Giant Methane Storms On Uranus

Most of the times we have looked at Uranus, it has seemed to be a relatively calm place. Well, yes its atmosphere is the coldest place in the solar system. But, when we picture the seventh planet in our solar system invariably the image of a calming blue hazy disc that the spacecraft Voyager 2 took in 1986 comes to mind.

Uranus as seen by NASA’s Voyager 2 NASA/JPL-Caltech

However, all we have previously known about the atmosphere of Uranus has been ’thrown to the wind’ with observations made last year.

In August 2014 a group led by Imke de Pater pointed the Keck telescope at Uranus and were a little bit surprised to see storms raging. It wasn’t as though clouds haven’t been seen before, but the clouds they spotted last year were very much brighter than any seen before. The fact that the storms are bright in the methane spectrum isn’t a surprise – Uranus, and its neighbour Neptune, are pretty much just big balls of methane, water and ammonia (but it does make for a snigger-worthy headline).

Light from Uranus, as captured from my backyard in Sydney. The dips in the spectrum mainly correspond to methane (positions of the methane absorption is shown by the blue lines). Andy Casely

The storms are described in a paper recently published in Icarus, with the pre-print available here. After the first observations, the group put out a call to amateur astronomers to see if they could also observed this unusual activity too. They did, and with this information the group built a case to point the Hubble Space telescope at Uranus, which happened in October. Again, they saw large storms, showing that what they had seen in August hadn’t been a one off event - the weather report on Uranus is looking rather unsettled.

The storms on Uranus, as seen from the Keck telescope. Imke de Pater (UC Berkeley), Larry Sromovosky and Pat Fry (U. Wisconsin), and Heidi Hammel (AURA)

Uranus was the first planet to be discovered in the ‘recent’ era of science. All the planets up to Saturn were observed to be different ‘wandering’ stars by many ancient cultures – so we’ll never know who first spotted them. But Uranus was first observed in 1690 by John Flamsteed. He plotted it six times – but didn’t realise it was different from any other star (he catalogued it to be 64 Tauri). The French astronomer Pierre Lemonnier also observed Uranus, but didn’t distinguish it from the other stars he was watching. It was William Herschel who realised, in 1781 after thinking it was a comet, that he’d seen a planet orbiting further from the sun than Saturn.

Despite knowing where it was for over 300 years, we’ve only in the last decade started to take a detailed view of the Northern hemisphere of Uranus. The observations made by de Pater and her team are the first time this giant region of our solar system has been surveyed by modern telescopes from Earth. This is because of the very strange rotation, which makes Uranus pretty unique.

Our Earth rotates on its axis tilted only slightly from being straight up (if we define up as being perpendicular to a planet’s orbital plane). It is this tilt that drives our seasons.

Uranus has the most extreme tilt of axis in the whole solar system, it is inclined 98° from up. This means Uranus has the most extreme seasons – as each hemisphere of the planet faces the sun as it orbits (a cycle that take 84 years). The upshot is that as the Northern hemisphere has been in winter until recently, and from Earth we have been unable to see it. In 2007 Uranus reached it’s equinox, with the equator pointing at the sun and each of the two hemispheres illuminated.

Uranus' strange orbit explained. M. Showalter/M. Gordon/SETI Institute

The group observed Uranus with the Keck telescope as it past equinox seven years ago. They expected to see storm activity, as parts of the planet that haven’t seen the sun in 20 years started to come to light. They thought it has gone quiet again, which is why 2014’s storms took them by surprise. Added to this is the fact the storms are flaring up in the Northern hemisphere, the part of Uranus that is entering its spring, and thought not to have warmed up from its prolonged winter yet.

Where is the energy to drive these storms coming from? That’s the mystery. Storms on the other gas giants are thought to be fed by energy from their dynamic interiors. Voyager 2 saw that Uranus should have a dynamic interior (it has an active magnetic field like Neptune) but that little of this energy is reaching the atmosphere. This is why Uranus is the coldest planet in our solar system, parts of the atmosphere were observed to be a chilly -224°C.

What this observation of these giant storms really does highlight, is just how little we know about our solar system’s giant icy planets Uranus, and its neighbour Neptune. In the light of the fact that missions like Kepler are finding many other similar planets orbiting distant stars, we really need to sort this out. Hopefully knowing more about our ‘local’ planets will mean that we can understand much more about those further away.

Meanwhile, while we in Australia may have past our summer – think of the Southern hemisphere of Uranus where a 20-year winter is coming….

http://en.wikipedia.org/wiki/Uranus

Sir William Herschel announced its discovery on March 13, 1781, expanding the known boundaries of the Solar System for the first time in history. Uranus was the first planet discovered with a telescope.

Herschel notified the Astronomer Royal, Nevil Maskelyne, of his discovery and received this flummoxed reply from him on April 23: “I don’t know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it”.[27]

Although Herschel continued to describe his new object as a comet, other astronomers had already begun to suspect otherwise. Russian astronomer Anders Johan Lexell was the first to compute the orbit of the new object[28] and its nearly circular orbit led him to a conclusion that it was a planet rather than a comet. Berlin astronomer Johann Elert Bode described Herschel’s discovery as “a moving star that can be deemed a hitherto unknown planet-like object circulating beyond the orbit of Saturn”.[29] Bode concluded that its near-circular orbit was more like a planet than a comet.[30]

The object was soon universally accepted as a new planet. By 1783, Herschel acknowledged this to Royal Society president Joseph Banks: “By the observation of the most eminent Astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System.”[31] In recognition of his achievement, King George III gave Herschel an annual stipend of £200 on condition that he move to Windsor so that the Royal Family could look through his telescopes.[32]

Naming
Maskelyne asked Herschel to “do the astronomical world the faver [sic] to give a name to your planet, which is entirely your own, [and] which we are so much obliged to you for the discovery of”.[33] In response to Maskelyne’s request, Herschel decided to name the object Georgium Sidus (George’s Star), or the “Georgian Planet” in honour of his new patron, King George III.[34] He explained this decision in a letter to Joseph Banks:[31]

“In the fabulous ages of ancient times the appellations of Mercury, Venus, Mars, Jupiter and Saturn were given to the Planets, as being the names of their principal heroes and divinities. In the present more philosophical era it would hardly be allowable to have recourse to the same method and call it Juno, Pallas, Apollo or Minerva, for a name to our new heavenly body. The first consideration of any particular event, or remarkable incident, seems to be its chronology: if in any future age it should be asked, when this last-found Planet was discovered? It would be a very satisfactory answer to say, ‘In the reign of King George the Third’.”

Herschel’s proposed name was not popular outside Britain, and alternatives were soon proposed. Astronomer Jérôme Lalande proposed that it be named Herschel in honour of its discoverer.[35] Swedish astronomer Erik Prosperin proposed the name Neptune, which was supported by other astronomers who liked the idea to commemorate the victories of the British Royal Naval fleet in the course of the American Revolutionary War by calling the new planet even Neptune George III or Neptune Great Britain.[28] Bode opted for Uranus, the Latinized version of the Greek god of the sky, Ouranos. Bode argued that just as Saturn was the father of Jupiter, the new planet should be named after the father of Saturn.[32][36][37] In 1789, Bode’s Royal Academy colleague Martin Klaproth named his newly discovered element uranium in support of Bode’s choice.[38] Ultimately, Bode’s suggestion became the most widely used, and became universal in 1850 when HM Nautical Almanac Office, the final holdout, switched from using Georgium Sidus to Uranus.[36]

Uranus is named after the ancient Greek deity of the sky Uranus, the father of Cronus (Saturn) and grandfather of Zeus (Jupiter), which in Latin became “Ouranus”.[1] It is the only planet whose name is derived from a figure from Greek mythology rather than Roman mythology. The adjective of Uranus is “Uranian”.[39] The pronunciation of the name Uranus preferred among astronomers is ,[2] with stress on the first syllable as in Latin Ouranus, in contrast to the colloquial, with stress on the second syllable and a long a, though both are considered acceptable.[e]

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