VENUS

Earth’s lead-melting twin sister.

At certain times throughout the year, our twilight skies are graced with the radiant white light of what looks like a luminous star. It’s not only brighter than Sirius but often brighter than Jupiter, too. In fact, ancient observers began referring to it as the “morning star” or the “evening star” depending on whether it appeared at dawn or dusk. Of course, we know that stars don’t make these dramatic movements across the sky. This “star” is actually the planet Venus. It was this radiance that inspired the ancient Romans to name it after the goddess of beauty (the Greek equivalent being Aphrodite). We see Venus make these back-and-forth swings from one side of the Sun to the other because its orbit is interior to Earth, just like Mercury. As the second planet from the Sun, Venus comes closer to Earth than any other planet (within 0.3 AU), hence why it appears so luminous.

Venus is often referred to as Earth’s twin, mostly due to it being a terrestrial world of similar size. It is ~95% of Earth’s diameter and ~80% of its mass. Both planets have a layered internal structure, with cores composed primarily of iron and nickel surrounded by rocky mantles rich in silicates and other minerals, with thin crusts on their exteriors. Venus even has a thick atmosphere. This close proximity, size and composition suggest that Venus and Earth formed around the same time and from the same building blocks.

These are, however, where the similarities end.

Because its orbit is interior to Earth’s orbit, Venus always appears relatively close to the Sun in our sky, though its invisible leash is not as tight as Mercury’s. Venus moves away from the Sun until it reaches its greatest elongation, the maximum apparent separation it can achieve. Then, it appears to turn around and head back toward the Sun. It eventually passes between the Earth and Sun, reappearing on the other side before beginning the cycle again.

With a telescope, you’ll see Venus not only change in apparent size but also pass through different phases as its angle to the Sun changes, much like our Moon! It appears full when it is on the far side of the Sun, then becomes increasingly gibbous as it moves toward greatest elongation, where it appears half-lit. As Venus approaches Earth, its illuminated portion shrinks from gibbous to crescent while its apparent size grows. It reaches its greatest brightest as a crescent before disappearing into the Sun’s glare. Later, it reemerges in the morning sky and repeats the cycle in reverse, eventually returning to a full phase.

Also like Mercury, it occasionally transits the Sun, though it is much more rare. They happen in a repeating cycle starting with a pair of transits eight years apart, then a long gap of 121.5 years, followed by another eight years and then 105.5 years. If you were hoping to see one of these with your own eyes you unfortunately have a long wait ahead of you. The last two transits took place in 2004 and 2012 so we are early into one of the long intervals. The next transits of Venus will occur in 2117 and 2125.

Warning: Never look directly at the Sun without approved solar glasses or a solar telescope filter. Otherwise, you could risk permanent eye damage and/or blindness.

Venus has an incredibly thick and toxic atmosphere. It is almost entirely made up of carbon dioxide which traps heat from the Sun really well and has created a runaway greenhouse effect on the planet. Temperatures can get as high as 900°F, hot enough to melt lead. Despite being the second closest planet from the Sun, this makes Venus the hottest planet in the solar system. Even hotter than Mercury. The atmosphere also features opaque and highly reflective clouds of sulfuric acid, giving Venus the highest albedo in the solar system. It is the third brightest object in Earth’s sky. The atmospheric pressure at the surface of Venus is 92 times that of Earth at sea-level. This is equal to the pressure about 1 km below the Earth’s ocean surface. To stand on the surface would mean enduring 1,350 lbs. of pressure crushing down on every square inch of your body.

The thick atmosphere of Venus makes optical observation of the surface impossible from both Earth-based or orbital vantage points. Fortunately, probes like the Magellan spacecraft (1989-1994) have been able to use various forms of radar to cut through the opaque clouds and map the surface. Radar uses pulses of electromagnetic waves that reflect off distant objects, return back to the source and record the time it took which reveals how far away the target object is.

Radar mapping reveals numerous geological features like volcanoes, mountain ranges, great chasmic rifts, impact craters and wind erosion. The Venusian surface is dry and rocky with 80% of it covered by rolling plains, and much of it characterized by broad, gently-sloping lowlands that can stretch thousands of kilometers across. Visible flow patterns on these plains indicate volcanic lava flows as the origin while dunes and yardangs indicate strong wind erosion and wind deposits. Contrasted against the plains are three massive continent-sized regions of mountainous highlands:

• Aphrodite Terra along the equator (roughly the size of South America) which is characterized by ridges, deep rift valleys and mountain ranges.

• Ishtar Terra in the northern hemisphere (roughly the size of Australia) which is characterized by a high, flat plateau bounded on most sides by the planet’s four main mountain ranges.

• Lada Terra at the southern pole is characterized by an oval-shaped bulge called a corona surrounded by volcanic rifts and plains.

Venus is dominated by volcanic features with the most volcanoes of any planet in the solar system (over 80,000). Volcanism is believed to have played a major role in shaping the surface via lava flows. In fact, Venus may have undergone a mass global resurfacing event about 500 million years ago. Observation of Magellan space probe images have confirmed that Venus remains volcanically active to this day.

The surface and atmosphere of Venus together create a nightmarish environment and render unmanned-exploration incredibly difficult and manned-exploration improbable. However, humans have managed to put probes in orbit around Venus and landers on the surface. On October 8, 1967, the Soviet Union successfully landed the first man-made object on another planet. The Venera 4 probe arrived at the planet via a fly-by spacecraft which dropped the lander. After parachuting to the surface, the lander analyzed the Venusian atmosphere’s composition, temperature and pressure, then relayed the data up to the fly-by spacecraft which in-turn relayed it back to Earth. It lasted about 23 minutes before succumbing to the hostile elements.

Fun fact: During entry, the temperature of the Venera 4 lander’s heat shield rose to almost 20,000°F.

In total, only four spacecraft have ever returned images of the planet’s surface. These Soviet probes were only able to withstand Venus’ scorched, crushing atmosphere for a matter of minutes to a few hours before being destroyed.