There's a Little Black Spot on the Sun Today

Early this morning, astronomers saw Venus slide across the sun. Back in the 18th and 19th centuries, astronomers circled the globe to gain views of Venus's transits. By comparing their measurements, they aimed to accurately size up the entire solar system. Nowadays, science has better ways to measure solar space, but Venusian transits are still a good excuse to think about Venus--Earth's mysterious sister.

Meet Venus

Like the Roman goddess of love for which it is named, the planet Venus is a compelling mix of beauty, mystery, and danger. In many ways, no other planet is more like Earth. Some even call it Earth's sister planet. It was formed at the same time. It's roughly the same size. And it has a complex atmosphere and a similarly composed rocky core.

But that's where the similarities end. Unlike Earth, Venus has no moons and no magnetic field, and in spite of a volcanic nature, exhibits no plate tectonics. Venus also moves in unusual ways. It has the most circular orbit of the planets. More surprisingly, it spins on its axis in the opposite direction of all the other planets--and so slowly that, on Venus, one day lasts longer than a year.

Botticelli portrayed the goddess Venus as a nude, but Venus the planet is eternally clothed in a dense and deadly shroud. Composed primarily of carbon dioxide, Venus's atmosphere creates crushing surface pressure, 90 times that of Earth. Imagine the pressure faced by today's submarines at "crush depth"--about 1,500 feet (450 meters) below the waves. Surface pressure on Venus is akin to the pressure of twice that depth.

The upper layers of Venus's atmosphere blow with hurricane force, while several lower layers are full of toxic sulfuric acid. Worse still, the presence of so much carbon dioxide creates a powerful greenhouse effect, trapping the sun's energy and creating temperatures even hotter than those on Mercury. The average temperature is about 460 degrees Celsius, or 860 degrees Fahrenheit--hot enough to melt lead.

This deadly mix of pressure, toxicity, and temperature makes Venus a difficult planet to study. Standard instruments on orbiting probes can't penetrate to the surface, and surface landers have survived for just a few hours. It wasn't until the use of powerful radio waves in the 1960s that Venus's surface became observable, and it wasn't until the arrival of the Magellan probe in 1990 that we could more fully examine and map the planet.

Magellan used a sophisticated radar imaging system to produce detailed maps of 98 percent of Venus's surface. Then, after four years in orbit, Magellan became the first satellite to be intentionally crash-landed onto a planet's surface. Collecting data until the end, it sent its last transmission in 1994. Only flybys have happened since.

See Venus stripped of her atmospheric vestments,
with Magellan's radar images