Timely story this week (November 10, 2015) from the ongoing AAS Division of Planetary Sciences meeting happening at National Harbor, Maryland. Astronomers there are presenting results of a study showing that our sun’s innermost planet, Mercury, has a recurring meteor shower during which bits of dust from an ancient comet regularly pelt its surface. The story is timely not just because of the meeting announcement, but also because this same comet – Comet Encke, whose orbit around the sun is only 3.3 years – is the one causing this year’s amazing display of Taurid fireballs, visible from Earth. For us, the North Taurid meteor stream should produce meteors most heavily on the nights of November 11 and 12, 2015.
Earth has one thing in relative abundance that Mercury lacks, and that’s air. It’s the cometary debris burning up in our atmosphere that makes the ongoing Taurid meteor shower – or any strong meteor shower – such a joy to behold.
Mercury does have a tenuous atmosphere, in which – this new study suggests – the incoming meteors from Comet Encke might have a “discernible” effect. A November 10 statement from NASA said the meteors streaking through the nearly airless space above Mercury:
… may lead to a new paradigm on how these airless bodies maintain their ethereal envelopes.
The new study – by Apostolos Christou at the Armagh Observatory in Northern Ireland, Rosemary Killen at NASA’s Goddard Space Flight Center, and Matthew Burger of Morgan State University in Baltimore, working at Goddard – also pointed to the meteor shower on planet Mars a year ago, caused by the close pass of Comet Siding Spring:
When that comet came within 100,000 miles (160,000 km) of Mars in October, 2014, it loaded Mars’ thin upper atmosphere with several tons of cometary material.
Several Mars-orbiting spacecraft sent back data indicating a spectacular meteor shower for Mars at that time. See the image below.
The NASA statement continued:
Bodies such as the moon and Mercury are typically thought of as airless, yet we have known since the time of the Apollo moon landings that they are surrounded by clouds of atomic particles either launched from the surface or brought in by the solar wind. Though tenuous by comparison to the dense atmospheres of the Earth or Mars, the observational record has revealed these surface boundary exospheres to be complex and dynamic entities, fascinating to study in their own right.
NASA’s MErcury Surface Space ENvironment, GEochemistry, and Ranging (MESSENGER), the first spacecraft to orbit Mercury, measured how certain species in the exosphere vary with time.
Analysis of the data by Burger and colleagues found a pattern in the variation of the element calcium that repeats from one Mercury year to the next. To investigate, Killen teamed up with Joe Hahn of the Space Science Institute, based in Austin, Texas, to understand what happens when Mercury ploughs through the so-called zodiacal cloud of interplanetary dust around the sun and its surface is pelted by high-speed meteoroids.
The researchers found that both the observed amount of calcium and the pattern in which it varies could be explained in terms of the material thrown off the planet’s surface by the impacts. But one feature in the data did not make sense: the peak in calcium emission is seen right after Mercury passes through its perihelion — the closest point of its orbit to the sun — whereas Killen and Hahn’s model predicted the peak to occur just before perihelion. Something was still missing.
That ‘something’ arrived in the form of a cometary dust stream. Discovered in the 18th century, Comet Encke is named after the German mathematician who first computed its orbit. It has the shortest period of any comet, returning to perihelion every 3.3 years at a distance of 31 million miles (nearly 50 million km) from the sun.
Its orbit, and that of any dust particles thrown off it, is stable enough so, over millennia, a dense dust stream would have formed. Killen and Hahn proposed that Encke dust impacting Mercury could kick up more calcium from the surface and explain what MESSENGER was seeing.
There are many more details to this story, which you can read about at NASA’s website if you’re interested.
To me, what’s most profound is the increasing knowledge that we – and our machines – can now detect and study meteor showers on other worlds. The details of a meteor shower on Mercury or Mars always turn out to be more intricate than we might have imagined. They’re similar to earthly meteor showers, but also different from them, and different from each other.
That would surely be true for all planets in our own solar system that have meteor showers. They’d all be as unique as the physical characteristics of the planets and their atmospheres themselves. And so it’s fun to think of distant meteor showers that surely occur in other solar systems, in whatever atmospheres exist around the 1,977 exoplanets, or planets orbiting distant suns, reported so far – as well as for the billions of exoplanets now suspected to lie undiscovered as yet in the vast space of our Milky Way galaxy.
That’s a mind-boggling thought, for those who love nature and admire its intricacies.
Bottom line: Comet Encke is now known to cause recurring meteor showers on Mercury. The data come from the MESSENGER spacecraft, which orbited Mercury from 2011 until a planned crash landing on Mercury’s surface earlier this year. Encke is the same comet causing the amazing display of Taurid fireballs on Earth this year.