I'm sure you're well aware of the "look up this day because Mars will be the size of the full moon" emails which come and go every year... my usual response is to just shares the Snopes link with whomever forwarded the email & note that if Marsreally were that size, we'd probably be doomed. Well I decided to try and think through what might actually happen if this were somehow the case. I'm not an astronomer, physicist, meteorologist, nor geologist... all I have is the partially-educated fascination of an engineer, and we at least practice an ever-so-slightly related manner of thinking.
What Mars Looks Like
First, let's consider what exactly Mars would look like. Mars does not naturally come so close to Earth as to appear so large, hence whatever force has caused it do so must have been quite powerful.
If we see Mars at sunrise, this would probably indicate that the most extreme of forces has acted upon Mars... to the points that there's a good chance it'll be but a mere breadcrumb of its original size. This is because it's hurtling straight at us in the opposite direction... think of two runners on a track: one runner decides to do a U-turn and head straight at the other one. Something BIG had to knock Mars not just out of its orbit, but in the totally opposite direction.
If we see Mars at mid-day or nighttime, it's approaching us from the side... it's either headed toward the sun or returning from having just passed it by (don't worry, odds are extremely good that it didn't hit Mercury or Venus along the way). It still would've taken a strong force to knock it sideways out of its natural orbit, though, and it'd probably be missing a hefty chunk out of it from whatever smacked into it first.
A Mars at sunset would be the most plausible & would probably yield the most Mars-looking Mars of these scenarios. This means it's catching up to us from behind -- less force is required to shift Mars into this tighter orbit, though whatever did it would still probably leave a hefty mark. Nonetheless, in this scenario we might still actually recognise it as Mars... in all the others, there's a good chance it wouldn't look at-all familiar.
The "Sunset Mars" would more likely take eons to reach that point... a small planetoid could collide with present-day Mars, causing it to shift into a tighter orbit, and over many years it may keep spiraling in toward the sun until it eventually crosses paths with Earth's orbit.
If we are assuming that this full moon-sized Mars is within our immediate future, however, then we're back to our "unrecognisable Mars" scenario whereby the force necessary to speed up Mars such that it can come into a tighter orbit AND catch up to us... that sort of force would probably have totally destroyed much of what we now recognise as Mars.
There're actually two more options: Mars could approach from the north or south, heading toward our poles. The northern hemisphere would see it day & night if it's headed toward Santa, and the southern hemisphere would get 24-hour viewing pleasure if it's Antarctic-bound. This would probably be similar to somewhere between the more Mars-like Sunset Mars and the unrecognisable fragment of a Sunrise Mars: the force needed to put Mars into this polar approach would be such that we probably wouldn't recognise Mars anymore, but at least it wouldn't be as bad as if it were a Sunrise Mars.
As it nears & is within a couple days distant, gravitational effects would have a profound impact on Earth well before it appeared to be the size of a full moon. The effects of Mars’ gravity are such that the odds are that a large portion (but certainly not all) life on Earth will have perished over the hours and days before hits. First the planet's gases would shift, then the liquids, then the solids, and then everything sitting unattached on top of the Earth (that includes you).
It'd start with the atmosphere. We wouldn't notice much at first, but weather patterns would rather quickly begin to change on a global scale. The side of Earth facing away from Mars will experience lower pressure as the atmosphere is gravitationally pulled to the side nearest to Mars, where higher pressures form.
...Except since the Earth is spinning, that means that the atmosphere is always moving; trying to stay on the side nearest to Mars. This effect won't be too pronounced at first, but eventually it'll overcome Earth's own natural weather patterns. At that point, harking back to my previous descriptions of a Sunset Mars, Sunrise Mars, Daytime Mars, Nighttime Mars, and Polar Mars:
A sunset Mars would mean the wind flows west at night and east by day. At dawn it'd be relatively calm, albeit rather low pressure. This is in contrast to the usual meteorology whereby low pressure tends to be equated with storms. At dusk there'd be high pressure, with the cooler nighttime winds coming from the east and the warmer daytime winds coming from the west: massive storms would erupt every sunset such that one wouldn't even be able to view Mars. However, Earth's rotation might also have some influence on dispersing or weakening these storms... I'm not entirely sure. On the average, sunsets would be warmer than normal; sunrises would be cooler.
A sunrise Mars would be the reverse: east by night, west by day, and calm at dusk. Sunrises would be warmer, sunsets cooler, and at sunrise there would be powerful storms moving ever eastward along with the sunset; unless, again, Earth's rotation manages to dissipate the storms somewhat by reducing the capability of the hot & cold air to circulate.
A nighttime Mars would mean that wind blows toward the night -- east at sunset; west at sunrise. Daytime would be the low pressure calm & cooler than is typical today. People would get less sleep thanks to the everlasting nighttime storms formed by the warmer air coming from each side of the planet with the cooler effects of facing away from the sun. Again, the Earth's rotation might help dissipate the storms.
A daytime Mars would mean that wind blows toward the day -- west at sunset; east at sunrise. Nighttime would be the low pressure calm & cooler than what is typical today. Less work would be done, with construction being particularly difficult thanks to daytime storms, though as before: Earth's rotation may help keep the storms weaker or more isolated.
Lastly, the polar Marses: if approaching the North Pole, all winds would flow northward; and southward for the South Pole. The winds would be particularly pronounced in summertime as warm equatorial winds rush to fill in the colder polar air. Massive storms would likely form at the Mars-facing pole, this time aided by Earth's rotation such that they're likely to circulate more readily. Meanwhile, the pole furthest from Mars will see temperatures plunge even further in both summer and winter. Temperatures in between will cool as compared to typical temperatures, with its warm air heading toward one pole & cool air from the other filling its place.
Waterfront folk would notice awkward tides. For the non-Polar Mars approaches: sometimes there are strong tides, sometimes they seem normal, and other times there's no tide at all. As Mars nears along its path, at some points it'll be on the same side as the moon -- causing higher tides; and sometimes it'd be on the other side of Earth from the moon -- reducing the Moon's effect on tides.
As it nears ever closer – within days of impact – tides would become more pronounced: higher, faster, stronger, as Daft Punk would say. Pretty much everyone within a couple hundred feet of sea level would experience not just one tidal wave, but repeating tidal waves pretty much every 12 hours that get stronger and stronger each time. If the storms didn’t get you, the tides probably will.
While these non-Polar approaches might disrupt the natural circulation of our oceans (which have a major effect on our weather), if Mars approaches the poles it’ll alter these processes to a massive degree. As Mars’ gravity begins to rival that of the Moon, water will shift toward whichever pole faces toward Mars on a more continuous level.
Floods will come and will remain, as the ocean swallows all low (and even not-so-low) land in the hemisphere that gets to see Mars, and the Moon’s gravity will continue the regular tidal forces of higher and lower water tables.
Those on the far hemisphere will lose their waterfront properties not to the sea, but rather to the land, as the ocean will have receded vast distances. Land bridges of yore (as well as some new ones) will be likely to reopen once again, potentially reunited continents with their lost Pangaean brethren. Food will be quite plentiful at first for these folk, as they can just walk out on the former sea floor and collect beached fish and other marine life, though in the coming months (assuming there are months left in Earth’s fate) food could overall become scarce as crops fail & animal/plant life dies owing to the weather changes.
At some point we'll probably start getting rained on by space debris from whatever collision knocked Mars out of its orbit, especially if we see Mars at sunrise, and possibly also if it's a mid-day or nighttime Mars; but less likely if a sunset Mars. These could cause massive disruption if crashing into land, but even moreso if crashing into the sea – just as would any other asteroid. It’s a near-certainty that if Mars is heading for us, there are even more resulting asteroids & debris also headed our way well in advance of the planet itself.
As Mars gets ever nearer – likely within days or hours of impact – the land will also start to shift. Over the time span, however, you’re not going to see a shift of landmasses to form the raindrop shape that the atmosphere & oceans take. While that does happen, the process takes eons; not the days or hours that are still remaining by the time the land starts to move. Rather, slight contortions in the plates will cause rocks to slip, shifting existing plates & forming new ones, and ultimately resulting in a multitude of both small & large earthquakes.
Volcanism is likely to increase, particularly on the side facing Mars, as the molten core attempts to form into the teardrop shape that the sea and air both share. However, the rocky crust pushes back, attempting to keep the core constrained into its roughly spherical shape. The resulting pressure releases itself through volcanoes: active volcanoes are first to erupt followed by dormant ones, then the magma finds its way through chambers of old & reignites long-dead volcanoes, and lastly brand new volcanoes form where none were before. The volume and power of lava erupting forth puts everything we’ve ever experienced in our history to shame.
The fumes & gases released from the volcanoes poison the area... perhaps not killing people outright, but certainly not benefiting plant life & also adding to the greenhouse gases – giving a boost to global warming’s pace.
If you’re on the side facing Mars, I’ll dispel one Looney Tunes myth right off the bat. Let’s just assume Mars got so close you could reach out and touch it. Just imagine it’s sitting right there, inches away from the surface or perhaps even resting softly on it. You won’t be able to put one foot on it & then start walking up its surface, changing your gravitational allegiance from Earth to Mars. The fact of the matter is that Earth is still the more massive object, hence you’ll still be bound to Earth: your feet will still be on the same soil.
However, as it gets nearer, your weight will start to vary ever so slightly. If Mars is approaching on a non-Polar orbit, you’ll be a bit lighter when you’re on the side of Earth facing Mars and a bit heavier when you’re on the side facing away. So if you ever wanted to feel good about that diet and exercise regimen: measure yourself when Mars is at its zenith.
But don’t worry… you’re not going to get squished into a pancake if you’re facing away from Mars. The very last split-second of impact, multiply your weight by 0.62 to get how much you’d weigh on the Mars-facing side of Earth; or multiply by 1.38 to get your weight if you’re on the side facing away from Mars. That’s your worst-case scenario (or best-case if you’re a bit self-conscious) as far as your weight is concerned, but as I’ll delve into later… your weight is the least of your concerns at that moment.
If Mars is approaching toward a pole, odds are you won’t feel much difference until we’re probably seconds or minutes from complete annihilation. Then you might start to feel a bit wobbly, but even then you’d still likely be able to walk more-or-less normally… or at worst at a skill level as if you’d had a bit too much to drink. Only at the last couple moments of life will you feel yourself being pulled toward Mars – it’d be as if someone were pushing you from the side… not a “whoosh sucked sideways” sort of thing; just as “this person keeps leaning into me more and more” sort of thing. But again, this is the least of your concerns.
For a bit of comparison: Mars has 38% the gravity of Earth and the Moon is 17% that of Earth. So essentially, Mars is roughly double the gravity of the moon. Now compare how high you can jump during high tide versus how high you can jump at low tide. The difference between the two is likely to be pretty much zero. Now double that… and you still have zero.
If you did a finely-controlled scientific experiment with very precise instruments: yes, you’d see a difference; but for practical purposes you’re still going to be grounded… even when Mars is the same distance away (in which case it’ll actually appear twice as big as the Moon).
So basically, even when the Red Planet is – even by a layperson’s opinion – definitely very close, we still wouldn’t be feeling much difference. Harking back to the air, sea, and land: if you look at an individual particle or even a bucket of dirt or water: you’re not going to see any difference. It’s when you aggregate it all together when its shape change actually becomes apparent. Taken individually, it’s just one small dot… just like you can’t tell with the naked eye that the Earth is round, yet one would be hard-pressed these days to dispute it just because you can’t see it.
Swing and a Miss
The moment everyone’s been waiting for, and unfortunately not too many are probably still around to witness it; and those that are will probably have their view blocked by storm clouds. Bummer. They’ll still have a story to tell, though, since the odds are that Mars will miss entirely, anyway. This is especially true of a Daytime, Nighttime, or Polar Mars, where what may appear to be something aimed directly at us is actually more likely to zoom by behind Earth, as Earth continues around its orbit. A sunrise Mars is also somewhat unlikely, as it’s like a tiny bullet aimed at a tiny target in a very large room.
Really the only thing a bit more likely to hit us (and still the odds are in Earth’s favor of a miss) is a sunset Mars, whereby Mars is catching up from behind. Since it would presumably be approaching at a slower rate than any of the others (since just like a relay team, both runners are headed in the same direction), it therefore has a longer time to be attracted by Earth’s gravity. Similarly, Mars is also pulling ever-so-slightly on Earth.
In the likely event of a miss, remaining life (which will likely include a decent amount of humans, but certainly a vastly reduced population) will quickly return to a more-or-less “normal” state, apart from the societal damage, infrastructure destruction, and pending famines probably facing the remaining human population. The rest of animal and plant life will probably be able to cope pretty well so long as they weren’t inundated with water, uprooted from winds, or asphyxiated or incinerated by volcanism.
Water levels will return to pre-Mars conditions, apart from some new salty lakes which will increase in salinity as they evaporate over time. With time, ocean currents will reform and the atmosphere will return to more typical levels, though this resettling could take several years (at least) before it’s bit more like “normal” conditions.
It’s possible we may be missing a moon. Mars gravity, if aligned properly, could’ve boosted the Moon out of here… and similarly Mars could be missing Phobos &/or Deimos. Note that while it’s unlikely we would be able to pickup Mars’ moons & keep them, it’s very possible that one or both could collide with Earth… more likely, actually, than a collision with Mars proper.
There’s a slim chance the Moon could also be pulled into Earth… slim, but at least possible (especially considering everything written thus far is already about the slimmest of slim chances). Or Mars could’ve smacked right into the Moon & kept on going, missing Earth’s bulk. That would be a really cool sight… but one which would also have devastating long-term consequences for Earth, as we’d lose the engine behind our oceans.
A miss probably wouldn’t pose much issue to Earth’s orbit. At worst, it’d only perturb it enough that over eons (as in: longer than humanity has ever existed) would the planet start to register warming or cooling effects, depending on which way it was pushed. Really the only long-term concern for survivors is in hoping that we still have a moon (and even then: life might still survive).
If Mars actually smacks into Earth, it’d be like David striking Goliath. While Earth could probably take the hit, life would almost certainly be annihilated. Only the hardiest of organisms at the microscopic level might survive, in which case the evolutionary chain starts over. It’ll be hundreds of millions – perhaps billions – of years before complex life takes hold again, and even then it may look nothing like we know it today.
Those on the side facing Mars in those last few seconds will meet their end instantaneously. Those on the sides of the planet will probably die just as quickly, albeit ever-so-slightly delayed. Massive earthquakes would be an understatement as Earth struggles to hold itself together in the first place: large clumps will be lost to space, likely forming a ring of debris that will persist for millions of years before eventually falling back or coalescing into a new moon.
If you think you’d be able to hang onto a fragment flying away from Earth: note that it’s unlikely that someone will survive the trip – nor even hang on as a corpse – on an ejected fragment of Earth, as the force of the collision will send anything on the fragments – no matter how hard you’re holding on – off on its own trajectory into space. And if you’re ejected into space, the force of the wind will fry you just as thoroughly as when a meteor or the Space Shuttle enters into the atmosphere. You’ll vaporise. And if you’re still living a couple seconds later, the vacuum of space will finish you off.
Some fragments from the collision will fly out into space, just as others won’t be ejected fast enough to overcome the gravity of both worlds: causing brimstone to rain from the sky as pieces come hurtling back to the surfaces of Earth and Mars. A number of these will be too large or too near to vaporise in the atmosphere, and hard hats won’t be too effective at keeping these from punching a hole through anything they land on.
The atmosphere will be punched just as thoroughly as the ground. While a portion of the atmosphere will be lost to space, this probably won’t represent a significant portion of it. Of concern to those still alive is the coming conflagration: a superheated high-pressure wind that will expand in all directions from the impact site.
If the heat of the collision itself doesn’t get you, the friction from the wind will fry you as noted above; or the force of the wind will be like skydiving without a parachute; or the wind pressure itself could quite possibly cause you to explode. Not necessarily explode as if you were a bomb… but your organs would rupture and your body could blister, and liquids & gases within your body could be placed under pressure to a degree that they cease to do any of the life supporting things they’re supposed to do.
Next up are the tsunami waves. There are two types… one is the usual sky-high wave of water we can all envision from so many disaster movies about asteroids hitting Earth. Another is the also significant wave of molten rock also heading out from the collision point. And there’s also a wave of solid rock – all those aforementioned fragments – ejected more to the side rather than necessarily upward into space. All of those will destroy anything in the way, with the molten & solid rock waves reaching out for hundreds of miles; and the water waves likely impacting the entire planet. If you survived the flooding prior to collision, these waves are your final exam.
All that volcanism in the last couple hours before collision will pale in comparison to what occurs afterward, as the force of the collision ripples straight down to the core and out the other side. The force into the core causes its molten portion to burst out in every direction it can, igniting old and new volcanoes in all the corners of the globe. Most affected are those on the opposite side, where the force of the collision pushes the molten core the hardest.
Everyone on the sides of the planet met an instant end from being shot into space, burning one way, burning another way, burning a couple other dozen ways, being whapped by a variety of waves, exploding ever so slightly, and so forth… the people on the other side of the planet might just get to enjoy their lives for a tad longer… long enough to experience many of the same things, except with more volcanoes, to think the thought “Hey, this hurts” before they die. And so it goes with all the rest of plant, animal, and almost every bit of microbial life.
The remaining Earth will be almost lifeless. It’ll also likely be alone, as odds are that Mars won’t stick with Earth; instead it’ll ricochet off in its own direction. Similarly, Earth will be bounced in its own way. Its orbit may still be similar, but it will have likely been put on a path either somewhat further or nearer to the sun. That won’t be of much concern in the near-term (especially since there won’t be much left, anyway), but as any microbial survivors mature into more complex organisms: they’ll have to be adapted to a warming or cooling world than what their distant ancestors may have once enjoyed.
One small hope is if Mars doesn’t hit us directly, but instead skips across Earth’s surface. With such an occurrence, many of the same things mentioned above will occur; but at least they’ll be of a slightly smaller scale. It’s still a near-guarantee that all animal and plant life will go extinct, but at least a larger share of microbial life might survive – giving evolution a boost of a couple hundred million years with repopulating the planet.