Monday, June 25, 2007

Dust devils on Mars and New Mexico

Sorry for the hiatus - I'm traveling quite a bit this summer. This week, literally jet-setting at the JETSET summer school in the Açores, where R is one of the invited lecturers and I'm just tagging along for the week. We're on São Miguel Island, it's raining, he's lecturing, and I'm working. Later in the week we'll have a look around at the volcanic features of this island, which is one of several here at the triple junction of the African, Eurasian, and North American plates in the middle of the Atlantic Ocean. North America, Europe and Africa are all moving away from each other and spreading is occuring along the rifts in between the plates - producing the volcanoes that make up the Açores. All of the islands are still geologically active; the last eruption on São Miguel was in the 1600s. There are hot springs and other geothermal features here, plus the islands get much of their energy from geothermal plants - how cool is that?!

While I've been traveling around, Spirit has been putzing around the northeastern part of Home Plate. We traversed up the side to get the stratigraphy, then came down again to revisit the silica-rich targets of Silica Valley - as shown in this false-color image, the nubby Si-rich rocks seem to be underlying the valley rather extensively. I know those of you following along at home see pictures of Home Plate, then Silica Valley, then Home Plate, then back again, etc. and it's probably hard to figure out what the team is planning to do! Truthfully, the plan is guided by what the rover sees and how long it takes the team to process the information. So while we have a long-term plan to get up on top of Home Plate and head south, we keep seeing interesting things nearby and want to take the time to investigate them before leaving the area permanently. Better to backtrack 10m now then miss out on important science later!

But putzing around one area has had its benefits - particularly lying in wait for dust devils. Both rovers regularly monitor the Martian atmosphere, like little weather stations on the surface. They keep track of the opacity of the atmosphere by seeing how much light is let in at specific times of day and the team can translate that into how much dust is in the atmosphere. They have sequences that look for clouds in the Martian mornings. We are also tracking the percentage of argon in the atmosphere, which is related to atmospheric pressure - a funny thing about Mars is that the atmospheric pressure is small, so when carbon dioxide condenses or sublimes at the polar caps, the entire atmosphere deflates or inflates a little bit, changing the percrntage of other gases like argon. Most interesting on a day-to-day basis are the dust devils, which are small vortices of hot air rising from the ground and carrying fine particles of dust and sand. Here's a great article about dust devils on Mars.

Because the atmosphere is thin, the dust devils don't have a lot of pressure in them, so they can't do anything like tip the rovers over. But they can blow dust around - and off the solar panels of the rover as seen in this true-color image of Spirit's solar panel from sol 1231! The solar panels on the rover are flat to be able to catch maximum sun rays. But this also means dust settles on them from normal operations. The design team know this would happen and in fact it was one of the contributing factors to the expected rover lifetime - that the solar panels would dust up and not be able to generate any more power. But unexpected dust devils have come along every so often and blown the dust off of both rovers and allowed them to keep on generating energy for themselves. Spirit hasn't had its solar panels cleaned since sol 420, more than two Earth years ago, and was pretty dirty. But in the last few weeks, Spirit has been buffeted by at least two dust devils and is now generating as much power as her frequently-cleaned sister, Opportunity. In fact, because of a large dust storm on Mars affecting Opportunity, this week Spirit is actually outperforming Opportunity in energy generation!

To people living in the American southwest, dust devils are a familiar sight and form in the same way as on Mars, though because of the denser atmosphere, terrestrial dust devils aren't necessarily gentle. I had the good fortune recently to witness a highly unusual dust devil phenomenon about 10 km from my own house! The rising warm air from the ground forms the dust devil funnel, and the sinking cold air from the low-hanging clouds gets sucked into the vortex, creating a full funnel cloud. R and I witnessed this for about 15 minutes, seeing the dust on the ground rise up toward the clouds and the peculiar limb-darkening of the upper funnel. The ground speed was only about 60 mph and it did no damage, unlike a true tornado (fortunately for me and my tornado-phobia!). Eventually the ground funnel moved so far from the connecor in the clouds that the thin funnel broke apart and the whole thing collapsed. I'll never forget one of the things I learned in Randy Jokipii's planetary physics class during my first semester of grad school - the mathematics of vortex formation are such that they are impossible to start, but once started are impossible to stop - and are therefore, by nature, random events. Cool!

Tuesday, June 12, 2007

Water on Mars?

Hey Science Girl, any comments on this story? Sure, my comment is: don't believe it - it was retracted, chiefly because the rocks in the photos are actually on the inside of a crater on a 20-30 degree slope. Scientists can make mistakes. But it does bring up a good opportunity to talk about false-color images!

Many of the rover pictures, and other planetary photos, appear in "false color." We're used to looking with our eyes and seeing all the colors at once. But you've done the experiment where you pass light through a prism and make a rainbow, right? That shows you that the light we see, whether in a source like the sun or reflected off objects, is made of light of many wavelengths. Something that looks green to your eye is reflecting a lot of 510-nanometer light back to you (green), and not so much 650-nanometer light (red). The MER camera - Pancam - only sees one color at a time. Basically, it has filters that only let a specific wavelength of light through, so it takes a grayscale photo showing the intensity of the scene in one color - or filter - at a time. The Pancam has 13 different filters, which means it can see the scene in 13 different wavelengths. Some of the wavelengths are familiar to us, like blue and red, and others are beyond human vision, like infrared.

When we humans get the data, each photo is a grayscale image taken at a different wavelength. If we had an infinite number of filters that covered the whole visible spectrum, we could combine them into what our eyes would see. But instead we have to be a little more clever and combine the filters in combinations to approximate what our eyes would see. These are called "approximate true color" or "true color" images. When you look at Mars in true color, you see that it's pretty much red. The rocks are red, the soil is red, the dust is red, even blueberries are red. It's pretty hard to make out differences in true color. So, we get tricky and make "false-color" images, where we combine filters in ways the eye would never see in order to bring out differences among rocks and soils and features. For example, many geologic features are distinct in the Pancam filters L2, L5 and L7, which correspond to wavelengths of 753, 535, and 432 nm, or infrared, greenish-yellow, and indigo. These get combined so L2 represents red, L5 represents green, and L7 represents blue - meaning that much of the visible red wavelengths that dominate all of Mars are missing in this representation and the blues become more prominent.

Now let's look at the photo in the article again, knowing now it is in false color (L257 actually). The blue in this photo means the material reflects more 432 nm light than 535 and 753 nm light, making it appear blue. We can't tell from this image whether our eye would actually see it as some other color. In fact, when we look at the true-color images of the area where this picture comes from (Burns Cliff), we can see that in fact, the "blue" stuff in the cracks reflects a LOT of red light, appearing reddish brown when we're able to collect images using the red filter. We can also see the slope of the area, making it impossible for water to pool.

False-color images are really useful for a couple of reasons. They allow us to take less data and still have a reasonable sampling of the target in wavelenghts that span our available filters. They allow us to discriminate more readily amonf the reddish rocks of Mars, including the famous blueberries, which are actually grayish-red hematite. And, when combined in less intuitive ways, they can make some really spectacular and colorful and amazing views of another planet.

Friday, June 08, 2007

Silica Valley

You might have seen this Press Release last week describing a little patch of soil that turned out to be a really incredible find. Since Spirit's right front wheel stopped working, the rover drives backwards dragging the stuck wheel around. When there's soft soil or dust, this effectively digs a small trench and churns up subsurface material. Most times, the soil on Mars (that's soil in the planetary sense - no organics like soil on Earth) is pretty uniform and looks the same from place to place to place. Sometimes though, Spirit has turned up light-colored material like Tyrone, Arad, and Paso Robles. Those bright materials have turned out to be sulfate salts - generally Ca-sulfate (gypsum) - which forms on Earth as a precipitate from liquid water. Those deposits show us that liquid water played a role here in the soils somehow, maybe as a shallow lake or ephemeral playa (not too different from New Mexico actually).

As we headed to Home Plate, Spirit's wheel churned up another bright patch. By this point, most of the team wasn't freaking out about every white patch and it was actually the APXS team leader who pointed it out and asked for a picture. When we looked at it in Pancam color and with the Mini-TES, it was immediately clear it was something new. The Mini-TES is a remote-sensing instrument that collects light in the near-infrared spectrum, beyond where our eyes can see. The TES stands for Thermal Emission Spectrometer. You've probably seen infrared cameras on TV, and we just had someone come up and image our home in infrared to detect water leaks, and the TES works in a siliar way - it passively collects thermal emission from the objects around it. The shape of the thermal emission spectrum reflects the mineralogy of the rock, so we use Mini-TES to remotely sense the mineral makeup of nearby rocks. When Mini-TES looked at the white patch, it showed something totally new - the presence of free SiO2.

SiO2 is silica or silicon dioxide, better known on Earth as quartz, sand, opal, chert, amethyst, jasper, or agate, just to name a few. If you're a rockhound, you know that quartz minerals form where there's lots of water, like fluids flowing through a vein or vapors depositing on springs. We took better Mini-TES spectra to distinguish what form the SiO2 was in and it looks most like opal, which means the SiO2 is not like quartz crystals but rather kind of amorphous, more like deposits found at hot springs. So that's not to say there was a hot spring here, but since we're right next to Home Plate - a structure we think was caused by hot lava interacting with groundwater - we really think they might be linked!

OK, that's cool you think, but why the heck do we need to put a press release every time there's something that might be water-related? Well, first it *is* kind of cool. No other mission has ever found anything like SiO2, even though it's been speculated to exist for decades. Second, remember that this continues to be a publicly-funded mission and continues to fullfill its objectives. Every public program needs to report its progress - you can be cynical in saying it's to continue to get funding, and while obviously I adore working on this mission, I would find funding somewhere else, so I'll take the non-cynical view that we have an enthusiasm as well as an obligation in reporting to the public all the good things we're doing, as do all missions.