Mars May Have Once Been a Ringed Planet — and It Could Be Again

Mars’ moons were likely formed by a ring of debris blasted into space after the Red Planet was hit by a massive impact and, when the moon Phobos disintegrates in 70 million years, another ring may form.

Sunrise over Gale Crater as seen by NASA’s Mars rover Curiosity and how it might look if the Red Planet had a ring system (NASA/JPL-Caltech-MSSS, edit by Ian O’Neill)

Mars is currently known as the “Red Planet” of the solar system; its unmistakable ruddy hue is created by dust rich in iron oxide covering its landscape. But in Mars’ ancient past, it might have also been called the “Ringed Planet” of the inner solar system and, in the distant future, it may sport rings once more.

The thing is, we live in a highly dynamic solar system, where the planets may appear static over human (or even civilization) timescales, but over millions to billions of years, massive changes to planetary bodies occur frequently. And should there be a massive impact on a small rocky world — on Mars, say — these changes can be nothing short of monumental.

In new NASA-funded research published in the journal Nature Geoscience, planetary scientists have developed a new model of Mars when it was hit by a massive impact over 4 billion years ago. This catastrophic impact created a vast basin called the Borealis Basin in the planet’s northern hemisphere and the event could be part of the reason why Mars lacks a global magnetic field — it’s hypothesized that a powerful impact (or series of impacts) caused massive disruption to the Martian inner dynamo.

But the impact also blasted a huge amount of rocky debris from Mars’ crust into space, ultimately settling into a ring system, like a miniaturized rocky version of Saturn. Over time, as the debris drifted away from Mars and settled, rocky chunks would have formed under gravity and these “moonlets” would have clumped together to form larger and larger moons. So far, so good; this is how we’d expect moons to form. But there’s a catch.

Phobos as imaged by Europe’s Mars Express mission (ESA)

After forming in Mars orbit, any moon would have slowly lost orbital altitude, creeping toward the planet’s so-called Roche Limit — a region surrounding any planetary body that is a bad place for any moon to hang out. The Roche Limit is the point at which a planet’s tidal forces become too great for the structural integrity of an orbiting body. When approaching this limit, the closest edge of the moon to the planet will experience a greater tidal pull than the far side, overcoming the body’s gravity. At some point, something has to give and the moon will start to break apart.

And this is what’s going to happen to Phobos in about 70 million years. Its orbit is currently degrading and when it reaches this invisible boundary, tidal stresses will pull it apart, trailing pieces of moon around the planet, some debris falling onto the Martian surface as a series of meteorite impacts, while others remain in orbit.

The research, carried out by David Minton and Andrew Hesselbrock of Purdue University, Lafayette in Indiana, theorizes that mysterious deposits of material around Mars’ equator might have come from the breakup of ancient moons that came before Phobos and Deimos.

“You could have had kilometer-thick piles of moon sediment raining down on Mars in the early parts of the planet’s history, and there are enigmatic sedimentary deposits on Mars with no explanation as to how they got there,” said Minton. “And now it’s possible to study that material.”

According to their model, each time a moon broke apart to create a ring, the next moon would be five times smaller than its predecessor.

In short, Mars and its moon may appear to be pretty much unchanged for billions of years, but the researchers think that up to seven moon-ring cycles have occurred over the last 4.3 billion years and Mars is on the verge (on geological timescales) of acquiring rings once more. Fascinating.


Venus is Lonely. Very, Very Lonely


Venus is a hellish world. Although the planet is nearly the same size of Earth, that’s where the similarities end. Having said that, it does have an atmosphere, but it’s not the kind of atmosphere you would ever want to spend time breathing in. Composed of a dense carbon dioxide/nitrogen mix where clouds are made from sulphuric acid, you can forget about Venus as a tropical holiday destination. Even if you found a way to ‘breathe’ on Venus, you’d need to prepare yourself for the scorching 470°C surface temperatures and bone crushing pressures 100 times the pressure we are used to on Earth.

Doesn’t sound like a very nice place does it? Certainly an interesting world, providing us with invaluable science (after all, the reason for the extreme temperatures on Venus is due to a run-away greenhouse effect, it could help us understand the growing problems we are facing with our comparatively mild global warming woes), but an unlikely candidate for human colonization (unless we lived in the clouds).

Venus might not be a popular world for mankind to live on, but it doesn’t seem to be a popular world for natural satellites to orbit around either. It doesn’t have any moons, and astronomers are a little confused as to why this is the case. The only other planet without moons is the innermost terrestrial planet, Mercury. Every other planet in the Solar System has at least one natural satellite.

For hundreds of years, astronomers have been on the lookout for anything orbiting Venus but they’ve had little luck. However, some of the earliest observations of Venus appeared to indicate the presence satellites (in 1645, F. Fontana mentioned the possibility of a satellite discovery, followed by further observations in the late 1600’s and 1700’s). Since 1768, there have been no further reports of any satellite sightings. 1956 was the last published survey for Venusian satellites, using photographic plates, and that survey (published by Gerard Kuiper in 1961) drew up blanks for any satellites measuring over 2.5 km wide.

The lack of Venusian moons is puzzling, as a Venus-moon interacting mechanism has often been invoked as the reason why Venus has a retrograde spin (i.e. viewed from the ‘top’ of the Solar System plane, Venus has a clockwise rotation, whereas the rest of the planets, apart from Uranus — that spins on its side, bizarrely — have an anti-clockwise, or prograde, spin). Perhaps Venus once had a moon, but it has since been lost due to gravitational interactions with other Solar System bodies, or due to tidal instabilities, the innermost terrestrial planets collided with their large satellites a long time ago.

This is where Scott Sheppard from the Carnegie Institution of Washington and Chadwick Trujillo from the Gemini Observatory (Hawaii) step in. In a recent publication titled, “A Survey for Satellites of Venus,” Sheppard and Trujillo pick up where Kuiper left off, and carry out a systematic survey searching for any natural satellites around Venus. Only this time, by using the cutting-edge 6.5 meter telescope and IMACS wide-field CCD imager at Las Campanas observatory in Chile, they looked for objects only a few hundred meters in diameter.

The researchers scanned the interior of the Venusian ‘Hill Sphere’ to see if any undiscovered tiny moons were lurking. The Hill sphere is the volume of space surrounding a planetary body where natural satellites can orbit without being destabilized by the gravitational effects of the Sun. If there are any unforeseen moons, they should be found in stable orbits within the Hill sphere.

Sheppard and Trujillo have drawn blanks. Although a few errant asteroids were detected, no natural satellites down to a diameter of 600 meters were discovered. They surveyed 90% of the Venusian Hill sphere, and 99% of the inner Hill sphere (0.7rH) — the volume of space predicted to contain the stable orbits of natural satellites.

This new survey improves the non-detection of satellites down to a factor of 50 on previous studies, thereby proving Venus either, a) never possessed any satellites over 1km in diameter, or b) the orbits of past large satellites have become unstable and crashed into Venus or flung into space.

Either way, Venus remains alone, with only the ESA Venus Express for company

Source: A Survey for Satellites of Venus, Sheppard & Trujillo, 2009. arXiv:0906.2781v1 [astro-ph.EP]