A 2012 transit of Venus across the Sun has yielded valuable information about the Venusian atmosphere, a reminder of past glories and a stepping stone to future visits.
Since the orbits of Venus and Earth (or indeed any of the Solar System's planets) are not in exactly the same plane, we don't see Venus pass across the face of the Sun at every inferior conjunction. In fact, there have only been two transits of Venus in over a century: one in 2004 and the other in 2012.
Transits of Venus have a long and important scientific history, but when millions gathered around the world to watch the shadow of our nearest planetary neighbor cross the Sun, most believed it was just a spectacle. In the days of interplanetary spacecraft, it was thought there was nothing we could learn from such an event that we didn't already know.
However, a team at the University of Palermo, Italy, have proved this view wrong with a study published in Nature Communications, although they needed off-planet telescopes to do it.
In 2012, when Venus crossed in front of the Sun, sunlight shone through its atmosphere – just as it passes through the Earth's atmosphere during a lunar eclipse. When this happens, ultraviolet and X-ray light are absorbed by carbon dioxide and oxygen ions. This allowed Dr. Fabio Reale to use NASA's Solar Dynamics Observatory and the U.S./Japanese Hinode mission to see which wavelengths were absorbed, providing a fingerprint of the contents of the Venusian atmosphere.
By looking at Venus in a range of wavelengths of light, Reale got images that appeared to vary in size. Wavelengths absorbed by ions high in the atmosphere produced a shadow 140-200 kilometers (87-24 miles) wider than in optical light, which is only blocked by the lower atmosphere. This allowed Reale's team to work out which molecules and elements exist at different altitudes.
Venus and the Sun as seen at optical wavelengths (a) and extreme ultraviolet (b). Credit: Reale et al, Nature Communications.
The findings will help NASA plan future missions to Venus. "Learning more about the composition of the atmosphere is very important for understanding the braking process for spacecraft when they enter the upper atmosphere of the planet, a process called aerobraking," Reale said.
Both Venus' day and night sides were hidden from us, with Reale instead getting a picture of the atmosphere over the parts of Venus experiencing dawn and dusk. Given the long Venusian day, the atmosphere on the evening side is a lot hotter than where the day is starting. It was thought this might lead to an elongation of the atmosphere. However, to the limit of our observing capacity, the planet looked perfectly symmetrical.
NASA is excited by the potential to use the measurement of Venus' transit to perfect our observations of planets in other systems transiting their parent stars, which is now the main way of finding exo-planets, and possibly learn something of their atmospheres.
"In the future, there might be missions that have enough sensitivity to detect the difference in radius in different wavelengths," said Reale. "In particular, if there are exoplanets with an extremely thick thermosphere, the size difference in different wavelengths will be larger and there will be a better chance of detecting the change."