360 degree panorama from NASA's Phoenix Mars lander at Vastitas Borialis, Mars. At the top just left of the center is the Telltale wind sensor provided by Aarhus University and in the green frame one of three radiometric calibration targets provided by the Niels Bohr Institute, University of Copenhagen is shown just after landing. | University of Arizona, NASA/JPL, Texas A&M University, NBI/UCPH
During a design review meeting in Denmark with NASA officials, Danish scientists were asked if they could build some kind of wind indicator onto these calibration targets, but discussions showed that one such sensor on a high mast on the lander would be preferable. Haraldur Páll Gunnlaugsson, Institute for Physics and Astronomy at Aarhus University volunteered to take responsibility for development and delivery of a Telltale wind sensor for the Phoenix mission.
Using the MECA suite of instruments among many other results, the oxidant, which since the Viking missions was known to be present in the Martian soil, was very surprisingly shown to be a perchlorate, at high temperatures an aggressive oxidant. Using the Telltale wind sensor a set of very good wind measurements was acquired. Investigation of airborne dust on this mission provided indications that airborne dust in the North Polar region is brighter than that found by the rovers in the equatorial regions. In spite of the very close proximity of water-ice investigation of soil particles brought to the (magnetic) microscopy substrates of the MECA microscopes revealed a distinct set of sub-populations with a particle size distribution indicative of extremely little previous interaction with liquid water. And last, but not least: Phoenix did succeed in proving that water-ice is present below a thin layer of soil in the polar regions of Mars.
The scientific principal investigator, Steven Squyres, had invited Denmark to provide a small suite of magnet instruments for the investigation of airborne dust on this mission. Positioned close to the radiometric calibration targets of the rovers were sweep magnets designed to answer the question of the abundance of magnetic – and maybe in particular non-magnetic – particles in the Martian dust. As it turned out, surprisingly little dust was able to enter into a small area "protected" by the magnetic field from a strong ring magnet just below the aluminium surface of the sweep magnet instrument.
Magnetic Properties Experiments on board NASA's Mars Exploration Rovers, Spirit and Opportunity. Magnets in front of the camera mast collected dust for analysis by the main camera, Pancam, and by instruments on the robotic arm. Next to the radiometric calibration target (right) is a very strong ring magnet which collected magnetic dust and thus kept a small area in the center relatively free of dust. | NASA/JPL, Cornell University, Mainz University, Honeybee Robitics og NBI/UCPH
Results from measurements by the Mössbauer spectrometers on dust collected on Danish magnet instruments on the rovers disclosed that the rather strong magnetic susceptibility of the dust (and soil) on Mars is caused by the presence of magnetite in the dust particles. Also present in the particles were crystallites of the mineral olivine, a mineral which is not stable in the presence of water during the long time scales characteristic of planetary evolution.
So, although the rovers were set to “follow the water” and indeed succeeded in finding rock minerals with unequivocal proof of being formed in liquid water, it was concluded that the magnetite in the dust was of dry (volcanic) origin and that the formation of the dust had little if anything to do with water. Analysis of the elemental composition of dust collected by the magnets showed a strong correlation between iron, titanium and chromium in the dust corroborating the interpretation of a volcanic origin of the magnetite present in the dust. The simple interpretation is that major components of the dust are formed by comminution of rocks on the surface by eons of temperature fluctuations and frost.
After Tycho Brahe's original, meticulous and careful observations of Mars the planet has been systematically observed since the advent of the telescope. Christian Huygens was the first to measure the rotational period of Mars to about 24 hours and he crudely sketched the presence of polar ice cap on the southern part of the planet. Cassini calculated the rotational period to just under 24 hours and 40 minutes and discovered and identified the polar ice caps on Mars. Later William Herschel suggested the caps be made of water-ice or snow.
During some very favorable oppositions of Mars in 1887 – 1891 (periods of time, where the Earth and Mars are very close to each other in their orbits) the close proximity allowed him to do very detailed observations of what he described as “canali”, i.e., natural water channels. This was erroneously translated into English as “canals” and the idea arose that intelligent life there tried to survive by salvaging water by use of vast canal-systems. Although not accepted by most professional astronomers it was not until NASA's flyby in 1965 by Mariner 4 that craters similar to those on the Moon were seen in numbers which would prohibit the possibility of extensive erosion by wind and weather (and hence water) and with this the idea of intelligent life on Mars was gone. However, microbial life could not be excluded and in the late 1960ties and early 1970ties NASA developed the Viking missions; two twin space probes each with a very capable orbiter and a lander equipped to look for biology immediately below the top surface of the soil on Mars.
Danish meteorologist Søren E. Larsen was the first Dane to become heavily involved in the interpretation of data from Mars. Søren Larsen worked on wind data from the Viking landers launched in 1976 by NASA and later also on NASA's Mars Pathfinder mission, the first Mars-mission to land using airbags. In the 1980ies Jens Martin Knudsen, astrophysicist from the Niels Bohr Institute, became interested in a group of meteorites, SNC-meteorites, now called Mars-meteorites because they come from Mars. Because of this interest, Jens Martin Knudsen read all he could find about minerals on Mars, including results of a Magnetic Properties Investigation on the Viking landers. This investigation inspired him to use permanent magnets in an attempt to improve our knowledge about the mineral(s) responsible for the magnetism of soil and dust on Mars.
Jens Martin Knudsen played a leading role in putting Denmark back as a participant in the exploration of Mars through his inspiration of NASA scientists, who as a result invited him to participate in missions to Mars. The impact of Jens Martin Knudsens work was acknowledged by NASA in 2015 when a spectacular ridge in Marathon Valley at the edge of Endeavour crater on Meridiani Planum, Mars, was named after him: The image shows Knudsen Ridge as seen by the Opportunity rover.
Establishment of the Mars-group at the Niels Bohr Institute and NASA's Mars Pathfinder Mission.
As an expert in the technique of Mössbauer spectroscopy Jens Martin Knudsen in 1992 suggested using this technique on the surface of Mars. The aim would be to learn about the influence of water on the development of the Martian surface as reflected in the phases of (oxidized) iron-compounds present in soil and dust. It was during a presentation of the potential of using Mössbauer spectroscopy on Mars that the idea of the magnet array caught the attention of NASA scientists. When asked how this idea could be of practical use Jens Martin Knudsen referred to Robert B. Hargraves, who was the principal investigator on the Magnetic Properties Investigation on the Viking landers. A fax from Robert Hargraves from Princeton University initiated some laboratory simulation experiments, the formulation of an official proposal for and development of a prototype instrument, a new magnetic properties experiment, to be used on future missions to Mars.
Eventually a further developed version of this prototype instrument was flown on NASA's Mars Pathfinder mission in 1997 as an addition to the stereo camera, Imager for Mars Pathfinder (IMP).
Magnetic Properties Experiments on board NASA's Mars Pathfinder mission in 1997. Two “magnet arrays” collected atmospheric dust. Results indicated that the dust was formed by precipitation in liquid water. | Mars Pathfinder: NASA/JPL, University of Arizona og NBI/UCPH
Initial results from magnetic properties experiments on Mars Pathfinder showed that the dust particles captured on the magnetic properties instruments were moderately magnetic, spectrally indistinguishable from bright surface material and presumably composite of nature (i.e. each particle seemed to consist of numerous crystallites), some of which were iron oxides or oxy-hydroxides responsible for the color while other iron oxides were responsible for the magnetic properties. This structure of the particles pointed to an origin resulting from precipitation in liquid water.
Referencer
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