September 23, 2010
Despite the great differences between the atmospheres of Venus and Earth, scientists have discovered that very similar mechanisms produce lightning on the two planets. The rates of discharge, intensity, and spatial distribution of lightning are comparable. Findings will help scientists understand the chemistry, dynamics, and evolution of the atmospheres of the two planets.
Previous missions, such as the Venera, Pioneer Venus, and Galileo spacecraft, have reported evidence for optical and electromagnetic waves from Venus that could be produced by lightning. Ground-based telescopes capturing lighting flashes at Venus have also supported this. Yet the differences in the two atmospheres have led some to claim that lightning on Venus would be unlikely, and the topic has been controversial. The launch of Venus Express with its magnetometer built by the Space Research Institute in Graz, Austria, has provided an opportunity to confirm the occurrence of lightning on Venus and to study in detail its magnetic field at altitudes between 125 and 300 miles (200 and 500 kilometers).
"Short, strong pulses of the signals expected to be produced by lightning were seen almost immediately upon arrival at Venus," said C. T. Russell of the University of California, Los Angeles, "despite the generally unfavorable magnetic field orientation for entry of the signals into the Venus ionosphere at the altitude of the Venus Express measurements."
When clouds form on Earth or Venus, the energy that the Sun has deposited in the air can be released in a powerful electrical discharge. As cloud particles collide, they transfer electrical charge from large particles to small, and the large particles fall while the small particles are carried upward. The separation of charges leads to lightning strokes. This process is important for a planetary atmosphere because it raises the temperature and pressure of a small portion of the atmosphere to a high value so that molecules can form, which would not otherwise occur at standard atmospheric temperatures and pressures. This is why some scientists have speculated that lightning may have helped life to arise on Earth.
On our planet, about 100 lightning discharges occur per second, but from any one location we see far fewer. Similarly on Venus, we do not see the entire planet, and we have to estimate the total occurrence rate with some assumptions about how far one can see. Thanks to the new datasets from Venus Express, Russell and colleagues were able to show that lightning is similar in strength on Earth and Venus at the same altitudes.
"We have analyzed 3.5 Earth years of Venus lightning data using the low-altitude Venus Express data (10 minutes per day)," said Russell. "By comparing the electromagnetic waves produced at the two planets, we found stronger magnetic signals on Venus, but when converted to energy flux, we found very similar lightening strength."
It also seems that lightning is more prevalent on the day side than at night and happens more often at low venusian latitudes where the solar input to the atmosphere is strongest.
"Venus and Earth are often called twin planets because of their similar size, mass, and interior structure," said Russell. "The generation of lightning is one more way in which Venus and Earth are fraternal twins."