I was reading up on Mars thin atmosphere and was wondering how could Earth becomes like the red planet except it only has only 100 pascal (air pressure) at imaginary sea level? Mars is 600 pascal on average.
Earth's atmosphere is held by force of gravity and by magnetic field. The gravity is not going to change anytime soon but magnetic field is bit unstable and its source is not fully understood. We know it's generated by the iron-nickel Earth's core but we don't know all the details, including how and why it sometimes switches the poles.
If the Earth gets hit by unusually strong cosmic event (Solar megastorm, gamma ray burst) during the magnetic reversal period it could potentially lose large fraction of its atmosphere. But whatever happens the pressure will never get lower than on Mars. The Earth is much heavier, so even if most of our atmosphere gets somehow blown off it will be rebuilt by evaporating oceans and by volcanoes producing carbon dioxide and some other gases.
A sufficiently nearby supernova might do it.
I've read (don't recall where to offer a citation) that a supernova within 100 light years would produce very noticeable climate effects, and one within fifty light years would result in an extinction event -- both from direct radiation effects and via destruction of the ozone layer (which would take centuries to return to normal).
A significantly closer supernova might result in significant atmosphere loss, by heating the upper atmosphere enough to greatly accelerate the normal slow loss. Whether it would be enough (even during a magnetic field reversal) to get down to 100 Pa, I seriously doubt -- but if combined with other events, perhaps a "perfect storm" of solar activity and a pre-existing runaway greenhouse, it might contribute the "last straw".
The bad news here is that there are no genuinely nearby stars that are supernova candidates -- the nearest is probably Betelgeuse, which is far enough away to have only small climate effects. Vega is quite close, and is the right kind of star, but isn't in its senescence. Eta Carina is in the "any day" stage, but is much too far away to matter.
It all depends on how quickly you want this done. Earth's atmosphere is slowly leaking away each time an air molecule in the upper atmosphere reaches escape velocity (Earth's magnetic field and ionization of molecules in the upper atmosphere complicates things a little, see Atmospheric escape). The speed of air molecules is dependent on the square root of the absolute temperature so you need a rather large temperature increase to make the molecules go faster so the atmosphere will leak away more rapidly. The current mean speed of air molecules at room temperature (300K) ~ 347 m/s and speeds follow the Maxwell-Boltzmann distribution. The escape velocity is 11186 m/s. You only need to increase the fraction of air molecules with velocities greater than the escape velocity. Increasing the temperature to 1500K will do the trick, although this will increase the pressure at first due to evaporation of volatile materials such as the oceans and the earth crust.
A good way to increase the temperature substantially is something like the Theia event wherein a Mars-sized planet collided with the proto Earth forming both the Earth and the Moon as we know them. As there are no Mars-sized bodies with a remote chance of a collision course with Earth to be found in the solar system at this point in time, it would have to be a rogue planet. For your purposes, this would have the added benefit of ripping away a substantial part of the atmosphere in the collision process. Still, it would take a few dozen megayears for cool down and evaporation.
An altogether different method is cooling down sufficiently such that the nitrogen and oxygen molecules freeze out of the air, which occurs 54.36K (the melting point of dioxygen). Moving Earth's orbit beyond Neptune would achieve this. Of course a collision with a rogue planet and as a result of momentum transfer, shifting the orbit of the resulting pair to a much wider, probably highly eccentric, orbit around the Sun will make all of this go a lot faster, but still a megayear for the end result.
One obvious answer to come up with is a CME (Coronal Mass Ejection). You haven't given a science tag so it's imaginable (perhaps a series of them) even if NASA says it can't happen to Earth.
Solar storm can strip a planet of its atmosphere, says NASA study
"Solar flares, for instance, affect Earth's upper atmosphere and may disrupt satellite communication systems. Another form of the sun's activity called coronal mass ejections (CMEs) may have more dramatic effect. A CME, according to NASA, could disrupt GPS signals and radio communications. But not even the most powerful CME can bring about the end of the world, NASA assures everyone."
In The Dark Forest, the author describes a deliberate attempt to decay the orbit of Mercury such that it falls into the sun. This event expels a significant amount of dust, etc. into the solar system which gradually widens to beyond Earth's orbit. The resulting friction and disruption of all this dust & matter gradually strip away Earth's atmosphere.
While in the story Mercury was pushed into the sun artificially, you could postulate that an interstellar and sufficiently large body could enter the solar system by ordinary gravitational attraction and result in similar consequences.