As excitement builds for Tuesday’s Venus transit, astronomers hope to catch a rare and exciting glimpse of the planet’s atmosphere. What’s more, by analyzing the sunlight refracted through the Venusian upper atmosphere, we may also gauge its temperature.
During historic Venus transits, the celestial event has been used by astronomers as a scientific tool to precisely measure the distance from Venus to Earth, and Earth to the sun. In this modern age, these distances are well known — we also have robotic spacecraft orbiting all the inner planets, Mercury, Venus and Mars taking in-situ observations — but remote observations from Earth are a great means of validating these measurements.
Although Venus currently has a spacecraft in orbit — the European Space Agency’s Venus Express — some rather exciting observations of the Venusian atmosphere will also be made.
Just as the transit begins at 3:09 p.m. PDT (6:09 p.m. EDT) on June 5 (Tuesday), astronomers will be looking for a phenomenon that has been reported for centuries during previous Venus transits. In 1761, French astronomer Chappe d’Auteroche was first to record an observation of a bright arc around the dark edge of the planet as it sunk into the sun’s limb, beginning the transit — a period known as “ingress.” The brightening was also visible as Venus completed its transit, emerging from the limb — a period known as “egress.”
d’Auteroche correctly deduced that this brightening, or “aureole,” was caused by the presence of an atmosphere refracting sunlight around the planet.
Since those pioneering days, Venus aureole have been repeatedly recorded by observers on the ground and by space telescopes — like observations by NASA’s Transition Region And Coronal Explorer (TRACE) solar observatory during the 2004 transit, shown top. As it turns out, far from just being a curiosity, science can be done by observing this beautiful event.
Paolo Tanga, an astronomer from the Observatory of Côte d’Azur in Nice, France, describes the “Aureole Effect” in an entry for the Transit of Venus Project website, pointing out that the luminescent crescent isn’t uniform and that, until recently, it was a bit of a mystery. Often, just as Venus is about to pass into the solar limb (at ingress) and after it has passed out of the limb (at egress), a bright spot emerges from the aureole. So what causes it?
“Only in the 20th century, when the rotational properties of the planet were known, the astronomers were able to verify that this spot coincides roughly with one of the planet’s poles,” says Tanga.
Basically, as Venus begins to pass in front of the sun, some sunlight is refracted around the planet’s atmosphere, creating the luminescent crescent of the aureole. The physics of refraction is very well known, as is the composition of Venus’ atmosphere. The amount of refraction applied to a beam of light is greatly influenced by the density of the medium (in this case atmospheric gases), which is affected by atmospheric temperature.
As we know the rotational properties of Venus, astronomers are able to deduce that the persisting bright spot of the aureole is located over the poles. This means that some property of the atmosphere above Venus’ poles is changing the angle of refraction through the atmosphere. As it turns out, this modification is caused by a temperature difference in Venus’ upper polar atmosphere — in a mysterious region known as the mesosphere.
During the 2004 Venus transit, the temperature of the mesosphere 110-120 kilometers (68-75 miles) above the surface of Venus was measured. Venus’ mesosphere is warmer above the poles when compared with other latitudes. In a presentation on the topic, Tanga describes an aureole brightening that coincided with a region of the polar mesosphere at a temperature of 215 Kelvin (-73 Fahrenheit), whereas lower latitude aureole corresponded to a mesospheric temperature of 135 Kelvin (-217 Fahrenheit). These transit measurements agreed with modeled data.
After Venus Express arrived in Venusian orbit in 2007, our knowledge of the planet’s atmosphere has increased, but the 2012 transit will be an invaluable tool to compare the results gained by the spacecraft and validate these modern planetary measurements.
Source: Transit of Venus Project
Image credit: NASA