It gets hit by more meteorites for the same area than Earth because it is closer to the asteroid belt. It’s hard to estimate the average ratios though, especially as Mars’s orbit continually changes in eccentricity and when it is more eccentric it spends more time closer to the asteroid belt so may be hit more often. It’s orbit is currently more eccentric than usual.
However you can estimate the average rates approximately using models combined with observations. This paper works out a ratio of 2.6 of Mars relative to the Moon (which has similar impact rate to Earth). This more recent paper confirms those conclusions.
It does seem to get hit by many meteorites large enough to form craters. This is the largest spotted recently by our Mars orbiters:
It is 159 feet across. NASA Mars Weathercam Helps Find Big New Crater
By comparison this is a similar sized crater in the Sahara desert (147 foot wide), found by satellite observation. It’s amongst the freshest craters we have. They think it formed after humans stop cultivating the area so that would mean more recently than 5,000 years ago.
PHOTOGRAPH COURTESY MUSEO NAZIONALE DELL'ANTARTIDE UNIVERSITÀ DI SIENA "Fresh" Crater Found in Egypt; Changes Impact Risk?
Mars also gets hit by many meteorites which would be far too small to leave anything at all on Earth as they would burn up in our atmosphere. HiRise is able to resolve features well below meter scale and the Mars atmosphere is far too thin to make them burn up in its atmosphere.
Mars gets hit by meteorites large enough to create a small craters meters to tens of meters in size 200 times a year.
One of 200 fresh craters that form on Mars every year as a result of its thin atmosphere and higher impact rate than Earth. Pow! Mars Hit By Space Rocks 200 Times a Year - academic paper here http://www.lpl.arizona.edu/~shan...
This doesn’t mean that our rovers are at imminent risk of being hit by a meteorite though. They have discovered meteorites on the ground
Iron meteorite found on Mars by Opportunity in 2005 - this is the first meteorite to be found on another planet.
However 200 impacts a year isn’t much compared to the size of Mars. It’s surface area is 144.8 million km² so that means one small crater forms each year on average every 720,000 square kilometers. In an area the size of Alaska or France, you’d get just under one small crater a year.
Let’s do a “back of the envelope” rough calculation - probably slight over estimate but suppose that a rover has to be within 10 meters of one of these craters on average to be damaged (the larger ones are tens of meters in diameter, but the smaller ones are more numerous, however they will produce debris that they kick up as they hit).
Then that’s 314 square meters or 0.000314 square kilometers of Mars each year. So the chance of a rover being hit each year is probably of the order of 1 in 720,000/0.000314 or 1 in 2.2 billion. That may be out by an order of magnitude or so, I don’t know if anyone has done a detailed assessment of this, but it is too small a probability to matter, of the order of 1 in a billion or so for a very very rough first guess. Even if we made the distance of likely damage for a rover that is near to one of those impacts as large as 100 meters, that would make it one chance in 10 million per year.
Whatever the chance is, it is more than on Earth at any rate, where the chance is zero of course.
Towards the other end of the scale, it gets hit by asteroids large enough to send material all the way to Earth roughly every one or two million years.. It turns out that you need an oblique impact powerful enough to form a crater about 10 km in diameter, or a direct hit causing an impact crater about 100 km across. The impactor would be about 1 km across, and you would get impacts like that roughly every one or two million years.
The search has turned up some interesting rayed craters on Marsthat might perhaps be the sources of Martian meteorites.. Here is a detailed study of the formation of the young Zumil rayed crater, including impact simulation of a 1 km impactor at an oblique angle. They found that some of the material from this impact would eventually reach Earth. It is a strong candidate for a source for some of our Martian meteorites..
You can also look at the cosmic radiation ages of the Martian meteorites we have on Earth. This tells you how long they were exposed to cosmic radiation during the crossing from Mars to Earth (while geological age shows the age since the rock first formed). Thee youngest meteorite, EET 79001, is from 730,000 years ago. Then there is a cluster of meteorites all from 1.2 million years ago, and there are other clusters every one or two million years going back about 20 million years.
Mars has about 385,000 craters with a diameter of 1 km or more. Mars Crater Catalog by Stuart Robbins (20120821). Though many of those would come from earlier times in the solar system when it was hit by asteroids more often.
By comparison with these figures of 200 fresh smaller craters a year on Mars and 385,000 in total of diameter 1 km or larger remaining for all time, there are only 176 confirmed impact craters on Earth, that’s including all sizes including small ones. But the smaller ones erode quickly and there are only 15 known craters smaller than 300 meters in diameter. Larger ones can get erased by geological processes. (details of the numbers for Earth here).
However the smaller Mars craters are from meteorites that would burn up in the atmosphere on Earth, and many of the larger ones are from billions of years ago during the late heavy bombardment, making it hard to make an accurate comparison.
I’m surprised actually that the theoretical ratio is as low as 2.6 for Mars relative to the Moon. (Though of course it is understandable that the smaller meteorites too small to hit the ground on Earth are more numerous for Mars). Could it be partly because Mars has a more elliptical orbit and so is encountering more impacts than usual at present? If anyone knows more about this do say.
Comet Siding Spring did a close flyby of Mars but missed by 140,000 km or 41 Mars radii. That’s a long way away but from time to time a comet or large asteroid is bound to hit Mars.
Artist's concept of comet Siding Spring (C/2013 A1) - Mars Science Laboratory
Calculations suggest that a big enough comet, if it hit the polar regions could create an ice covered lake that would stay liquid for a thousand years, which is one of many possible habitats for life on Mars.
We don’t know of any such lake at present. But there have been lakes in the very recent geological past formed similarly due to volcanic processes.
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