Venus — the divergent twin
Almost exactly Earth's size, almost exactly Earth's mass, sitting one orbit closer to the Sun. And yet: the surface is 460°C, the atmospheric pressure is that of a kilometre of ocean, the clouds are made of sulfuric acid, and the planet turns backwards. Every close-up of Venus is a close-up of what Earth might have been, or might yet become.
The names
Venus is the third brightest object in the sky after the Sun and the Moon, and it has been watched, named, and mythologised by every civilisation that ever looked up.
In ancient Mesopotamia it was Inanna to the Sumerians and Ishtar to the Babylonians — goddess of love and war, whose myth of descent into the underworld mirrored the way Venus periodically vanishes from the sky and returns. Her symbol, the eight-pointed star, became the emblem of the planet itself. In Greek antiquity the planet was recognised as two separate bodies before it was one: Phosphorus, the morning star, and Hesperus, the evening. In Latin the morning aspect became Lucifer, "light-bringer". The Romans finally named the whole planet Venus, goddess of love and beauty — the name modern astronomy kept.
In Vedic astronomy Venus is Shukra — "clear, pure, bright" — one of the nine celestial influencers of Hindu tradition. For the Yolngu of northern Australia, Venus is Barnumbirr, a creator-spirit whose morning rise draws a rope of light back to Earth. For the Mapuche of southern Chile she is Wünelfe, "the First"; the eight-pointed star form of that symbol was adopted, simplified, into the flag of Chile.
For the Maya, Venus was Chac Ek — "the Great Star" — the most closely tracked celestial body in their astronomy. The four surviving pre-Columbian bark-paper codices include one, the Dresden Codex, that contains a full Venus table: five 584-day synodic cycles, laid out to the day, corrected across centuries. The Maya believed Venus governed rainfall and agriculture; they timed major military campaigns and ritual sacrifices to specific stations of its cycle. All from naked-eye observation.
The divergent twin
Venus is close to Earth in every dimension that doesn't matter. It has 95 percent of Earth's diameter, 82 percent of Earth's mass, roughly the same bulk composition, and orbits one step closer to the Sun at 0.72 AU. Two planets built from the same material at roughly the same distance from the same star.
Then the numbers start to diverge. Venus rotates backwards — the only major planet in the solar system that does so — and it rotates almost impossibly slowly. A single sidereal day on Venus, one full turn on its axis, takes 243 Earth days. That's longer than a Venusian year, which completes in 224.7 Earth days. On Venus, a solar day is longer than an orbital year.
Venus has no moon and no ring system. No natural satellite of any kind. From Earth it is the third brightest object in the sky after the Sun and the Moon, but in visible light through a telescope it is a serene, almost featureless yellowish-white sphere. Everything interesting about Venus is hidden under the cloud deck.
The gaseous shroud
The Venusian atmosphere is a 250-kilometre-thick envelope of carbon dioxide — 96.5 percent CO₂, 3.5 percent nitrogen, and small quantities of sulfur dioxide, water vapour, argon and hydrogen chloride. It is the most extreme greenhouse in the solar system.
At the surface, that atmosphere exerts a pressure of 92 bar — 92 times Earth's sea-level pressure, equivalent to being one kilometre deep in Earth's oceans. Under that shroud, heat cannot escape. The surface temperature is nearly the same everywhere on the planet — day, night, equator, pole — and it sits at roughly 460 to 475 °C. Venus is hotter than Mercury despite receiving less sunlight, because Mercury has no atmosphere and Venus has all of one.
Between 48 and 70 kilometres up sits a permanent global cloud deck made of droplets of concentrated sulfuric acid. At the surface the wind barely moves — under one metre per second. In the cloud deck the wind moves at over 100 m/s, circling the whole planet in four Earth days: a phenomenon called atmospheric superrotation. Nobody fully understands what drives it. There is also an "unknown UV absorber" in the upper clouds — an unidentified chemical species that absorbs more than half the sunlight Venus receives and heavily shapes the planet's weather. Fifty years of continuous observation, and the atmosphere is still full of open questions.
The limitless canvas
Volcanic landforms cover at least 90 percent of the Venusian surface. Shield volcanoes rise kilometres above the plains; long sinuous channels — carved not by water but by super-heated, low-viscosity lava — run for thousands of kilometres. The single longest, Baltis Vallis, is over 6,800 kilometres long, one-sixth of the planet's circumference.
The impact-crater record is peculiar. Craters on Venus are scattered almost randomly, with little of the concentration or progressive degradation seen on other rocky planets. The most parsimonious reading is that Venus underwent a global resurfacing event somewhere between 300 and 600 million years ago — either as one catastrophic overturn of the lithosphere, or as a more gradual, steady-state process. Which of the two is still an open argument in Venus geology.
The oldest exposed rock is thought to be the tessera terrain: highly deformed, fault-crossed highland blocks like Ovda Regio and Alpha Regio. Some readings of the tessera evidence suggest an ancient, water-rich era on Venus, long before whatever runaway process shut the water out.
Phosphine, and the argument about it
In September 2020, a team led by Jane Greaves reported the tentative detection of phosphine gas (PH₃) in Venus's temperate cloud deck at altitudes of 53 to 62 kilometres. The observations were made with the James Clerk Maxwell Telescope in Hawai'i and the Atacama Large Millimeter Array in Chile.
The reason the result made news is that phosphine is a highly reduced gas, and Venus's atmosphere is highly oxidised and acidic — chemistry that ought to destroy phosphine on short timescales. Every known abiotic source (volcanic outgassing, lightning, photochemistry, meteoritic delivery) fell short of the reported ~20 parts-per-billion concentration by three to fifteen orders of magnitude. Under the standing assumption that unknown chemistry hadn't been overlooked, the surviving explanation was biology: an aerial microbial biosphere living in the sulfuric-acid cloud droplets themselves.
The result has been contested since. Several independent teams re-analysed the same JCMT and ALMA raw data and concluded that the 267 GHz spectral line attributed to phosphine was either a statistical artefact of the noise-reduction pipeline or the overlapping spectral line of sulfur dioxide in the upper mesosphere. A follow-up campaign with the SOFIA airborne observatory set a stringent upper limit on phosphine abundance. The Greaves team responded with reprocessed detections at lower reported concentrations. The debate has not resolved, and it is unlikely to resolve from remote observation. What settles it is in situ chemical sampling — a probe descending through the cloud deck with a mass spectrometer. That is exactly what one of the missions coming next will do.
The record
More spacecraft have been sent to Venus than to any other planet. Almost all of them are Soviet.
Two planets built from the same material at roughly the same distance from the same star. One turned into the ocean. The other turned into a furnace.