JOIN USThe colourful world around us seems to lose luster in the night; the bright red roses fail to attract us even in the bright full moonlit night. What we perceive as colour is directed by the physiological responses of the rods and cones in our eye. The colour, as a result, can be sensed only when the brightness levels have reached an optimum value.
Consequently, all solar system bodies (except Venus and Mars) look grey when seen through telescopes. The colours are hardly noticeable. The information that we gather about astronomical object is all based on the light we receive from them, which can be in a wide range. They look different in different wavelengths – a vital key to understand their physics. Today in the era of space observatories, the handicap of the human eyes are easily eliminated.
Varied Hues
Let us consider the images from the Hubble Space Telescope (HST), which uses special filters that allow only a certain range of light wavelengths. Since the cameras can detect light outside the visible light, like the ultraviolet and infrared, the “invisible” features of objects can be studied. The black and white images from the camera are subsequently given the colour as per the specification of the filter used. Today, many HST images are
attracting scientists and artists to produce interesting results and beautiful
artworks.
For example, the colour bands of Saturn are often understood as chemical differences in cloud layers and they are able to reflect and reemit sunlight. Blue colour signifies shorter wavelengths, while red depicts longer wavelengths. Yellow colour suggests that the upper cloud layers reflect the infrared light represented by red and the visible is represented by green; and the combination of the two they make yellow. At the poles, colour difference will depict the difference in reflectivity. Another example is Mars
reconstructed from different black-and-white images recording red, green and blue light reflected from the planet. The brightest image naturally is in red; the white ice cap is bright in all colours – very faithfully reproduced in the reconstruction.
The images of other planets likewise depict their chemical composition. Jupiter’s bands show gases including hydrogen and helium. The strong yellow touch is provided by the sulphur, which dominates in Venus giving it the yellow colour. Uranus was depicted in green colour based on the first ever image sent by Voyager in 1986; however Hubble
images show it as blue green, a signature of methane, which strongly absorbs red. Neptune also has plenty of methane, however the darker shade indicates the overall decrease in the brightness (incident sunlight) as compared to Uranus.
Mariner 10 depicted a good picture of Mercury, which showed its grey sphere, which is similar to our moon. The rocky surface and the colour were attributed to the iron compounds on the soil easily captured by the cameras in the absence of atmosphere in both cases. As more and more spacecrafts started sending images of Mercury its colour posed a bigger puzzle. Its extremely low albedo (fraction of reflected sunlight) makes it quite a dull object in spite of its proximity to the sun. The close up views provided by Messenger hinted on organic compounds (rich in carbon) on its surface whose origin was a puzzle.
The interplanetary space is not empty. Particles from primordial nebula, solar wind, meteorites and debris of asteroids and comets hover around rather freely. The lack of atmosphere on bodies like the moon and the mercury provide a perfect landing space which can get a “black” coating over millions of years. Spectroscopic studies should have revealed the iron rich nano particles resulting from such continuous bombardments on the moon and similar bodies. However the presence of organic compounds on Mercury has a different story to tell. Carbon rich nano particles provide the hint – that comets must have showered their dust on to Mercury rather generously. By the time comets reach the distance of Mercury, on their way to perihelion, they develop very broad tails of dust and plasma.
The rate of production of dust also increases tremendously; the ejection velocities surpass the escape velocity, which is a very small value for the small mass of the comet. The dust ejected therefore is lost from the comet once for all. They are left behind in their orbit which is frequented by Mercury. The impact velocity of the particles on to Mercury is quite low so that they settle on to the surface. They may melt owing to the impact or may not. Either way they produce a dark coating on the surface. A quantitative estimate of the cometary dust can tell a lot about the past history of bombardment over the last several billion years.
(The author is the Director of Jawaharlal Nehru Planetarium, Bengaluru)