DID VENUS ONCE HAVE A MOON?

MYSTERIES OF THE UNIVERSE

It’s well known for having no natural satellite today, but did an ancient companion have a big influence on the planet?

Venus is the second-brightest observable object in the night sky after the Moon

It’s long been established that Venus, like Mercury, does not have a moon. But does that mean the celestial body has always been entirely alone? Not quite. In 2002, a quasi-satellite was discovered by Brian A. Skiff at Lowell Observatory. Recently named Zoozve because the original designation of 2002 VE68 was misread by artist Alex Foster when he was designing a space poster, it was found to be an asteroid with an orbit dictated by Venus’ gravitational pull. It’s termed quasi because it primarily heads towards and around the Sun, passing within the orbit of Mercury as it travels. But what if Venus once had a proper, full-blown moon? What would it have been like, and what effect could it have had on the hottest planet in our Solar System?

According to Valeri Makarov at the US Naval Observatory in Washington DC and Alexey Goldin at Teza Technologies in Chicago, there’s every chance that Venus had an ancient moon that has long since been destroyed. The pair have gone as far as giving the hypothetical body a name – Neith – and they reckon it may well have played a big part in the current state of Venus, which could be an interesting explanation for one of the planet’s most mysterious phenomena.

Venus is similar to Earth in many ways. Its mass, density, volume and size are closely matched, and this has led to a belief that both planets share a common origin, having formed out of a condensing nebulosity around 4.5 billion years ago. Yet Venus is also very different. It has a scorching surface temperature of 475 degrees Celsius (887 degrees Fahrenheit), an atmosphere rich in carbon dioxide and thick clouds of sulphuric acid. Its atmospheric pressure is 90 times that of Earth’s, too.

“Venus is so close to Earth in main physical parameters, but it’s absolutely uninhabitable,” Makarov says. “Is that a catastrophic accident or a reflection of our future as a species?” To answer such questions, he has developed a keen interest in the planet over many years of research. “Venus is important for understanding the origin of life, existence of civilizations beyond the Solar system and our own destiny, perhaps,” he adds. “In the Soviet Union, Venus was very high on the priority list for space exploration, with some outstanding achievements not nearly surpassed to this day.”

Aside from pondering why Venus has no moon, Makarov is also intrigued that Venus rotates in the opposite direction to every other planet in the Solar System except Uranus. The pervading explanation for this has long been that Venus collided with another object in the chaos that was the early Solar System, with a good number of researchers suggesting such wayward objects could have been comets, asteroids or meteoroids – the likes of which have been known to smash into planets.

But Makarov is not convinced. “There have been a few conflicting theories of the present-day retrograde spin of Venus,” he tells All About Space. “Each of them is plausible in principle, but not very likely in my opinion.” Instead, he has wondered whether an object didn’t actually crash into Venus, but merely influenced it. And he reckons that object could actually have been the aforementioned moon. “The problem [of Venus’ retrograde motion] caught my attention several years ago when I read a paper by Jacques Laskar about the impact of atmospheric tides in the reversal of Venus’ spin,” Makarov explains. “A few points in that paper were sticking out, and I started to think about alternative explanations.

Venus is uninhabitable now, but research suggests that may not have been the case a billion or so years ago

“As far as collisions are concerned, I am not a devout believer in this theory. Too many features about the Solar System have been attached to this rather improbable scenario: our Moon presumably formed from a giant collision. Venus does not have a moon but spins in the opposite direction because of a giant collision. Mercury experienced a giant collision but it spins regularly and does not have a moon. The moons of Mars are split because of a collision, but have prograde rotation. I think there’s too much stuff on that plate.”

To put their theory of an ancient moon’s influence to the test, Makarov and Goldin ran advanced computer simulations that introduced Neith to see what would happen to Venus in various situations. The pair focused on Venus’ Hill sphere, which describes the region around a planet where orbiting objects can be gravitationally bound. The Hill radius in this case is about 160 Venus radii, as opposed to our planet which is 230 Earth radii. As they stated in their paper, published in Universe, they looked at how “external minor bodies in trajectories loosely co-orbital to a planet can be stochastically captured by the planet without any assistance from external perturbative forces”.

In other words, they were studying the circumstances in which a body could end up in a chaotic orbit around Venus and what the effects of that would be for both the object and the planet. As such, the idea behind the research was to see if they could get to the point where Venus would end up spinning in the opposite direction to other planets in the Solar System. To do this, they tested hundreds of thousands of simulations on the assumption that Venus did once spin in the same direction as Earth, Mars, Jupiter and the rest of its neighbours. Their findings were very interesting to say the least.

“Venus is important for understanding the origin of life and our own destiny”

Valeri Makarov

They found that if Neith was sufficiently large and had been in a retrograde orbit as it travelled past Venus around the time the Solar System formed, it was just as likely to have ended up within the Hill sphere than if it was travelling in a regular forwards orbit. “This could only happen at the very early stage following the oligarchic growth of the surviving planets, when the solar disc was still packed with a large number of smaller bodies,” Makarov says.

If the moon was captured in retrograde then it would have a different effect on Venus than if it was captured in prograde. “The difference is that a retrograde moon interacts with the prograde debris disc orbiting Venus, moving against the flow of particles, which increases the rate of dissipation of kinetic energy via dynamical friction,” Makarov explains. “This shortens the characteristic time of the initial moon’s descent towards Venus.”

The scientists noted that the planetesimal would have hung around for quite some time as it became Venus’ temporary moon – anything from 100,000 to a million years. “The time scale is not known to us; this requires more detailed computations and knowledge of basic rheological parameters of Venus’ mantle. It also depends on the initial mass of the moon, which is not known,” Makarov says.

That would have been more than enough time for the moon to have affected Venus, since it would have acted in much the same way as Earth’s Moon. Just as our Moon gains angular momentum from tidal drag, the effect of which is a slowing down of Earth’s rotation to the point that it extends a day by just under two milliseconds every 100 years, so Venus’ temporary moon would have caused that planet’s rotation to slow. And it was able to hang around because it would have made frequent incursions into the closer area around the planet. “The fast shrinkage of the initial chaotic orbit is important to avoid stochastic ejection of the captured moon,” Makarov explains.

According to the new theory, as the moon created tides which slowed the spin of Venus, the planet eventually came to a standstill for a brief period before reversing direction, explaining how Venus would have begun to spin the other way. “It’s possible that such retrograde captures happened many times and with other planets, but the candidate moons could not break their orbital velocity fast enough to become permanent satellites,” Makarov says.

Over time, with the process continuing, the captured moon would have frequently collided with the disc material around Venus, which combined with the planet’s gravity would have caused the moon to break up. Bits of the moon would have rained down on Venus’ surface, and that would also explain why the planet does not have a natural satellite today. “Venus’ moon would have descended to the Roche radius and been torn apart by tidal forces,” Makarov explains. “The remaining material would most likely have been deposited on Venus’ surface.”

By that point, the ‘damage’ would have been done. In this situation, Venus would have been forever doomed to rotate in the opposite direction – which is exactly what we see – but we won’t know for certain if this situation is correct because there has long been an absence of data, and it may yet be some time before we ever get sufficient evidence. But what consequences are there for a planet that ends up capturing a retrograde moon? According to Makarov, they could be dire. “Our Moon is perhaps crucially important for the emergence of advanced biological life on Earth. When life appeared on Earth, the Moon was much closer, at around six Earth radii,” he says. “If it were retrograde, life would not have a chance because of the excessive tidal heating, catastrophic destruction and loss of atmosphere.”