The Week in Space and Physics: The Great Drying of Venus
On the lack of Venusian water, Chang'e 6, the first crewed flight of Starliner and Orion's heat shield
Once upon a time, Earth, Mars and Venus all had plenty of water. Earth still has oceans, of course, which puts their past existence in little doubt. Studies of Mars have found clear signs of ancient shorelines, flowing rivers and even of tsunamis striking coastlines. The Red Planet, we can say with some confidence, was once Blue.
But what about Venus? Today that planet is utterly dry, with no water on its surface and only a trace amount in its atmosphere. Neither is there much sign of past water. Even if we could peer through its thick toxic clouds and examine its surface, we won’t see relics of long ago rivers. The Venusian surface is geologically young, with all traces of ancient oceans obliterated by catastrophic volcanic eruptions.
Still, the same processes that brought water to Earth and Mars should have acted on Venus, and many planetary scientists believe that it too once had plenty of water. Venus, in its youth, may have been a world of steamy tropical seas, rather than the literal vision of hell it is today. But if this really was the case, then where did all that water go?
In general there are two possible answers to this question. One is that water is still there, but trapped in Venus’ rocks or under its surface. This, indeed, is what happened to much of Mars’ water. Water-absorbing minerals seem to have soaked up vast amounts of it, drying out the surface. What was left either froze into ice scattered in craters, or escaped into space.
That points to the second possibility: that Venus lost its water to space. In most scenarios this happens as water is chemically split up in the atmosphere. The lighter hydrogen atoms tend to rise, and since Venus is not a big planet, they can often escape its pull altogether. As the hydrogen vanishes, so does the planet’s water.
To prove this idea, however, scientists need to come up with a convincing way to split up water molecules fast enough to drain an ocean. In a recent paper, from the University of Colorado Boulder, researchers think they have found such a way. Venus’ atmosphere is rich in carbon dioxide they note, and this can react with water to form molecules made of a single atom each of hydrogen, carbon and oxygen (HCO+).
Initially these molecules have a positive charge, so they pull electrons towards them. But when they get hold of one they break apart, forming a molecule of carbon monoxide and an atom of hydrogen. Sometime this hydrogen atom flies away, escaping the atmosphere altogether. Over time, they say, this process can lose enough hydrogen to explain why Venus is so dry today.
If this picture is indeed correct, it should be testable. Even though it is now dry, Venus’ atmosphere should still contain lots of these molecules of HCO+. A future satellite around Venus could check if this really is the case. And if it is, then it should explain why Venus is so dry today.
China Targets the Dark Side of the Moon
On Friday China launched the Chang’e 6 probe towards the Moon. The mission, which should touch down on the lunar surface in early June, aims to be the first to bring back samples of rock from the far side of the Moon.
Chang’e 6 is the latest in a series of Chinese missions to the Moon. The first, Chang’e 1, entered lunar orbit in 2007. Six years later Chang’e 3 landed on the Moon, becoming the first spacecraft to do so since the Soviet robotic missions of the 1970s. Chang’e 4 followed in 2019, and Chang’e 5 - which returned rocks from the near side of the Moon - came in 2020.
Put together these missions form part of China’s Lunar Exploration Program, a project that aims to expand Chinese technology and capabilities on the Moon. Two more missions - Chang’e 7 and 8 - are expected to lift off before 2030. Both will focus on exploring the resources found around the Lunar south pole.
China also seems serious about building out a base on the Moon. At first this base would be robotically operated, with initial missions focused on deploying power and communications infrastructure to support the base. Later missions would install scientific instruments, explore the Moon’s geology and demonstrate the technology needed for a long term presence on the Moon.
But China is also looking at human missions. Chinese officials say they are on track to put astronauts on the surface by 2030, a date that is starting to worry some at NASA. Whether China can really do that is unclear, but it is worth remembering that America was able to land astronauts on the Moon just eight years after Kennedy set it as a goal in 1961. If China has the will, a human landing in the early 2030s is by no means unlikely.
Either way, the latter half of this decade will see plenty of excitement on the Moon. America, too, is focused on building infrastructure, though it is targeting an orbiting space station rather than a dedicated base on the surface. As geopolitical rivalry between the two superpowers heats up, lunar exploration may be a welcome beneficiary.
Starliner
After years of development and delay, Boeing’s Starliner capsule is finally poised to carry a pair of astronauts to the space station. If the flight succeeds it will give NASA two independent means of sending humans to Earth orbit for the first time in decades. NASA’s other option, of course, is SpaceX’s Dragon capsule, which has been flying astronauts since 2020.
The first test flight of Starliner came way back in 2019. That mission, which was uncrewed, ended in failure after a software error put the capsule in the wrong orbit and drained much of its propellant. With no way of reaching the space station, Starliner was instead forced to return to Earth early.
A second flight, in 2022, passed more smoothly, though Boeing did then find problems with the capsule’s parachutes and electrical tape used throughout the cabin. Fixing those issues took up much of 2023, and so delayed the first crewed test flight to this year.
If things had gone to plan, that flight would have lifted off on Monday this week. Yet once again things have been delayed. This time the fault lies not with the capsule, but with the rocket it sits on, the Atlas V. A faulty valve in that rocket must be replaced, engineers said, and this will take at least a week to do. The launch will now take place on May 17, at the earliest.
Orion’s Heat Shield
NASA’s other capsule for human spaceflight is Orion. Unlike Dragon or Starliner, Orion is designed to support astronauts on flights into deep space and will, the agency hopes, carry future explorers on their way to the Moon. In 2022 a demonstration flight proved the capsule could do this, as it looped around the Moon and returned to Earth.
Unfortunately, a recent report showed the capsule’s heat shield suffered serious damage as it re-entered. Large chunks of the shield seem to have been stripped away, creating debris that could have put the capsule’s parachutes in danger. Even worse, some of the bolts holding the shield in place melted, and could have let hot gases enter the structure of the capsule. That, if it happens again, could destroy Orion during re-entry.
Engineers are not yet sure why the heat shield was damaged in this way. But NASA seems to regard the issue as a critical one to solve before Orion flies again. That flight, Artemis II, will take astronauts around the Moon for the first time since 1972.
I wonder what would happen if a series of large, Shoemaker-Levy size ice comets impacted Venus? Cool the place down? Change the atmospheric chemistry? Effect the volcanism? Most likely very little, but it might be a first step in terraforming the place