Saturn's rotation period has been a mystery. An estimate based on its meteorology comes with implications for our understanding of the planet's atmospheric jet streams and interior structure.
The rate at which a planet rotates is a fundamental property that informs our understanding of its formation, evolution, internal dynamics and meteorology. For planets with solid surfaces, the spin rate can simply be determined by tracking the motion of landforms as they rotate across the surface. But for the gas giants Jupiter, Saturn, Uranus and Neptune, which lack any solid surfaces, determining the rotation rates of their interiors is more difficult. Saturn has proved the most enigmatic, and in recent years our imprecise understanding of its rotation rate has become obvious1. On page 608 of this issue, Read and colleagues2 use clues from Saturn's dynamic meteorology to derive a new estimate for its rotation rate.
Tracking cloud motions over time shows that Saturn's atmosphere, like all atmospheres, does not rotate as a solid body but contains several east–west jet streams. Air at the equator circles the planet once every 10 hours 12 minutes, whereas air at higher latitudes can take up to 30 minutes longer to do so3. These cloud-tracked wind measurements imply that Saturn's atmosphere contains a broad equatorial jet — extending from 30° N to 30° S latitude — that flows eastward at speeds that are up to 450 m s−1 faster than air at higher latitudes. High-latitude atmospheric regions (outside the equatorial jet) are further subdivided into differentially rotating latitude bands whose relative speeds typically differ by 100 m s−1.
But what is the rotation rate of Saturn's interior? Despite the planet's fluid nature, electromagnetic forces in the electrically conducting interior should keep the interior rotation at nearly a single value. But is this interior rotation rate faster, slower or intermediate between the wide range of atmospheric rotation rates determined by cloud tracking? Answering this question has broad implications not only for the planet's interior structure4, but also for our understanding of whether the jet streams are primarily eastward or westward relative to the interior — and hence for the degree of angular-momentum exchange between the atmosphere and the interior, for the thermal structure below the clouds, and for the formation mechanisms of the jet streams.
For Jupiter, Uranus and Neptune, the key to unlocking this puzzle lies in the radio emission from sources in the planets' magnetospheres (the region of space near the planet where the planetary magnetic field dominates over that of the solar wind). Because the magnetic dipoles of these planets are tilted relative to their axes of rotation, magnetospheric emissions exhibit a periodicity that allows the rotation period of the magnetic field — and therefore of the planetary interior where the field is generated — to be determined. Saturn's magnetic dipole does not seem to exhibit such a tilt, however. Although Saturn does emit radio waves whose periodic modulations were long assumed to define the rotation rate5, recent measurements1 show that this period varies by about 1% over intervals of months and so cannot represent the interior rotation.
Read et al.2 adopt a radically different approach. They propose that Saturn's rotation rate can be determined by considering the dynamical stability of the planet's jet streams. Using observational estimates of winds and temperatures at and above the clouds, they expand on previous work6 in which they showed that, at many latitudes, the pattern of the jet streams is almost neutrally stable — lying very near the boundary between stability and instability — according to a stability theorem developed by Vladimir Arnol'd. In the neutral configuration, this theorem relates a flow's east–west wind speed to the latitudinal gradient of a quantity called the potential vorticity — essentially, the rate at which individual air columns spin, divided by a measure of the vertical thickness of the columns. Because the potential vorticity is independent of the reference frame, but the east–west wind speed is not, knowledge of the potential vorticity can be used to determine the reference frame in which the east–west wind speed must be evaluated for the neutrality condition to be valid. This allows an estimate of the interior rotation rate.
Read and colleagues' analysis2 builds on previous theoretical and observational work suggesting that such a near-neutral configuration is plausible for both Jupiter and Saturn7,8,9,10. But why would a flow adopt a state that is neutrally stable? Under appropriate conditions, Saturn's loss of heat to space, and the turbulent transports of momentum that help to pump the jet streams, may force the jets to become unstable. But because Saturn's radiated heat flux is meagre, the timescales for radiative cooling and jet pumping — and hence for the jets to gradually become unstable — are probably years to decades. By contrast, the natural timescale for a strongly unstable jet to naturally develop eddies that rob the jet of energy — thereby making it less unstable — is typically days to weeks. Because of this mismatch in timescales, these competing processes could drive the flow into a configuration that is almost neutrally stable. Analogous arguments have been put forward to explain the configuration of the large-scale air flow in Earth's mid-latitudes, but in that case the instability timescales are not well separated from the radiative timescales, weakening the argument for such a neutral configuration.
Interestingly, the planet's interior rotation rate of 10 hours 34 minutes proposed by Read et al.2 — like that suggested in another recent attempt to estimate the rotation rate11 — is intermediate between the fastest and slowest atmospheric rotation rates determined from cloud tracking. Such a value suggests that Saturn's winds exhibit an alternating pattern, with eastward-flowing jets at some latitudes and westward-flowing jets at others (Fig. 1). This is significantly different from the original estimate from Saturn's radio emission5, which implied a slower interior rotation period of 10 hours 39 minutes for which all the observed winds would be eastward. Because of the dynamical linkage between winds and temperatures, the new rotation rate has additional implications for the latitudinal gradient of temperatures below the clouds, as well as for the mass of Saturn's putative rocky core.
Because Jupiter's jet streams also alternate between eastward and westward, the revised rotation period gives Saturn a more Jupiter-like countenance than previously appreciated. Nevertheless, Saturn's winds are stronger than Jupiter's, its banded cloud patterns and populations of hurricane-like vortices differ considerably, and its magnetic field, which is almost symmetrical about its axis — a puzzle in its own right — contrasts with Jupiter's tilted dipole. These contrasts indicate that the planets are cousins rather than twins, whose intriguing mix of similarities as well as differences will keep planetary scientists engaged for years to come.
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