September 2015

The Origin Of Life On Earth

Written by Jesse Emspak

In November of 2014, for the first time, a spacecraft landed on a comet - and in the process revealed much about the early solar system, as well as offering clues to the origin of life on Earth.

Launched in 2004, the European Space Agency's Rosetta mission arrived at 67P/Churyumov– Gerasimenko, or simply 67P, in August of 2014. Rosetta is the name of the orbiting spacecraft, while Philae is the lander. Philae was to probe the comet's surface while Rosetta analyzed the gas and dust that surrounds the comet, called the coma. The coma and cometary tail form as a comet approaches the sun and the ices that compose large parts of it sublimate into gas. The coma and tail are what observers see from Earth when a comets is bright enough.

After some two and a half months of scouting out the comet's surface, Philae detached from Rosetta to land on the rubber-duck-shaped body. Philae's landing was rough; it bounced off the comet twice. That's easy to do because comets are small, with little mass, so their gravity is minimal - a person standing on 67P could easily jump off it and launch into space. In Philae's case a rocket that was supposed to fire "up" didn't, and the anchors the probe carried didn't deploy. Philae came to rest partially in shadow at the foot of a small cliff on the 67P's surface. Since Philae wasn't in the sun - at least not consistently -- it couldn't charge its batteries enough to run continuously, but it has managed to charge enough for a few runs of datagathering. Comets are special because unlike larger bodies like planets, geological processes haven't transformed them over time. Many originate in a region beyond Neptune called the Kuiper Belt, while others come from the Oort cloud. The Oort (named for the astronomer who discovered it) is so far from the sun that comets from there won't return to our skies for thousands of years. In both regions, though, the distance from the sun means that bodies that stay there are effectively deep-frozen.

67P is probably from the Kuiper belt. It orbits the sun every six and a half years, taking it from 186 million to 850 million kilometers from the sun, or between Earth and Mars out to just beyond Jupiter. In fact 67P is called a "Jupiter family" comet. Unlike bright (and famous) comets like Halley, 67P isn't visible without a telescope - it's about one hundred fifty times fainter than the dimmest naked-eye stars. Rosetta confirmed that the dust getting blown off the surface is the same as that which falls to Earth every day from space. That means that the interplanetary dust probably comes from comets - which fits the idea that comets are primordial objects.

Philae, meanwhile, showed us that the comet's surface varies quite a bit. Parts of 67P are covered in a thin dust layer, which itself sits on water ice. Other areas, like the one Philae ended up in, have deeper layers of dust.

As importantly, Philae found evidence of organic molecules. Organic molecules are compounds that contain carbon, so they include everything from methane (one carbon and four hydrogen atoms, or CH4) to complex molecules like benzene (C8H6).

Most scientists expected to see carbon dioxide, carbon monoxide, and even formaldeyde. They also expected ammonia and water. In addition to those, Philae found methyl isocyanate, acetone, propionaldehyde and acetamide, which had never been seen in comets before. Such molecules are the building blocks for amino acids and other compounds that make up complex chemistry of life on Earth. This is important because the surface of the Earth was likely molten before it cooled enough for a crust to form. The current thinking is that organic molecules - which break up when temperatures are high -- came from comets that bombarded the planet's surface billions of years ago, after the crust had cooled below the boiling temperature of water. There's another twist. Water was an important part of the origins of life, as it acted as a solvent. Previously many scientists assumed that the water came from comets. But the Rosetta orbiter found the water on 67P differs from that on Earth.

Water is made of two atoms of hydrogen and one of oxygen. Hydrogen has a single proton in its nucleus. But some water molecules contain deuterium, which is a hydrogen atom that has a neutron and a proton. On Earth this is about one in every 10,000 molecules or so. Yet the ratio of heavy water molecules to light water on 67P was much higher than that. Previous studies - including some flyby missions of comets - had found that most comets differ from Earth in his way, but not Jupiter family comets like 67P.

This prompted a rethink about the origin of Earth's oceans, and led some to speculate that the water came from asteroids, not comets. Asteroids are usually thought of as rocks flying trough space, but in the early solar system they might have contained much more water. Over billions of years the water - at least on those bodies closer to the sun -- sublimated away, leaving the rocky portions behind.

The orbiter's mission will be extended to September of 2016. At that point mission control will send it into 67P for a crash-landing, joining Philae. By that time the sun will be too far away for Rosetta's solar panels to power the orbiter. Like a lot of really great science the mission has raised as many questions as it answers. And Philae might send more data; as 67P rotates its "day" might bring Philae back into the sun long enough for the probe to charge its batteries. Or as Rosetta's project manager at ESA, Stephan Ulamec, told Science magazine: "The lander surprises us again and again."