Siding Spring was a joke. At least compared to the Event. The Change. Whatever name they were giving it.
It began with a stream of comets. They crashed into the Northern Martian Ice Cap from a polar solar orbit. Or at least, a number of them did. It was insane and the world flipped out. Undetected, a hundred comets, plus or minus, came streaming in and smashed into Mars' North Pole. The Mare Boreum was smashed repeatedly by over 22 teratons of cometary ice and rock.
The blasts were tremendous, beyond anything humanity could ever hope to match with many times the total nuclear arsenal at its peak of the Cold War. It made every person feel small. And excited.
Half the volatiles of from the comets would be lost to space. The energy of the blast was too great, the gravity too small. The shockwave crashed across the small world, dust thrown into the sky everywhere. Mars became opaque. We could not see the surface. Not that there was anything left orbiting Mars after the impacts. Or on its surface.
Opportunity finally met its match. Curiosity burned. Quest ignited. Spirit was long gone, but would be reborn, in a sense, in its own, unique way. Exomars and the Chinese rover, Hui Lu, were smashed as well.
Amazingly, for all the sound and fury, the first impacts increased the mass of Mars' atmosphere only increased in density by .05%, even if it radically altered its composition: no longer just CO2 with traces of anything else, Mars had far, far more water and ammonia and methane than it had ever had before.
The world waited. Excited. In time, the atmosphere would clear and the world rushed to build orbiters to see down onto the surface and be ready for the next launch opportunity. While the world rushed, it ought to have waited: one Mars year later, another set of impacts took place. This time a Kuiper Belt Object the mass of Makemake came crashing down on the same northern plain. The devastation must have been awesome. We could only see the brightest flash the solar system had seen since Theia's impact on Tellus. And definitely not the surface.
This stunned the world further. Mars now seemed to have an atmosphere as dense as Earth's, but...vastly different. Nitrogen, to be sure, but no free oxygen. Lots of water. Far more ammonia. Far more methane. It was cold, but warming rapidly. At least on a geological, excuse me, areological time frame.
The northern plain, Mare Boreum, became an ocean in truth as well as name. High, strange shaped waves lapped at feet of Olympus Mons. The Hellas Basin became the Hellas Sea. The Valles Marineris became the Mariner Sea. Even the Argyre became yet another sea.
For twenty years, Mars was bathed in a constant torrent of rain. Washing away its old rusted face, baring a new exciting one. Mars was the Red Planet no more. It was a blue and muddy brown. It became very bright in the skies of earth. So bright and so changed, some suggested we change its name, from the angry god of war, to the bright eyed one, Athena.
We shall see. In the mean time, bots fly in swarms to the surface and to its orbit and to watch and observe. The nations of the world and the ridiculously wealthy are racing to plant flags and take lands. After all, Mars is now a new world.