Tuesday 27 September 2011

Outer planets ablaze at 1.5 microns

Astronomer Mike Brown at Caltech hunts in the outer reaches of the solar system. He usually concentrates on what is called the Kuiper Belt which extends from the orbit of Neptune out to about twice as far as Neptune's orbit. He recently used the Keck telescope in Hawaii with the intention of imaging Neptune's moon Triton, but the views of the planet itself, and closer neighbour Uranus, were too interesting to ignore. The Keck's adaptive optics system, which compensates for turbulence in the earth's atmosphere, produces stunning results.

Neptune, glowing like a demonic cricket ball, shows the two distinct bands which are glowing at the imaging wavelength of 1.5 microns (1500 nm). Of course these are not really 'hot' since the surface of Neptune is -200 C and the bright bands will only be a few degrees warmer. These correspond to faint but visible features on Neptune's 'surface'.

With Uranus the main features are the rings, which stand out distinctly given the low level of radiation from the planet itself, and the spot to the top left which is the moon Miranda. The bright spots on the surface are clouds. This is notable because the 'surface' of Uranus, in visible light, is almost devoid of any features (although, as the 'northern' uranian hemisphere warms up some banding is appearing).

There is more at space.com.

RW Wood took infrared photographs of gas giants Jupiter and Saturn at the end of October 1915. He had been granted use of the 60 inch reflector at Mount Wilson and took photographs through infrared, yellow, violet and ultraviolet filters. His infrared filter had a band-pass of 700 nm, with its upper limit being determined by the plate sensitivity, which was probably less than 800 nm. In this case infrared photographs showed fewer features than visible light whereas belts were clearly shown on the violet and ultraviolet plates. (It is not possible to record a wavelength as long as 1.5 microns using photographic emulsion, so an electronic sensor has to be used, although this is not thermal imaging. However, this wavelength is still within an atmospheric window for infrared.)

You can read Wood's 1916 paper on the SAO/NASA Astrophysics Data System (ADS). It is entitled Monochromatic Photography of Jupiter and Saturn.

[My thanks to Mike Brown for permission to reproduce his images here.]