Rome, Italy (SPX) Sep 23, 2010
Scientists now have firm indications that the Martian satellite Phobos formed relatively near its current location via re-accretion of material blasted into Mars's orbit by some catastrophic event. Two independent approaches of compositional analyses of thermal infrared spectra, from ESA's Mars Express and NASA's Mars Global Surveyor missions, yield very similar conclusions.
The re-accretion scenario is further strengthened by the measurements of Phobos's high porosity from the Mars Radio Science Experiment (MaRS) on board Mars Express. These results were presented by Dr. Giuranna and Dr. Rosenblatt at the European Planetary Science Congress in Rome, on Monday 20th September.
The origin of the Martian satellites Phobos and Deimos is a long standing puzzle. It has been proposed that both moons may be asteroids formed in the outer part of the main asteroid belt (between Mars and Jupiter) and were subsequently captured by Mars' gravity.
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.
"Understanding the composition of the Martian moons is the key to constrain these formation theories," says Dr. Giuranna of the Istituto Nazionale di Astrofisica in Rome, Italy.
Previous observations of Phobos at visible and near-infrared wavelengths have been interpreted to suggest the possible presence of carbonaceous chondritic meteorites, carbon-rich "ultra primitive" materials, 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," says Dr. 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 from a lander, or sample return would ideally help to settle this issue unambiguously," he added.
Other observations appear to match the types of minerals identified on the surface of Mars. Thus, the derived composition on Phobos appears more closely related to Mars than objects from other relatively 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," says Dr Rosenblatt of the Royal Observatory of Belgium.
The MaRS team, led by Dr. Martin Patzold of the Rheinisches Institut fur Umweltforschungh an der Universitat zu Koln, Germany, has 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. From this the team was able to reduce Phobos's mass.
"We obtained the best measurement of its mass to date, with a precision of 0.3%," relates Dr. Rosenblatt.
Phobos's volume past estimations were also improved thanks to the cameras onboard MEx. The MaRS team was thus able to derive the best-ever estimate of Phobos's density as 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's interior," says Dr. Rosenblatt. "High porosity is required in order to absorb the energy of the large impact that generated Stickney crater without destroying the body", confirms Dr. Giuranna.
"In addition a highly porous interior of Phobos, as proposed by the MaRS team, supports the re-accretion formation scenarios."
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 origin of both Martian moons is not, however, definitively elucidated since the density alone cannot provide the true composition of their interior.
The future Russian Phobos-Grunt mission (Phobos Sample Return), to be launched in 2011, will certainly contribute to our understanding regarding the origin of Phobos.
Share This Article With Planet Earth
Mars News and Information at MarsDaily.com
Lunar Dreams and more
Strong Robotic Arm Extends From Next Mars Rover
Pasadena CA (JPL) Sep 17, 2010
NASA's Mars rover Curiosity has been exercising its robotic arm since last month, when the arm was first fastened to the rover. In the long run, watch for this long and strong arm to become the signature apparatus of NASA's Mars Science Laboratory. After landing in August 2012, the mission will rely on it for repeated research activities. One set of moves crucial to the mission's success h ... read more
Watch Out For The Super Harvest Moon|
Water on Moon is bad news for China's lunar telescope
New Insights Into The Moon's Rich Geologic Complexity
Astrium Investigates Automatic Landing At The Moon's South Pole
Shuttle ready for move to launch pad
NASA To Ship Fuel Tank For Last Planned Shuttle Flight
NASA names 'last' space shuttle crew
Discovery starts first leg of final flight
Russian spacecraft lands safely after delays
International Partners Discuss ISS Extension And Use
Spacecraft with three cosmonauts undocks after delay
Glitch delays space station crew's return to Earth
Japanese Spacecraft Approaches Venus
Recreating Venus In The Lab
The Canadian Space Agency Invests In Concept Studies
Was Venus Once A Habitable Planet
New Views Of Saturn's Aurora
Cassini Gazes At Veiled Titan
Avoid Swimming In Interplanetary Lakes
Spring On Titan Brings Sunshine And Patchy Cloud
NASA's MODIS And AIRS Instruments Watch Igor Changing Shape And Warming Over 3 Days
A Growing La Nina Chills Out The Pacific
GOES-13's Family of Tropical Cyclones: Karl, Igor And Julia
ISRO To Launch Four Satellites In December
Synthetic Life Could Aid Space Exploration
Russia to take space tourists in 2013
Soyuz Spacecraft Upgrade Ups Payload By 70 Kg
Glitch delays space station crew's return to Earth
This Planet Smells Funny
Scientists looking to spot alien oceans
Deadly Tides Mean Early Exit For Hot Jupiters
Can We Spot Volcanoes On Alien Worlds
|The content herein, unless otherwise known to be public domain, are Copyright 1995-2010 - SpaceDaily. AFP and UPI Wire Stories are copyright Agence France-Presse and United Press International. ESA Portal Reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. Advertising does not imply endorsement,agreement or approval of any opinions, statements or information provided by SpaceDaily on any Web page published or hosted by SpaceDaily. Privacy Statement|