Sunday, 29 August 2010

Night Three of our Jupiter observations, Mauna Kea Observatory

Our second night's images are now available as a rough movie on YouTube, thanks to Tom Stallard, who is working back in Leicester on our data: if you search on "Jupiter through an Earthly veil" or "Jupiter IRTF", you will find it easily. The movie shows many of the familiar features of the planet, even though it is a set of infrared images, not pictures taken in normal, visible, light. Here's the movie:

The banded structure of Jupiter is clear, with the various belts and zones. About half-way through, the Great Red Spot (showing white in our movie) rises over the edge of the planet, and travels towards the noon meridian. Before that happens you can see the closest of Jupiter's big moons, Io, moving into eclipse on the western side of the planet (on the right of our movie); it comes out of eclipse towards the end of our image set.

Io is the most volcanic body in the Solar System, with plumes of sulphur that can sometimes be seen reaching far out into space. So it glows very brightly in our movie - it's the brightest thing we see.

The aurorae can also be seen clearly around the poles. In the north, the aurora starts as an oval that then turns into a bright line as it moves onto the edge of Jupiter. In the south, the southern aurora is only seen as a bright line at the pole.

In previous blogs I have mentioned that we sleep at the 3,000-metre level, at Hale Pohaku, whilst the telescopes are at 4,200 metres. That means a 12-kilometre drive up each night to observe and then back down again in the morning. And that means a drive of 6 kilometres at the start over a dirt road before you reach the paved road at the summit.

The dirt road has the tendency to develop the texture of an old-fashioned wash-board, with deep ridges. This can be dangerous, particularly on the way down, as vehicles only have grip when the tyre is in contact with the top of the ridge. People who have not taken this into account have had problems; tragically, sometimes the results have been fatal. So the drive to and from the telescopes is not without its challenges.

Tonight is another gloriously clear sky. We left Hale Pohaku with Scorpio setting and Taurus rising, two beautiful constellations. With Taurus come the Pleiades, that lovely cluster of small stars. At the summit, you can often see Nine, instead of the usual, Seven Sisters. The Pleiades are testimony to the global nature of astronomy - in Hawaii, they are the Makali'i, the Japanese have named their telescope - Subaru - for them. Wikipedia gives a total of 20 names for this group of stars. They have clearly inspired cultures around the world.

It's the turn of the southern aurora to strut its stuff tonight, after the northern lights last night. The reason we see the aurorae differently each night is because - like Earth - Jupiter's magnetic pole is offset from its rotational pole. Since they are centred on the magnetic poles, that makes the aurorae appear to wobble back and forth as the planet rotates.

One of our key objectives is to see if we can observe waves travelling down from the aurorae towards the jovian equator. This would be a sign that energy from the polar regions, where we have the strong winds and electric currents, can make it down to lower latitudes. That, in turn, would help us to understand a problem that has plagued planetary scientists since the arrival of the Voyager spacecraft in 1979 - why is Jupiter, and the other giant planets, Saturn, Uranus and Neptune, so hot?

For Voyager measured the temperature of the upper atmosphere around the equator of Jupiter to be around 900 degrees, several hundred degrees hotter than can be explained by the input of sunlight alone. In a previous blog, I mentioned that the energy generated by the winds and currents associated with the auroral/polar regions was 100 times that due to the absorption of sunlight. So an obvious solution would be to transfer some of that energy down from the poles to the equator.

But not so fast: Jupiter is spinning nearly two-and-a-half times faster on its rotational axis than is the Earth - a Jupiter day is just 9 hours 55 minutes. Jupiter's diameter is more than ten times that of Earth. The result is that very large forces, known as Coriolis forces, are generated. And that means that any energy-bearing wind that starts out going from the pole towards the equator tends to get turned westwards, and never reaches its original destination.

So planetary scientists still cannot account for Jupiter's high upper atmosphere temperatures. Our goal from these observations is to see if we cannot add a little bit more to our understanding of that puzzle - hence the search for waves.

When we are working at the summit, it is nice to have a bit of comfort food to keep us going. For Henrik, that means several cans of fizzy drink (sodas, US English) per night; for me, it's peanut butter sandwiches and cocoa - the perfect diet.

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