Neptune’s first orbit: a turning point in astronomy
Astronomers will celebrate a remarkable event on 11 July. It will be exactly one year since the planet Neptune was discovered. Readers should note a caveat, however. That year is a Neptunian one. The great icy world was first pinpointed 164.79 years ago – on 23 September 1846. And as Neptune takes 164.79 Earthly years to circle the sun, it is only now completing its first full orbit since its detection by humans. Hence those anniversary celebrations.
As to the nature of the planet, this was only revealed in full in 1989 when the US probe Voyager 2 swept past it and sent back images of a seemingly serene blue world – though later analysis revealed dark spots on its surface that are vast cyclonic storms.
But the real surprise came from Triton, says Professor Carl Murray of University College London. “We found it had a thin atmosphere and huge streaks of black material across its surface. These are created by geysers of dust and nitrogen erupting from under Triton’s icy surface as it is heated by the sun. In other words, even at the edge of the solar system, where temperatures are more than -200C, sunlight can still drive distinctive weather systems. It is quite extraordinary.“
Neptune’s weather is characterized by extremely dynamic storm systems, with winds reaching speeds of almost 600 m/s—nearly attaining supersonic flow.More typically, by tracking the motion of persistent clouds, wind speeds have been shown to vary from 20 m/s in the easterly direction to 325 m/s westward. At the cloud tops, the prevailing winds range in speed from 400 m/s along the equator to 250 m/s at the poles.Most of the winds on Neptune move in a direction opposite the planet’s rotation.The general pattern of winds showed prograde rotation at high latitudes vs. retrograde rotation at lower latitudes. The difference in flow direction is believed to be a “skin effect” and not due to any deeper atmospheric processes. At 70° S latitude, a high-speed jet travels at a speed of 300 m/s.
The abundance of methane, ethane and ethyne at Neptune’s equator is 10–100 times greater than at the poles. This is interpreted as evidence for upwelling at the equator and subsidence near the poles.
In 2007 it was discovered that the upper troposphere of Neptune’s south pole was about 10 °C warmer than the rest of Neptune, which averages approximately −200 °C (70 K).The warmth differential is enough to let methane, which elsewhere lies frozen in Neptune’s upper atmosphere, leak out as gas through the south pole and into space. The relative “hot spot” is due to Neptune’s axial tilt, which has exposed the south pole to the Sun for the last quarter of Neptune’s year, or roughly 40 Earth years. As Neptune slowly moves towards the opposite side of the Sun, the south pole will be darkened and the north pole illuminated, causing the methane release to shift to the north pole.
Because of seasonal changes, the cloud bands in the southern hemisphere of Neptune have been observed to increase in size and albedo. This trend was first seen in 1980 and is expected to last until about 2020. The long orbital period of Neptune results in seasons lasting forty years.
Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Several possible explanations have been suggested, including radiogenic heating from the planet’s core, conversion of methane under high pressure into hydrogen, diamond and longer hydrocarbons (the hydrogen and diamond would then rise and sink, respectively, releasing gravitational potential energy),and convection in the lower atmosphere that causes gravity waves to break above the tropopause.