A composite image of Mars and its two moons, Phobos (foreground) and Deimos (background).
Credit: NASA/JPL/University of Arizona
The origin of Phobos and Deimos, the two Martian moons, has been a mystery to astronomers. These two bodies are a fraction of the size and mass of the Moon, measuring just 22.7 km (14 mi) and 12.6 km (7.83 mi) in diameter. Both have a rapid orbital period, taking just 7 hours, 39 minutes, and 12 seconds (Phobos) and 30 hours, 18 minutes, and 43 seconds (Deimos) to complete an orbit around Mars. Both are also irregular in shape, leading many to speculate that they were once asteroids that got kicked out of the Main Belt and were captured by Mars’ gravity.
There’s also the theory that Phobos and Deimos were once a single moon hit by a massive object, causing it to split up (aka. the “splitting hypothesis”). In a recent paper, an international team of scientists led by the Institute of Space and Astronautical Science (ISAS) revisited this hypothesis. They determined that a single moon in a synchronous orbit would not have produced two satellites as we see there today. Instead, they argue, the two moons would have collided before long, producing a debris ring that would have created an entirely new moon system.
Running the clock back on the enigmatic pair of Martian moons Phobos and Deimos gives researchers insight to their possible origin.
A recent study provides crucial clues on the possible ‘origin story’ for the two tiny moons of Mars, Deimos and Phobos.
Modern astronomy provides us with a snapshot, a look at the present state of affairs across the solar system… but what were things like in the distant past? The existence of the two tiny moons seen orbiting Mars presents a particular dilemma for astronomers. Close up, Phobos and Deimos resemble tiny misshapen captured asteroids… but how did they get into the neat, tidy orbits that we see today?
Image of the Martian Moon of Deimos, as imaged by the Mars Reconnaissance Orbiter. Credit: HiRISE/MRO/LPL (U. Arizona)/NASA
Mars and Earth have several things in common. Like Earth, Mars is a terrestrial planet (i.e. composed of silicate rock and minerals). It also has polar ice caps, a tilted axis, and evidence of liquid water on its surface. On top of that, Mars and Earth are the only terrestrial planets in the Solar System to have natural satellites.
In fact, Mars has two satellites, which are appropriately named Phobos and Deimos (named after the Greek gods of horror and terror, respectively). Of the two, Deimos is the smaller moon and orbits at a greater distance from the planet. And like Deimos, it has the characteristics of an asteroid, which is a strong indication of where it may have come from.
Discovery and Naming:
Deimos was discovered in 1877 by American astronomer Asaph Hall, who was deliberately searching for Martian moons at the United States Naval Observatory (USNO). Its name was suggested shortly thereafter by Henry Madan, the Science Master of Eton College, and was derived from Homer’s The Iliad.
Phobos and Deimos, photographed by the Mars Reconnaissance Orbiter. Credit: NASA/JPL
Size, Mass and Orbit:
Deimos has a mean radius of between 6 and 6.38 km (3.73 – 3.96 mi). However, the moon is not a round body, and measures roughly 15 × 12.2 × 11 km (9.32 x 7.58 x 6.835 mi), making it 0.56 times the size of Phobos. At 1.4762 × 1015 kg, or 1.4762 trillion metric tons, Deimos is 1/49,735,808 times as massive as the Moon. As a result, Deimos’ surface gravity is very weak, just 0.003 m/s – or 0.000306 g.
Deimos’ orbit is nearly circular, ranging from 23455.5 km at periapsis (closest) to 23470.9 km at apoapsis (farthest) – which works out to an average distance (semi-major axis) of 23,463.2 km. With an average orbital speed of 1.3513 km/s, it takes 30 hours, 18 minutes and 43.2 seconds to complete a single orbit (or 1.263 days).
Composition and Surface Features:
Deimos, like Phobos, is similar in composition to carbonaceous chondrite and silicate/carbon-rich (C- and D-type) asteroids. Though the surface is cratered, it is considerably smoother than Phobos’ surface, which is due to its craters being filled with regolith.
Only two geological features on Deimos have been given names – the craters of Voltaire and Swift. These features take their names from the famous 17th/18th century French and English writers who speculated about the existence of two Martian moons before they were even discovered.
Image of Deimos captured by HiRISE, showing the craters of Voltaire and Swift in the upper left corner. Credit: NASA/JPL/University of Arizona
Origin:
The origin of Mars’ moons remains unknown, but some hypotheses exist. The most widely-accepted theory states that, based on their similarity to C- or D-type asteroids, they are objects that were kicked out of the Asteroid Belt by Jupiter’s gravity. They were then captured by Mars’ and fell into their current orbits due to atmospheric drag or tidal forces.
However, this theory remains controversial since Mars’ current atmosphere is too thin. As such, it is highly unlikely that it would have been able to cause enough drag to slow either moon down enough for them to have achieved their current orbits. A modified version of this hypothesis is that Phobos and Deimos were once a binary asteroid, which was then captured and separated by tidal forces.
Other popular hypotheses include that they were formed by accretion in their current orbits, or that Mars was once surrounded by many large asteroids which were ejected into orbit it after a collision with a planetesimal – like the one that formed Earth’s Moon. Over time, these would have fallen back to the surface until only Phobos and Deimos remained.
Exploration:
Overall, Deimos history of exploration is tied to that of Mars and Phobos. While no landings have been made on its surface, several have been proposed in the past. The first of these were made as part of the Soviet Phobos (Fobos) program, which involved two probes – Fobos 1 and 2 – that were launched in July of 1988.
If the first proved successful in landing on Phobos, the second would been diverted to make a landing on Deimos. However, the first probe was lost en route to Mars while the second managed to returned some data and images of Phobos surface before contact was lost.
In 1997-1998, NASA selected the proposed Aladdin mission as a finalist for its Discovery Program. The plan was to visit both Phobos and Deimos with sample return missions involving an orbiter and lander. After reaching the surface, the landers would collect samples and then launch them back to the orbiters (which would return them to Earth). However, the mission was passed over in favor of the MESSENGER probe, which was sent to study Mercury.
Other missions have been proposed with are still under study. These include the “Hall” concept proposed in 2008, which calls for a probe that relies on solar-electric propulsion (SEP) to reach Mars and return with samples to Earth. Another was the Gulliver mission, a concept proposed in 2010 which would attempt to retrieve 1 kg (2.2 lbs) of material from Deimos’ surface.
The planners behind the OSIRIS-REx mission have also proposed mounting a second mission that would return samples from Phobos and Deimos. And at the 2014 Lunar and Planetary Science Conference, a proposal was made for a low-cost mission based on the Lunar Atmosphere Dust and Environment Explorer. It is named the Phobos and Deimos & Mars Environment(PADME) mission, and would involve an orbiter being sent to Mars by 2021.
Deimos has been photographed from the surface of Mars by both the Opportunity and Curiosity rovers. And someday, actual astronauts may be able to look up at it from the Martian surface. From their point of view, Deimos would appear like a star to the unaided eye. At its brightest, it might look like Venus does from here on Earth.
For those watching over an extended period of time, Deimos would pass directly in front of the Sun quite regularly. It’s too small to cause a total eclipse, it would look like a black dot moving across the face of the Sun.
Astrophotographers are already getting some great images of Mars, such as this sequence captured by Efrain Morales Rivera on January 9th, 2014.
Get those telescopes ready: the coming months offer Earthbound viewers some great views of the planet Mars.
Mars reaches opposition for 2014 on April 8th. This is approaching season represents the best time to observe Mars, as the Red Planet is closest to us in April and rises in the east as the Sun sets opposite to it in the west. Mars reaches 10” in apparent size this week. Mars is already beginning to show surface detail through a moderate-sized telescope as it continues to grow. In mid-February, Mars currently rises at around midnight local, and rides high to the south at local sunrise.
Mars imaged by Leo Aerts on February 3rd. Shot using a Celestron 14″ scope, DMK 21AU618 webcam with a 2.5 powermate projection and a RGB Baader filter set.
The 2014 opposition of Mars offers a mixed bag for observers. Hanging around 5-10 degrees south of the celestial equator just east of the September equinoctial point in Virgo, viewing opportunities are roughly equal for both northern and southern hemisphere observers. At opposition, Mars will shine at magnitude -1.5 and present a 15.2” disk, only slightly larger than the near minimum apparition of 2012, when it appeared 13.9” across. This is a far cry from the historic 2003 appearance, when Mars nearly maxed out at 25.1” across.
Why such a difference? Because the planet Mars has an exceptionally eccentric orbit. In fact, the eccentricity for Mars is 9.3% compared to 1.7% for the relatively sedate Earth.
A decade plus of Mars oppositions, from 2012 through 2025. Graphic created by the author.
This guarantees that all oppositions of Mars – which occur roughly 26 months/780 days apart – are not created equal. In our current epoch, Mars can pass anywhere from 0.683 to 0.373 Astronomical Units (A.U.s) from the Earth. This year’s passage sees Mars overtake us at 0.62 A.U.s or over 96 million kilometres from Earth on the night of opposition. Mars is slightly closer to us at 0.618 A.U.s six nights later on April 14th.
Why the slight difference? Well, the speedier Earth is on the inside track headed towards aphelion in July, while Mars is lagging but headed slightly inward towards perihelion just afterwards in September. This combined motion makes for a slightly closer approach just after opposition until the Earth begins to pull away.
And this also means that Mars will make its apparent retrograde loop through Virgo on the months surrounding opposition:
The motion of Mars through Virgo from March 1st through July 31st. Created by the author using Starry Night Education software.
Now for the good news. Oppositions of Mars also follow a rough 15-year cycle, meaning that they get successively closer or more distant with every two year passage. For example, the 1999 opposition of Mars had a very similar geometry to this year’s, as will to the future opposition in 2029.
And we’re currently on an improving trend: the next opposition in 2016 is much better than this year’s at 18.6” in size, and during the 2018 opposition, Mars will present a disc 24.3” across and will be nearly as favorable as the one in 2003!
It’s also worth noting that Mars sits within four degrees of the rising Moon on the evening of April 14th. The bright star Spica also sits even closer to the Full Moon on the same evening, at less than two degrees away. This particular evening is also noteworthy as it hosts the first of two lunar eclipses for 2014, both of which favor North America.
Mars, the Full Moon and Spica rising in the east on April 14th. Created using Stellarium.
Can you catch Mars near the Moon before sundown on the 14th using binoculars? The Moon will also occult Mars on July 6th for viewers across central and South America.
Though Mars is nicknamed the Red Planet, we’ve seen it appear anywhere from a pumpkin orange to a sickly yellow hue. In fact, such a jaundiced color change can be a sign that a planet-wide dust storm is under way. Such a variation can be readily seen with the naked eye. What color does Mars appear like to you tonight?
On Mars, northern hemisphere summer starts on February 15th, 2014. This means that the northern pole cap of the planet is tipped towards us at opposition during 2014. The day on Mars is only slightly longer than Earth’s at 24 hours and 37 minutes, meaning that Mars will have seemed to rotated only an extra ~8 degrees if you observe it at the same time on each successive evening.
The white pole caps of the planet are the first feature that becomes apparent to the observer at the eyepiece. In February, Mars shows a noticeable gibbous phase in February as we get a peek at the edge of the nighttime side of the planet. Mars will be nearly “full” at opposition, after which it’ll once again take on a slightly distorted football shape.
A growing (& shrinking) Mars through the 2014 opposition season. Created by the author using Starry Night Education software.
Tracking the features of the Red Planet is also possible at moderate magnification. One of the largest features apparent is the dark area known as Syrtis Major. Sky & Telescope has an excellent and easy to use application named Mars Previewer that will show you which longitude is currently facing Earth.
Sketching the regions of Mars is a fun exercise. You’ll find that drawing planetary features at the eyepiece can sharpen your observing skills and give you a more critical eye to discern subtle detail. And this season also provides an excellent reason to turn that newly constructed planetary webcam towards Mars.
Up for a challenge? Opposition is also a great time to try and observe the moons of Mars.
The moons of Mars as seen on April 8th at around 9:00 Universal Time. Created by the author using Starry Night Education software.
Phobos and Deimos are a tough catch, but are indeed within range of amateur instruments. The chief problem lies in their close proximity to dazzling Mars: +11.5 magnitude, Phobos never strays 14” from the Red Planet in 2014, and 12.4 magnitude Deimos never travels farther than 45” away. Phobos orbits Mars once 7.7 hours — faster than the planet rotates beneath it — and Deimos orbits once every 30.3 hours. The best strategy for a successful Martian moon hunt is to either place Mars just out of the field of view at high power when a moon reaches greatest elongation or block it from view using an eyepiece equipped with an occulting bar.
Extra credit for anyone who nabs pics of the pair!
And opposition is also “Visit Mars season,” as MAVEN and India’s Mars Orbiter Mission arrive later this year. In 2016, NASA’s Mars InSight mission is slated to make the trip, and the window is fast-closing for Dennis Tito’s proposed crewed fly-by mission of Mars in 2018.
And finally, to aid you in your quest for those elusive Martian moons, reader and human astronomical calculator extraordinaire Ed Kotapish was kind enough to compile a list of favorable apparitions of the moons of Mars on the weeks surrounding opposition. (see below)
Good luck, and be sure to send in those pics of Mars and more to Universe Today!
ELONGATIONS OF THE MARTIAN MOONS
DATES AND TIMES IN UT
STARTING 3/30/2014
MAR 30
PHOBOS 0300 W
PHOBOS 0645 E
DEIMOS 0900 W
PHOBOS 1040 W
PHOBOS 1425 E
PHOBOS 1815 W
PHOBOS 2205 EMAR 31
DEIMOS 0005 E
PHOBOS 0155 W
PHOBOS 0545 E
PHOBOS 0935 W
PHOBOS 1320 E
DEIMOS 1515 W
PHOBOS 1715 W
PHOBOS 2100 E
APR 01
PHOBOS 0055 W
PHOBOS 0440 E
DEIMOS 0620 E
PHOBOS 0830 W
PHOBOS 1220 E
PHOBOS 1610 W
PHOBOS 2000 E
DEIMOS 2130 W
PHOBOS 2350 W
APR 02
PHOBOS 0340 E
PHOBOS 0730 W
PHOBOS 1115 E
DEIMOS 1235 E
PHOBOS 1510 W
PHOBOS 1855 E
PHOBOS 2245 W
APR 03
PHOBOS 0235 E
DEIMOS 0345 W
PHOBOS 0625 W
PHOBOS 1015 E
PHOBOS 1405 W
PHOBOS 1755 E
DEIMOS 1855 E
PHOBOS 2145 W
APR 04
PHOBOS 0130 E
PHOBOS 0525 W
PHOBOS 0910 E
DEIMOS 1000 W
PHOBOS 1305 W
PHOBOS 1650 E
PHOBOS 2040 W
APR 05
PHOBOS 0030 E
DEIMOS 0110 E
PHOBOS 0420 W
PHOBOS 0810 E
PHOBOS 1200 W
PHOBOS 1550 E
DEIMOS 1615 W
PHOBOS 1940 W
PHOBOS 2325 E
APR 06
PHOBOS 0320 W
PHOBOS 0705 E
DEIMOS 0725 E
PHOBOS 1055 W
PHOBOS 1445 E
PHOBOS 1835 W
PHOBOS 2225 E
DEIMOS 2230 WAPR 07
PHOBOS 0215 W
PHOBOS 0605 E
PHOBOS 0955 W
PHOBOS 1340 EDEIMOS 1340 E (Mutual)
PHOBOS 1735 W
PHOBOS 2120 E
APR 08
PHOBOS 0115 W
DEIMOS 0445 W
PHOBOS 0500 E
PHOBOS 0850 W
PHOBOS 1240 E
PHOBOS 1630 W
DEIMOS 1955 E
PHOBOS 2020 E
APR 09
PHOBOS 0010 W
PHOBOS 0355 E
PHOBOS 0750 W
DEIMOS 1100 W
PHOBOS 1135 E
PHOBOS 1530 W
PHOBOS 1915 E
PHOBOS 2305 W
APR 10
DEIMOS 0210 E
PHOBOS 0255 E
PHOBOS 0645 W
PHOBOS 1035 E
PHOBOS 1425 W
DEIMOS 1715 W
PHOBOS 1815 E
PHOBOS 2205 W
APR 11
PHOBOS 0150 E
PHOBOS 0545 W
DEIMOS 0825 E
PHOBOS 0930 E
PHOBOS 1320 W
PHOBOS 1710 E
PHOBOS 2100 W
DEIMOS 2330 W
APR 12
PHOBOS 0050 E
PHOBOS 0440 W
PHOBOS 0830 E
PHOBOS 1220 W
DEIMOS 1440 E
PHOBOS 1605 E
PHOBOS 2000 W
PHOBOS 2345 EAPR 13
PHOBOS 0340 W
DEIMOS 0550 W
PHOBOS 0725 E
PHOBOS 1115 W
PHOBOS 1505 E
PHOBOS 1855 W
DEIMOS 2055 E
PHOBOS 2245 E
APR 14
PHOBOS 0235 W
PHOBOS 0620 E
PHOBOS 1015 W
DEIMOS 1205 W
PHOBOS 1400 E
PHOBOS 1755 W
PHOBOS 2140 E
APR 15
PHOBOS 0130 W
DEIMOS 0310 E
PHOBOS 0520 E
PHOBOS 0910 W
PHOBOS 1300 E
PHOBOS 1650 W
DEIMOS 1820 W
PHOBOS 2040 E
APR 16
PHOBOS 0030 W
PHOBOS 0415 E
PHOBOS 0810 W
DEIMOS 0925 E
PHOBOS 1155 E
PHOBOS 1545 W
PHOBOS 1935 E
PHOBOS 2325 W
APR 17
DEIMOS 0035 W
PHOBOS 0315 E
PHOBOS 0705 W
PHOBOS 1055 E
PHOBOS 1445 W
DEIMOS 1540 E
PHOBOS 1830 E
PHOBOS 2225 W
APR 18
PHOBOS 0210 E
PHOBOS 0605 W
DEIMOS 0650 W
PHOBOS 0950 E
PHOBOS 1340 W
PHOBOS 1730 E
PHOBOS 2120 W
DEIMOS 2200 E
