A little geochemical laboratory on Mars

THE PHOENIX SPACE PROBE: A LITTLE GEOCHEMICAL LABORATORY ON MARS

* Un pequeño laboratorio geoquímico en Marte

As meticulously planned, at 23:54 on Sunday 25 May 2008 (Universal Time) a small robotic laboratory was installed on the surface of Mars, at 68 degrees north latitude and 233 degrees longitude. Named Phoenix, because it the successor of an earlier, failed project, it is a marvel of modern engineering and must answer several questions posed by scientists in relation to environmental conditions of that world, hundreds of millions of kilometers from home.

Our neighbor Mars, surely, will be the first planet to be visited by humans, mainly because it is the planet of the Solar System most similar to Earth.

Today Mars is a giant desert, but apparently it was not always so. In recent decades, various space probes (most of them orbiters) have sent us data showing that apparently in the distant past liquid water existed on its surface. This necessarily means that the Martian climate was more benign, with higher temperatures and a thicker atmosphere, able to hold water molecules together in liquid phase, not only as vapor or crystallized into ice as it is today.

If this pleasant climate was conducive to the emergence of life as we know it is something we still do not know. The answer depends on whether the necessary chemical conditions were also in place or not, such as the presence, in addition to H₂O, of complex carbon-based molecules (the so-called organic molecules) which unfortunately are very fragile and easily destroyed by ultraviolet light from the Sun (remembering that Mars has no ozone layer) or by possible adverse chemical reactions upon contact with minerals on the soil itself.

Enters Project Phoenix, the winner of a NASA grant to carry out part of this research in 2008. The enviable economic progress of the United States of America has allowed them the luxury of leading this show, seen around the world (and which have already had almost five years of preparation), at a price of 1 dollar and 38 cents for each inhabitant of this nation. It also has the support of Canada (which came with $ 1,11 per capita), and Germany, Switzerland, Finland and Denmark.

EVERYTHING READY FOR THE ARRIVAL OF THE PHOENIX PROBE TO MARS

Finally, after a trip of more than 10 months, on Sunday 25 May 2008 the U. S. space probe Phoenix would arrive to Mars. If all would go well, it would become the sixth device constructed by humans to convey important scientific data from the neighboring planet's surface, after (among a dozen failures) Viking 1 and Viking 2, in 1976, Mars Pathfinder in 1997, and Spirit and Opportunity in 2004. This was the first time a landing would occur on the icy polar regions, with the consequent expectation of what would be found.

The mission had an estimated cost of $ 457 million, funded primarily by the National Aeronautics and Space Administration (NASA). Scientific instruments were the responsability of the Jet Propulsion Laboratory of the California Institute of Technology (Caltech), United States of America; Washington University in Saint Louis, United States of America; the Max Planck Institute for Solar System Research, Germany; the University of Arizona, United States of America; Texas A & M University, Texas, United States of America; the University of Texas at Dallas, United States of America; the University of Neuchatel, Switzerland; the Search for Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, United States of America; NASA's Ames Research Center, United States of America; Tufts University, United States of America; York University, Toronto, Canada; the Finnish Meteorological Institute; the University of Copenhagen, Denmark; and the University of Aarhus, Denmark.

The direction of the project was the responsability of the Lunar and Planetary Laboratory, University of Arizona. Technical responsibility for the spacecraft was upon the private company Lockheed Martin Space Systems, and navigation and communications were by the Jet Propulsion Laboratory of Caltech.

The unmanned spacecraft, of 664 kg total mass and the size of a car, would be arriving at a relative velocity of 20 500 kilometers per hour, i.e., 5,7 kilometers per second. It should lose all this tremendous speed in just six and a half minutes, counted from the moment of contact with the first layers of the atmosphere of the alien planet.

A thick, heavy ceramic heat shield would do the first part, protecting the spacecraft from the brutal initial shock against the Martian air, absorbing the tremendous heat generated by the friction, at hypersonic speed, with the molecules of the atmosphere. The shield would burn white hot and lose several inches of its thickness, as if it were an artificial meteorite.

In its passage from the 125 km altitude to approximately 12,6 kilometers, the tremendous slowdown (up to 9,2 g) would drop the speed of the spacecraft to about 1,7 times the speed of sound, safe enough to deploy the parachute. Then, the loss of speed would continue in this way for another three minutes.

It was expected that at a height of 1 km above the surface the speed would had dropped to about 200 kilometers per hour. From this point on, the parachute would be insufficient to ensure a soft landing, because the atmosphere of Mars is actually very rarified, only 1% of the pressure that we have here on Earth. That's why computers would now assume the control of the spacecraft through 12 small rocket engines, which would do the rest of the braking.

The automatic landing on its three small legs would be, if all have gone well, at 68 degrees north latitude, in the zone of the "midnight sun", a region of planet Mars never before visited.

Because of the distance between Earth and Mars, which in the landing night would have been about 276 million kilometers in a straight line, any radio communication with the spacecraft would take 15 minutes and 20 seconds for the message to go and 15 minutes and 20 seconds for the answer to come back, so that necessarily, the spacecraft should make all these dramatic moves by itself, following a program previously stored in its computers. This also meant that we would have knowledge if all worked out just right (or wrong) only 15 minutes and 20 seconds after each event.

Below is a table with the sequence of the major milestones on the date of arrival. (All times are in Coordinated Universal Time [UTC], and refer to the time of reception of the radio signal here on Earth):

15:46 Last chance to correct the trajectory.

23:16 Pressurization (for pumping) of the engines' propellant tanks.

23:38 The U.S. probe Mars Odyssey (in orbit around Mars since 2001) begins to relay data from the Phoenix spacecraft to NASA antennas at Goldstone, California, United States of America.

23:38 The radio telescope in Green Bank, West Virginia, United States of America, begins to hear the signal from the UHF auxiliary antenna in the rear shield of the Phoenix.

23:39 The Phoenix's auxiliary cruise module (with tanks, engines, solar panels, transmitters, etc., used in the journey from Earth to Mars) separates and is discarded.

23:40 Computers rotate the spacecraft so that the heat shield is in the proper position for reentry.

23:47 The spacecraft begins to enter the atmosphere of Mars.

23:47 to 23:49 The plasma (hot electrified gas) formed around the spacecraft interrupts radio communications. Maximum heat and deceleration.

23:50 The hypersonic phase ends and the speed has fallen to supersonic. The parachute is deployed.

23:50 The heat shield separates, unusable after strong ablation.

23:51 The craft's 3 landing legs extend.

23:51 The onboard radar is activated, to measure height and speeds relative to the ground.

23:53 The spacecraft separates from the parachute and back shield.

23:53 A second auxiliary UHF antenna on the spacecraft goes online and takes over communications.

23:53 Descend engines accelerate.

23:54 Velocity stabilizes, spacecraft is now moving slowly.

23:54 Landing.

23:55 Radio transmitter is turned off temporarily.

00:13 Solar panels are opened; meanwhile, the radio is silent.

00:28 The U.S. spacecraft Mars Reconnaissance Orbiter (in orbit around Mars since 2006) relays data recorded by Phoenix during entry, descent and landing.

00:30 The European Space Agency's Mars Express (in orbit around Mars since 2003) relays data recorded by Phoenix during the entry, descent and landing.

00:30 Technicians determine the health of the spacecraft after landing.

1:43 to 2:02 In its second pass over the landing region, the Mars Odyssey orbiter relays engineering data, and if possibe, the first images of the surface taken by Phoenix's main camera. The first things programmed to appear in the monitors should be the solar panels, to confirm that they unfolded properly. Later, views of Mars.

4:00 Press conference by the team, which, fortunately, and as expected, occurred in a party-like ambiance.

Welcome to Mars, Phoenix.

TOOLS AND DETECTORS

Unlike the last three previous missions that landed on Mars in 1997 and early 2004, this latter probe had no wheels: was fixed in the same place without being transported over the Martian terrain. However, the object of its studies was not so much what was on the ground, but what was underneath. Since long ago has been speculated, and by that time there were already signs, that in the subsurface of Mars, especially in the subsurface of frozen polar regions, there are enormous amounts of H₂O (and who knows, maybe the famous complex carbon molecules were found there too).

That's why Phoenix was equipped with an aluminum and titanium, 2,35-m-long robotic arm (sterile, like the rest of the spacecraft), capable of digging small trenches up to half a meter deep. If a hard layer of ice is found, on the tip of the arm there was a scraping power tool that allowed it to loosen flakes.

There was also a miniature camera with sufficient magnification to detect details finer than a human hair, with a lighting system beaming different colors to highlight different substances in the subsoil. If higher magnification was needed, the robot arm loaded samples in a special carousel installed inside the spacecraft, so constructed that separated the different types of particles, and then passed them through two microscopes, one optical and one with a reliefs sensor by nanoneedle, with a 100-nm resolution.

Permanent magnets attached at different points in the deck of the spacecraft studied the magnetic properties of dust blown by the winds.

At the tip of the arm there were also four electrodes, arranged in a fork, that pierced into the soil at different levels in the corresponding pit, to measure the electrical conductivity, temperature, and the capability of the ground to absorb heat.

The works of the robot arm were monitored by a main camera that could take pictures in three dimensions. This camera was mounted on top of a folding, 2-meter-high mast, which rose from Phoenix's main body once landed. I had two "eyes", each with a resolution of 1024 pixels by 1024 pixels, similar to the sharpness provided by the human eye. It could rotate 360 degrees, to form panoramic images, and could also bend down to examine samples and up to study the Martian clouds. One peculiarity is that it was equipped with lenses with special filters, interchangeable, with 12 finely-calibrated different colors, to highlight different minerals and chemicals in the Martian soil and rocks, and even in the atmosphere.

CHEMICAL LABORATORY

The robot arm was tipped with a special spoon with which it was responsible for taking carefully-selected samples from the surface, in order to place them onboard the main body of the spacecraft, where special equipment that allowed detailed chemical analysis were installed.

One of these devices focused on studying the minerals. In it, the sample fell into a small container filled with liquid water (brought from Earth), where a paddle mixed them until it was dissolved as much as possible, at controlled temperature to combat freezing. On the inner walls of this "artificial tongue" 26 electrical sensors were installed, most of them coated with films or gels made of different chemicals. Each one of these different films let through only one type of chemical compound to the corresponding sensor; the activation of a given sensor or sensor group was an indication of the presence of a given chemical in the soil sample. Also, in a period of a couple of days, a mechanism throwed pills of different chemical compounds to the muddy water, to see what happened; so, by the chemical reactions that ensued, substances that have been missed in the first attemps could be identified.

Phoenix had only four of these containers, so unfortunately it could not analyze more than four different samples.

The robot arm also carried samples to another device, specialized in detecting organic compounds. Inside this, by means of miniature electric ovens, the ground was cooked very, very carefully. Special sensors measured exactly how much heat was absorbed by the sample as the temperature rose. Different substances absorb heat in different ways; also, in the melting or vaporization of any frozen substance, the heat absorption rate changes. This could give an idea of what was mixed in the Martian soil.

The vapors that were separating from each sample were taken to a mass spectrometer, which identified the molecules and even atoms types by observing their movements when subjected to electromagnetic forces.

On the other hand, by a careful re-examination of how these vapors were being released, one could tell much about the ground that contained them.

This "artificial nose" had a second inlet port, through which the composition of the Martian atmospheric air could be analized directly.

There were eight miniature ovens, each of which could be used only once, so scientists had to be very careful in choosing each of the samples.

WEATHER STATION

Phoenix also served as a weather station, reporting the weather in the Vastitas Borealis arctic plain where it was placed. A short and seemingly simple flexible tube, fitted on top of a thin mast 2,2-m in height, indicated by its deformation (recorded by photos taken by the main camera) the strength and direction of the wind. Along the vertical rod, near the top, in the middle and at the bottom, there were three very sensitive thermometers that did a profile of the heat distribution around the surface when the sun was shining warmly or when the night hours approached. A barometer measured atmospheric pressure, and the "fork" of four electrodes at the tip of the robotic arm measured the relative humidity of the Martian air, when the device was exposed to the wind.

To measure the cloud ceiling and the amount of suspended dust, a laser was fired vertically: any returning reflection was captured by a special electronic eye that calculated the transparency of the air at all times.

The spacecraft was designed for a minimum useful lifespan of 90 days on the surface of Mars, but it lasted five months. As the Martian winter approached, the daily hours of sunlight (at landing, with the "midnight sun", they were 24 h 40 min) were less and less, and the moment arrived when they were insufficient to generate a minimum level of electricity in the solar panels, needed by the heating system in order to keep Phoenix in operation. Thus, the end of the 6-month-long Martian arctic summer also meant the end of the mission. Scientists, however, still have many years of work to analyze and interpret all the accumulated scientific data collected during those months of intense activity. And surely, the answers they would find will bring new questions along.

Aldo Loup.

Originally published in ABC Color, on 27 July 2008. Illustration: Phoenix performed experiments until the arrival of the terrible Martian "polar night". Paradoxically, the presence of deposits of frozen H₂O could make the region habitable. Credit: Corby Waste for NASA / JPL-Caltech / University of Arizona.