A unique sample: how rocks from Phobos may reveal much about Mars.

Artist impression of the MMX spacecraft around Phobos and Deimos.

There is a myth of a meteorite from Mars that struck a dog when it fell in Egypt in 1911.

“This would be the world’s unluckiest dog,” declared Steve Squyres to the packed auditorium here at ISAS.

While the canine may have been fictitious, the meteorite was genuine. It became known as the Nakhla meteorite, one of the hundred examples we have of meteorites that originate from the red planet.

Such rocks are invaluable for understanding another world. Squyres describes samples as a “gift that keeps on giving” since analysis can be repeated as instruments and techniques improve. However, our current sources for Martian samples are not able to give us a full picture of the planet. It is a drawback that is making Squyres extremely excited about the Martian Moon eXploration (MMX) mission.

Squyres is a pioneer in Mars research. He is deeply involved with NASA missions to our neighbour, from global examinations with the Mars Reconnaissance Orbiter, to science based on the Martian surface with the Mars Science Laboratory and exploration of the terrain with the Mars Explorations Rover (MER) mission. The MER mission is perhaps where Squyres is most renowned, being the principal investigator for the science performed by the twin Spirit and Opportunity rovers that landed on Mars in 2004. Both rovers shot past their planned lifetime of 90 Martian days, with Spirit continuing to 2010 and Opportunity still active this year, holding the record for the longest wheeled drive on Mars. The rovers found geological evidence water once ran on Mars, from discovering “blueberries” of hematite minerals that typically forms in the presence of water to crossbeds of overlapping rock caused by watery flows. These projects have revealed a planet of incredible geologic diversity, with a plethora of rock types that can each tell a different story about Mars’s history.

Mechanically hard rocks born in volcanoes point to Mars’s geological past; a time when the silent planet may have been as internally active as the Earth. Meanwhile, peeling back the layers of softer sedimentary rocks reveal an ancient environment that might have been habitable.

To understand Mars’s past, we therefore need to study a representative sample of these rocks. This is where it gets tricky.

The rovers are able to analyse rocks on the Martian surface, providing information about the surrounding terrain. But while mobile, rovers can only cover tiny distances.

“No rover,” says Squyres. “has investigated rocks that are in any way representative of Mars.”

Martian meteorites such as Nakhla can originate from all over the red planet’s surface. However, the trip is not an easy journey.

To reach Earth, the rock must survive being ejected from Mars’s surface at more than 5 km/s; the speed required to escape Mars’s gravitational pull. It must then travel for millions of years in space before being shock-heated into a shooting star or fireball as it falls through our atmosphere.

“Only the strongest survive,” says Squyres, explaining why Martian meteorites are all examples of dense, hard rocks. The smaller rocks and softer sediments disintegrate en-route, leaving us with samples originally globally distributed on Mars but still not representative of the planet’s geology.

But while a meteorite would struggle to survive the trip to Earth, surviving the trip to the Martian moons is another story.

Our Martian Moons eXploration mission plans to visit both Mars’s small moons, Phobos and Deimos, and explore the environment surrounding their parent planet. This autumn, it was confirmed that the MMX spacecraft would gather a sample from Phobos to return to Earth.

While both moons were considered for the sample site, Phobos has a few advantages compared to Deimos. Orbiting closer to Mars than its sibling moon, Phobos feels a stronger pull from the planet’s gravity. Sitting deeper within Mars’s gravitational well makes landing on Phobos a greater technical challenge, but it allows an easy transition over to Deimos should any unforeseen problems arise during the mission.

In terms of the science, Phobos offers another big advantage. The moon should be regularly showered by Martian meteorites.

Impacts from meteorites on Mars eject material into orbit. While only the fastest and biggest rocks may make it to Earth, a far far greater range can make it to Phobos, a mere 6,000 km above the Martian surface.

Phobos itself is also subjected to meteorite hits. Numerical simulations conducted by researchers Kenneth Ramsley and James Head from Brown University in the US suggest these collisions eject material from the small moon, but most will not be able to escape Mars’s gravity and will instead orbit the planet and re-join Phobos.

This process does lead to a size sorting, but a far less dramatic one than caused by the journey to Earth. Grains less than about 300 micrometers feel a force from the Sun’s radiation (a process known as “Poynting-Robertson” drag) and spiral back to the Martian surface. Anything larger than these grains is liable to be found on Phobos’s surface and their origin can be anywhere on Mars.

The result is that when the MMX spacecraft gathers a sample from the moon, it will gather both Phobos and Mars material. As Phobos is made from a very dark substance compared to Mars’s reddish hue, the two rock types should be easy to distinguish. This will provide a geologically varied sample from Mars that originates from sites all over the planet. This —Squyres says— makes it unlike any other collection of Mars rocks.

“You’re going to have Mars rocks!” he concludes. “This will be a tremendously exciting mission.”