Watermelon planet

By Phil Plait | October 7, 2008 9:33 am


Mercury overview from MESSENGER’s 2nd flyby

Holy Haleakala. Look at those rays! They go all the way across the planet!

This is Mercury as seen by MESSENGER, which flew by the planet for a second time yesterday (out of three passes on its way to orbiting the planet in March 2011). This overview was taken when the probe was 27,000 km (17,000 miles) from Mercury, 90 minutes after closest encounter. What you’re seeing here is pretty much the opposite side of the planet as was seen last January at MESSENGER’s first pass, so most of this is territory never seen before in this detail (in this case, at about 5km/pixel).

The bright streaks or stripes are called rays. They’re material ejected from what appears to be a young crater first seen during the initial flyby. When an object impacts the surface of a planet, material can spray out in long rays; check out an image of the full Moon to see similar rays radiating out from the crater Tycho. Amazingly, the rays were known before MESSENGER; radar signals bounced off Mercury from Earth indicated the rays were there. The ray material lying on the surface reflects radar differently than rock, and that was detected even from Earth.

But this is the first time they’ve been seen.

The bright crater in the middle of Mercury is Kuiper, known since the Mariner 10 days of the 1970s. But not like this! It has a ray system as well. In fact, if you look closely, you’ll see that almost all the bright small craters can be seen to have rays, too. Over time, the solar wind and meteorite impacts erase ray systems, so they come from young craters, and young craters tend to have brighter floors.

Now check out this close-up:

MESSENGER’s Mercury. Click to embiggen.

That is not the Moon! This is a shot taken just about 9 minutes after closest approach, when MESSENGER was about 1800 km (1100 miles) from the surface. You can almost pick out the order in which events happened here, with fresher craters overlaying older ones. The big sucker at the top is called Polygnotus, and has an inner ring, common in larger impact events. Polygnotus is actually more of a basin than a crater; the floor is smooth, indicating it got filled with lava after impact. Another double-walled basin can be see on the left, called Boethius. You can see a cliff running up-and-down near the center of Boethius; that is a scarp that must have formed after the impact event. You can tell because of the short, bright feature that is cut in half near the top of the scarp (halfway from the center of the basin to the rim).

Reading a planet’s surface may seem a little bit like phrenology or palm reading. In a sense, it is; you look at the overview as well as the details laid out in front of you, and try to tease out information from the jumble. But unlike those pseudosciences, planetary surface geology tells you about the way the world really is. And it’s a different world! We can read Mercury like, well, like a map. We can look at the features and peel away the history of this planet, from massive impacts to the subsequent shrinkage of the crust, causing cracks to penetrate all over the surface. Huge cliffs, deep chasms, steep crater walls… all brutally baking under an unforgiving Sun much closer than it is from the Earth.

And this is almost all new. Our previous maps of Mercury were amazing for their time, have no doubt, but we are a restless bunch, and love to fling ever-better technology at the places we’ve only seen fuzzily in the past. Now our vision is sharper… but this is only a glimpse. MESSENGER will take a third swing at Mercury next year on September 29, and then, 18 months later, will settle down for a long stay, orbiting the planet and returning thousands of images such as these. There’s much we still don’t understand about the smallest planet, even with these fantastic images to help us. In fact, they are only whetting our appetite, but the feast will start soon enough.

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


Comments (41)

  1. TheManVersion

    Phil, that is most cool.

    Do we commonly see rays that stretch that far? Must have been hellacious, but not quite hellacious enough to shatter the planet.

    It looks like one of those Terry’s orange chocolate balls that you have to smack the bejeezus out of to make edible.

  2. Nice. That close-up pic alone has enough interesting nooks and crannies for Hoagland to write another three books.

    “If you draw a line directly from the middle of the big crater to an arbitrary point just outside the Mercurian greenhouses (plainly evident in this view), you’ll get the inverse ratio of dead to active brain cells in the average shnook who believes what I write…”

  3. Ala'a

    Visual confirmation of previous radar observations, WOW!

  4. Gorgeous. Thanks for sharing Phil!
    I really, really like this shot too: http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/EN0131766396M.png

  5. 01101001

    Cute what the trackback context is from my posted article in BAUT Forum referring to this blog article.

    The promises of which I wrote had nothing to do with Mercury or Phil Plait, but refer to the non-predictions of an advocate of non-mainstream physics.

    I contrasted those promises with the reality the Bad Astronomer provides.

    Mercury. Wow.

  6. Why do I almost feel as if I should be sitting in awe just going , “Oooohhhhhhh, ahhhhhh”? Maybe because I am? Would be cool to actually get a probe into a stable orbit around the planet so we can really give it the once over instead of these flybys. Anyone worked out the orbital mechanics involved in that?

  7. madge

    Saw this on the messenger website and immediately fell in love with it. Gonna be a week for flybys. on Thursday Cassini makes its closest flyby yet of Enceladus. 25 km folks! Don’t it just make you GRIN?!

  8. Chip

    There must be some interesting ideas from the Nasa-Messenger staff about how such long thin rays are formed over the planet. Maybe on impact there were many solid bits that left long dusty trails as their trajectory curved back in. Though Mercury is lunar-like, it is not like the moon. What a strange place.

  9. Doh! /facepalm I was so distracted by the pretty pictures I missed the part where Dr Plait says 2011… Um, never mind!

  10. Beautiful, just beautiful. And this is just the beginning… Messenger will bring us so much great images and science, it’s exhilarating! Looking forward to the next pass.

  11. Does the first picture put to rest the rumors concerning shield volcanoes on Mercury, or could they still be lurking there somewhere?

    That escarpment in the leftmost crater of the second crater reminds me of the famous Straight Wall (Rupes Recta) on the moon, although it’s not as straight. It’s probably the edge of a lava flow coming from the right side rather than a fault displacement since it seems to have a much more sinuous boundary than the Straight Wall.

  12. Sir Eccles

    Did you really just compare a picture from the Mercury flyby to “phrenology or palm reading”??????

    Oh look I see a future for you Mercury. This line says how many moons you will have and this feature shows where you’ll live look you’ll even have a swimming pool *spit*

  13. Max Fagin

    Are these images true color?

  14. I just had the most bizarre thing happen looking at the close of the craters. I was remembering you write about how craters can look like mounds without context. So I deliberately tried to see mounds, and I soon started seeing them.

    Now they’re all I can see! I seriously can’t get back to seeing craters again!

    Mom! Where’s the broom? I think I just shattered my perception!

  15. Alan Stern

    Nice, but of course there are many smaller planets in our solar system, and even smaller
    extra-solar planets known.

  16. And this is why I have no free time. . . . BTW, loved the reference to phrenology. – g^2

  17. Michael Parmeley

    I have a dumb question, if they were a relatively handful of miles away from the planet (the cnn.com article I read says 121 miles but considering the source I wouldn’t vouch for its accuracy) why didn’t they just enter orbit this time? Why do they have to leave and come back to enter orbit?

  18. It was moving too fast to brake into orbit. The purpose of the flybys is to slow the spacecraft enough so when it flys by Mercury in 2011, it will be going slow enough to brake into orbit.

    This is probably why Mercury’s rays are longer than the moon’s. Objects in Mercury’s vicinity travel much faster in their solar orbits than objects in Earth’s vicinity, hence impacts release more energy.

  19. changcho

    Thanks for the heads up; neat pictures. It’s funny that you mentioned:

    “There’s much we still don’t understand about the smallest planet, even with these fantastic images to help us. In fact, they are only whetting our appetite, but the feast will start soon enough.”

    I remember when I was a kid Mercury was considered the littlest planet – Pluto was though of as bigger back then.

  20. Wow, those are some dazzling images!

    Thank you, Dr. Plait!

  21. tony873004 Says: “It was moving too fast to brake into orbit. The purpose of the flybys is to slow the spacecraft enough so when it flies by Mercury in 2011, it will be going slow enough to brake into orbit.”

    Correct. Since we are more than halfway out of the sun’s gravity well, it takes more energy to negate orbital momentum and get to Mercury than it takes to raise orbital momentum enough to leave the solar system. Put another way, Messenger’s flight to Mercury (relatively right next door) is more difficult than New Horizon’s journey to Pluto and on out of the solar system.

    – Jack

  22. Grand Lunar

    When I looked at that image, I thought “Wow, that’s like Tycho!”

    GASP! There’s a monolith buried there! j/k

    Good to see the cool images from MESSENGER are making their rounds. I wonder what other cool things on this hot rock await.

  23. @The Chemist – don’t worry, just tell yourself which way the light is coming from, and they flip back – which I find interesting. This implies that our perception of convex/concave can be influenced by conscious thought, and isn’t just an arbitrary process.

    Anyway, beautiful picture! Made my evening :)

  24. You’re wrong! It’s not a watermelon – it’s a pumpkin! I have incontrovertible proof: http://alicesastroinfo.wordpress.com/2008/10/07/new-images-from-mercury/

  25. Naomi

    Whoa, that’s one hell of a ray system! And the second one is absolutely fascinating – you can build up a geological (er, mercurological? Hermilogical?) history just by looking!

    “on Thursday Cassini makes its closest flyby yet of Enceladus. 25 km folks! Don’t it just make you GRIN?!”

    OH yes 😀 (As a random bit of trivia, it was the Head of Geology at a local uni raving about how cool Enceladus was that cinched my decision to do geology, not astronomy. At any rate, it’s FAR easier to get in to planetary studies / astrogeology via geology!)

  26. dkary

    Just a quick answer for “Chip” on those crater rays: the usual explanation (for the Moon, and I would assume for Mercury as well) is that when the impactor hits a small part of the target area turns to a liquid which jets out of the forming crater. This liquid jet then rapidly cools into a long thin line of powder that falls back to the surface as a ray.

  27. “Watermelon planet” Mercury to go with Canteloupe moon – Triton! 😉

    Y’know they actually called certain areas on that Neptunean moon cantalopue terrain.

    So lessee now every planet has to have its own fruit or vegetable .. 😉

    Mercury = watermelon

    Venus = One of them orange -yellow capiscum’s (hot & sweet)

    Earth = basket of all of them & the Moon = cauliflower (grey)

    Mars = asparagus (Well according to one of the guys from Phoenix)

    Ceres = lychee (small & tasty with lots of water)

    Jupiter = Ginat Ki;ller Tomato (Great Red Spot)

    Saturn = Huge sweet rockmelon (Colour sort of works also very sweet & everyone’s fave!)

    Ouranos = cabbage (No odour jokes please, just because its the least popular -almost. Betetr be careful Ouranos or they’llkick youout too! 😉 )

    Neptune = Blueberry (Fairly small a bit sour, very blue, sorta how we all feel about it trying knocking Pluto from its last planet role.)

    & Triton = Cantelkoupe – (For its ‘Canteloupe terrain” as noted.)

    Pluto = raspberry (That’s what its giving back at the IAU & its pretty red -and, yes durnnit, it is too a planet! 😉 )

    Eris = Umm .. Nup all the fruit are taken the solar system stops here!
    😛 :-)

  28. Alan Stern Says: October 7th, 2008 at 12:25 pm :

    “Nice, but of course there are many smaller planets in our solar system, and even smaller extra-solar planets known.”

    May we presume this the Alan S.Stern of New Horizons, NASA and Pluto & ice dwarfs fame?

    If so :

    Awesome! 8) You, sir, are a legend! 😀 8)
    Great work & great to see you here, you are one of my heroes!

    BTW. Did you read my defence of Pluto’s rightful planetary status on this site on some earlier threads? If so, what did you think? Hope you appreciated it. I quoted your verdict of the IAU ‘s definition as “idiotic” and completely agree.

  29. Hmm.. Given the convincing image of Mercury as a pumpkin …

    AlicesAstroInfo (Great site btw & quick work! 😉 ) Said on Oct. 7th, 2008 at 5:19 pm :

    “You’re wrong! It’s not a watermelon – it’s a pumpkin! I have incontrovertible proof: http://alicesastroinfo.wordpress.com/2008/10/07/new-images-from-mercury/

    Okay. Good idea. I’ll alter my ‘planetary fruit’ accordingly :

    Mercury can now be a pumpkin,

    Venus an orange (Jupiter a pineapple – massive and colourful?)

    Eris can be the watemelon,

    Haumea (ex-2003 EL-61, 1/3rd Pluto’s size & egg-shaped by its fast spin but recently called a Dwarf planet – & people thinkof iot like this : dwarfs are still people, dwarf stars are still stars, dawrf planets therefore must still be planets!) fits in well as a grape.

    Makemake can be a potato

    Pallas a cucumber &

    Vesta a rambutan (red for its lava history and tough skin, likewise,
    sweet inside, tropical fruit for those who don’t know.)

    Okay is that uh ..every planet now!? Reckon so …


    “… he had left out a planet. It was not his fault; everyone leaves it out. I leave it out myself when I list the nine planets, because it is the four-and-a-halfth planet. I’m referring to Ceres; a small but respectable world that doesn’t deserve the neglect it receives.”
    – Page 63, chapter 5 “The World Ceres” in ‘The Tragedy of the Moon’ by
    Isaac Asimov, Mercury Press, 1973.

    “Still there is no rule that says a planet has to be larger than a certain size and despite its smallness, Ceres would certainly have entered the list of planets if Piazzi’s discovery had remained as it was.”
    – P. 63, Asimov, Isaac, ‘The Tragedy of the Moon’, Mercury Press, 1973.

    “I consider it quite conceivable that the day may come when Ceres will be the astronomical centre of the solar system.”
    – P. 66, Asimov, Isaac, ‘The Tragedy of the Moon’, Mercury Press, 1973.

  30. Tom

    Woo hooo! That’s very cool. So SHARP too.
    What a great time we live in. The economy may be falling apart, but then it seems almost every month there is something new discovered or photograped from space to realize that are problems here on Earth are just nothing on the big scale of the solar system.
    Snow on Mars, Rovers still going, Opportunity starting a 12 mile road trip, Cassini’s upcoming 16 mile pass over Enceladus, Rosetta passing Stein….I can hardly get work done with all the daily discoveries!
    Let’s hope the next Mars rover doesn’t get cancelled…..

  31. IBY

    Wow, it does kind of look like a watermelon. But then again, it could look like a lot of other stuffs too.


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