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djh_van t1_j96ljrc wrote

Reply to comment by Itis_TheStranger in This image of Mars shows the north polar ice cap, the border between highlands and lowlands, former river valleys, plains covered by dark sands and the large Hellas Planitia impact basin in the south. Credit: ESA/DLR/FU Berlin by MistWeaver80

  • Our atmosphere protects us from a lot of the small and medium impacts by burning them up. Other celestial objects have either no atmosphere to do this (moon) or a very thin atmosphere that doesn't burn them up before impact (mars), or the wrong composition of atmosphere.

  • Often they don't have a climate to weather the impact craters that were made.

  • our moon acts like a giant magnet or deflector shield orbiting our planet and scoops up a lot of the objects that might otherwise have hit us.

  • the gravity of some of the bigger planets (Jupiter, Saturn) actually helps to deflect some of the more energetic objects coming from outside our solar system

  • the old impact craters are there, but they are ancient and our planet's life has covered them with vegetation and millenia of human activity.

  • and lastly, we've been very very very lucky in the last few hundred years. Nothing major has got through that obstacle course. But in recorded human history there have been a few biggies get through. Ancient records describe them, and even as recently as during the explorer days we've had records of asteroids coming in over some remote island or oceans.

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OlympusMons94 t1_j96r48u wrote

Atmospheric composition doesn't matter. Density does some for small objects, but any rock big enough to make the large craters visible in images like this won't be stopped by an atmosphere.

The Moon provides negligible shielding. It covers only a tiny portion of the sky. Hold out your little finger at arm's length. The Moon is half as wide (that's wide, not long) in the sky as that little finger. Imagine how good a shield that tip of your little finger would be. Well, the Moon is smaller and it's not at arm's length. It's almost 400,000 km away. Ther eis a lot of room in between.

Earth is also a much bigger target with much stronger gravity compared to the Moon.

Jupiter is about as likely to send objects toward Earth as divert them away.

Weathering, erosion, and covering with water and sediment (as well as vegetation) because of our thick atmosphere and water are important.

Besides that, Earth has a lot of volcanism to resurface face cratered areas. That is also why the dark lunar maria we can see on the Moon are so lightly cratered compared to the lighter surrounding highlands. The maria are giant plains of frozen lava. (Much of the maria surfaces are still really old, though. A relate dlld point is that there were a lot more eimpacts very early in the solar system's history.)

Lastly, Earth also has active plate tectonics, which deforms craters on land, completely subducts craters on the ocean floor within a couple hundred million years, and is related to Earth's volcanic activity.

Because of geologic activity, Venus, Europa, Enceladus, Io, and Pluto all have surfaces with few large or obvious craters. Their surfaces have all been resurfaced by lava or ice within the past few hundred million years.

u/Itis_TheStranger

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Itis_TheStranger t1_j97jdbx wrote

Thank you for that reply. This is a great conversation and it's interesting to read the different information.

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vipperofvipp t1_j97qhlj wrote

I’ve read a description before of Jupiter being described as a massive offensive lineman that protects Earth.

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