giant planet consists of a rocky core, an icy mantle and an outer gaseous hydrogen-helium envelop. The icy mantle comprises the bulk of the planet’s mass and is what defines an ice giant planet. In contrast, a gas giant planet is almost entirely made of hydrogen and helium.

Figure 2: Zonal winds on Uranus and Neptune. The data points correspond to atmospheric wind speed measurements of Uranus and Neptune by the Voyager 2 spacecraft (circles) and the Hubble Space Telescope (squares). The solid lines for Uranus and Neptune are empirical fits to the data, constrained to zero at the poles. Credit: Yohai Kaspi et al. (2013).

Both Uranus and Neptune have fast atmospheric jets that flow westward near the Equator and flow eastward at higher latitudes. Neptune in particular, has the fastest planetary winds anywhere in the Solar System. A paper by Yohai Kaspi et al. (2013) show that these atmospheric jets extend to depths of not more than ~1100 km for both planets. On Uranus, this depth corresponds to a pressure of ~2000 bar or the outermost 0.15 percent of the planet’s mass. For Neptune, this depth corresponds to a pressure of ~4000 bar or the outermost 0.20 percent of the planet’s mass. When one considers that Uranus and Neptune have mean diameters of 50,700 km and 49,200 km respectively, a depth of not more than ~1100 km implies that the atmospheric dynamics on Uranus and Neptune do not extend deeply into the planetary interiors. As a result, the dynamics of these atmospheric jets are probably driven by shallow processes.

Figure 3: Artist’s impression of a Uranus/Neptune-like ice giant planet.

In the upper atmosphere of Uranus and Neptune, the temperature is cool enough for methane to condense to form clouds. Deeper down, at pressures of up to a few bars, are clouds of ammonia and hydrogen sulphide. Yet deeper, at tens of bars or more, water condenses into clouds. A plausible mechanism that can drive the atmospheric jets on Uranus and Neptune is by latent heat release from moist convection. Deep in the atmosphere at pressures of ~300 bars, the condensation of water to form clouds can release sufficient latent heating to drive the atmospheric jets. Moist convection on Uranus and Neptune cannot be powered by energy from sunlight because sunlight only penetrates to pressures of a few bars. Instead, moist convection on Uranus and Neptune can only be driven by internal heat radiating out from the planetary interior. The internal flux to solar flux ratios of Uranus and Neptune are 0.06 and 1.6 respectively. For Neptune, such a high ratio means that the internal heat radiating out from the planet’s interior is greater than the energy the planet receives from the Sun. As a consequence, Neptune’s high internal heat flux drives a more vigorous moist convection and probably explains why it has the fastest winds in the Solar System.

Yohai Kaspi et al., “Atmospheric confinement of jet streams on Uranus and Neptune”, Nature 497, 344-347 (16 May 2013)