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Mars Exploration – a long Danish tradition

Andet forskningsprojekt |19/01/2017

NASA/JPL og Malin Space Science Systems

The Carlsberg Foundation Special Research Project: “A Field Trip 250 million km from home – Salary for Essential Expert and Materials and Parts for Danish Experiments on NASA’s 2020 Mars Mission”.

The possible existence of life on Mars has always attracted scientific interest. It is now clear that vast amounts of water are present on Mars today in the form of ice and that, in the past, liquid water was present for substantial periods of time. The Niels Bohr Institute (NBI) has been an integral part of the research leading to this knowledge through contributions to all Mars landers and rovers since Mars Pathfinder. The crucial unresolved question is of course whether seemingly favorable conditions actually also did lead to the rise of life?

The NBI Mars-group has been invited to participate in three experiments with legal right to immediate access to all scientific data as it is returned from Mars by NASA’s next Mars rover mission. Its purpose is to find, characterise, collect and store the first set of samples that will be returned to Earth by a follow-up mission. The goal is to find samples which will tell us about the mineralogical context for possible early life on Mars and thus inform us of whether life ever evolved on Mars. With the grant from the Carlsberg Foundation the salary for a key group member is secured and also the delivery of space flight hardware to this 1.9 billion dollar mission is secured. If support is continued this will give Danish scientists and students a direct influence on surface operations as well direct access to its scientific results. 

Curiosity, NASA's Mars Science Laboratory Mission and a Habitable Environment on Mars

15 years of data from orbiting imagers, spectrometers and other instruments allowed the Curiosity Mission in 2012 to select a landing site with a history of complex fluvial activity. As soon as 27 sols into the mission the first conglomerate was found – a rock with the first rounded pebbles ever detected by any landed mission. Analysis of pebbles from this and other conglomerates showed that the rounding was caused by the pebbles being transported in a river, which once led into a lake. The work invested in landing site selection paid off with the monumental discovery of a past

habitable environment on Mars in the so-called Yellowknife Bay deposit in Gale Crater. The word habitable signifies an environment that could potentially have sustained bacterial life in a form familiar to us. 

”Selfie” by Curiosity at the John Klein drill site in Yellowknife Bay, Gale crater, Mars. The selfie is assembled by many individual images acquired by the MArs HandLens Imager, MAHLI around sol 177 of the mission. Analysis of drill sample showed presence of sulfates and phyllosilicates indicative of an ancient habitable environment in a now dry lake-floor at this location | NASA/JPL og Malin Space Science Systems

Moreover, analysis of measurements from a suite of instruments on board the rover showed that liquid water is possible on the present day Martian surface in the form of brines. Scientists on the Curiosity team have been wondering whether microbial life might be able to survive in such brines and this has already led to proposals for experiments to be flown on later missions. One such example is an experiment called HABIT for Habitability, Brine Irradiation and Temperature package now selected for inclusion on the Landing Platform of ESAs ExoMars 2020 Mission.

The crucial unresolved question about Mars' past habitability is of course whether these seemingly favorable conditions for the development of life in fact also did lead to the rise of life? 

NASA's Mars 2020 Rover Mission, the first mission to find and cache samples for later return to Earth.

In 2013 scientists Kjartan Kinch and Morten Bo Madsen, from the Niels Bohr Institute, were invited as Co-Investigators on three out of seven experiments on NASA's Mars 2020 Rover Mission. For the main scientific camera, Mastcam-Z, they were asked to deliver a Calibration Target and associated Calibration Routines. Interpretation of reflectance spectra relies critically on the comparison with observations of an on-board radiometric calibration target. A major challenge for correct calibration is the presence of Martian airborne dust. Dust deposition on color reference targets has to be mitigated and/or corrected to produce valid reflectance spectra. One of the most important tasks for our research group on the 2020 mission is to design and construct a “deep space flight qualified calibration target”.

NASA's Mars 2020 Rover with the Mastcam-Z radiometric calibration target supplied by Denmark. | NASA/JPL, Arizona State University, Cornell University og NBI/UCPH

Our expertise in this connection is the use of permanent magnets to mitigate problems with dust deposition and development of codes to correct for any dust present on these calibration surfaces.  This important contribution is now secured by the grant from the Carlsberg Foundation.

For the Calibration target for another instrument, SuperCam, the group will deliver a set of five calibration target substrates. SuperCam provides information about both elemental composition from Laser Induced Breakdown Spectroscopy (LIBS) and about mineralogy from Raman and infrared reflectance. The chance to employ this powerful combination in a dedicated analysis of air-borne dust is likely to strongly advance our knowledge about Martian dust. Finally, Morten Bo Madsen is also a Co-Investigator on the MOXIE experiment, which for the first time will produce oxygen directly from the Martian atmosphere by solid oxide electrolysis.

Proof of Water-Ice in the shallow sub-surface at the Phoenix Landing Site.

While the Mars Exploration rovers were being planned and built, results from a gamma-ray spectrometer and neutron spectrometers on board NASA's Mars Odyssey orbiter had shown that water in permafrost extended far beyond both northern and southern polar layered deposits on Mars. This led Peter Smith of University of Arizona to propose to pull the Mars Surveyor lander, which was cancelled in 2001, back out of the storage, refurbish it and fly it to the North Polar Region in order to confirm the presence of water ice and investigate its nature. 

Peter Smith won the competition for a Scout sized mission and the Phoenix lander was launched 4th of August 2007. Based on the surprising result of the sweep magnet experiment Peter Smith invited Denmark to supply a set of radiometric calibration targets, iSweeps, for the Surface Stereo Imager of the Phoenix lander as well as the magnetic substrates developed earlier for the microscopy station. 

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 robotic field geologists, Mars Exploration Rovers, Spirit and Opportunity, “Follow the Water”.

After Mars Pathfinder, The next opportunity to continue and extend investigation of Martian dust arose with the Mars Exploration Rovers, Spirit and Opportunity, landing on 4 January 2004 in Gusev crater, which is believed to be an ancient crater lake, and on Meridiani Planum on 25 January 2004, where the mineral hematite had been detected from orbit. Each of these rovers carried as part of their scientific payload a Mössbauer spectrometer built by Göstar Klingelhöfer from Germany and an elemental particle analyzer based on detection of X-rays excited by alpha-particles, an alpha particle X-ray spectrometer, APXS. 

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.

Mars Exploration – a long Danish tradition

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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