Mars as viewed with the AAT
Halfway through the night of June 18, the scheduled AAT observers (myself and Yeshe Fenner from Swinburne University) found ourselves with a dearth of suitable extragalactic targets for our program using the Taurus Tunable Filter and the new EEV 2K x 4K CCD. With only a slight amount of coaxing from the night assistant, Steve Lee, we decided to take advantage of the exquisite seeing that night (0.7 arcseconds!) and the ability to do direct imaging to take time out to observe Mars. Normally, the brightness of the planet (apparent mag -2.3) makes this impractical even with the shortest exposure times, but the narrow bandwidth of the TTF (12 Angstroms), coupled with some judicious adjustment of the mirror covers, made it possible to avoid saturating the detector in 0.5 second exposures. The results through 4 different blocking filters is shown in the figure at left, which has north at the top, and celestial east to the left.
The image at top left is taken through the TTF's B1 blocking filter, with a central wavlenegth of 3900 Angstroms. In blue light, Mars' polar caps of frozen carbon dioxide, and any clouds or hazes (particularly near the planet's limb) are prominent. In this image, the south polar cap of Mars is seen at lower right, while a bright "hood" cloud is all that remains of the north polar cap as autumn begins in the northern hemisphere. A bright limb cloud can also be seen on Mars' western (evening) terminator, which is almost certainly the recurrent "Syrtis Blue Cloud" as it lies atop Mars' most famous dark surface feature.
Going to redder wavelengths emphasises these darker surface markings, the so-called "maria" and "terrae" recorded by Schiaparelli (of Martian canal fame) and others. The image at top right is taken through the B4 filter centered on 5000 Angstroms, the one at lower left through the R0 filter (6680 Angstroms), and the one at lower right through the R1 filter (7070 Angstroms). The prominent fork-like feature running from the centre of the planet towards the west is made up of Sinus Sabeaus and Sinus Meridiani. The bright patch to the north is the Moab/Arabia basin. The darkest region to the northeast is Niliacus Lacus, while that in the southeast is made up of Aurorae Sinus and Erythraeum Mare.
Using these images, Steve Lee has attempted to reproduce a "true
colour" view of the planet, and the result is shown below:
As it happens, we were extremely lucky to get these images when we did. A week or so afterwards, a huge dust storm swept out of the Hellas basin (see the Hubble Heritage image), and has since engulfed half the planet!