Phobos Might Only Have 10 Million Years to Live

Article written: 13 May , 2008
Updated: 24 Dec , 2015

You can breathe easily. The Moon is slowly receding away from the Earth at a rate of 3.7 cm/year (1.5 in/yr). But the Martians aren’t so lucky. Their moon Phobos is known to be doing exactly the opposite. It’s spiraling inward, and in the distant future it will crash into the surface of Mars. Researchers originally thought that Phobos has about 50 million years to go, but an Indian researcher has re-run the calculations and thinks Phobos only has about a quarter of that time to live.

It was originally believed that Phobos would take about 50 million years to crash into the surface of Mars, but according to Bijay Kumar Sharma, an Assistant Professor at the National Institute of Technology in Bihar, India, it might happen much more quickly. Dr. Sharma has revised the calculations for Phobos’ destruction in his new paper, Theoretical Formulation of the Phobos, moon of Mars, rate of altitudinal loss.

According to Sharma, Phobos will actually be destroyed about 10.4 million years from now, and not the 50 million years the researchers had previously calculated.

Phobos is believed to be an asteroid that Mars captured early on in its history. it’s one of the least-reflective objects in the Solar System, and thought to be similar to a D-type asteroid. It currently orbits Mars at an altitude of about 9,380 km (or about 6,000 km above the Martian surface).

Why does the Earth’s moon spiral outward, while Phobos is spiraling inward to Mars?

The Moon formed billions of years ago when a Mars-sized object crashed into Earth and sprayed material into orbit. This material pulled back together from mutual gravity to form the Moon, and this debris received a gravitational slingshot from the Earth.

They key is that the material was tossed into a high enough orbit, above what’s known as the synchronous orbit. This is where the Moon completes an orbit slower than the Earth takes to rotate once. Since the Moon ended up higher than this orbit, it’s spiraling outward. If its orbit was less than the length of a day, it would spiral inward.

And this is what has happened to Phobos. It orbits below this synchronous orbit, where it completes an orbit around Mars faster than the planet itself turns. It’s spiraling inward instead of outward.

Once Phobos gets down to an altitude of only 7000 km above the center of Mars (or 3,620 km above its surface), it will enter what’s known as the Roche limit. At this point, the tidal forces of Mars will tear Phobos apart, turning it into a ring that will continue to spiral into Mars. According to Dr. Sharma, this will happen in only 7.6 million years from now.

To know exactly how long Phobos has to live, Dr. Sharma suggests that a mission should be sent to Phobos to land on its surface and then use radar to measure the changing distance to Mars.

Original Source: Arxiv

11 Responses

  1. Astrofiend (Syd, Aust) says

    That should make for a great show!

    I wonder – will that make for a crater ring encircling Mars as these ring pieces eventually get sapped of their angular momentum and de-orbit? That would be great!

  2. D-wreck says

    That’s going to be too bad for any Mars inhabitants, assuming we colonize the red planet.

  3. Craig says

    Except that we CAN’T breathe easily about the Moon having an outward-spiraling orbit. When the Moon eventually tears itself out of the clutches of Earth’s gravity, all Hell will break loose on Earth.

    There are significant negative impacts the loss of our natural satellite will have on this planet.

  4. pantzov says

    can someone tell me what effect this event, whenever it happens, will have on the orbit of mars and/or the planet’s rotation?
    a little?
    a lot?

  5. Patreon
    Steve says

    I always heard that tidal forces of the Earth’s oceans are the cause for the moon receding. Is this not the case? I’ve never heard the “slingshot” theory from when the moon formed.

  6. alphonso richardson says

    Wow. too bad we won’t be around to see it: I’d love to watch that

  7. Dave says

    I’ve always wondered if the tidal forces from the Moon played a role in plate tectonics here on Earth. Any thoughts?

  8. tacitus says

    If we’re still living on Mars in 7 million year’s time, I suspect we’ll have had the capability to stabilize Phobos’s orbit for a long, long time before that.

  9. marcellus says

    Seven million years is a LONG time in human terms. The future Martians won’t need to worry about Phobos while they get colonization up and running. Watching Phobos from the surface of Mars would be a nice distraction from the grind of terraforming that planet to human sustainability.

  10. Cosselbray says

    Why not try to bring Phobos down early to help warm up Mars? A benefit now rather than a problem in the future.

  11. Ricky Diaz says

    We have ways we can send Phobos on a collision course with Mars in our lifetime, but it would be sad to lose a moon in the Solar System and Mars is a rocky planet, not a gas planet, so moons are more rare. Mars has only 2 captured by Mars’ gravity. Mars is considered a moon destroyer. Many craters have been found on Mars that are basically oval shaped, meaning the impactor came at a very low angle, very likely moons. Phobos might form a crater over 850 miles long. The low angle might be enough for the collision to knock Mars off it’s axis, but maybe not change it’s rotation speed much. For example, Venus didn’t used to spin backwards. And the rotation period used to be 224 days, because Venus used to be tidally locked to the Sun, having a rotation period equal to the orbital period. Then sometime, Venus was struck by a massive asteroid, maybe 375 miles across and at a low angle, which caused the collision to knock Venus upside down (relative to the plane of the Solar System.) But the rotation speed didn’t change. The reason why it now spins once every 243 days, which is longer than 1 orbital period, is because it’s gradually becoming tidally locked to the Sun again. It spins backwards, so for it to become tidally locked to the Sun again, Venus’ rotation speed must slow down to zero and then begin rotating the right way again and again gradually- become tidally locked to the Sun again. So Venus’ rotation period will eventually become much much longer. A period of more than 100 years. The questions are: By the time it reaches that speed, how fast will the rotation speed be changing? Will it make a rotation in a complete 1,000 years? 10,000 years?

    I understand I explained more about Venus than Mars. I like teaching that fact about Venus’ past and future.

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