The vapor pressure of any substance increases as temperature rises. Solids too, not just liquids. Sublimation happens when the total gas pressure of the local atmosphere is less than the vapor pressure, and melting has not happened yet because it is not hot enough. Melting temperature is built into a substance. It depends only weakly on the outside world. But boiling point depends radically on the surrounding pressure. It's not so much a property of the substance, as a property of the combination of the substance and the local pressure. So by changing the local pressure in factors of 2, over the range of 1/1000 to 100 atmospheres, one can adjust the boiling point up or down, just like turning a knob. Can't do that to melting point. (It would take thousands of atmospheres to matter much). So at low pressures, for some substances, the boiling point may have moved to temperatures lower than the melting point. Then it's called sublimation. In other words, as the temperature slowly increases from very cold to very hot, in a high-pressure atmosphere, things melt and then boil, in a vacuum or low-pressure atmosphere, things sublime and never get to melt. Start in normal earth atmosphere and temperature. If evaporation occurs by raising the temperature, most things melt and then boil. But if evaporation occurs by reducing the surrounding pressure, many things sublime and never get to melt. Particularly our favorite substance, water. Water on Earth boils. Ice-comets in space sublime. Small things space almost always sublime, even if they are made of rock. Big things are different, they have gravity, and gravity holds an atmosphere, and the atmosphere holds in small pressures of vapor, so then there is a planetary environment within which things can melt without disappearing quickly. Of course, many things evaporate quite slowly even when melted, such as most rocks, so temporary melting can occur during collisions of asteroids in a vacuum. If an asteroid hits the moon, well, it has a vacuum, no atmosphere to speak of, but there still might be a puddle of molten rock in the bottom of the crater for seconds or days, depending on the size of the asteroid and crater. While that rock is molten, it will be evaporating slowly from its surface, and one could say it is boiling, even though there may not be much in the way of bubbles. Then when it cools enough to solidify, it will still continue evaporating, more slowly because the temperature is lower. During that period one could say it is subliming, though perhaps too slowly to measure. Hope that clarifies it.
