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Tuesday 6 August 2013

Celebrating 10 years of Mars Express

At the beginning of June, the European Space Agency (ESA) celebrated the 10 year anniversary of the Mars Express mission. The spacecraft, launched in 2003, has had ten successful years of operation and has resulted in the publication of over 600 scientific papers. To help mark this occasion, our intern Cait interviewed Olivier Witasse, the mission's Project Scientist, about some of the key discoveries the spacecraft has made and what the future of the mission looks like.

Artist's impression of ESA spacecraft Mars Express in orbit around Mars
Mars Express has had ten very successful years in orbit around Mars. Image credit: ESA
One of the key themes of space exploration is the search for life. We base our search for past or current life on the presence of liquid water – it's vital for life like ours so it's one of the first conditions we look for. Direct evidence of past water on Mars is one of the key findings and surprises of the Mars Express mission:

"...from what we have been discussing with the scientists over the last year, there is one result which I think is really on top of the others - the discovery of the so called hydrated minerals."

Before the discovery of hydrated minerals, scientists at ESA were expecting to find carbonates on what may have once been a sea bed. Finding carbonates would imply that there were once large oceans on Mars. Carbonates can form in one of two ways - a purely chemical reaction where carbon dioxide in the atmosphere is dissolved by the surface water of an ocean or as a result of the shells of past marine creatures. If the oceans were absorbing atmospheric carbon dioxide then further reactions with minerals (i.e. magnesium, calcium) in the ocean would produce carbonates which would settle onto the ocean floor. The atmosphere on Mars is mostly carbon dioxide, so if there were once oceans on Mars we would expect the formation of these carbonates. The shells of some marine animals also contain carbonates, so when the creatures die, their shells sink to the bottom of the ocean to form carbonate deposits with the possible preservation of fossils. These carbonates would remain even when the ocean faded away.

Mars hydrated mineral map made by combining data from ESA Mars Express OMEGA instrument with NASA Mars Reconnaissance Orbiter CRISM instrument
The dots in the map show sites where minerals which require the presence of water to form were detected. More than ten different such minerals have been found on Mars, with many occurrences found together. The detections were made by combining observations made by the OMEGA instrument with those made by the CRISM instrument on-board NASA’s Mars Reconnaissance Orbiter. Image credit: ESA/CNES/CNRS/IAS/Universit√© Paris-Sud, Orsay; NASA/JPL/JHUAPL; Background image: NASA MOLA

Instead, they found hydrated minerals, which only form when liquid water is present, confirming that there was a time when liquid water was stable on Mars. When this occurred was also a surprise; these minerals were only detected in the more ancient parts of the planet's surface.

"We can determine the age of the surface of Mars by counting the number of craters. When we study a particular area we can know if it is an older area or young area and these hydrated minerals have been found only on the surface that is very old, 4 million years ago or something like that."

This discovery has also shown that liquid water was only on the planet for a very short time in the planet's history. ESA have turned their data into maps of the planet's surface which indicate where the different minerals have been found, and produced a short video to show them off.
Video ©ESA. Hydrated mineral map: ESA/CNES/CNRS/IAS/Universit√© Paris-Sud, Orsay; NASA/JPL/JHUAPL; Olivinepyroxeneferric oxide and dust maps: ESA/CNES/CNRS/IAS/Universit√© Paris-Sud, Orsay; Video production: ESA.

Another unexpected discovery and possible sign of microbial life was the detection of methane in the planet's atmosphere. The discovery still needs confirmation as the detection of methane was at the very limits of the capability of the instrument.

Olympus Mons, on Mars, is the highest volcano in the Solar System
The highest volcano in the Solar System, Olympus Mons, rises 25 kilometres above the surrounding plain:
Three times higher than Mount Everest.
"Methane in the atmosphere is quickly destroyed by ultra-violet radiation. In fact, the origin of methane is linked to microbiological activity, volcanic or geological activity of the planet and we think that there is no life at the moment on Mars and we think, well we thought, that the planet was not active anymore."

The discovery of methane suggests that Mars may not be the dormant planet we expected it to be - it may still be geologically active. These pieces of evidence propel us into the next step of unlocking the secrets of the early climate on Mars.

"The next question is 'if liquid water was there, could Mars have been habitable at the beginning of its evolution?' and if it could have been habitable, 'could there have been life on the surface of Mars?'."

Mars Express is paving the way for the next ESA Mars mission - ExoMars. ExoMars consists of two missions. The first is an orbiter, due for launch in 2016 which is aiming to measure the atmospheric content so will be able to either confirm or deny the existence of methane. The next, due for launch in 2018, will be the ExoMars Rover which will look for signs of past water on the Martian surface. The maps and surface discoveries of Mars Express will be vital in choosing the landing site for the ExoMars Rover.

Artist's impression of the ExoMars Trace Gas Orbiter
Artist's impression of the ExoMars Trace Gas Orbiter.
Image credit: ESA
But that doesn't mean it's the end for Mars Express; recently it received some very good news.

"In fact, after 10 years we have still not finished our adventures, we will continue... we just got the information that the mission is extended until the end of 2016."

So, over the next 3 years Mars Express will be keeping busy. Starting with a joint ESA-NASA set of observations, Mars Express will start its extended mission by working in tandem with the NASA Rover, MSL (The Mars Science Laboratory) or 'Curiosity'.

"We have a programme of joint observations because it's always interesting to compare what the rover measures on the ground and what an orbiter can measure from high altitudes."

Its next target will be the Martian Moon, Phobos. Mars Express has an elliptical orbit which means that every 5 months it's closer to the larger of the two moons. Equivalent to the mystery of methane on Mars, the question as to how Mars actually got its moons is still very much open.

"There are different scenarios to explain their presence around Mars so with Mars Express we are studying this and at the end of the year, we have the closest ever Phobos fly by which will be at something like 42-45km from the surface. It's very very close so we need to make sure that Mars Express and Phobos will not collide, of course, otherwise it's the end of the mission"

The leading theory is that the moons of Mars are captured asteroids that were nudged out of the outer asteroid belt by Jupiter's gravity. They are made of similar (yet slightly less dense) rock to those in the outer asteroid belt, are very cratered and are very oddly shaped. However, their orbit is slightly too circular compared to what we would expect from a captured object and some scientists do not believe that the Martian gravity or atmosphere would have been sufficient to slow the objects enough to capture them. Another school of thought is that the moons were created (in a similar way to that of planets) by the gravitational attraction between rocks and dust in space - either as a random occurrence or as a result of a large object colliding with Mars and the resultant debris forming the moons. This flyby will enable Mars Express to measure the gravity field of Phobos very accurately which will reveal much more about the structure and composition of the interior of the moon.

Photograph of Martian moon Phobos by Mars Express
This image of Phobos was obtained by the High Resolution Stereo Camera on-board Mars Express on 30 August 2008. The distance from the moon’s centre was 2366 km, and the image resolution is 22 m/pixel. The original image has been corrected for mirror distortion. Image ©ESA/DLR/FU Berlin (G. Neukum)
In the following year, Mars Express will observe the close encounter of a comet and Mars. The 'Siding Spring' comet was originally expected to collide with Mars, which would have led to a very exciting event. However, closer predictions suggest that it will pass by only 100,000km from Mars (that's ¼ of the distance between the Earth and Moon!). This will not only give the opportunity for some cometary science but also some amazing pictures of the comet and Mars.

Then, in late 2014, Mars Express will gain a partner in its observations- MAVEN, the Mars Atmosphere and Volatile Evolution satellite from NASA. MAVEN will be collecting data about the upper atmosphere of Mars. The martian atmosphere is slowly being lost to space, in part due to interactions with the solar wind. The solar wind is a constant flux of charged particles being sent out from the Sun. On Earth, the solar wind is deflected by our magnetosphere (which is due to Earth's magnetic field) which protects our atmosphere and the life on Earth. Mars lost its global magnetic field over 4 billion years ago, resulting in the loss of its main atmosphere. The martian ionosphere extends hundreds of kilometres out into space where particles within it are ionised by solar radiation, and now magnetised, they can be easily swept away by the solar wind. MAVEN will measure the rate of this loss and, by measuring the current atmosphere, it will collect data that will lead to new information about the past climate on Mars and how it's linked to the loss of the atmosphere.

The open questions around Mars are being thoroughly investigated, from Curiosity running around on the surface to Mars Express in orbit, but what are the differences between the two?

Video example of a polar orbit (creative commons)
"With a polar orbit, because Mars is rotating you can see everything if you have sufficient time so the good thing with Mars Express is that we have, after 10 years, a global view of the planet, we have seen the same area sometimes many times, in different seasons, in different illumination conditions, sometimes during the day, sometimes during the night so if you have an orbiter, and if you have a good orbit, you can see all the planet so that is the main advantage over a rover for example.

The advantage of the rover is that you can do observations of the surface where you are in quite high detail. You have a microscope, you have a high resolution camera so what you see on the surface is of course excellent."

So rovers and orbiters work well individually and can provide complementary results – both are necessary for the full exploration of the planet. By comparing the findings of the rover to what we see from the orbiter, we can find locations with similar properties to where the rover is to use as possible landing sites for future missions. Mars Express can use infra-red to detect hydrated minerals - once they've been detected we can use the information to pick out a good landing spot for the rover, i.e. the landing site for Curiosity was chosen to aim to explore an area with hydrated minerals.

Over the next 10 years scientists will explore the early climate of Mars and hopefully reveal how water existed on the planet's surface. We are waiting for another methane detection to disprove what we thought about the activity of the planet and at some point we may understand how Mars captured its moons. Mars is the only planet we have been able to explore first hand with rovers (for more than a minute) and there's still a lot that we don't know about it - so the final question is: what secrets might the other planets be hiding?

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