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When looking at topographical pictures of Mars it seems like there are craters far more frequently than here on Earth. This statement, of course, is just pure speculation on my part. However, it makes me want to ask, is Mars more at risk for meteor and asteroid collisions than Earth due to its location? Why or why not? A bonus question to this would be "why might the craters on mars be more readily visible then?" Here's an example:

enter image description here

I can easily identify 20+ craters in that picture alone, is earth the same way just covered by surface features and vegetation making it less noticable?

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    $\begingroup$ There are a lot of historic still undiscovered asteroid craters on Earth. Geological processes did hide smaller craters. A lot of asteroids did a splash down into the oceans. $\endgroup$
    – Uwe
    Commented Dec 22, 2018 at 17:30
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    $\begingroup$ Octopus Um Most of the area of the oceans is much too deep for exploration by scuba teams. Mud and sediments will hide asteroids. $\endgroup$
    – Uwe
    Commented Dec 22, 2018 at 17:48
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    $\begingroup$ Earth has a higher gravitational cross-section for impacts than Mars, but Mars has no erosion going on, which leaves all impacts visible forever. $\endgroup$
    – AtmosphericPrisonEscape
    Commented Dec 22, 2018 at 19:25
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    $\begingroup$ Another point re the oceans is that oceanic crust is being continually recycled, so any craters there get erased fairly soon (in geologic terms). Unlike continents, where the crust can be several billion years old, basically none of the ocean crust is more than ~180 million years old, and most of it is under 100 million: ucmp.berkeley.edu/tectonics/crustages.jpg $\endgroup$
    – jamesqf
    Commented Dec 23, 2018 at 4:39
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    $\begingroup$ It's also instructive to compare other bodies. For instance, of Jupiter's Galilean satellites, Io & Europa have few craters because they are geologically active, while Ganymede & Callisto have many. Likewise Saturn's moon Titan has few, while the other moons have many: upload.wikimedia.org/wikipedia/commons/thumb/4/4b/… Similarly with e.g. Venus & Mercury. $\endgroup$
    – jamesqf
    Commented Dec 23, 2018 at 18:42

2 Answers 2

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There are three factors which contribute to the difference in “apparent crateredness” between Earth and Mars.

By far the most significant is ongoing erosion from weather. In Mars’ thin atmosphere, a crater can last for hundreds of thousands of years if not millions. On Earth, small craters are buried or worn away relatively quickly.

The second factor is that small meteors burn up in Earth’s thick atmosphere, so many fewer small craters are made to begin with.

The third factor would be the number of meteors each planet encounters; I think this is somewhat higher for Mars than for Earth (but I could be wrong). However, the surface of the moon demonstrates that Earth’s neighborhood receives plenty of meteor bombardment; Mars would look much like the moon, crater-wise, if not for its own atmosphere and weather erosion.

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  • $\begingroup$ Ahhhhh! I'm going to assume our moon would be another reason we see far less too. Mars not having a large moon would probably affect the number of meteors reaching the surface. Nice answer didnt even think about the moon and the instantaneous atmospheric erosion effect that Earth has. $\endgroup$
    – Magic Octopus Urn
    Commented Dec 22, 2018 at 18:11
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    $\begingroup$ @MagicOctopusUrn No, the Moon, being completely covered in craters layer on layer, shows that Earth too has been completely bombarded all over by millions of asteroids. It's just that the Moon has no atmosphere or active geology to remove the craters. So the number of craters visible on atmospheric bodies today is far from representative of the impact history. $\endgroup$
    – LocalFluff
    Commented Dec 22, 2018 at 18:26
  • $\begingroup$ @LocalFluff oh I was simply trying to say the moon being covered in craters implies that it took some hits on behalf of Earth. If the moon didnt exist we may have had many more impacts over the years- right? Guess i phrased that weirdly. $\endgroup$
    – Magic Octopus Urn
    Commented Dec 22, 2018 at 20:14
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    $\begingroup$ @MagicOctopusUrn Of course it might have, but hardly much. The mass of the Moon is only 1.2% that of Earth and it is 30 Earth diameters away (although it was much closer at formation), so it hasn't been much of a shield. As the Moon has been battered hard all around, so certainly was the Earth. Mars has two small moons that are either captured asteroids or formed by impact ejecta from Mars. Mars' northern hemisphere is a lowland apparently reformed after an impact removed material. Perhaps Mars' crater history is only burried in dust because it doesn't have tectonics to erase them permanently? $\endgroup$
    – LocalFluff
    Commented Dec 22, 2018 at 21:01
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    $\begingroup$ The Earth gives much more protection to the moon (being larger) than vice versa. $\endgroup$
    – Russell Borogove
    Commented Dec 22, 2018 at 21:22
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This report shows that Mars is hit over 200 times a year by meteorites big and/or fast enough to leave a crater of typically 12.8 feet diameter.

Earth strikes of this size are thankfully much rarer, so yes: Mars is more at risk.

Possible reasons for this are:

a) Mars is "only" 100 million km from the asteroid belt, whereas Earth is 180 million km away;

b) The Earth is protected by its atmosphere, which causes most incoming nasties to burn up;

c) The Earth's moon is colossal compared to Mars's largest moon Phobos (and even more so compared to Deimos) and so will offer more protection, either physically or gravitationally. Note that this is a possible rather than probable effect :)

vsz ponders on whether there were more asteroids flying around in the past - check out the Late Heavy Bombardment.

There's an implication in the question that the asteroid strikes should be sizeable enough to leave an obvious crater but of course there are many more smaller strikes which leave little or no evidence. The Earth grows in mass by 37,000 to 78,000 tons per year from falling space material:

How many meteorites hit Earth each year? (Intermediate)

The factors which cause over 200 craters on Mars must also apply to the small and medium particles, so it's fun to speculate on how much weight Mars puts on per year...

The bonus question asks “Why might craters on Mars be more readily visible?”

1) The first possibility is that the meteor lands in the ocean. The Earth is quite badly named, as 78% of the surface area is actually not earth but water, so we can assume that 78% of our meteors land here. Water acts as really thick air and slows the meteor comparatively gently and it will fall to the seabed.

We don’t yet know how hot meteors are; a small, rocky meteor will be extremely hot on the surface due to air friction, but it won’t conduct this heat to its core. One with a greater metal content will conduct and so will hold more heat. In either case there are very few reports of meteors causing fires at the impact site, so they won’t create a noticeable amount of steam when they hit the sea. Unless you’re in the area it’s unlikely that 78% of our meteorites will be detected.

2) Of those that hit the land, the most probable fate is to be worn away by the weather, and only comparatively recent craters still exist.

3) As always, there’s “misc” category. The Manson crater in Iowa, USA (which was thought to have killed the dinosaurs) is covered by glacial till; the Chicxulub crater in Mexico (which really did kill the dinosaurs) is also buried (by limestone sediment) but has a tell-tale series of sinkholes (“cenotes”) around its rim.

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    $\begingroup$ It is good that this is an answer and not a comment. It is literally citation for the claim that Mars is more at risk than Earth, aka the original question. Especially on a hard science SE like Space, citations are critical! Bravo! $\endgroup$
    – corsiKa
    Commented Dec 23, 2018 at 3:12
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    $\begingroup$ Thank you, corsiKa. I originally wanted to expand on Russell's third reason: "The third factor would be the number of meteors each planet encounters; I think this is somewhat higher for Mars than for Earth (but I could be wrong)." $\endgroup$
    – Boodysaspie
    Commented Dec 23, 2018 at 4:39
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    $\begingroup$ There's a lot of good in this answer, +1. I feel it would be good to also mention that the appearance of craters would be greater on Mars than on Earth even if the exact same number of meteors were to fall on both in a given year, because of erosion and differences in the surfaces of the two planets. I only suggest that because the visibility of craters on the surface of Mars seems to be the genesis of the question, so expanding to note that the number of visible craters is not merely a function of the number of impacts. $\endgroup$
    – Todd Wilcox
    Commented Dec 23, 2018 at 20:27

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