Mars’ moon Phobos is a pretty fascinating customer! Compared to Mars other moon Deimos, Phobos (named after the Greek personification of fear) is the larger and innermost satellite of the Red Planet. Due to its rapid orbital speed, the irregularly-shaped moon orbits Mars once every 7 hours, 39 minutes, and 12 seconds. In other words, it completes over three orbits of Mar within a single Earth day.
It’s not too surprising then that during a recent observation of Mars with the Hubble space telescope, Phobos chose to photobomb the picture! It all took place in May of 2016, when while Mars was near opposition and Hubble was trained on the Red Planet to take advantage of it making its closest pass to Earth in over a decade. The well-timed sighting also led to the creation of a time-lapse video that shows the moon’s orbital path.
During an opposition, Mars and Earth are at the closest points in their respective orbits to each other. Because Mars and the Sun appear to be on directly opposite sides of Earth, the term “opposition” is used. These occur every 26 months, and once every 15 to 17 years, an opposition will coincide with Mars being at the closest point in its orbit to the Sun (perihelion).
When this happens, Mars is especially close to Earth, which makes it an ideal occasion to photograph it. The last time this occurred was on May 22nd, 2016, when Mars was and Earth were at a distance of about 76,309,874 km (47,416,757 mi or 0.5101 AU) from each other. This would place it closer to Earth than it had been in 11 years, and the Hubble space telescope was trained on Mars to take advantage of this.
A few days before Mars made its closest pass, Hubble took 13 separate exposures of the planet over the course of 22 minutes, allowing astronomers to create a time-lapse video. This worked out well, since Phobos came into view during the exposures, which led the video showing the path of the moon’s orbit. Because of its small size, Phobos looked like a star that was popping out from behind the planet.
This sighting has only served to enhance Phobos’ fascinating nature. As of 2017, astronomers have been aware of the moon’s existence for 140 years. It was discovered in 1877, when Asaph Hall – while searching for Martian moons – observed it from the U.S. Naval Observatory in Washington D.C. A few days later, he also discovered Deimos, the smaller, outer moon of Mars.
In July of 1969, just two weeks before the Apollo landing, the Mariner 7 probe conducted a flyby of Mars and took the first close-up images of the Moon. In 1977, a year after the Viking 1 lander was deployed to the Martian surface, NASA’s Viking 1 orbiter took the first detailed photographs of the moon. These revealed a cratered surface marred by long, shallow grooves and one massive crater – known as the Stickney crater.
Asaph Hall named this crater after Chloe Angeline Stickney Hall (his wife) after discovering it in 1878, a year after he discovered Phobos and Deimos. Measuring some 10 km in diameter – almost half of the average diameter of Phobos itself – the impact that created Stickney is believed to have been so powerful that it nearly shattered the moon.
The most widely-accepted theory about Phobos origins is that both it and Deimos were once asteroids that were kicked out of the Main Belt by Jupiter’s gravity, and were then acquired by Mars. But unlike Deimos, Phobos’ orbit is unstable. Every century, the moon draws closer to Mars by about 1.98 meters (6.5 feet). At this rate, scientist estimate that within 30 to 50 million years, it will crash into Mars or be torn to pieces to form a ring in orbit.
This viewing is perhaps a reminder that this satellite won’t be with Mars forever. Then again, it will certainly still be there if and when astronauts (and maybe even colonists) begin setting foot on the planet. To these people, looking up at the sky from the surface of Mars, Phobos will be seen regularly eclipsing the Sun. Because of its small size, it does not fully eclipse the Sun, but it does make transits multiple times in a single day.
So there’s still plenty of time to study and enjoy this fearfully-named moon. And while you’re at it, be sure to check out the video below, courtesy of NASA’s Goddard Space Center!
Mars’ natural satellites – Phobos and Deimos – have been a mystery since they were first discovered. While it is widely believed that they are former asteroids that were captured by Mars’ gravity, this remains unproven. And while some of Phobos’ surface features are known to be the result of Mars’ gravity, the origin of its linear grooves and crater chains (catenae) have remained unknown.
But thanks to a new study by Erik Asphaug of Arizona State University and Michael Nayak from the University of California, we may be closer to understanding how Phobos’ got its “groovy” surface. In short, they believe that re-accretion is the answer, where all the material that was ejected when meteors impacted the moon eventually returned to strike the surface again.
Naturally, Phobos’ mysteries extend beyond its origin and surface features. For instance, despite being much more massive than its counterpart Deimos, it orbits Mars at a much closer distance (9,300 km compared to over 23,000 km). It’s density measurements have also indicated that the moon is not composed of solid rock, and it is known to be significantly porous.
Because of this proximity, it is subject to a lot of tidal forces exerted by Mars. This causes its interior, a large portion of which is believed to consist of ice, to flex and stretch. This action, it has been theorized, is what is responsible for the stress fields that have been observed on the moon’s surface.
However, this action cannot account for another common feature on Phobos, which are the striation patterns (aka. grooves) that run perpendicular to the stress fields. These patterns are essentially chains of craters that typically measure 20 km (12 mi) in length, 100 – 200 meters (330 – 660 ft) in width, and usually 30 m (98 ft) in depth.
In the past, it was assumed that these craters were the result of the same impact that created Stickney, the largest impact crater on Phobos. However, analysis from the Mars Express mission revealed that the grooves are not related to Stickney. Instead, they are centered on Phobos’ leading edge and fade away the closer one gets to its trailing edge.
For the sake of their study, which was recently published in Nature Communications, Asphaug and Nayak used computer modeling to simulate how other meteoric impacts could have created these crater patterns, which they theorized were formed when the resulting ejecta circled back and impacted the surface in other locations.
As Dr. Asphaug told Universe Today via email, their work was the result of a meeting of minds that spawned an interesting theory:
“Dr. Nayak had been studying with Prof. Francis Nimmo (of UCSC), the idea that ejecta could swap between the Martian moons. So Mikey and I met up to talk about that, and the possibility that Phobos could sweep up its own ejecta. Originally I had been thinking that seismic events (triggered by impacts) might cause Phobos to shed material tidally, since it’s inside the Roche limit, and that this material would thin out into rings that would be reaccreted by Phobos. That still might happen, but for the prominent catenae the answer turned out to be much simpler (after a lot of painstaking computations) – that crater ejecta is faster than Phobos’ escape velocity, but much slower than Mars orbital velocity, and much of it gets swept up after several co-orbits about Mars, forming these patterns.”
Basically, they theorized that if a meteorite stuck Phobos in just the right place, the resulting debris could have been thrown off into space and swept up later as Phobos swung back around mars. Thought Phobos does not have sufficient gravity to re-accrete ejecta on its own, Mars’ gravitational pull ensures that anything thrown off by the moon will be pulled into orbit around it.
Once this debris is pulled into orbit around Mars, it will circle the planet a few times until it eventually falls into Phobos’ orbital path. When that happens, Phobos will collide with it, triggering another impact that throws off more ejecta, thus causing the whole process to repeat itself.
In the end, Asphaug and Nayak concluded that if an impact hit Phobos at a certain point, the subsequent collisions with the resulting debris would form a chain of craters in discernible patterns – possibly within days. Testing this theory required some computer modeling on an actual crater.
Using Grildrig (a 2.6 km crater near Phobos’ north pole) as a reference point, their model showed that the resulting string of craters was consistent with the chains that have been observed on Phobos’ surface. And while this remains a theory, this initial confirmation does provide a basis for further testing.
“The initial main test of the theory is that the patterns match up, ejecta from Grildrig for example,” said Asphaug. “But it’s still a theory. It has some testable implications that we’re now working on.”
In addition to offering a plausible explanation of Phobos’ surface features, their study is also significant in that it is the first time that sesquinary craters (i.e. craters caused by ejecta that went into orbit around the central planet) were traced back to their primary impacts.
In the future, this kind of process could prove to be a novel way to assess the surface characteristics of planets and other bodies – such as the heavily cratered moons of Jupiter and Saturn. These findings will also help us to learn more about Phobos history, which in turn will help shed light on the history of Mars.
“[It] expands our ability to make cross-cutting relationships on Phobos that will reveal the sequence of geologic history,” Asphaug added. “Since Phobos’ geologic history is slaved to the tidal dissipation of Mars, in learning the timescale of Phobos geology we learn about the interior structure of Mars”
And all of this information is likely to come in handy when it comes time for NASA to mount crewed missions to the Red Planet. One of the key steps in the proposed “Journey to Mars” is a mission to Phobos, where the crew, a Mars habitat, and the mission’s vehicles will all be deployed in advance of a mission to the Martian surface.
Learning more about the interior structure of Mars is a goal shared by many of NASA’s future missions to the planet, which includes NASA’s InSight Lander (schedules for launch in 2018). Shedding light on Mars geology is expected to go a long way towards explaining how the planet lost its magnetosphere, and hence its atmosphere and surface water, billions of years ago.