By Steve Gorman
LOS ANGELES, Sept 14 (Reuters) - A California-based astrophysicist presented a new theory on Monday to explain why Venus, the planet most like Earth in size, mass and structure, lacks its own moon. Call it a case of celestial cannibalism.
The extremely slow speed at which Venus rotates on its axis would have doomed any natural satellite that may once have orbited that planet to gradually spiral closer to its parent and ultimately be consumed in a cataclysmic smash-up.
So goes the theory
offered by scientists at the University of California, Riverside, and published on Monday in The Astrophysical Journal, marking a departure from past explanations for why Venus, the second planet from the sun, is without a lunar companion.
"If it had a moon, and it likely did at some point, it could not have kept it," astrophysicist Stephen Krane, lead author of the study, told Reuters on Monday. "That moon would have crashed down on the planet in a relatively short time scale."
Krane said data from computer models based on planetary physics and simulating the gravitational mechanics of hypothetical Venusian moons of various sizes showed them all meeting the same fate - sooner or later collapsing into their mother planet.
Such a presumed moon-gobbling event likely occurred within the first billion years in the history of Venus, which like the rest of the solar system dates back about 4.5 billion years, Krane said.
Scientists previously theorized that Venus, known as Earth's "twin," either never acquired its own moon to begin with, or possibly had a moon that was obliterated in the massive impact from another celestial object.
EARTH SPINS MORE SWIFTLY THAN VENUS
The UC Riverside study, focusing on Earth-moon dynamics, found that planetary rotational speed was the key factor.
In contrast to Venus, which takes 243 Earth days to complete a single rotation - slightly longer than its orbit around the sun - Earth spins on its axis every 24 hours, more than 200 times more rapidly than Venus, Krane said.
The transfer of energy from Earth's faster rotation is gradually pushing our moon farther away, causing it to recede from Earth at the rate of 4 centimeters (1.6 inches) a year - precisely measured by mirrors left on the lunar surface in 1969 by Apollo 11's astronauts.
Venus, the third brightest object in the night sky after the sun and moon, looms large as point of study for scientists seeking to learn more about Earth's history, and its future.
Similar in its rocky structure but slightly smaller than Earth, Venus is enveloped in a thick, toxic atmosphere that has created a runaway greenhouse effect, scorching its surface at temperatures as high as 880 degrees Fahrenheit (471 degrees Celsius), hot enough to melt lead.
While there is no proof a Venusian moon ever existed, it is unlikely that Venus, in its infancy, somehow avoided the kind of world-shattering collision with another proto-planet that is widely believed to have formed Earth's moon, according to Krane.
The dawn of the inner solar system was crowded with large rocky objects hurtling through space and smashing into one another, with Venus probably taking more celestial hits than Earth because Venus was closer to the sun.
Scientists hope to unlock more secrets of Venus with two forthcoming NASA missions set for launch in the late 2020s or early 2030 dubbed DAVINCI+ and VERITAS to study the planet's atmosphere and geologic features.
(Reporting by Steve Gorman in Los Angeles; Editing by Lincoln Feast.)
