There isn't a distinct, hard boundary where Earth's atmosphere is "constrained" by gravity. Instead, the atmosphere gradually thins out as you move away from the Earth's surface.

Here's a breakdown of how this works:

* Exosphere: The outermost layer of the atmosphere, extending from about 700 km to 10,000 km (435 to 6,200 miles). Here, the atmosphere is extremely thin and atoms and molecules can escape into space.

* Gradual Thinning: As you move higher into the exosphere, the density of the atmosphere decreases significantly. This means there are fewer particles per unit volume, making the atmosphere less and less "constrained" by gravity.

* No Defined Boundary: There's no specific altitude where gravity suddenly stops holding onto the atmosphere. It's a gradual process.

Key Points:

* Gravity still acts on the atmosphere in the exosphere, but its effect weakens with increasing distance from Earth.

* The exosphere is considered the transition zone between Earth's atmosphere and the vacuum of space.

* While we often talk about the "edge of space," it's more accurate to describe it as a gradual transition rather than a clear-cut boundary.

Therefore, rather than a distinct boundary, it's more accurate to say that the Earth's atmosphere gradually fades out as it interacts with the vacuum of space.