Landing Phoenix
Published on May 25, 2008 at 2:00 pm.
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Filed under Mars, space exploration.
In a few hours, the Phoenix will land on Mars. The chosen landing site is far into the northern plains. This is farther from the equator than any other spacecraft has successfully landed on Mars. Of course, that doesn’t mean that it can’t safely be done, just that it hasn’t been done before. Mars is an odd world. It is almost like two planets stuck together. The northern hemisphere is largely smooth and low elevation. The southern hemisphere is largely higher elevation and heavily cratered. All those craters in the south mean a lot of ejecta — material thrown out from the impacts that created the craters in the first place. But, the northern plains are mostly smooth. That does not mean that there have been no impacts there, of course. Rather, it means that something is covering up evidence of those impacts. The seas on the Moon are smooth and low elevation, too. In that case, though, the seas are great lava plains filling giant impact basins. Some of the earliest ideas for why the Martian northern plains are so smooth is that they, too, are lava plains. However, that may not be the case. More and more, we are finding that the northern hemisphere is covered in ice. This may have been a giant ocean at one point. Impacts in this area would have been into an ocean in Mars’ earliest days. The water would cover the craters. And, since that time, impacts would be into a vast field of ice. That, too, could result in the craters eventually being covered up, too.
All that ice in the northern hemisphere is the goal for the Phoenix lander. The idea is to study the ice to find whether or not it may have ever had life, or whether life is even possible today. The ice is believed to be only inches below the surface in places. Hopefully, that is the case where the Phoenix lands. Then, it can scrape aside the uppermost layers of soil to expose the ice below. Now, it is likely not just soil on top of ice. The ice below may be mixed with the soil, similar to the permafrost in Earth’s tundras. But, of course, we don’t really know. We’ve never landed and studied this area before.
When Phoenix lands, it will land with retrorockets. It will slowly descend to the surface. This is how the Viking landers landed on Mars over three decades ago. It differs from how our last three successful landings occurred, though. Pathfinder and the two Mars Exploration Rovers landed using airbags. They slowed to a reasonable speed above the surface of the planet and dropped the rest of the way. They bounced and rolled along until coming to rest. The airbags deflated, and the landers opened up. The advantage of that way of landing is that the spacecraft can right itself if it lands upside down by simply opening up the lander in a predetermined manner. If it lands in a rocky or rough terrain, or on a steep hill, it can roll around until it finds a relatively level spot to come to rest (assuming that landing on the rocks doesn’t puncture an airbag). But, with the Vikings and with Phoenix the lander comes down with limited ability to avoid a bad landing situation. If there is a big boulder under one leg of the lander as it lands, then it flips over and is destroyed. If the landing site is sloped too much, a similar danger exists, not to mention the problems that would created in trying to get an antenna lock on Earth and getting good sunlight to the solar panels. So, the folk over at JPL and the University of Arizona are going to be biting their fingernails waiting to find out if the lander survived.
But, there are things that can be done to maximize the chances of success. For one thing, the Mars Odyssey, one of the orbiters circling Mars now, has been sending back data on potential landing sites. The Odyssey’s Thermal Emission Imaging System (THEMIS) has surveyed the potential landing sites looking for places with the fewest rocks. The rocks heat up in the day, and they show up as greater thermal emissions at night. So, Odyssey has been looking for thermally dark places. That would be places where fewer rocks are expected to be found. That also may be where the ice is closest to the surface (under the least soil). So, that makes such landing sites particularly good.
And, that also is the subject of the last part of this particular post. The Phoenix mission’s public outreach office, in cooperation with spacEurope, has announced the winner of a space art competition: Through the Eyes of Phoenix (organized by spacEurope). There were some really good entries. The winner, though, was Doug Ellison with his entry So Few Rocks, seen below.
But, the other entries shown on the competition’s link seem all be wonderful, too. By tomorrow, we’ll hopefully know if Mr. Ellison’s ideas are anywhere close to what the landing site looks like.
-Astroprof
Mars topography map courtesy NASA
So Few Rocks courtesy Doug Ellison, spacEurope
