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!