So where did life on comets come from in the first place?

There's an increasing body of evidence that suggests organic molecules that formed the origin of life on Earth got their start on meteors — but then how did the organic compounds get there to begin with?

Top image: Michael Jacger

According to research published in Science Express, it may all go back to ice, radiation, and protoplanetary disks. A protoplanetary disk is the slowly congealing mass of stellar gas that will one day form a planetary system. Bits of dust and ice form in these disks, and in some cases stick together and begin to form planets. It's these tiny pellets of ice that researchers Fred Ciesla and Scott Sandford focused on, and can provide a possible origin of life.

Their research involved tracking the creation and evolution of these ice grains, and what environments they would have been subjected to, and if organic compounds could have been formed this way. What they found is that conditions in these protoplanetary clouds are similar to those that have produced complex organics in laboratory conditions.

These ice pellets are made up of H2O, CO, CO2, NH3 and CH3OH, and around the edges of a disk are kept relatively cool, or less than 30 K. Between their own star and those around them, they're periodically bombarded with UV light. This causes the molecular bonds in the ice to break producing reactive ions and radicals. They then can reform into new, sometimes organic, compounds. This organic growth boosts as the pellet warms, even if it isn't being irradiated just then. So as the pellets travel and go through temperature changes, the warming and irradiation can cause the production of organic molecules.

This is one possible model for the origin of organic compounds — and it could have happened on a meteor passing through a nascent solar system, which then carried the spark of life to a planet. With the research showing ~5% of the mass of ices being converted to organics, and protoplanets not being at all short on mass, that's plenty of opportunity for organic compounds to form.