25 July 2008
I have a pretty cool job. I look at data coming down from spacecraft orbiting around and driving on another planet, and try to figure out what it used to be like there. To be able to interpret images of Mars, it helps to first be able to interpret images of our own planet. And that’s why I’m going to Hawaii in a couple of days!
On Saturday, I head to Hawaii to participate in a NASA Planetary Volcanology Field Workshop. Over the past few months, the workshop participants have been receiving emails containing data from earth-observing satellites that is roughly analogous to the data available from satellites around Mars. Our assignment is to assemble a geologic map of a small portion of Kilauea volcano. Then, we all head to Hawaii, maps in hand, and actually walk around on the lava flows and compare what we thought we were seeing from orbit to what is really there. The workshop is run by several volcanology experts, who I’m sure have lots of fun laughing at what we inexperienced planetary scientists come up with in our geologic maps!
If you want to play along at home, here are the images that we were provided. You can try your hand at interpreting them! Then once I’m back I’ll tell you how wrong I was!
The first data is simple visible imagery from the IKONOS satellite. This shows a medium-resolution image, with several higher-res “windows”.
You can see that there is a big, black lava flow that came from the top of the image, and exits out the bottom. The varying darkness of the lava might mean that this is actually several lava flows on top of one another. In some of the high-res frames, there are patches of brighter terrain that are difficult to make out. Maybe they are vegetation? Maybe rocks?
We also have topography data, which confirms what we saw in the visible image: the slope runs from the top of the image to the bottom. This data is similar to data from the Mars Orbital Laser Altimeter (MOLA).
The shading is a bit confusing (to me at least; I think the lighting is coming from the upper right), but there is pretty clearly a steeper slope toward the bottom, and there is also a ridge running right down the middle.
Next up is a radar map. Radar is a very useful tool for looking at planetary surfaces, but also can be confusing. Unlike visual imagery, radar is an “active” sensing method: the satellite sends out radio waves, and then detects what gets bounced back. I like to think of it as a big spotlight shining down on the surface at an angle, and the image you get is the light scattered back. Dark spots are smooth, so that the light is reflected off the surface like it is a mirror, away from the spacecraft. Rough spots scatter the signal in all directions, including back toward the spacecraft. This radar map used three different wavelengths, so locations that are red are rough at shorter wavelengths of a few cm, locations that are green are rough at longer wavelengths of ~24 cm, and blue spots are rough at long wavelengths (68 cm).
My interpretation of this is that the black area is a smooth lava flow (called pahoehoe), while most of the yellow is a rougher lava flow called A’a (the joke is that that’s the sound you would make if you had to walk on it barefoot). The bluish is probably partly vegetated old flows, and the darker green may be more thickly vegetated. Orange may be old a’a with some plants on it. The pinkish band at the bottom is where the slope gets steeper, and radar data can get untrustworthy on slopes…
Next up is a false-color infrared image. This is a “principle components” image, based on a set of six infrared filters. That means that, mathematically you can find out what is causing the most variation in the infrared, and assign that variation to show up as red. Find the next most significant contribution to the change between IR images, and call it green, and the third most significant goes to blue. Don’t worry about the details; the bottom line is, it shows differences!
You can see that most of the “smooth” lava flow shows up as pink or magenta, with varying shades probably indicating several successive flows from the same eruption. The yellow is thick vegetation. Reddish green is (I think) a’a. Brighter green may be older a’a, or sparse vegetation. (The blue things are clouds) Note that you can see the ridge that splits the image pretty clearly here.
So there you have it: my interpretation of this data. I’ve never done this before, so I may be totally wrong. I will be at the field workshop for a week, and then am taking some extra time off because, hey, I’ll be in Hawaii. So unfortunately, you will have to wait until I am back on August 12 to hear just how wrong my interpretations were. I promise there will be lots of pretty pictures though!
So, until the 12th, aloha!