Oh come on! That could be anywhere!

Is the body a satellite of Jupiter or Saturn?

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There are two significant features about the image that should help you solve this.

I'm not going to dole out any hints until someone has had a few cracks at it.

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This isn't twenty questions! You'll have to work this one out on your own.

The answer can be deduced through logic.

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Venus

A little ambiguity can go a long way.[/quote]

I have a hunch the object is somewhat small. Perhaps something like Tethys.

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Hmm. You might be right. I was going with the view that we probably haven't mapped that many bodies to that degree of accuracy. Toss-up between Mars and Venus.

I'm just going to think out loud, here.

With craters twelve km across or larger, the body has to be pretty big, so
I'd say it has to be either a planet or one of what I classify as the "large"
moons. Those large moons are, in descending order of size, Ganymede,
Titan (surface), Callisto, Io, Luna, Europa, and Triton. All the others are
too small to have so many craters that large -- I think.

Of the planets, craters should be difficult to identify on Venus, since we
can only see them with radar, so I would expect very few to have been
identified. I'm quite sure that nowhere near that many craters have been
found on Earth, and certainly not in the ocean beds, so the distribution
is too even for it to be Earth. The northern polar region of Mars is very
smooth compared to most of the planet, very sparse craters there, so
Mars is out. The gas giants are of course out. I'm guessing that the
recent MESSENGER flyby of Mercury imaged the remaining unseen parts
of the planet. If not, it is eliminated, too, but I'll go with the guess that
Mercury has now been imaged, making it the only contender planet.

Of the large Moons, Titan's clouds prevent us from seeing craters there,
and I haven't heard of them being found with radar, and the number is
way to big for that, so Titan is out. Io is constantly resurfaced, so it has
no big craters. Europa's surface is smooth like ice because... it's ice! So
no big craters there. I doubt that the entire surface of Triton has been
imaged. If not, Triton can be eliminated. That leaves Ganymede, Callisto,
and Luna as the only contenders among the moons.

Four possibilities.

Just need to look at some spacecraft images and compare. However,
the possibility that Mercury has finally -- and just recently -- been
completely imaged makes it a particularly attractive guess.

Mercury?

Here's my pre-MESSENGER image of Mercury on my website:
http://www.freemars.org/jeff/planets/Mercury.htm

-- Jeff, in Minneapolis

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Shouldn't the Moon have noticeably fewer craters on the Nearside due to the distribution of maria?

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That's what I figured. Where I've probably gone wrong, is in assuming:
1. Venus has been well mapped, and
2. Nothing else has.

ok, since the map is complete it cannot be a body we only know a little about.

This excludes Mercury, and any moon past Saturn (and most of Saturn's moons). It's not earth because of the oceans, and it's not Mars - I know that one well enough. This leaves Venus and some moons. Venus is mostly a new surface.


Io and Europa have few craters. This leaves me with our moon, Ganymede and Callisto.

I'm going with our moon.

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The image seems to show several craters that are in a neat row. That makes my amateur mind think of Roche limits and Shoemaker-Levy 9 and such, and Jupiter. That would exclude our moon and, going from crosscountry, put Ganymede and Callisto back in favor.

ETA: And since the spread of the craters is pretty even across the surface, I think that would favor Callisto over Ganymede.

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The correct answer has actually been mentioned, but I want to know why it has to be the correct answer before I award the palm.

Think of crater distributions and what they tell us about a surface...

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An 'old' surface, geologically inactive for a long time. And no oceans of course.

How old?

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you seem to be insinuating that earth is the answer to your question, but no one here believes that could be true.

but this image disagrees

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1) an even crater distribution shows that a body can be hit from all sides. 2)It also means that the surface doesn't reshape itself very often. 3)Another thing it tells us is that there is no liquid on the surface.

#1 suggests that our moon isn't correct because one side always faces the earth leaving it less available to outside impacts.

#2 rules out Venus, Io, Europa, and some moons farther out.

#3 means Earth and Titan won't show their craters or that they won't be there at all.

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4 billion years?

The fact that one side of the Moon always faces Earth shouldn't make any
significant reduction in the size or number of impacts there. About as many
bodies must be deflected toward the Moon by Earth's gravity as deflected
away, and the number actually blocked would be miniscule. Earth is much
too far from the Moon to be a significant shield.

-- Jeff, in Minneapolis

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"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

A 4 billion year old surface would be saturated with craters.

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I don't follow the logic of this statement. A saturated distribution would imply this, but an even crater distribution implies something quite different - something that virtually gives the game away!

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True, true. I was thinking that there would also be a lot of craters <12km is size.

Then it has to be Venus!


No?

That image has entirely suspicious concentrations of craters in Baltica, Laurentia, and Australia compared to other cratons. One suspects it says almost as much about the distribution of geologists as of craters.

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Without knowning the size of the body, one can't very well tell how saturated it is from that image, can one?

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Yes. You were right all along, though you still haven't spelled out precisely why.

The two things in the image I was hoping you would notice were:

1: Even crater distribution. This implies that the entire surface is all of the same age. If there were some very old regions and some relatively young regions (as on the Moon or Earth), the oldest regions would be saturated with overlapping craters while the youngest regions would be relatively devoid of craters.

2: Low crater frequency. This implies that the surface is relatively young. If it were very old, there would be many more craters.

So, we are looking for a body whose entire surface was recently resurfaced on a global scale. There are only two such bodies:

Io, which is being globally resurfaced by volcanism today; but its surface is so young it has no craters! So that rules out Io.

Venus, which experienced lithospheric turnover about 500 million years ago. Venus it is.

When I posted the question I thought, "No one will think of Venus", so I was a little peeved when PraedSt guessed the right answer right away.

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Actually those cratons are the oldest surviving pieces of the Earth's crust, and that is why they have more craters.

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Really? As Wikipedia would have it, the African cratons are mostly of older rock than Fennoscandia, frex.

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Lol!