Neptune and Uranus – ice twins that fell out

The mystery of Uranus and Neptune is as thick as their atmospheres. Though visited by Voyager 2 in the 1980s, little is actually known about them. Now a new study is urging another look.

Images of Uranus and Neptune taken in natural light by the Hubble Space Telescope.

Jupiter – king of the planets – has epic storms, Saturn has majestic rings, Mercury is being mapped like never-before, Venus is a vision of hell and Mars continues to haunt our imaginations. But what of Uranus and Neptune?

For too long these ice giants (so-called because they contain ‘ices’ such as ammonia, methane and water) have seemed like the forgotten twins of the Solar System. It’s time for some redress. In a paper published this week (arXiv:1208.5551v1), two scientists from Tel Aviv University have pointed out discrepancies in the way the properties of the two planets are derived.

It is not hard to see how it could have happened. The Voyager probes took advantage of a serendipitous arrangement of the Solar System that allowed them to visit all the outer planets in the swiftest time. After feasting on Jupiter and Saturn, Voyager 2 was then sent to hunt down Uranus and Neptune. When it reached its new quarry, the kills were swift. As Voyager 2 flung by at speed there wasn’t enough time to gather ever-more data and so scientists had to make extrapolations from what they got.

What they found was that the planets’ equatorial radii differed by less than four per cent and possessed broadly similar masses and gravitational moments (the distribution of mass within the planets). The paper also states that, “even their rotation periods are within 10 per cent of each other.” These periods were determined from radio and magnetic field data from Voyager 2. However, in a previous study conducted with other colleagues, Dr Ravit Helled suggested that the radio data may not correlate to the movement of the deep interiors.

The magnetic fields may also originate close to the surface (Stanley and Bloxham 2006) and thus may not be indicative of deep layer movements, which may be different from that of the thick atmospheres. Interior models for Jupiter and Saturn (which are better understood) just don’t work when applied to Uranus and Neptune. Even the water composition of these planets is not really known. The extrapolations seemed flawed.

Thus Helled and her colleague, Professor Morris Podolak, urge caution when making assumptions about Uranus and Neptune. Interpret the data in the right way, however, and the pieces fit together better. For instance, although the masses and radii of both planets may be similar, they are not the same: Neptune is smaller, but more massive. This subtle difference gives it a significantly higher density than Uranus. Which brings us to that planet’s bizarre rotational axis – tilted 97.7 degrees from its orbital plane. The perception of unanimity between these ice twins now really starts to fall away.

Unlikely as it may seem, Uranus’s weird, horizontal rotation may be the link between mass, gravitational moment and the planets’ observed thermal emissions. Uranus doesn’t seem to radiate heat from its core once solar radiation is accounted for. Neptune, lying further from the Sun, receives 60 per cent less warmth, but is a similar temperature to Uranus: -221 Celsius. This suggests that Neptune has a more prominent internal heat source.

How does this link with Uranus’s axial tilt? In the past planetesimals could have collided with both planets. An oblique collision with Uranus could not only have knocked the planet sideways, but also created a thin shell over the interior that acts as a heat barrier (Stevenson 1986). A more radial collision with Neptune could have just churned up its interior, releasing more heat.

The model used by Podolak and Helled suggests that Uranus’s core could easily have survived – unlike Neptune’s. Thus Uranus’s axial tilt suggests a more centralised, hidden interior (one that may skew density calculations), which fits in with the observed thermal emissions. The scientists say that testing their models will require one thing: further measurements.

Kulvinder Singh Chadha is a freelance science writer and author based in the UK. He is the former Assistant Editor of Astronomy Now magazine and has an astrophysics degree from the University of Hertfordshire.

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