Milestone on Mars

Print edition : September 07, 2012

Landing ellipse in the Gale Crater, an overhead view with north at the top.-NASA/JPL-CALTECH

NASA lands a robotic rover precisely and softly on the red planet to study past and present processes on its surface.

On August 6, at 05:17:57.3 hours UTC (Coordinated Universal Time), the National Aeronautics and Space Administrations (NASA) Mars mission, called the Mars Science Laboratory (MSL), spectacularly delivered to Mars a robotic rover, Curiosity, by landing it precisely and softly within a kilometre of the targeted landing site in the Gale Crater of the red planet. The touchdown not only was very precise but was achieved by using a hitherto untested sky-crane technique (see box). Launched on November 26, 2011, aboard the launch vehicle Atlas V 541 from Cape Canaveral, Florida, at 15:02:00.0 hrs UTC, its journey across the earth-Mars distance (at the time of landing) of 248 million km had lasted about 254 days. The expected operational lifetime of the primary mission on Mars surface is a full Mars year, which is equivalent to 687 earth days, or 669 Martian days, which are called Sols. But this could be more thanks to the long-lasting power supply based on a radioisotope thermoelectric generator (nuclear battery) that produces electricity from the heat of radioactive decay of 4.8 kg of plutonium-238.

According to a NASA fact sheet, the overarching scientific objective of the mission is to assess whether the landing area and the area Curiosity will explore have ever been a potential habitat for Martian life (essentially microbial) their habitability and evidence of any life in rock record. These studies form part of a broader investigation of past and present processes in Mars atmosphere and on its surface. These will include the study of the Martian climate and geology, while at the same time gathering information for a future manned mission to Mars.

Every environment on the earth containing liquid water has supported microbial life, and microbes form most of the living matter on the earth. So, scientists expect that any life, if it exists or existed at all, will be microbial. Follow the water remains the essential strategy of the MSL mission as well, as it has been for all NASAs Mars exploration missions since the mid-1990s. Since organic or carbon-containing compounds are important for life, the mission is designed to detect these and categorise them as well. Thus it adds follow the carbon component to follow the water theme.

The red "X" marks the spot where NASA's Curiosity rover landed on Mars. Early estimates made immediately after the rover landed (green diamond) indicated that it touched down about 2.4 km from the point it was targeting, to the left and out of sight on this graphic. This is well within the targeted landing region, called the landing ellipse, marked by the light blue line. After landing, images from the High Resolution Imaging Science Experiment on NASA's Mars Reconnaissance Orbiter were able to pinpoint the rover's precise location, as shown by the red "X".-NASA/JPL-CALTECH

According to NASA, six main areas of study will contribute to the overall scientific objective: (1) Determining Martian surface mineralogy and near-surface geology; (2) Detecting essential biochemical building blocks of life (biosignatures); (3) Interpretation of the evolutionary processes responsible for the observed nature of Martian rocks and soils; (4) Profiling the evolution of the Martian atmosphere which at present chiefly comprises carbon dioxide (95.3 per cent), nitrogen (2.7 per cent) and argon (1.6 per cent) over four billion years since the formation of the solar system; (5) Establishing the current state and distribution of water and carbon cycles on the planet; and (6) Characterising the different components of surface radiation, including galactic radiation, cosmic radiation, solar proton events, and secondary neutrons, and determining its broad spectrum. (Towards this, on-board instruments have already measured the radiation exposure to the spacecraft during its journey to Mars.)

Curiosity is almost twice as long (about three metres) and at 899 kilogram is five times as heavy as NASAs twin Mars Exploration Rovers (MERs), Spirit and Opportunity, which were launched in 2003. It has inherited many design elements and technologies from them, including the six-wheel drive, a rocker-bogie suspension system, and cameras mounted on a mast for the mission teams on the earth to choose targets for exploration and driving routes. Curiosity carries the most advanced payload of scientific equipment ever used on Mars surface. The payload is equipped with a suite of 10 scientific instruments and a 2.1 m robotic arm that can drill into rocks, scoop up soil and deliver samples to the test chambers inside the rover.

In addition to its sample-acquisition capabilities, Curiosity will carry an RTG power pack, computers, inertial measurement unit (IMU) for navigation and devices for communication (in both X and ultra high frequency, or UHF, bands). It has the capability to drive for 20 kilometres or more during the missions lifetime of 98 weeks. Besides the science payload on board, sensors on the heat shield that were discarded during the entry, descent and landing (EDL) phase of the spacecraft has already gathered data about Mars atmosphere and the spacecrafts performance during its descent.

This image shows changes in the target landing area. The larger 25 km x 20 km ellipse was the target area before early June 2012, when the project revised it to the smaller 20 km x 7 km ellipse centred closer to the foot of Mount Sharp.-NASA/JPL-CALTECH

In April 2004, NASA invited proposals for scientific investigations and instruments to be put on board the MSL. In accordance with the planned objectives of the mission, eight proposals were selected and, in addition, NASA entered into an agreement with Russia and Spain for carrying instruments provided by them. The 10 instruments on the MSL together weigh about 76 kg compared with the combined weight of 5 kg of five instruments aboard Spirit and Opportunity. The mass of one of the instruments, called sample analysis at Mars (SAM), alone is 40 kg, four times the total weight of Sojourner, NASAs first rover used in its 1997 mission.

SAM is a suite of instruments meant to analyse atmospheric samples and materials collected and delivered by the robotic arm. It includes a gas chromatograph, a mass spectrometer and a tunable laser spectrometer. With their combined capabilities, a wide range of organic compounds can be identified. Isotope ratios are signatures to the evolution of water and atmosphere on Mars. The SAM instruments can also determine the ratios of different isotopes of key elements.

Samples delivered by the robotic arm will also be analysed by the chemistry and minerology (CheMin) X-ray diffraction and X-ray fluorescent instrument. It is designed to measure bulk composition and identify and quantify the minerals in rocks and soils. ChemCam is a suite of remote sensing instruments, which include a laser-induced breakdown spectroscopy (LIBS) system, being flown for the first time for planetary exploration, and a remote micro-imager (RMI). ChemCam can use a laser pulse to vaporise thin layers of material from Martian rock and soil targets up to 7 m away, whose constituent atoms LIBS can identify. A detailed image of the area illuminated by the laser beam can be taken by the RMI. The laser and the RMI are placed on top of the rovers mast. This data will be used by researchers, together with data from the other cameras, to choose targets to be analysed by other instruments.

This full-resolution image, obtained about three seconds after the heat shield separation and about two and a half minutes before touchdown, shows the heat shield during the rover's descent to the surface of Mars on August 6. The image was obtained by the Mars Descent Imager instrument known as MARDI and shows the 4.5-metre diameter heat shield when it was about 16 metres from the spacecraft.-NASA/JPL-CALTECH

The Alpha particle X-ray spectrometer (APXS), also located on the arm, is designed to quantify the relative abundance of different elements in the Martian rocks and soils. The radiation assessment detector (RAD) is an instrument for assessing the radiation environment on the Mars surface. This information will form the input for planning human exploration of Mars and assessing the planets ability to sustain life.

The Russian Federal Space Agency provided the dynamic albedo of neutrons (DAN) to measure subsurface hydrogen up to one metre below the surface. Detection of hydrogen is a clue to the presence of water molecules in the form of ice or bound to other minerals. The Spanish Ministry of Education and Science has supplied the rover environment monitoring station (REMS), which is a kind of meteorological package to measure atmospheric pressure, temperature, winds and ultraviolet radiation levels. An equipment called sample acquisition/sample preparation and handling system includes tools to remove dust from rock surfaces, scoop up soil, drill into rocks and gather powdered samples from rocks and sort samples by particle size and finally deliver these to the analysing instruments on board Curiosity.

In all Curiosity actually carries 17 cameras, which include MastCam, Mars Hand Lens Imager (MAHLI) and MSL Mars Descent Imager (MARDI). Of these, MARDI has been used during the descent. Its high-definition colour images and video, taken with 1.3 millisecond exposure time starting from a height of 3.7 km to 5 m from the ground, are already available. MAHLI is mounted on the rover arm and is designed to take extreme close-ups of rocks, soil and, if present, ice, with a resolution smaller than the width of a human hair (50 micrometre). It will also be able to focus on objects that hard for the rover and its arm to reach. The MastCam mounted at about the height of the human eye will take pictures of the rovers surroundings in high resolution stereo and colour. It also has the capability to take and store high-definition video images. It will also view materials collected by the arm. In addition to the imaging requirements for science, the rover also has a black and white stereo navigation camera (NavCam) and a low-slung stereo hazard avoidance camera (HazCam).

The Gale Crater, which lies in the equatorial region at 4.50 S latitude, was chosen by NASA scientists to serve as the laboratory for the mission from a list of 60 potential sites. The site offers a visually dramatic landscape and also great potential for significant science findings, said Jim Green, Director of NASAs Planetary Science Division. There were some engineering constraints as well that led to Gale as the choice.

The crater is believed to have materials washed down from its wall. From the data obtained from earlier orbiter missions, the landing site is also known to contain a very bright-coloured dense type of rock, which is unlike any rock previously studied on Mars. It may be the first target for investigation. Within the crater is a mountain named Aeolis Mons (Mount Sharp), which rises about 5.5 km above the crater floor. It consists of layered rocks, and its stratification suggests that the mountain is a surviving remnant of an extensive series of deposits that were laid down after a massive impact that excavated Gale Crater more than three billion years ago. The layers may contain a historical record of environmental conditions when each stratum was deposited, including minerals that form in water.

An artist's concept shows the sky-crane manoeuvre during the descent of Curiosity. The rover was connected to the descent stage by three nylon tethers and by an umbilical providing a power and communication connection. The bridle extended to full length, about 7.5 metres, as the descent stage continued descending. When touchdown was detected, the bridle was cut at the rover end, and the descent stage flew off to stay clear of the landing site.-

An area of great interest for scientists lies at the edge of the landing site. Instruments carried by past orbiters have detected signs of clay minerals and sulphate salts. The region also has an alluvial fan that is likely to have been formed by water-carried sediments. According to one hypothesis about Martian geology, these minerals reflect changes in the Martian environment, the amount of water on the Martian surface in particular. Perhaps, these minerals are traps for organic compounds potential biosignatures of life and are protecting them from oxidation.

The guided entry technology that enabled Curiosity to land more precisely than previous Mars missions, coupled with its navigating ability, meant that the main science destination for the mission could be outside of the area that would have otherwise been considered safe for landing. The Gale Crater is about 154 km in diameter and the targeted landing area within it was a 20 km x 7 km ellipse. The landing ellipse, with a 99 per cent probability of landing within it, had originally been set as 20 km x 25 km, which is about one-third the size of landing ellipses for earlier MERs.

However, during the MSLs journey to Mars, continuing analysis of variables of the EDL phase led to confidence in even higher precision in landing. This allowed mission planners to shrink the target area to an almost 20 km x 7 km ellipse. Even with the smaller ellipse, Curiosity could touch down at a safe distance from steep slopes at the edge of Mt. Sharp. In the coming months, Curiosity will drive to science destinations on Mt. Sharp, outside of the landing ellipse. The trimming of the landing ellipse has meant that the landing was closer to Mt. Sharp and the distance that the rover will have to travel to reach Mt. Sharp is reduced by almost half.

The science targets identified for the rover to investigate initially lie in the lower layers of the mountain. According to NASA, getting to key targets at the lower layers of Mt. Sharp may consume a large part of the prime mission period. The route may require the rover to move through some difficult terrains such as sand dunes, hills and canyons. The rover has been engineered to roll over obstacles up to 65 cm high and travel up to 200 m a day. If Curiosity continues to work properly after the 98-week period, it might begin to explore the younger layers of Mt. Sharp in an extended mission.

The MSL mission by itself, as the NASA background document notes, is not designed to answer the open question whether life has existed on Mars. Curiosity will not carry out investigations to detect processes indicative of present-day biological metabolism. It also does not have the ability to image micro-organisms or their fossil equivalents.

In some sense, by assessing whether the Gale Crater has had environmental conditions suitable for habitability and preservation of evidence of life, the MSL is a prospecting mission for future missions that may carry advanced instruments for detecting life.

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