October 25, 2013
The Moon. Credit: NASA/JPL-Caltech
The Moon, Earth's natural satellite, is the only astronomical object beside our planet that humans walked on. It is so close that we can see its surface textures with the naked eye. Yet, we don't exactly know how it came to be. This is where Mercury steps in.
Comparing the Moon to other planets' natural satellites, our companion is very large compared to its planet. It is the fifth largest moon in the solar system, but other moons of similar size orbit gas giants. It is too big and too close to the Earth to have formed by simply accumulating materials in protoplanetary disk. The current leading theory is that Mars-sized planet named Theia collided with Earth some 4.5 billion years ago. Part of it merged with the Earth, the rest was blasted into orbit to form the Moon.
That's the theory that most agree on, but not the one without its shortcomings. The most significant problem are the isotopic similarities between the Earth and the Moon. If the Earth and Theia were different, why are the Earth and the Moon so alike?
Mercury's surface taken by Messenger. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
There are many similarities between the two – both are without atmosphere and rocky bodies, not too different in size either. Most importantly, however, they seem to have similar geological history. Scientists came to this conclusion thanks to Messenger spacecraft that finished mapping Mercury's surface in March this year. The spacecraft was launched in 2004 and arrived in orbit around the planet in 2011.
"Mercury and the moon [seem to] have followed very similar tracks," said Sean Solomon, NASA's Messenger mission to Mercury main investigator and also the director of the Lamont-Doherty Earth Observatory of Columbia University in New York, USA. "It is striking, given that Mercury is thought to have formed from the material that made up the early disk of gas and dust spinning around the sun, and not due to a giant impact like the moon," he added.
About 27% of Mercury's surface is relatively smooth terrain, probably made from low-viscosity lava from vulcanic eruptions that cooled down to produce pyroclastic deposits. On the Moon, about 16% of surface is covered in similar way. The lava was able to flood low-lying regions and "iron out" old impact craters on both bodies and the interesting thing is that these volcanic eruptions happened on both Mercury and the Moon around the same time.
Both Mercury and the Moon have two different hemispheres. The side of the Moon permanently facing Earth is where the geological activity was present, while the dark side is almost completely covered in craters. Mercury, similarly, has most of its volcanic plains in the northern hemisphere.
Although processes inside the two bodies have been very different during the formation of the solar system and the said bodies, Solomon feels it is important to look at similarities in geological processes near the surface in order to find answers.
"It means that many aspects of the Moon are common to rocky bodies that are similar in size despite being different in bulk composition," he said. "Therefore, we have to focus on those aspects of the Moon that are special, if we want to gain traction on the question of what made the Moon as opposed to what made the planets."
As mentioned above, the problem with the current theory where Earth collided with another planet is that the Moon and the Earth are isotopically similar. We face this problem, because Mars is isotopically nothing like the Earth. If Mercury turns out to be different as well, then we are missing something. Future missions, where sample from Mercury could be procured, or meteorites from the planet could give us the answer.
If the isotopic composition of Mercury turns out to be similar to Earth, then there was possibly some process in the early stages of solar system formation that homogenized the isotopes of all planets. Venus is for a moment out of question when it comes to collecting samples due to extremely high pressure at the surface of the planet. We can, however, with some certainty conclude that both Venus and hypothesized Theia had similar isotopic composition to that of the Earth.