Expert: Courtney Seligman - 7/28/2011

Question
Sir, If there is any orbital changes between earth and mars and mars comes close to earth. Is any changes in atmosphere due to low gravitation of mars.

Answer
It would depend upon how close it came. At the moment, the closest it can get is about 35 million miles. But let's suppose it was 200 times closer, or about 170 thousand miles away. That would be a bit closer than our Moon, and Mars has almost 10 times the mass of our Moon. Between the two, Mars would affect us about 20 times as much as the Moon.

That wouldn't produce any noticeable change in the atmosphere, or in the gravity we feel at the surface of the Earth. The Moon exerts a gravitational force on things at the surface of the Earth which is about 300 thousand times weaker than the gravity the Earth exerts on them, which we call their weight. If Mars exerted a force 20 times as large as the Moon, it would still only produce a force on things at the Earth's surface that was about 1500 times less than their weight. That's so small that you couldn't even tell there was a difference, without sensitive measuring tools. As a result, there would be no noticeable effects on the body of the Earth, or on our atmosphere, or on us, save for a couple of things.

First, being about twice the size of the Moon and in the example quoted a bit closer, Mars would look nearly 3 times the diameter and ten times the surface area of the Moon, so it would be a very impressive reddish ball, in the sky.

Second, since tides are related to the distance and mass of bodies like the Moon, tides raised in the body of the Earth and in the oceans (and in the atmosphere) would be about 20 times bigger than the tides raised by the Moon (which are about 2/3 of the tides that we experience, the other third being due to the Sun). So ocean tides would increase from an average of a foot and a half or so, to about 20 feet in size, which would be a very impressive (and on coastlines, disastrous) effect.

Ground tides would also increase, which would undoubtedly have some effect, but the size and nature of the effect would be more complicated. Right now, tides raised by the Sun and Moon have no effect on things such as earthquakes, but a much bigger object, a bit closer than the Moon, might produce enough of an effect to trigger at least some earthquakes.

So there would be visual effects, obvious oceanic tidal effects, and most likely some solid-ground tidal effects. Similarly, there would be tides in the atmosphere, which would slightly change the barometric pressure, relative to its normal value, as the Earth rotated beneath the Martian disc. Odds are that sensitive measurements would be able to detect the daily change in pressure caused by the effect, but whether they would noticeably affect the weather is another matter. That is so complex that the supposed effects would, I am sure, be hotly debated if anyone thought there was any chance of such a thing happening.

And of course, if Mars could come even closer, effects would be even larger.

However, under the scenario proposed above, there would be no change in the composition of our atmosphere, or our ability to hold onto an atmosphere. Only if Mars were close enough to practically collide with the Earth could you have those kinds of effects.

And fortunately, there is no way for Mars to actually change its orbit significantly, and get much closer than the 35 million miles which is currently its absolute closest approach. Over a million years or so, changes in its orbit due to interactions with the Earth and Jupiter can gradually change its closest approach, making it as small as 30 million miles, or as large as 50 million miles; but there is no way for it to ever come even as close as 20 million miles, let alone the 170 thousand miles imagined above.

So to summarize, if you want to imagine the possibility of Mars getting much, much closer to Earth, you could create most any sort of effect, but it would have to be VERY close to have substantial effects, and hundreds of times closer than it can ever come to have even relatively minor effects.