More than 125 planetary scientists will gather in Pasadena, California, next week to begin planning NASA's next steps on Mars — or rather, where that step should fall. The workshop will rank more than 40 candidate landing sites for the Mars Science Laboratory (MSL), a rover slated to depart for the planet in September 2009. There will be no hasty decisions: NASA plans three more workshops after this one, and will choose the final site a month before launch.

Top dog: the Mars Science Laboratory (left) will dwarf today's rovers (right). Credit: NASA

NASA says the MSL will improve in every way on the Spirit and Opportunity rovers now exploring Mars. Its ten instruments weigh a total of 75 kg, compared with five instruments weighing 9 kg on the current rovers. It will be able to land within a 20-km-diameter target circle, anywhere within 60° north or south of the martian equator, instead of being restricted to a narrow band around the planet's middle. And it will be able to negotiate rougher and steeper terrain than Spirit and Opportunity and travel further — at least 20 kilometres.

With so many options for where to land, workshop co-chairman John Grant of the Smithsonian National Air and Space Museum's Center for Earth and Planetary Studies feared scientists would suggest thousands of candidate sites. “We were quite relieved to find out that the number of sites is manageable,” he says. The first workshop's goal is to rank the sites as high, medium or low priority for detailed study using cameras and other instruments orbiting Mars.

The MSL will continue the work of its predecessors in tracing the history of water on Mars. Geologists would prefer to explore an area where many layers of rock are exposed, to give a large cross-section through time. The less dust covering the rocks, the better. The site must also be navigable for a wheeled rover, without too many obstacles. And particularly prized will be areas where scientists believe water once settled in quiet pools that may have provided habitats for ancient martian life.

Another improvement on past rover missions is that scientists will know more about the landing sites in advance. Since Spirit and Opportunity arrived on Mars in January 2004, Mars Express and the Mars Reconnaissance Orbiter (MRO) have begun their own study of the surface. The MRO's High Resolution Imaging Science Experiment may be able to spot outcrops from orbit of the kind that Spirit and Opportunity have visited.

Candidate landing sites

Sedimentary Fans


A rover in the Eberswalde crater could roam (black line) among fan-shaped sedimentary deposits at the end of flow channels, pausing at rock exposures. Eberswalde's features suggest that there was once standing water there. Plus, the crater floor is flat, making for easy travel.


Crater floors

At least 50 metres of water is believed to have pooled in some parts of Holden crater, one of a string of large, ancient craters in Mars's southern hemisphere. Fan-shaped sediments near the crater's centre suggest calm waters where life might once have taken hold.


Sulphate Deposits

The OMEGA spectrometer on the European Mars Express has found minerals such as sulphate and phyllosilicates that form in water, making them candidates for further exploration. The sulphate deposits pictured here are in a canyon called the Juventae Chasma.