Tag Archives: Mars

It’s Been A Busy Week For Space Science, Part II: Mars

There were four major announcements regarding various spacecraft last week, but the last post (on Messenger finding water ice on Mercury and Voyager 1 discovering a new region of the heliosphere) got long. So, let’s talk about Mars! Curiosity found percholates in a soil sample used to calibrate some of the instruments and make sure everything’s in working order, and NASA announced a return to Mars in the coming years.

Martian soils are complex (or at least that one patch of them is)

This announcement, regardless of what it was going to be about, was bound to be at least a bit of a letdown, since due to a misinterpretation of some ambient scientific enthusiasm, many people were expecting A Big Announcement. But, given the complexity of the rover and the array of complex analysis tools on board, news that everything is in fine working order is good news. Curiosity analyzed some loose soil in an area called Rocknest, which appears similar to other environments encountered by Pathfinder, Spirit, and Opportunity. The soil analysis was done with the APXS (alpha-particle x-ray spectrometer), the CheMin (Chemistry and Mineralogy), and the SAM (Sample Analysis at Mars), and along with verifying that everything was in good working order, the SAM detected perchlorates in the soil. Percholrates are salts with a (Cl O4) ion, and they’ve been found in other soil samples by the Pheonix lander. Perchlorates are very reactive, and there’s speculation that they may be accompanied by carbon compounds. Carbon was detected during the analysis, but it’s not entirely clear if it was from the soil sample or from a small contamination from Earth. Sulphur compounds were also discovered in the soil.

On an unrelated note, am I the only person who keeps hearing “Loch Ness” instead of “Rocknest”?

NASA Mars rover analyzes first soil samples

NASA plans to return to Mars in 2020

After the interesting but still relatively minor announcement about the soil analysis on Mars, NASA announced Monday that a new rover modelled on Curiosity would be landing on Mars in 2020. Much of technology will be reused, including the elaborate landing gear and the much of the chassis, which helps keep the cost (relatively) low given NASA’s continual budget pressures. To support this new mission, NASA is planning support missions too, including:

  • MAVEN, launching in 2013, which will monitor the Martian atmosphere
  • InSight, launching in 2016, which will look at the deep interior of the planet using seismologic and geologic analysis
  • significant participation in the European Space Agency’s Electra (2016) and ExoMars (2018) missions.

Data and insight from these missions will inform the scientific direction of the 2020 rover.

I understand why NASA is taking this tack: funding is tight, to say the least, and so far Curiosity is a raging success. The landing went smoothly, all the instruments are in good working order, and the rover is immensely popular. That last bit is really important, because have a popular, very visible mission not only gives NASA a lot of good press — it gives NASA a lot of political capital, which is critical to maintaining funding. NASA has played the PR game very well, and it’s paying off. Couple a very popular and so far successful mission with a plan for another mission that is budgeted to cost less, and that’s a much easier sell to politicians who control the funding than saying “we want to go try something new again.” It’s a safe option, and there’s plenty of good reasons to take that safe option.

But it’s safe, and while there’s plenty we still don’t know about Mars, there’s plenty we still don’t know about lots of other objects in the solar system too (including oceans on our own planet, but that’s another post entirely.). Concentrating on Mars makes sense if we’re heading towards sample return (ie, having a component of the craft gather rocks or soil and return them to Earth) or eventually putting a human on Mars, and that’s what NASA is gunning for. But given the immense gap between landing a rover on Mars and landing a human on Mars, the increasing funding pressures, and the lack of a driving cultural pressure like the Cold War to spur on extreme exploration, I’m not entirely convinced that we’ll see sample return or a human on Mars in the not-wildly-distant future. I understand why NASA is trying to shoot for that, and there’s good economic and political reasons to build on Curiosity’s success — I’m just not entirely convinced that it’s a line of exploration that’ll come to it’s ultimate fruition.

However, I’d love to be proven wrong.

New NASA Rover to Launch in 2020
A 2020 Return to Mars? (National Geographic)
Mars Beyond 2009

It’s Been A Busy Week For Space News

In the past week, there’ve been four major announcements from various space exploration missions. Messenger found solid water ice on the surface of Mercury, Voyager 1 has not left the solar system like initially thought, and the structure of the heliosphere is different than what we thought, Curiosity found organic molecules on the surface of Mars, and NASA is sending another rover to Mars in 2020. That’s a lot of big news for one week! This post covers the Messenger and Voyager announcements, and the next post (coming soon!) covers the two Martian announcements.

Messenger finds solid ice on Mercury

Out of all of these announcements, this is the one that astounds me. Mercury is the innermost planet, sitting in an eccentric orbit (ie, the orbit is shaped like an ellipse rather than a circle) 0.30 to 0.46 AU from the Sun, and it is also very small. It has virtually no atmosphere, since it doesn’t have much gravity to hold one in place, and the solar wind pushes gases off the planet. Because it has such a thin (and highly variable) atmosphere, there’s little insulation on the planet, and the temperature varies wildly from day (~700 K at the equator, ~550 K near the poles) to night (~100 K near the equator, ~50K near the poles). This is not planet where one’d expect to find what amount to glaciers! But there are two things that make solid surface ice possible: the presence of craters, and the planet’s spin.

Spin axes of Mercury and Earth

Spin axes of Mercury (left) and Earth (right). Poles on Mercury do not experience the relatively extreme seasonal shifts in light that Earth’s poles do.

Like any rocky planet, the surface of Mercury is not smooth, and there are craters and canyons from impacts with asteroids and comets. These craters can have steep, deep walls, which act as sunshades for the bottom of the crater. Additionally, Mercury is a very upright planet — the axis around which it rotates is nearly vertical. This means that in the polar regions, there are regions at the bottom of craters and cayons that never see direct sunlight. Because the surface temperature is so strongly dependent on incoming heat from the sun (ie, when the sun goes down, the temperatures fall ~600 degrees), the bottoms of these canyons stay cold (often ~120 K).

Shadown line due to lip of the crater.

Shadow line due to lip of the crater. At the poles, sunlight comes in at a much more oblique angle; the angle is exaggerated here since picture is not to scale. Region shaded in blue is shaded by the lip of the crater, and remains cold.

Ice vs frozen volatiles

Ice vs. frozen volatiles. Blue line indicates the shadow line, black material is frozen non-ice volatiles (including carbon compounds), and red is water ice. Note that there is a sliver of red under the black material outside of the shadow region, and in the shadow region ice dominates (though other materials may be present).

When Mercury is hit with comets or asteroids, the impact deposits not only the rocky material, but often frozen ice and volatile compounds, including carbon compounds. (Note that most carbon compounds are called “organic” — this does not mean that they were created by living organisms, or are indicative of the presence of organisms.) The ice and other volatiles migrate around the planet, vapourizing in the hot regions and condensing out in the cold, and some of the material winds up in the polar regions; material that lands in the cold patches sticks around. Additionally, the organics and non-water volatiles can act as a blanket in the transition regions, where it’s cold, but it’s not quite cold enough (or consistently enough) to sustain surface ice. In these regions, the volatiles protect a layer of ice that sits just below the surface.

NASA confirmed this with a variety of instruments. Messenger mapped the topography of the polar regions of Mercury to high precision, which allows NASA to calculate which craters have cold bottoms. The reflectivity of the surface was mapped; since ice reflects a lot of light, bright spots at the poles are evidence of solid surface ice. In contrast, organic material has a very low reflectivity, and shows up dark. The bottom of several of the polar craters had a large dark area with a smaller bright patch hugging the crater wall, which is exactly what you’d expect to see if ice and organic material were forming what amount to glaciers on the surface of Mercury. The evidence is further confirmed by the signature of hydrogen in the neutron background of the planet. A background field of neutrons is present around planets, due to interactions between the material of the planet and cosmic rays. As cosmic rays hit the surface of the planet, they interact with the material of the planet (often penetrating below the surface), and neutrons are released in the reactions. These neutrons bounce around and out of the surface, and can be detected from an orbitting spacecraft. Hydrogen (bound in water) is a very good absorber of neutrons, so patches where few neutrons are detected are regions where hydrogen (water, or ice) is likely to be found. Messenger found dark spots (so indications of high hydrogen concentrations) in the same places where the reflectivity was high and the surface temperature was consistently cold. Adding this all up means that there is frozen water in the polar regions on the surface of Mercury!

Messenger finds new evidence for water ice at Mercury’s poles
Map of the permanently shadowed areas

Voyager 1 finds that the edge of the solar system is not where we thought it’d be

Voyagers 1 and 2 were launched way back in 1977, and have been slowly and steadily making their way to the outer edge of the solar system. (These are the spacecraft with the golden records, etched with samples of images and sounds of life on Earth, compiled by a committee headed by Carl Sagan.) Both Voyagers passed by Jupiter in 1979 and Saturn in 1980, and Voyager 2 passed by Uranus and Neptune in 1986 and 1989 respectively (Voyager 1 missed both planets); they’ve been drifting out of the solar system for the past 23 years.

There is a boundary between the heliosphere (which is the region where there is a solar wind, or a field of charged particles emitted by the Sun) and outer space (perhaps better called interstellar space), and earlier this year, there were indications that Voyager 1 was pretty much at what we thought that boundary was. Previously, this is what we thought the edge of the heliosphere looked like:

Schematic of the heliosphere

Schematic of the heliosphere. The solid red line indicates the termination shock (where the solar wind abruptly slows to subsonic speeds), and the solid blue line outside it indicates the heliopause, or the edge of the heliosphere (defined as the region where solar wind exists). The region between these two boundaries is the heliosheath, where both solar wind and cosmic rays can be found. Outside the heliosphere (ie, outside the blue ring) is the interstellar medium.

Solar wind zips along at 400 km/s, but it slows when it starts to interact with the interstellar medium (ie, outer space). As the wind interacts more and more with the interstellar medium, the wind slows down. The point where it slows to below the speed of sound is called the termination shock, and it marks the beginning of the edge of the heliosphere. Voyager 1 passed it in December 2004, and Voyager 2 in May 2006. The region between the termination shock and the heliopause (ie, the border between the heliosphere and outer space, or the outermost edge of the solar system) is the heliosheath, and it’s a region where the solar wind interacts strongly with the interstellar medium, and becomes turbulent. Voyager detected all this as expected, and reported in 2010 that the solar wind had no more outward velocity, that is, the particles were not pushing outwards towards outer space, but were moving around laterally. This isn’t the edge of the heliosphere, since there are still particles of solar origin: they’re just not (collectively) moving outwards anymore.

What NASA expected to see was as the charged particles of solar origin dropped off, the cosmic rays form interstellar space would increase. The particle data from Voyager 1 shows this pattern: there are extremely well defined sharp dips in the amount of solar particles which correlate to increases in the number of detected cosmic rays, and then there’s a sharp, longer, drop-off in solar particles.

Particles and magnetic field detected by Voyager 1

Particles and magnetic field detected by Voyager 1. As the solar particles dip, the cosmic rays increase, and the strength of the magnetic field increases. Note that the direction of the magnetic field does not shift at all. (Click for full size.) Image credits: Left panel: NASA/JPL-Caltech/GSFC Right panel:NASA/JPL-Caltech/GSFC/University of Delaware Images available from http://www.nasa.gov/mission_pages/voyager/telecon20121203.html

What wasn’t expected was the magnetic field data. Magnetic fields have a magnitude (or strength) and a direction; NASA expected that when Voyager hit interstellar space, the direction of the magnetic field would shift from the solar magnetic field the structure of the interstellar magnetic field. The magnitude would likely shift as well, but the shift in direction is the key indicator of the edge of the heliosphere. This wasn’t observed: while the magnitude of the magnetic field increases after the steep drop-off in solar particles (and there are noisy increases during the brief windows leading up to the eventual drop-off), the direction of the magnetic field remains constant. This indicates that the solar magnetic field is still present, and Voyager is not yet out of the heliosphere. But this increase in the strength of the magnetic field, along with the changes in the particles detected, leads scientists to conclude that the two magnetic fields (solar and interstellar) are connected in this region, which allows the charged particles of both origins to zip along more quickly. Solar particles can stream outwards into interstellar space, and cosmic rays can stream inwards towards the sun. The edge of this region (dubbed the “magnetic highway”) fluctuates, and the initial, short dips and peaks in the data are due to the edge of the magnetic highway fluctuating inwards towards the sun. Voyager appears to now be beyond the fluctuating edge of the highway.

Voyager is the first object to reach the edge of the heliosphere, and scientists suspect that this is the last stage before it finally exits the heliosphere all together, probably some time in the next year or two. But this magnetic highway stage wasn’t anticipated, so who knows what other surprises Voyager has in store for us!

Voyager 1 Encounters New Region in Deep Space
Voyager mission site at JPL

Robots in Spaaaaaaaace!

I realize that most of the hubbub about Curiosity, the newest Martian rover mission, was in August after the successful landing and subsequent initial instrument tests, but is it ever too late to talk about robots on other planets? No! Consider this my tiny endeavour to keep space science in the public’s eye, even when NASA’s not just landed a small car right side up on another planet with perfect precision.

Curiosity successfully landed on August 5th, and after a perfect landing, and all the instruments are in excellent working order. The instruments on board are all described on the NASA Jet Propulsion Lab’s site, with a blurb about how they work and what sort of things they’re looking for. For this post, I’m going to focus on the Canadian instrument on board.

The Alpha Particle X-ray Spectrometer, or APXS, is the only all-Canadian contribution to the mission. It’s designed by Ralf Gellert, a professor at the University of Guelph (previously known best for cows with windows in their stomachs and pigs with phosphate-reduced poop), built by Macdonald-Dettwiler Associates (who also built the Canadarm), funded by the Canadian Space Agency, and the research involves a group at Guelph as well as people at a few other Canadian universities. I’ve got some thoughts about what the APXS represents to Canadian science and science policy, but I’ll save that for a second post. I’ve heard press about various other instruments on board, including the laser spectrometer, the array of cameras, and the meteorological station, but not very much about how the APXS works and what it can do. So let’s talk about how it works!

The APXS is a pop-can sized instrument mounted on the robotic arm on Curiosity that uses two types of radiation to determine the elemental composition of rocks, soils, and dirt on Mars. Six radioactive Curium-244 sources covered with a thin titanium foil sit around an X-ray detector. Curium (element 96), like all elements heavier than uranium (element 92), radioactively decays by emitting an alpha particle, which is 2 protons and 2 neutrons bound together. When curium emits an alpha particle, it decays to plutonium-240, which then decays with a number of daughter atoms and x-rays. The x-rays from plutonium and alpha particle from curium are the radiation used to analyze a sample; the other by-products are typically of lower energy and are filtered out from the radiation beam by a foil over the sources. This radiation streams out of the sources in all directions, but to focus the beam, a metal ring surrounds the sources and the detectors to absorb radiation radiating at a wide angle.

Photo and schematic of the APXS

Left: Photo of the APXS. The detector itself sits in the middle of of the ring behind the sources. Photo from JPL. Right: A schematic of how the APXS works. The instrument is shown as if it were pointing at a table-top. Picture from the University of Guelph.

Two kinds of radiation are used so that a wide range of elements can be detected. The alpha particles are used for Particle Induced X-ray Emission, or PIXE, which is used to detect elements with a low atmoic number. The X-ray radiation is used in X-ray Fluorescence, or XRF, and detects elements with a higher atomic number. Combining the the two technique means that elements from sodium (atomic number 11) to zirconium (atomic number 40) can all be clearly detected in the combined spectrum.

In PIXE analysis, particles (alpha particles here, but other particles like protons can also be used) are beamed at a sample. The particles are emitted with a known, fixed energy and collide with the atoms in the sample, where some of the particles will interact with the inner shells of the electron structure. Electrons in an atom sit in a series of increasingly energetic levels, each with a fixed energy. If a particle hits the atom and interacts with an electron, it imparts energy to the electron. Since the electron can only have a fixed amount of energy at any given level, if it gains energy it must jump up to a higher level, which leaves a gap in the original level it started at. An electron from a higher energy level (not necessarily the one that originally jumped up to a higher level) then falls down to fill the gap in the lower energy shell, and emits an X-ray with the difference in energy between the two levels to conserve energy. This is clearer with a picture!

Schematic of PIXE process.

Alpha particle comes in, excites electron and scatters off, then an electron (either the one that was bumped up in energy or another one) fall to a lower energy and the atom emits an X-ray carrying the difference in energy between the starting and finishing energy levels of the second electron (ie, the electron that drops down in energy).

The process is dependent on the inner (ie lower energy) shells of electrons interacting with the incoming particle. The particles are most likely to interact with the outermost shell or two of the electron levels of an atom, so if an atom has several shells, there’s too many electrons shielding the lower energy electrons for many interactions to occur. PIXE is then most effective for atoms with a low atomic number.

XRF does much the same thing, but instead of using a particle to induce the electron’s transition, it uses X-rays. X-rays are useful because they can interact more easily with a inner-shell electrons in heavier elements than the massive alpha-particles. XRF is then more useful for detecting heavier elements that PIXE cannot, but because the efficiency of the process is low at low atomic number, PIXE (which is a more efficient process at low atomic number) is needed to gather a robust spectrum.

But doing PIXE and XRF on Mars poses problems: even the very thin Martian atmosphere will absorb low energy X-rays, so the lightest element that the APXS can detect is sodium (atomic number 11). Hydrogen, carbon, nitrogen, and oxygen (atomic numbers 1, 6, 7, and 8 respectively) are all invisible to the APXS, and oxygen in particular is abundant in the samples. Most of the heavier elements are present as oxides, so the mineralogy becomes very important in the elemental analysis. Plus the atmosphere varies, like any atmosphere, and the temperature fluctuates, and generally the conditions are not as pristine as they are in a controlled lab setting. This means that there’s more calibration work needed on Mars than in the lab, and also that the invisible elements need to be carefully treated.

So when a sample is being analyzed, Curiosity’s robotic arm extends (slowly and carefully, to avoid any collisions) out towards the sample rock. A pressure sensor on the front of the instrument trips when the instrument touches the rock, and the protective doors that cover the aperture of the instrument when it’s not in use open. The electronics are switched on, and the detector starts to gather a spectrum of X-rays emitted at various energies from the sample. The instrument is left in place for a between a few minutes and a few hours to gather data, which is stored and then beamed back to Earth in the daily data dump. When it’s finished collecting data, the arm moves back from the rock, the doors close, and the next sample is chosen.

The spectra that come back look something like this:

A typical spectrum from the APXS, taken from Spirit

A typical APXS spectrum. The horizontal axis is energy of the detected x-ray, and the vertical axis is the number of x-rays of a given energy detected. Image from JPL.

The horizontal axis is the energy of the detected X-ray, and the vertical axis is the number of X-rays detected at that energy. The spectra consist of peaks at specific energies, and the individual elements are identified by the energies of a series of characteristic electron transitions. The emitted x-ray of a given transition has a fixed energy, equal to the difference in energy between the two states of the electron. Since different elements have different atomic structures and different amounts of energy between the electron shells, the x-rays emitted by different elements undergoing either PIXE or XRF will have different energies. Working backwards, then, if a spectrum is collected and there is a peak at an energy corresponding to a transition of a given element, then that element is present in the sample. The precise composition of the sample is determined by calculating the relative area under each of the peaks.

The invisible elements complicate the analysis considerably, especially since there’s so much oxygen in the rocks. The mineralology of the rocks must be accounted for, and information from other instruments (including the cameras — visual identification is very helpful) is used to make initial estimates of the proportion of each element in its various oxidation states. For example, iron oxide may be ferrous (FeO) or ferric (Fe2O3), though to the spectrometer, it’s all just iron. So to do the analysis, knowledge about similar rocks on Earth is used to estimate the proportion of iron in each oxidation state. After all the expected oxygen from the bound minerals is accounted for, any “invisible element” left over may be bound water. Many minerals have water bound within the crystals, and Spirit and Opportunity found evidence of bound water in Martian rocks using this analysis.

Spirit and Opportunity were sent to Mars to find water in some form (and did!), while Curiosity was sent to examine whether or not at some point in its geological history, Mars could’ve sustained life. It’s important to note that it’s not looking for *life*, just evidence that conditions hospitable to life once occurred. Due to the raging success it’s had on Mars (the instrument on Opportunity is, to the best of my knowledge, still functioning, eight and a half years into a 90 day mission), other APXS’s are planned for missions to the Moon by the Indian and Russian space associations (the Chandrayaan-2 rover, landing hopefully in 2016) and to Comet 67P/Churyumov-Gerasimenko by the European Space agency (the Philae lander, landing in 2014).

More info:

  1. A summary of the APXS by the Canadian Space Agency
  2. One of the papers outlining the design of the first APXS used on Mars on the Pathfinder mission
  3. Major paper with results from the instrument on Spirit, one of the two Mars Exploration Rovers