Mercury's hot and cold south pole

By Phil Plait | September 28, 2011 6:30 am

The MESSENGER spacecraft, orbiting Mercury for nearly a year now, took this pretty nifty shot of the tiniest planet’s south polar region, showing deep, dark craters in the Goethe basin:

This region is about 300 km (180 miles) from the true south pole of the planet. On Earth that might be a cold spot, but on Mercury, cold spots are hard to come by.

… however, see how dark those craters are? Since they’re near the pole, the Sun never gets far above the horizon for them, and the crater floors are shrouded in perpetual darkness. That does make them cold! Well below the freezing point of water, it’s thought. Interestingly, radar observations of Mercury have indicated something in the crater floors is highly reflective, and water ice fits that bill. It’s not at all confirmed, but it’s entirely possible Mercury — a planet hot enough in the open Sun where zinc can exist as liquid lakes on the surface — might have frozen lakes of ice locked in crater bottoms near its poles!

While gazing idly at this picture, another thought popped into my head. Mercury is small, and has lower gravity than the Earth. The impact speed of an asteroid depends partly on the gravity of a planet, since the asteroid will accelerate as it falls to the surface. Earth has more gravity, so you might expect impact speeds to be higher here than on Mercury. The impact speed of an object falling to Earth would be 11 km/sec, but only 4.3 km/sec for Mercury.

However, that’s not the entire case. Remember, everything is in motion in our solar system. Mercury orbits nearer the Sun, so it moves a lot faster in its orbit than Earth does; about 48 km/sec as opposed to Earth’s 30 km/sec. So in reality the impact speeds of objects can be much higher on Mercury: 48 km/ sec + 4.3 km/sec = 52 km/sec, compared to 30 km/sec + 11 km/sec = 41 km/sec for Earth.

Moreover, the energy released on impact — which is what carves out the crater and causes widespread devastation — increases as the impact velocity squared. So for a given size/mass of an impactor, hitting Mercury yields an explosion 1.6 times greater than it would on Earth!

That’s on average; the direction the asteroid comes in makes a difference (it might "catch up" with a planet, making a slower impact speed, versus coming in from head on and doubling (or more) the speed), but still, it’s interesting to me. I guess real estate agents are right: it’s not the size that matters, it’s the location.

Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington


Related posts:

Jaw-dropping mosaic of Mercury’s battered, beautiful face
More Mercury
MESSENGER’s family portrait
Watermelon planet (a personal favorite of mine)

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (27)

  1. Trikester

    Does the depth of the Sun’s gravity well have an effect on impact speeds at Mercury’s distance vs Earth’s distance from the sun?

  2. Peter Eldergill

    Are craters always circular? I would figure that if an meteroid were to hit the planet at an angle, would it leave an elliptical crater? It seems all the craters I’ve seen are circular

    Just curious

    Pete

  3. Chief

    exactly the question I was thinking, closer to the sun faster orbital speed as the body rounds the sun.

  4. Childermass

    It is really not Mercury’s velocity, but the difference between the velocity of Mercury and the impactor that counts. Anyone know that affects the picture? I would assume that the impactor who be orbiting in the same general direction that most everything else in the Solar System does (i.e. clockwise or counter-clockwise depending which of the Sun’s poles you are pretending to be over).

  5. anb

    The Goethe basin is near the north pole of Mercury. Due to MESSENGERs elliptical orbit no high-resolution images (better than a few hundred meters per pixel) are expected from the south polar region.

  6. Ian

    Also, does Earth’s atmosphere have a significant impact (heh) on the terminal speed of meteorites?

  7. Chris

    So where do you store water on Mercury?
    You stick it where the sun don’t shine! :-)

  8. RwFlynn

    @2. Peter Eldergill

    You figured correctly! I’ve seen a few elliptical craters posted here on the BA Blog every so often. A Google image search yields some nice examples as well.

  9. Isn’t the tilt axis of Mercury near zero? In that case, the planet’s orbital speed can’t make any difference in impact velocity at the poles.

  10. Trebuchet

    This post reminded me of an early Larry Niven story titled, if I recall correctly, “The Coldest Place”. Set on the dark side of Mercury — it was written when it was believed that Mercury was tidally locked and presented just one face to the sun.

    @ #2 and #7: I wonder if most craters are circular because the planet’s gravity tends to cause an approaching asteroid to curve into it and strike more perpendicularly? Phil??

  11. Nigel Depledge

    Apparently, even an impact that is quite oblique leaves a circular crater. I’m not sure under what circumstances an impactor will leave an elliptical crater.

  12. Peter B

    Peter Eldergill @ #2 asked: “Are craters always circular? I would figure that if an meteroid were to hit the planet at an angle, would it leave an elliptical crater? It seems all the craters I’ve seen are circular.”

    As a general rule craters are circular. I understand this is because craters are mostly caused by the release of energy at impact – it’s effectively a bomb going off at that point – rather than the angle of impact. What many people miss is the effect of the speed at impact is so high that the meteor is vaporised, and that material then spreads out evenly in all directions.

    Yes, there are some elliptical craters on the Moon, but I understand they’re very rare.

  13. Chris

    Google “elliptical craters” and you can see some images of them.

  14. Beau

    Thanks for the daily physics lesson!

  15. anb (#5) seems to be correct. The Goethe Basin is at 78.5N.

    http://en.wikipedia.org/wiki/Goethe_Basin

  16. NASA discloses Peltier life!

    NASA scientists, with managerial guns held to their paychecks, today announced the likelihood of Peltier life at planet Mercury’s south pole. “We observe a temperature gradient of 450 C between shadow and sunlight. Assuredly life evolved was created to derive energy from this abundant thermodynamic gradient.” NASA immediately demanded a Bernanke Buck ($1 trillion at -0.25% interest) for studies.

    “We forsee an end to all Earth’s energy shortages from manned exploration of Mercury. Retrieval of Peltier life followed by a century of quarantine under vigorous Enviro-whiner litigation is the solution to all our energy needs.”

  17. Joseph G

    Fascinating!!! And I hadn’t realized the bit about impacts having on average more energy on Mercury. That certainly makes sense, though, considering the Caloris basin and its antipodal weirdness. Wiki has the skinny on it, but basically, the Caloris basin is an impact crater over 1500 km across. At the exact antipode is a region of jumbled, fractured crust. The prevailing theory is that the seismic shock wave from the impactor traveled around Mercury and reconverged there, creating the “chaotic terrain”. Just mind-boggling!

    The bit about perpetual shadow reminds me of an old sci-fi story I read about a mission to the “coldest place in the solar system”. The story sets it up to make you think of Pluto, but at the end of the story the location is revealed to be the “dark side” of Mercury. This was back before it was discovered that Mercury rotates in a 2:3 resonance to its orbit with the sun, as opposed to being tidally locked.

    Edit: @#10 Trebuchet:

    This post reminded me of an early Larry Niven story titled, if I recall correctly, “The Coldest Place”. Set on the dark side of Mercury — it was written when it was believed that Mercury was tidally locked and presented just one face to the sun.

    D’oh!! That’ll teach me to read the thread before posting :(

  18. Indy

    @ #9 Daniel Clements , the axial tilt of Mercury is 0.01 degrees. So, yes, very nearly zero. :)

  19. Pete Jackson

    Although the crater floor is shielded from direct sunlight, it won’t be in total darkness because of reflected light from the crater walls. To test that, I downloaded the picture above and raised the gamma and contrast to the hilt and could just make out a few craters on the floor. That says that the imagery is excellent with lots of low order bits, and that the NASA project scientists will eventually be making analysis of these areas to test whether they have higher albedo (which you might expect if there were ice there) than the rest of Mercury.

    Of course, they may be covered with regolith and only the penetrating power of radar can reach to the ice.

  20. Randy Owens

    @10. Trebuchet & @17. Joseph G: Yes, back when I first heard this news, Niven’s “The Coldest Place” was the first thing that came to mind. And, it was especially ironic because that was his first published story, and it almost wasn’t, because after he’d written it but before it was published, came the discovery that Mercury wasn’t tidally locked after all, so presumably wasn’t really cold anywhere. They decided to let it go as an interesting counterfactual.

  21. vince charles

    19. Pete Jackson Said:
    September 28th, 2011 at 2:18 pm

    “Although the crater floor is shielded from direct sunlight, it won’t be in total darkness because of reflected light from the crater walls.”

    Yes, the Kaguya orbiter tried this angle at the Moon:

    http://lunarscience.nasa.gov/articles/kaguya-satellite-has-shown-no-exposed-ice-in-lunar-crater

  22. Joel Younger

    Phil said: “So in reality the impact speeds of objects can be much higher on Mercury: 48 km/ sec + 4.3 km/sec = 52 km/sec, compared to 30 km/sec + 11 km/sec = 41 km/sec for Earth.”

    A note from the Department of Mathematical Pedantry: 11.2 km/s for Earth and 4.3 km/s for Mercury are the minimum velocities of bodies colliding with the two planets, not an addition to the orbital velocity of the planets. So, say a rock is dropped from just at the radius of influence of a planet’s gravity, that’s what you see.

    If, however, the rock has an initial relative velocity (= planet velocity + rock velocity) of its own, the new impact velocity is given by v(impact) = sqrt[v(original) + 2*G*M/r]. The addition to the original velocity is the infall acceleration, but it is not the same as simply adding the escape velocity to an existing orbital velocity.

    Interestingly, if you apply the vis-viva equation to the same sort of bodies that strike Earth, their orbital velocities are more than double by the time they get to Mercury’s neighborhood around 0.4 AU, as compared to what they are crossing Earth’s orbit at 1 AU. For impactors with retrograde orbits, that can mean relative velocities in excess of 120 km/s!

  23. Ebbs

    What are the chance of Earthlings being able to survive (assuming that it is water ice reflecting from inside the craters), inside those craters?

    Because Mercury is the closest in to the sun planet, it will probably consist of a much higher concentration of heavy metals such as Gold, Platinum etc. That would make mining such a planet a possible, well, goldmine opportunity!

    If it is considered too dangerous or expensive for humans to survive and mine Mercury, why not robotic miners then?

  24. Voltaire2

    Didn’t anybody here see Apollo 18? You do NOT go into the perpetually dark craters on Mercury or the Moon! Unless you want to meet THEM….

  25. Joseph G

    @21 Randy Owens: And, it was especially ironic because that was his first published story, and it almost wasn’t, because after he’d written it but before it was published, came the discovery that Mercury wasn’t tidally locked after all, so presumably wasn’t really cold anywhere. They decided to let it go as an interesting counterfactual.

    Interesting! I didn’t know the timing was that close.

  26. So in reality the impact speeds of objects can be much higher on Mercury: 48 km/ sec + 4.3 km/sec = 52 km/sec, compared to 30 km/sec + 11 km/sec = 41 km/sec for Earth.

    What if the object is going in the same direction as Mercury? It’s the difference in velocity between the two objects, not their absolute velocities.

    [edit] Though, re-reading the above, I realize that you did say can be.

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