Scientists unravel mystery of planet’s retrograde rotation
Venus, often called Earth’s sister planet due to their similar sizes and compositions, has long puzzled astronomers with its extremely slow and backward spinning motion. Of all the planets in our solar system, Venus is the only one that rotates retrograde, or clockwise when viewed from above. This backwards “topsy turvy” rotation means that on Venus the sun rises in the west and sets in the east.
Now, new research published in the journal Nature Astronomy points to a likely cause for Venus’s backwards spin – a collision with an ancient moon billions of years ago.
Moon Grazing Incident to Blame
Using computer simulations, astronomers from NASA, UC Berkeley and other institutions examined various scenarios that could have caused Venus to flip upside down. Their findings suggest that billions of years ago, Venus may have had a moon that orbited the planet in the opposite direction of its rotation. Over time, gravitational forces between the two celestial bodies caused the moon’s orbit to decay, setting it on a collision course with Venus.
About 10-30 million years after its formation, the wayward moon would have slammed into Venus in a grazing collision that not only changed the planet’s spin but altered its tilt as well.
“This grazing but significant moon/Venus collision event could have created Venus’ current topsy turvy spin state,” said lead researcher Sam Jacobson, an undergraduate student at UC Berkeley. “The energy transfer and angular momentum change was enough to have caused a dramatic spin flip.”
- Venus currently rotates retrograde (backwards relative to its orbit) with a 243 day rotational period
- Computer models show a past collision with a large moon could have flipped Venus’s orientation
- The ancient moon likely orbited Venus in the opposite direction it was spinning
- Over millions of years, the moon’s orbit would have decayed, resulting in a crash into the planet
- The grazing impact would have been enough to alter Venus’s tilt and reverse its spin direction
Comparison of Venus and Earth
|95% as large as Earth
|117 Earth days
The discovery of this ancient cosmic crash helps explain not only why Venus spins differently than most other planets, but also how its tilt ended up almost completely upside down in relation to its orbit.
Jacobson suggests the Venus/moon collision is similar to the Earth/Theia impact that scientists believe formed the Moon about 4.5 billion years ago.
“This concept of planetary scale collisions that dramatically transform spin states has historical precedent in our own Moon’s formation and its effects on Earth,” he explained.
What’s Next For Venus?
While this gravitational mishap occurred billions of years in the past, astronomers speculate that evidence of the collision may still exist within Venus’s landscape. In upcoming space missions, researchers hope to gather higher resolution surface images that could reveal anomalies supporting the moon impact theory.
Additionally, the bizarre retrograde spin means that Venus has no internally generated magnetic field. Without this protective shield, charged particles from the solar wind continue eroding Venus’s atmosphere – an ongoing process that may have robbed Venus of conditions hospitable for life.
By further analyzing the properties of Venus’s atmosphere, scientists aim to model whatEarth’s twin planet was like in its early history and whether it could have sustained liquid water oceans when the ancient moon was still intact.
Understanding how Venus diverged onto such a different evolutionary path can give us greater insight into the past habitability and current status of rocky planets across our galaxy. This research highlights that while separate worlds may begin quite similar, their futures can be altered forever by chance catastrophes across the cosmos.
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