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Planetary News: Mars (2007)

The Empire Strikes Back

Europe’s First Trip to Mars Brings Home "The Gold"

By A.J.S. Rayl
March 16, 2007

Just a few years ago, Mars Express seemed to many Americans to be little more than an ambitious dream for Europe. Now, three and a half years later, the mission has become an undeniable part of Mars exploration history and the European Space Agency (ESA) recently announced it will orbit on for at least another couple of years.

Mars Express
Mars Express
Artist's conception of Mars Express in orbit
Credit: Courtesy ESA

"This is the first European planetary mission ever and we're all a little surprised everything is still working so well," Mars Express Project Scientist Agustin Chicarro told The Planetary Society last week. "With every big mission to Mars, it seems more is gained than incremental knowledge," he added, reflecting on the last few years. "Now with Mars Express -- and the Mars Exploration Rovers to provide ground truth -- we are taking the next step.”

Europe's first planetary mission is a stellar success. Mars Express has earned its place in the elite club of Mars explorers, returning textbook-changing data and hundreds of breathtaking images revealing the Red Planet in a way we've never quite seen before. It has also inspired future European pursuits, including a rover mission called ExoMars set to look for life in 2011.

"We're not in any mood to shut it down," ESA Director of Science David Southwood hinted prior to the vote. Actually, considering all of the mission's achievements, it was hardly surprising that ESA’s Science Programme Committee on February 23 unanimously approved funding Mars Express into May 2009. The mission is currently in the middle of its first extension period, which pushed its lifetime from November 2005 to October 2007. The second extension just approved begins when the first extension ends.

Mars Express, which was approved by the European Space Agency's Science Programme Committee in 1997, is the first in a series of “express” ESA ventures -- developed more quickly than typical large NASA mission -- faster, better, cheaper, done right, with technologies that would transfer to future European missions. The plan for this mission was to study the Red Planet from the top of its atmosphere to below the surface. It would be the first mission ever to take scientists thousands of meters beneath the surface of Mars to look for water and ice with state-of-the-art radar and the first recent mission to carry instruments designed to look specifically for life, past or present for at least one Martian year (687 Earth days).

Mars Express and Beagle 2
Mars Express and Beagle 2
Artist's conception of the Beagle 2 lander leaving the Mars Express spacecraft for transfer to the surface of Mars.
Credit: ESA. Illustration by Medialab.

Like the American and Russian missions before it, Mars Express adopted the "Follow the Water" theme. Understanding the history and fate of water on Mars is a key to learning whether Mars ever supported life or could support life, because all known life depends on liquid water.

At a projected cost to ESA of 165 million Euros, however, Mars Express seemed almost too financially reasonable, even though the instruments were to be developed and financed independently by member countries. There was also the fact that Europe’s primary claim to fame in space exploration was its mission to Halley’s comet in 1985 and the reality that more spacecraft to Mars have failed than succeeded.

But Europe had the one asset it absolutely had to have: desire.

Earth's last view of Beagle 2
Earth's last view of Beagle 2
The bright spot on the left side of this image is Beagle 2, which had been released by Mars Express and was drifting ahead of Mars Express in its trajectory toward Mars. Taken on December 19, 2003 at 08:33 UTC, just minutes after the release, this is the last view of Beagle 2 before its crash landing on Mars.
Credit: Courtesy ESA

By 2003, the project -- having suffered more than its share of modifications -- was clearly a "go," and with a lander. Somehow, it met all its deadlines. "It was a bloody fast mission," recalled M-Ex Project Manager Rudi Schmidt as the mission was readied for launch. Right on schedule, Mars Express was launched on June 2, 2003, aboard a Soyuz rocket from Baikonur, Kazakhstan. It carried seven science instruments and the United Kingdom’s lander. Named for the expeditionary ship that ferried Charles Darwin to the Galapagos, Beagle 2 was to search for signs of extraterrestrial species on the surface of Mars.

As the spacecraft neared the Red Planet more than six months later, Beagle 2 separated from the mothership to coast for five days, then enter the Martian atmosphere. It was scheduled to land Christmas morning and signal with a riff from a rock song. Anticipation was global. Beagle 2 and its "father," Principal Investigator Colin Pillinger, of the Open University, had captured hearts around the world. But no signals were ever heard. In January 2004, the “puppy” was declared lost. Although Beagle’s silence left many grieving that day, ESA’s mothership pressed on.

Just hours later, Mars Express slipped into orbit flawlessly, and the world cheered.

Unlocking Clues to Past Water

Even before Mars Express reached its final science orbit at the end of January 2004, the mineralogical mapping spectrometer -- known as OMEGA (Observatoire pour la Mineralogy, l’Eau, les Glaces et l’Activité) -- confirmed the presence of vast fields of perennial water-ice stretching out from Mars' south pole, contributing to the continually evolving story of water on Mars.

OMEGA discovers water-rich minerals in Marwth Vallis
OMEGA discovers water-rich minerals
In this HRSC 3-D image of the Marwth Vallis area (shades of grey), OMEGA has mapped the water-rich minerals, denoted here in blue. No hydrated minerals or sediments have been detected, either in the channel or in its opening. However, the outflow was so violent that it eroded and exposed ancient hydrated clay-rich minerals, dating back to an early era when water was present.
Credit: Courtesy ESA / OMEGA / HRSC

OMEGA went on to uncover hydrated sulfate salts in various areas around the planet's surface, findings that support the Mars Exploration Rover Opportunity's discovery that the Meridiani Planum area of ancient Mars had liquid water on or immediately beneath the surface. Then, at the American Geophysical Union (AGU) meeting in December 2005, OMEGA scientists announced their discovery of evidence of past water on Mars' surface in deposits of clay minerals and other phyllosilicates. The data “unambiguously” revealed the presence of hydrated surface minerals that contain water in their crystalline structure, according to Principal Investigator Jean-Pierre Bibring, of the Institut d’Astrophysique Spatiale, in Orsay, France.

OMEGA only detected the clay minerals, known to form during long-term exposure to water, in the oldest regions of Mars. That led the team to hypothesize that any large bodies of standing water were gone within half a billion years of the planet’s formation, either disappearing from the surface by seeping underground or being lost into space. During the evaporation process, sulphates were created, and when this process naturally concluded and the remaining water became permanently frozen, the atmosphere gradually turned the soil red by creating another mineral the instrument detected—ferric oxide.

From the discovery of these minerals and their locations, Bibring and some of his colleagues hypothesized, controversially, that Mars has undergone three distinct geologic eras and concluded that the planet became a cold, dry, hostile environment a long, long time ago. It is possible the clay beds formed underground by another mechanism, such as volcanoes or, perhaps, the natural cooling of the planet. If that’s the case, “the surface conditions may always have been cold and dry,” Bibring said.

Marwth Vallis: a choice site to search for life
Marwth Vallis: A choice site to search for life
The clay beds observed by OMEGA once may have been supportive environments for the development of life, making them a logical, choice site for future rovers to explore int he search for life. This HRSC image is a false-color view of Mawrth Vallis region, with an artist's depiction of a future rover superimposed.
Credit: Courtesy ESA / OMEGA

Either way, the identification of these clay beds on Mars provides high-priority targets for future landers looking for evidence of life, because where there is water on Earth, there is life. “If living organisms formed, the clay material would be where this biochemical development took place,” he rationalized, “and the cold Martian conditions could have preserved most of the record of biological molecules up to the present day.”

Where Did the Water Go?

What happened to all the water that once flowed across the surface remains one of the biggest Martin mysteries. To help unravel that mystery and to find out how strongly the interplanetary plasma and electromagnetic fields affect the Martian atmosphere, one team of Mars Express scientists is taking measurements with ASPERA-3 (the Analyzer of Space Plasma and Energetic Atoms) to study how the solar wind (the stream of ions and electrons racing outward from the Sun) interacts with the Martian upper atmosphere and the planetary wind (the outflow of particles from the atmosphere and ionosphere).

ASPERA-3 scientists use a technique known as energetic neutral atom imaging to visualize the charged and neutral gas environments around Mars, thus actually see plasma escaping the planet. By measuring ions, electrons, and energetic neutral atoms in the outer Martian atmosphere to find the number of oxygen and hydrogen atoms (the constituents of water) that interact with the solar wind, the scientists can observe the slow, “invisible” escape of volatile gases and liquid compounds that make up the atmosphere and hydrosphere of a planet. The data from this “solar wind scavenging” allows estimation of how much water the planet would have lost over its 4.5 billion years at the current rate of loss.

Water-ice at Vastitas Borealis
Water-ice at Vastitas Borealis
This HRSC image, which is centred at 70.17º North latitude and 103.21º East longitude, provides a perspective view of residual water ice on the floor of Vastitas Borealis Crater on Mars.
Credit: Courtesy ESA / DLR / FU Berlin (G. Neukum)

In its first year of operation, ASPERA-3 confirmed that a very efficient process is at work in the Martian atmosphere, which actually could one that could help resolve the mystery of the missing water, according to Rickard Lundin, ASPERA-3's principal investigator from the Swedish Institute of Space Science, Kiruna, Sweden. Mars no longer has a global magnetic field to deflect the solar wind, so the deluge of charged particles from the Sun is free to interact unhindered with atoms of atmospheric gas and sweep them out to space. The solar wind penetrates Mars' ionosphere deeply into the atmosphere, Lundin and colleagues found, down to an altitude of 270 kilometers (168 miles). That may be the reason for the acceleration processes that cause the significant loss of atmosphere on Mars, they suggest, and probably is a key factor in the loss of a lot of water from the planet some 3.8 billion years ago, assuming the magnetic field was as low there then as it is today.

A Keen Eye on Mars

Amid all the new data are High Resolution Stereo Camera (HRSC) pictures that reveal the Martian topography in exquisite detail. The camera has returned a bounty of breathtaking views never before shot, from an orbit of 260-300 kilometers (161-186 miles). Originally developed for the Mars ’96 mission, the updated HRSC features an unprecedented pointing accuracy, achieved by combining images at two different resolutions, and a capacity to image in 3-D.

A frozen sea?
A frozen sea?
This HRSC image shows a perspective view of a possible dust-covered frozen sea near the Martian equator. The image is centred at 5.46º North latitude and 150.30º East longitude.
Credit: ESA / DLR / FU Berlin (G.Neukum)

By late winter of 2007, the HRSC had imaged 35 percent of Mars’ surface in stereo and in color at a resolution of 10 to 20 meters. “By the end of 2007, we will have achieved approximately 50 percent,” said HRSC Principal Investigator Gerhard Neukum, of the Freie Universität Berlin, Germany. His team has also used the HRSC to take selected close-ups of chosen areas at a two-meter resolution, allowing scientists to pick out "great detail" on the surface.

HRSC data are aiding in the “Follow the Water” quest, offering visual support for OMEGA’s findings and adding to the increasing body of evidence pointing to bodies of liquid water in Mars’ distant past. The pictures of Kasei Valles, one of the biggest outflow channel systems on Mars, for example, indicate this feature was carved by a gigantic Martian glacier that persisted for a billion years, during the time when the temperature of Mars had dropped too low for liquid water to flow across the surface.

The HRSC has also returned compelling pictures of volcanic areas at the north pole, where cliffs more than a mile high encompass fields of dark volcanic ash, and of other fields with cones up to 600 meters tall that hint of fairly recent volcanic activity. These images helped lead the team to propose that there is “a clear link” between volcanic regions and water flows. It appears that when volcanic activity occurred on Mars, it melted ice and heated water deep inside the planet that then flowed to the surface. Intriguingly, some of these flows are recent, at least in geological terms -- within the last 30 million years, according to Neukum.

Some of the camera’s images have revealed current residual patches of frozen water, homing in on water-ice at the planet’s south pole, and even a frozen expanse of water. At ESA’s Mars Express conference in March 2005, scientists announced they had found a five-million-year-old frozen sea near the planet’s equator on the flat, dust-covered plain known as Elysium Planum. John Murray and colleagues at the U.K.'s Open University reported a geographic feature that looks something like an Antarctic ice pack, measuring 810 by 900 kilometers (500 by 560 miles), covered by a layer of volcanic ash that has kept it from evaporating. The images will be scrutinized by future mission leaders looking for scientifically rich landing locations, if only because it is remotely possible that the first Martian life to be found made or makes ice its habitat.

Looking Beneath the Surface

Water underground
Water underground
Artist's impression of water under the Martian surface. Credit: Courtesy ESA / Illustration by Medialab

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument is finding other choice sites for future rovers to explore as it characterizes the surface and searches for signs of water and geologic formations below the surface. "The first sounder and the only instrument designed to investigate the surface and the subsurface of another planet," as MARSIS Principal Investigator Giovanni Picardi, of the University of Rome ‘La Sapienza,’ describes it, the radar looks for water by picking up reflections of radio waves in the upper 4–5 kilometers (2.5–3 miles) of Martian crust, using techniques similar to those used in oil prospecting on Earth.

MARSIS uses the reflections to map the distribution of water and ice in the upper portions of the crust, which scientists then interpret as subsurface structure. Deployment of the radar’s three antennas was delayed by about a year for safety reasons, but within days of the instrument's checkout in June 2005, the radar vicariously took scientists thousands of meters beneath Mars’ surface to look for ice, water-ice, permafrost layers, or even liquid water.

As the spacecraft passed over the northern lowlands in the Chryse Planitia region at Mars’ mid-latitudes, MARSIS uncovered a buried impact crater at a shallow depth of 1.5 to 2.4 kilometers (0.9 to 1.5 miles). Inside this circular structure, about 250 kilometers (155 miles) wide, is a thick layer of material rich in water-ice. Previous observations revealed water-ice in the area, but MARSIS data suggest that this is a rather large chunk --1.8 kilometers (1.1 mile) thick. Continued research has uncovered more intriguing layers. "We are finding reservoirs of ice that have never been seen before," Jeffrey Plaut, co-principal investigator, of Jet Propulsion Laboratory (JPL) reported. "But we are still puzzling out when and where the water on Mars was liquid."

Bright lower echo from Mars' south-pole
Bright lower echo from Mars' south-pole
The upper image of this composite is a ‘radargram' from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). It shows data from the subsurface of Mars in the water-ice-rich layered deposits that surround the south pole of the planet. The lower image shows the position of the ground track of the spacecraft (indicated by a white line) on a topographic map of the area based on data from the MOLA laser altimeter on board NASA's Mars Global Surveyor. The images are 1250 kilometers wide. MARSIS Principal Investigator Giovanni Picardi, of the University of Rome 'La Sapienza', and Co-Principal Investigator Jeffrey Plaut, of JPL, and 22 other researchers report analysis of the instument's south polar observations in the online edition of the journal Science this week. Credit: Courtesy NASA / JPL / ASI / ESA / Univ of Rome/ MOLA Science Team

When the radar probed the layered deposits that surround the north pole of Mars, it detected what scientists believe is a nearly pure water-ice layer thicker than 1 kilometer (0.6 mile) over a deeper layer of basaltic regolith. While polar-layered deposits hold most of the known water on modern Mars, other areas of the planet do appear to have been very wet at times in the past.

MARSIS is also looking at geologic features underground. "It's almost like having X-ray vision," Thomas R. Watters, of the National Air and Space Museum's Center for Earth and Planetary Studies, said of MARSIS after studying the data to better understand the planet's history.

In contrast to Earth, Mars shows a striking difference between its northern and southern hemispheres. Almost the entire southern hemisphere has rough, heavily cratered highlands, while most of the northern hemisphere is smoother, lower in elevation, and more plains like. It appears now, in the MARSIS data, that ancient impact craters lie buried beneath the smooth, low plains of Mars' northern hemisphere and that some of the subtle topographic depressions mapped previously in the lowlands are related to impact features. The number of buried craters that MARSIS has detected beneath Mars' smooth northern plains indicates the underlying crust of the northern hemisphere is extremely old, "perhaps as ancient as the heavily cratered highland crust in the southern hemisphere," Watters noted.

MARSIS map of south pole ise deposit
MARSIS map of south pole ice deposit
This map shows the thickness of the south polar layered deposits of Mars, an ice-rich geologic unit that MARSIS probed. The thickness of the layered deposits is shown by colors, with purple representing the thinnest areas, and red the thickest.The total volume of ice in the layered deposits is equivalent to a water layer 11 meters deep, if spread evenly across the planet. The boundary of the layered deposits was mapped by scientists from the U.S. Geological Survey. The dark circle in the upper center is the area pole-ward of 87 ° south latitude, where data cannot be collected. The map covers an area 1670 by 1800 kilometers.
Credit: Courtesy NASA / JPL / ASI /ESA / Univ of Rome / MOLA Science Team/ USGS

Just this week, Plaut, Picardi and 22 other researchers reported their analysis of the radar instrument's observations at the south pole in the online edition of the journal Science and yesterday ESA made an official announcement. MARSIS has detected a substantial amount of water trapped in frozen layers over Mars' south polar region, equivalent to a liquid layer about 11 meters deep covering the planet.

The polar layered deposits extend beyond and beneath a polar cap of bright-white frozen carbon dioxide and water at Mars' south pole. Dust darkens many of the layers. MARSIS made more than 300 virtual slices through the layered deposits covering the pole, looking through the icy layers to the lower boundary to map the thickness of the dusty ice. In places, the layers extend as deep as 3.7 kilometers below the surface. "The south polar layered deposits of Mars cover an area as wide as a big portion of Europe. The amount of water they contain has been estimated before, but never with the level of confidence this radar makes possible," said Plaut.

The strength of the echo that the radar receives from the rocky surface underneath the layered deposits suggests the composition of the layered deposits is at least 90 percent frozen water. One area with an especially bright reflection from the base of the deposits puzzles researchers. It resembles what a thin layer of liquid water might look like to the radar instrument, but the conditions are so cold that the presence of melted water is deemed highly unlikely.

Detecting the shape of the ground surface beneath the ice deposits provides information about even deeper structure of Mars. "We can see now that the crust has not been depressed by the weight of the ice as it would be on the Earth. The crust and upper mantle of Mars are stiffer than the Earth's, probably because the interior of Mars is so much colder."

A new view of the Face on Mars
A new view of the Face on Mars
The High Resolution Stereo Camera (HRSC) onboard Mars Express took this image of a remnant massif, viewed from the southwest. The massif became famous as the Face on Mars in a photo taken on July 25, 1976 by the American Viking 1 Orbiter. Rather than being built by some extraterrestril intelligent lifeform as some people proposed in the late 1970s, the massif is thought to have formed via landslides and an early form of debris apron formation. It is characterized by a western wall that has moved downslope as a coherent mass (on the near side of the massif in this view). The image is based on data gathered over the Cydonia region, which lies at approximately longitude 350.54° E and latitude 40.75° N. Credit: Courtesy ESA/DLR/FU Berlin (G. Neukum), Malin Space Science Systems

"Our radar is doing its job extremely well," added Picardi. "Not only is MARSIS is providing us with the first ever views of Mars’ subsurface at those depths, but the details we are seeing are truly amazing. We are expecting even greater results when we will have concluded an on-going, sophisticated fine-tuning of our data processing methods. These should enable us to understand even better the surface and subsurface composition."

Methane on Mars—What Does It Mean?

Although Beagle 2 didn't survive, the Planetary Fourier Spectrometer (PFS), within weeks of arriving at the Red Planet, found possible signatures of constituents in the atmosphere that could be emanating, theoretically, from some source of biological life.

The PFS looks at the global atmospheric distribution of both major and minor constituents with greater accuracy than any previous mission. It measures the vertical pressure and temperature profile of carbon dioxide, which makes up most of the Martian atmosphere, while also monitoring for minor constituents, such as water, carbon monoxide, ammonia, methane, or formaldehyde.

Another new view
Another new view
This is another new view of the Face on Mars in the Cydonia region. A few days after the Viking 1 released the now famous image, on July 31, 1976, a NASA press release noted that the formation "resembles a human head." However, JPL and NASA scientists had already correctly interpreted the image as an optical illusion caused by the illumination angle of the Sun, the formation's surface morphology, and the resulting shadows, giving the impression of eyes, nose and mouth. Nonetheless, the Face on Mars has been the subject of widespread speculation about artificial structures on Mars, with the face being the most talked-about formation. In April 1998, additional data from the Mars Orbiter Camera on Mars Global Surveyor finally caused the controvesy to wane. Data from MGS in 2001 added more visual evidence that the face is a figment of human imagination in a heavily eroded surface.
Credit: ESA/DLR/FU Berlin (G. Neukum), MSSS

The instrument made headlines in March 2004 when it became the first instrument ever to make direct in situ measurements of methane in the atmosphere of Mars. The discovery was confirmed independently around the same time by two ground-based experiments. Subsequently, PFS Principal Investigator Vittorio Formisano, of the Istituto Fisica Spazio Interplanetario, in Rome, reported concentrations of methane and water vapor in the atmosphere above Elysium Planum, as well as finding “signatures” that appeared to be from formaldehyde and ammonia.

The methane could be produced by some sort of geologic out-gassing, but Formisano has been leaning toward the theory of a biologic origin. As Formisano  pointed out, even Earth volcanism, which is more active than on Mars, would not account for the 10 parts per billion of methane in the atmosphere that his team is finding (and which the two ground-based teams also reported).

If methane, formaldehyde, and ammonia are confirmed to be in the atmosphere and emanating from the same area, the combination could bolster the case for biologic origin or a source unknown on Earth. The signatures of formaldehyde and ammonia, however, have not been shown unequivocally and therefore are controversial. The team continues to make measurements.

Strange Aurorae and CO2 Clouds

As the search for water and signatures of life continue in studies with MARSIS, the HRSC, OMEGA, PFS, and ASPERA-3, the mission is also opening new windows on Mars’ atmosphere and ionosphere. SPICAM (Spectroscopy for the Investigations and the Characteristics of the Atmosphere on Mars) is a dual ultraviolet/infrared spectrometer that measures the composition of the atmosphere over smaller volumes than the PFS. Using the technique of stellar occultation, the SPICAM team is also measuring the vertical profiles of carbon dioxide, temperature, ozone, aerosols, and clouds.

Strange aurorae
Strange aurorae
Artist's impression of how the 'green' aurorae may look to an observer orbiting on the night-side of Mars. Credit: Courtesy M.Holmström (IRF)

In June 2005, SPICAM Principal Investigator Jean-Loup, of Bertaux Service d'Aeronomie, Verrières-le-Buisson, France, and his team announced the first discovery of an aurora on Mars. The mid-latitude aurora -- unlike any other ever seen in the solar system -- appeared to be corresponding to a paleomagnetic signature or remnant of Mars’ former magnetic field. The anomalistic fields, which appeared in observations taken late in August 2004, were nearly as strong at the surface as Earth’s magnetic field and were laid out in east-west bands of alternating polarity, extending for more than 1,000 kilometers (620 miles) north to south.

According to Mars Global Surveyor (MGS) data, the strongest magnetic anomalies on Mars are related to Terra Cimmeria and Sirenum in the heavily cratered uplands of the southern hemisphere. In analyzing the map of crustal magnetic anomalies compiled with MGS data, SPICAM team members observed that the region of nighttime light emissions (corresponding to 177 degrees east and 52 degrees south) is right where the strongest magnetic field is localized. The SPICAM observations will help define the role that the Martian crustal magnetic fields play in producing these distinctive, cusp-like magnetic structures, which concentrate fluxes of electrons into small regions of the atmosphere and catalyze the formation of highly concentrated auroras.

High clouds drifting over Mars
High clouds drifting over Mars
NASA's Sojouner rover took this true-color picture of clouds in the Martian eastern sky (30 degrees above the horizon) one August sol in 1997 before sunrise. Observations of the atmosphere by the SPICAM spectrometer on Mars Express revealed for the first time that carbon dioxide clouds form and exist at very high atmospheric layers, between 80 and 100 kilometers above the surface. This makes them the highest clouds ever observed above any planetary surface and they may be of the same type observed by Sojouner.
Credit: Courtesy NASA

In August 2006, the SPICAM team announced another discovery -- the highest clouds above any planetary surface. At an altitude between 80 and 100 kilometers (50 and 60 miles), where the temperature is around –193 degrees Celsius (–315 Fahrenheit), the clouds are presumed to be composed of carbon dioxide.

SPICAM made the discovery by observing distant stars just before they disappeared behind Mars, looking at the effects on the starlight as it traveled through the Martian atmosphere, and building up a picture of the molecules at different altitudes. The first hints of the new cloud layer came when certain profiles or “sweeps” through the atmosphere showed that the star dimmed noticeably when it was behind the atmospheric layer. Although this happened in only one percent of the profiles, by the time the team had collected 600 profiles, they were confident the effect was real.

"If you wanted to see these clouds from the surface of Mars, you would probably have to wait until after sunset," said Franck Montmessin, a SPICAM scientist with Service d'Aeronomie du CNRS, Verrières-le-Buisson, France, and lead author of the cloud research report. This is because the clouds are very faint and can only be seen reflecting sunlight against the darkness of the night sky. In that respect, they look similar to the mesospheric clouds, also known as noctilucent clouds, on Earth.

The clouds form, the team hypothesizes, when minuscule dust grains, just 100 nanometers across (1 nanometer is one billionth of a meter), "microscopic chippings" from the rocks on the surface on Mars that have been blown to extreme altitudes by the winds or the debris from meteors that have burnt up in the Martian atmosphere, which are above 60 kilometers in the Martian atmosphere, serve as the "nucleation centers" around which crystals of carbon dioxide coalesce. The new high-altitude cloud layer has implications for landing on Mars, because it suggests the upper layers of Mars' atmosphere can be denser than previously thought. This will be an important piece of information for future missions that aerobrake or use the friction in the outer atmosphere to save propellant and slow down the spacecraft for orbit insertion, or for future lander missions.

Phobos -- up close and personal
Phobos -- up close and personal
This image, taken by the High Resolution Stereo Camera (HRSC), is one of the highest-resolution pictures so far of the Martian moon Phobos. It shows the Mars-facing side of the moon, taken from a distance of less than 200 kilometers with a resolution of about seven meters per pixel during orbit 756, on August 22, 2004. Due to geometric reasons the scale bar is only valid for the centre of the image. Credit: Courtesy ESA / DLR / FU Berlin (G.Neukum)

The new high-altitude cloud layer has implications for landing on Mars because it suggests that the upper layers of Mars’ atmosphere could be denser than previously thought. This is important information for future orbiters that aerobrake (use the friction in the outer atmosphere to slow down the spacecraft for orbit insertion) or future lander missions. If Beagle 2 had such information, its fate may well have been different.

Examining the Ionosphere

In addition to looking below the surface of Mars, MARSIS also has been used to examine the upper part of the atmosphere. It has revealed a complex structure in Mars’ ionosphere and found a number of unexpected features, according to P.I. Picardi. Team members are analyzing the data to explore the connection between the ionosphere’s structure and the behavior of remnant magnetic fields permeating the crust of the Red Planet.

When comparing the MARSIS map with maps of the Martian crustal magnetic field from MGS data, MARSIS scientists realized that their findings corresponded to areas of strong magnetic fields in the crust of the planet. Intriguingly, the crustal magnetic fields cannot account for all the distinctive echoes MARSIS observed, even if they are responsible for the majority of them; therefore, the team is investigating other possible mechanisms for the production of these echoes, including wind-driven atmospheric waves excited by topographic features and various types of wavelike structures in the atmosphere caused by interaction with the solar wind.

MARSIS has recorded other unexpected echoes, including unusual reflections recorded in the night side of Mars that would be impossible in a horizontally stratified atmosphere and may indicate the presence of low density "holes" in the ionosphere, much like those observed in the night side of Venus.

MaRS, the radio instrument on which Mars Express relies to communicate with Earth, is also being used to study the atmosphere and the ionosphere of the planet. By measuring local variations in gravity over the surface of the planet and providing pressure and temperature profiles of the atmosphere, MaRS found a third, noncontinuous layer in the ionosphere that had been predicted but never before detected.

Kasei Valleds
Kasei Valleds
This HRSC image, which is is centerd at 26.97º North latitude and 67.68º West longitude, shows the northern main channel of Kasei Valles, which probably formed from gigantic flood events. Credit: ESA / DLR / FU Berlin (G.Neukum)

During two Mars Express occultation seasons -- April to August 2004 and December 2004 to January 2005 -- MaRS detected the presence of regions in the atmosphere where the density of charged particles (electrons) was about one tenth of the density of the main ionospheric layer. These regions, localized between 65 and 110 kilometers (40 miles and 68 miles) altitude, "unambiguously characterized the third ionospheric layer below the first two," respectively situated at 135 kilometers and 110 kilometers (84 miles and 68 miles) altitude, reported MaRS Principal Investigator Martin Pätzold, of the University of Koln, Germany, and colleagues. Because MaRS observed these higher density regions only in a few ionospheric areas, the team concluded that this third layer is sporadic and geographically localized.

The presence of the layer was observed at different times of the day and at different latitudes -- early morning and afternoon in the northern hemisphere of the planet, in the equatorial region, and at mid-southern latitudes. "In any case, we do not exclude the existence of the third layer also in the night-time portion of the ionosphere, even if we believe such occurrence may be rare," says Pätzold.

Pätzold and colleagues believe that the origin of this third layer may be due to the interaction of the ionosphere with incoming meteorites, a mechanism similar to that occurring in Earth's ionosphere. “We favor the idea that the third layer is created by the influx of meteors, like on Earth,” Pätzold said in November 2005. On Earth, however, the low ionospheric layer is nonsporadic and created by electrical charge exchange between the atmosphere and magnesium and iron atoms present in meteors. Since the atmosphere is thinner and less dense on Mars, meteors require more kinetic energy to exchange charged particles above 75 kilometers (45 miles) altitude where the Martian atmosphere becomes really thin. "The population of meteors with this energy is more limited," Pätzold pointed out. "This could explain the sporadic occurrence of the third ionospheric layer on Mars."

Mars Express Orbits On

Mars Express
Mars Express
Artist's conception of Mars Express
Credit: Courtesy ESA / Medialab

Mars Express has made important discoveries with each of its scientific instruments, enriching the database on the planet with which we have had a centuries-old love affair. "The HRSC is showing us geological processes on Mars, such as volcanism, that may be much younger than people previously thought. OMEGA has written a history of Mars in more detail, and a chronology of the alteration of minerals on the surface. And ASPERA-3 is telling us how the water leaves the planet," Project Scientist Chicarro offered by way of summary.

"SPICAM has shown the existence of auroras that were totally unexpected and completed full profiles on density and temperature of atmosphere. The PFS discovery of methane could lead to something important concerning life on Mars, if we could confirm the results," Chicarro continued. "MARSIS is the first working radar on any planet [other than Earth] and has shown delicate layering in the polar regions and buried impact basins in northern plains that indicates all of Mars has the same scars of the early bombardment -- that means the whole of Mars is a pretty old surface. Radio science is finding interesting things like the third transient layer in the ionosphere, and the gravity profiles that are rather unique are like the cherries on the cake, because they provide additional information to all the other fields we are studying," he concludes. "Of course, it was a shame for Beagle 2. But we knew it was an experiment and it was not built to the same standards of reliability because of time and money constraints, but its instruments were all first class."

Despite the loss of the Beagle, by all accounts now, Europe's first mission to another planet has delivered on its promises and secured its place in history. For ESA, the mission has been -- in space exploration jargon -- one giant leap. “Just 5 years ago, we were viewed by many as a flash-in-the-pan called Giotto,” mused ESA’s Director of Science Southwood. Mars Express, however, changed all that.

From the top of the atmosphere to below the surface, the mission has added new chapters to the ever-evolving history book of Mars and more discoveries are, no doubt, on the way. "You might say," Southwood offered with a smile, "the Empire has struck back."