New Theory Says Phobos Formed From Re-Accretion of Impact Debris

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Most theories on the formation of Phobos and its sister moon of Mars, Deimos, hold that the two moons did not form along with Mars, but were captured asteroids. However, new research indicates that Phobos formed relatively near its current location via re-accretion of material blasted into Mars’ orbit by some catastrophic event, such as a huge impact. This could be an event similar to how Earth’s moon formed. Thermal infrared spectra data from two Mars missions, ESA’s Mars Express and NASA’s Mars Global Surveyor have provided independent researchers similar new conclusions of how Phobos formed.

The origin of the two Martian satellites has been a long standing puzzle. Previous researchers have postulated that because of Phobos small size and highly cratered surface, as well as the fact that Mars is reasonably close to the asteroid belt, that Phobos was a captured asteroid. Recently, alternative scenarios suggested that both moons were formed in-situ by the re-accretion of rocky-debris blasted into Mars’s orbit after a large impact or by re-accretion of remnants of a former moon which was destroyed by Mars’s tidal force.

Today, Dr. Giuranna from the Istituto Nazionale di Astrofisica in Rome, Italy, and Dr. Rosenblatt from the Royal Observatory of Belgium presented their new findings at the European Planetary Science Congress in Rome, saying that the thermal data from the two spacecract, as well as the measurements of Phobos’ high porosity from the Mars Radio Science Experiment (MaRS) on board Mars Express, supports the re-accretion scenario.

“Understanding the composition of the Martian moons is the key to constrain these formation theories,” said Giuranna.

Spatial locations of TES and observations of Phobos used for the compositional analysis. Credit: Giuranna and Rosenblatt

Previous observations of Phobos at visible and near-infrared wavelengths suggest the possible presence of carbonaceous chondritic meteorites, carbon-rich and likely from the early formation of the solar system, commonly associated with asteroids dominant in the middle part of the asteroid belt. This finding would support the early asteroid capture scenario. However recent thermal infrared observations from the Mars Express Planetary Fourier Spectrometer, show poor agreement with any class of chondritic meteorite. They instead argue in favor of the in-situ scenarios.

“We detected for the first time a type of mineral called phyllosilicates on the surface of Phobos, particularly in the areas northeast of Stickney, its largest impact crater,” said Giuranna. “This is very intriguing as it implies the interaction of silicate materials with liquid water on the parent body prior to incorporation into Phobos. Alternatively phyllosilicates may have formed in situ, but this would mean that Phobos required sufficient internal heating to enable liquid water to remain stable. More detailed mapping, in-situ measurements froma lander, or sample return would ideally help to settle this issue unambiguously.”

But other observations appear to match up with the types of minerals identified on the surface of Mars. From that data, Phobos appears more closely related to Mars than objects from other locations in the solar system.

“The asteroid capture scenarios also have difficulties in explaining the current near-circular and near-equatorial orbit of both Martian moons,” said Rosenblatt.

The MaRS instrument used the frequency variations of the radio-link between the spacecraft and the Earth-based tracking stations in order to precisely reconstruct the motion of the spacecraft when it is perturbed by the gravitational attraction of Phobos, and from this, the team was able provide most precise measurement of Phobos’ mass, with a precision of 0.3%.

Additionally, the team was able to give the best estimate yet of Phobos’s volume, with a density of 1.86±0.02 g/cm3.

“This number is significantly lower than the density of meteoritic material associated with asteroids. It implies a sponge-like structure with voids making up 25-45% in Phobos’ interior,” said Rosenblatt.

“High porosity is required in order to absorb the energy of the large impact that generated Stickney crater (the large crater on Phobos) without destroying the body,, said Giuranna. “In addition a highly porous interior of Phobos, as proposed by the MaRS team, supports the re-accretion formation scenarios”.

The researchers said a highly porous asteroid would have probably not survived if captured by Mars. Alternatively, such a highly porous Phobos can result from the re-accretion of rocky-blocks in Mars’ orbit. During re-accretion, the largest blocks re-accrete first because of their larger mass, forming a core with large boulders. Then, the smaller debris re-accrete but do not fill the gaps left between the large blocks because of the low self-gravity of the small body in formation. Finally, a relatively smooth surface masks the space of voids inside the body, which then can only be indirectly detected. Thus, a highly porous interior of Phobos, as proposed by the MaRS team, supports the re-accretion formation scenarios.

The researchers said they would like more data on Phobos to verify their findings, and the upcoming Russian Phobos-Grunt mission (Phobos Sample Return), scheduled for launch in 2011, will help to provide more understanding regarding the origin of Phobos.

Source: Europlanet Conference

Mars Webcam Provides Astronaut-like View of Red Planet

What would it be like to approach Mars in a spacecraft? In one of the coolest movies ever, we now know! Using the the Visual Monitoring Camera (VMC) on board Mars Express, science teams put together 600 individual still images to create a movie of descending towards and then moving away from Mars. It shows the spacecraft’s slow descent from high above the planet, speeding up as closest approach is passed and then slowing down again as the distance increases.
Continue reading “Mars Webcam Provides Astronaut-like View of Red Planet”

New Images of Phobos from Mars Express Flyby

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ESA released new images of Mars’ moon Phobos, taken during the Mars Express March 7, 2010 flyby, showing the rocky moon in exquisite detail and also in 3-D. Mars Express orbits the Red Planet in a highly elliptical, polar orbit that brings it close to Phobos every five months, and it is the only spacecraft currently in orbit around Mars whose orbit reaches far enough from the planet to provide a close-up view of Phobos. Like our Moon, Phobos always shows the same side to the planet, so only by flying outside the orbit is it possible to observe the moon’s far side. Mars Express did such flybys on March 7, 10 and 13. Get out your 3-D glasses for a great look at Phobos, below.

Phobos in 3-D. taken by the High Resolution Stereo Camera (HRSC) onboard the ESA spacecraft Mars Express. Credits: ESA/DLR/FU Berlin (G. Neukum)

Phobos is an irregular body measuring some 27 × 22 × 19 km. Its origin is debated. It appears to share many surface characteristics with the class of ‘carbonaceous C-type’ asteroids, which suggests it might have been captured by Mars. However, it is difficult to explain either the capture mechanism or the subsequent evolution of the orbit into the equatorial plane of Mars. An alternative hypothesis is that it formed around Mars, and is therefore a remnant from the planetary formation period.

The Phobos/Grunt landing site, as seen by Mars Express on March 7, 2010. Credits: ESA/DLR/FU Berlin (G. Neukum)

In 2011 Russia will send a mission called Phobos–Grunt (meaning Phobos Soil) to land on Phobos, and an experiment will collect a soil sample and return it to Earth for analysis.

Phobos-Grunt will also carry with it The Planetary Society’s LIFE experiment which will test the survivability of microorganisms in the conditions of deep space. The experiment is a study of the panspermia hypothesis, which posits that microorganisms have traveled between planets sheltered deep inside space rocks.

For operational and landing safety reasons, the proposed landing sites were selected on the far side of Phobos within the area 5°S-5°N, 230-235°E. But new HRSC images showing the vicinity of the landing with better illumination from the Sun that previous images, which will provide valuable views and information for mission planners.

Mars Express will continue to encounter Phobos until the end of March, when the moon will pass out of range. During the remaining flybys, the high-resolution camera and other instruments will continue to collect data.

Source: ESA

Could Phobos Be Hollow?

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Back in the 1950s and 1960s, there was some speculation that Mars’ moon Phobos could possibly be hollow due to the its unusual orbital characteristics. While scientists now agree that the moon is very likely not hollow, vast caverns may exist within the moon, and it might be a porous body instead of solid. The Mars Express spacecraft made a close flyby of Phobos on Wednesday to help provide more data on the interior of Phobos, and all indications are the event was a big success. The spacecraft skimmed smoothly over the odd-shaped moon at just 67 km, the closest any manmade object has ever been. No images were taken from this flyby. Instead all the instruments were turned off so that ground stations could listen for a pure radio signal of how Phobos “tugged” on the spacecraft. Scientists say the data collected could help unlock the origin of Phobos and other ‘second generation’ moons.

“Phobos is probably a second-generation Solar System object,” said Martin Pätzold, Universitat Koln, Cologne, Germany, and Principal Investigator of the Mars Radio Science (MaRS) experiment. Second generation means that it coalesced in orbit after Mars formed, rather than forming at the same time out of the same birth cloud as the Red Planet. There are other moons around other planets where this is thought to have been the case too, such as Amalthea around Jupiter.

Previous flybys of Phobos have shown that it is not dense enough to be solid all the way through. Instead, it must be 25-35% porous. This has led planetary scientists to believe that it is little more than a ‘rubble pile’ circling Mars. Such a rubble pile would be composed of blocks both large and small resting together, with possibly large spaces between them where they do not fit easily together.

The March 3rd flyby was close enough to give scientists the best data yet about the gravitational field of Phobos.

The radio waves travel at the speed of light and took 6 minutes 34 seconds to travel from Earth to the spacecraft on Wednesday night, and by analyzing the data on Phobos’ gravity field, scientists should be able to estimate of the density variation across the moon and detect just how much of Phobos’ interior is likely to be composed of voids.

This flyby was just one of a campaign of 12 Mars Express flybys taking place in February and March 2010. For the previous two, the radar was working, attempting to probe beneath the surface of the moon, looking for reflections from structures inside. In the coming flybys, the Mars Express camera will take over, providing high resolution pictures of the moon’s surface.

Source: ESA

Follow Closest Flyby of Phobos in Real Time

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Mars Express will skim over the surface of Mars’ largest moon on Wednesday, making the closest flyby of Phobos by any spacecraft. Passing at just 67 km above the surface, precise radio tracking will allow researchers to virtually peer inside the mysterious moon. You can follow the flyby in “real time,” — allowing for the current 6 minute and 30 second light time delay from Mars (13 minutes round trip) – on the Mars Express blog. The flyby will take place on March 3, at 20:55 GMT.

The straight-line distance between Mars Express and Earth is now about 116 million km.

Flying by at such close range, Mars Express will be pulled ‘off-course’ by the gravitational field of Phobos. This will amount to no more than a few millimeters every second and will not affect the mission in any way. However, to the tracking teams on Earth, it will allow a unique look inside the moon to see how its mass is distributed throughout. Phobos’ shape is 27 km × 22 km × 19 km, and has a mass of 1.072 x 1016 kg, or about one-billionth the mass of Earth.

To make the very sensitive measurements of Phobos’ interior, all the data signals from the spacecraft will be turned off. The only thing that the ground stations will listen out for is the ‘carrier signal’ – the pure radio signal that is normally modulated to carry data.

With no data on the carrier signal, the only thing that can modulate the signal is any change in its frequency caused by Phobos tugging the spacecraft. The changes will amount to variations of just one part in a trillion, and are a manifestation of the Doppler effect – the same effect that causes an ambulance siren to change pitch as it zooms past.

Two dress rehearsals for this exacting operation have already taken place, allowing ground station personnel and spacecraft controllers to practice.

Originally, the closest flyby was going to only 50 km above the surface, but a slight ‘over performance’ during a maneuver last week had put the spacecraft on a trajectory that included an occultation by Phobos. This meant that Mars Express would pass behind Phobos as seen from Earth. As this would jeopardize the tracking measurements, it was decided to perform another maneuver to position the flyby at a slightly higher altitude than originally planned.

An illustration showing the ESA's Mars Express mission. Credit: ESA/Medialab)

Mars Express will zoom past Phobos seven more times after Wednesday’s closest approach. The first planned High Resolution Stereo Camera (HRSC) observations will be on March 7, when the spacecraft will be at 107 km altitude above Phobos.

In addition to the tracking experiment, known as MaRS for Mars Radio Science, the MARSIS radar has already been probing the subsurface of Phobos with radar beams. “We have performed a preliminary processing of the data and the Phobos signature is evident in almost all the data set,” says Andrea Cicchetti, Italian Institute of Physics of Interplanetary Space, Rome, and one of the MARSIS team.

Source: ESA