Earth and Venus

One kept its water and its Moon. The other lost the water and paved itself in lava. The Sun ends both stories.

From the pair notes by Rowen Groeneweg and Jaxson Foreman.

Earth

Moon: Mars-sized impact
Oceans: In place by ~3.8 Gyr ago
Oxygen: Great Oxidation, ~2.4 Gyr ago
Late hit: Chicxulub, ~10 km, 66 Myr ago

Venus

Surface: 462°C (864°F)
Air: 96.5% CO₂, 3.5% N₂
Pressure: 92 bar
Volcanoes: More than 80,000

The beginning. Dust, then protoplanets. About 100 million years of collisions leave four large bodies. Earth and Venus are two of them.
The collisions. A Mars-sized body makes Earth’s Moon. Venus’s own giant impact, and a possible fallen moon, stay hypotheses.
Water and air. Earth’s oceans stay and oxygen arrives. Venus boils, or was always dry, and keeps a carbon-dioxide atmosphere.
Pangaea, then Chicxulub. Continents gather and split. A 10 km asteroid strikes 66 million years ago.
The Sun inflates. In about 5 billion years the Sun becomes a red giant. Earth is likely engulfed. Venus shares that ending.

Question 1

Formation pathway

While the young Sun was still gathering hydrogen, dust in the disk stuck together. In a few years those clumps reached hundreds of meters. In several thousand years they reached kilometers, big enough to pull on each other. In under a million years that made Moon-sized protoplanets. The inner disk was too warm for water and other volatiles to condense, so the planets grew from rock and metal. Up to hundreds of those worlds collided for about 100 million years, until four large bodies were left. Earth and Venus are two of them.

Earth

Same hot inner disk. Same rock and metal. The path splits at the Moon-forming impact.

Venus

Same start. The notes then give Venus a higher-energy collision, a single plate, and a different spin.

Evidence. The shared opening is the inner-disk accretion sequence. The Planetary Society is the source they name for that history.

Question 2

Collisions and captures

Earth’s Moon formed when a Mars-sized body hit the planet. Apollo rocks put that impact about 100 million years after the solar system began. Much later, Pangaea assembled and then broke, and the Chicxulub asteroid, about 10 km across, struck 66 million years ago. Venus’s notes lead with a protoplanet collision: higher speed, a superheated interior, long-lived volcanism, and one continuous plate. More than 80,000 volcanoes, about 60 times Earth’s count, resurfaced the planet in lava. A second hypothesis gives Venus a moon of its own that later fell inward, because the slow spin pulls a moon in. That is the opposite of Earth’s Moon, which is moving away. Either collision is also their candidate for the reversed spin.

Same kind of event, different result. Tap a collision. The short answer above stays either way.

Earth’s Moon Venus impact A fallen moon Chicxulub

Earth’s Moon

A Mars-sized body struck Earth. The Moon formed from the fragments. Apollo samples date the rocks to about 100 million years after the solar system began. NASA Science is the source they name.

A protoplanet hits Venus

One leading hypothesis for the heat and the volcanoes. Higher-speed, higher-energy impacts than Earth’s plate collisions. A superheated interior, floods of lava, and a single plate. An angled hit could have tipped the spin past 90° or reversed it.

Venus’s moon falls in

A second hypothesis: Venus once had a moon. The slow spin hands angular momentum inward, so the moon’s orbit collapses. Earth’s Moon does the opposite and recedes. The notes also offer the Sun’s pull on atmospheric tides, in a 92-bar air, as a way to slow and reverse the spin.

Chicxulub

About 66 million years ago a roughly 10 km asteroid hit Earth at tens of kilometers per second. Shock, fire, and tsunamis followed. The notes call the aftermath the opening of the age of mammals. This is an Earth event. Venus’s resurfacing in the notes is volcanic, not this impact.

Earth after the Moon

Plates keep moving. Pangaea’s assembly began about 355 million years ago. It broke in stages about 175 million years ago. Continents may meet again in 200 to 250 million years.

Venus after the impacts

One plate, not a shifting set. Arachnoids, volcanic ovals with radiating fractures, are part of the surface they describe. Venus is one of the two planets without a moon.

Evidence. Apollo rocks for the Moon’s timing. Chicxulub at about 10 km and 66 million years. Venus volcano count above 80,000. Pangaea dates are the ones in the notes: assembly from about 355 million years ago, breakup about 175 million years ago.

Still open. The Venus protoplanet collision and the fallen-moon story are hypotheses in the notes. So is which of those, or the atmospheric tides, reversed the spin. Pangaea Proxima and Novopangaea are two sketches of the next supercontinent, not one map.

Question 3

Volatiles and air

Earth took on water from the first dust, from nebular gas, and from asteroids and comets through its first several hundred million years. By the time the heavy bombardment eased, about 3.8 billion years ago, oceans were established, and they have persisted. For roughly the first two billion years the air had almost no free oxygen: nitrogen, carbon dioxide, methane, and water vapor. Around 2.4 billion years ago cyanobacteria triggered the Great Oxidation Event. Venus’s water is the contrast. One reading gives it shallow oceans for 1 to 2 billion years, then a runaway greenhouse. Another says the interior was always too dry. Volcanic gases hold at most 6% water, much drier than Earth’s mantle melts. Today the surface is 462°C. Water vapor is about 0.002% of the air. The clouds are sulfuric acid. The atmosphere is 96.5% carbon dioxide and 3.5% nitrogen, at 92 bar. Hydrogen still escapes from the top of the atmosphere.

Earth

Oceans kept. Oxygen added, and kept. The early hydrogen and helium were lost because they were too light for Earth’s gravity while the surface was hot.

Venus

In the magma-ocean stage, hydrodynamic escape removed most primordial water within about the first 100 million years. Volcanic degassing continued, with very little water. Sulfur dioxide feeds the acid clouds. Super-rotation moves the heat.

Evidence. Oceans by about 3.8 billion years ago. Great Oxidation Event around 2.4 billion years ago. Venus: 462°C, 92 bar, 96.5% CO₂, 3.5% N₂, about 0.002% water vapor, volcanic gas at most 6% water.

Still open. The notes carry both stories of Venus water: oceans for 1 to 2 billion years, and an interior that never had enough water for oceans. They also note recent evidence for the dry reading. The page does not pick one.

Question 4

A constrained future

Earth’s plates are still moving. Models put the next supercontinent in roughly 200 to 250 million years. Volcanism, a brighter Sun, and a vast interior far from the ocean would push temperatures past what most complex land animals can survive. About 5 billion years from now the Sun exhausts the hydrogen in its core and swells into a red giant, hundreds of times its present size. Mass loss lets Earth’s orbit expand, and for a time that looked like a way out. The notes conclude that tides from the swollen Sun still drag Earth inward. Engulfment near the tip of the red-giant phase is the likely end. Venus, already closer in, meets that same inflated Sun. The shared ending is the red giant, not a rescue.

Earth

Next supercontinent, then likely engulfment. Orbital expansion does not, in these notes, save the planet.

Venus

No separate escape is offered. The Sun’s inflation is listed as the matching final entry for both worlds.

Evidence. Next supercontinent in 200 to 250 million years. Red giant in about 5 billion years. Earth’s likely engulfment is stated after the debate over whether mass loss would spare it.

Still open. The shape of the next supercontinent is a model choice, Pangaea Proxima or Novopangaea or something rotated, not a forecast of coastlines. The engulfment conclusion is “most likely,” after that orbital debate.