1.   First of all the radar signal was received and retransmitted by the Viking Lander sitting on the Martian surface.

2.   They were designed principally to search for direct and indirect evidence of life on the Martian surface.

3.   An unfueled ascent craft, a fuel production plant, nuclear electric power generators, a crew habitat and nonperishable supplies would be sent to the Martian surface.

4.   A jarring parachute descent and a final drop of nearly a mile controlled by untried pulsing thrusters must unfold precisely to ensure the spacecraft reaches the Martian surface intact.

5.   A direct flight to the Martian surface and back to Earth might be simpler and more straightforward, but could be more expensive.

6.   A seemingly hardy line of evidence of color changes on the Martian surface emerged from observations by astronomers.

7.   But if a rover on the Martian surface wants to send a file back to earth, this is an inefficient way of doing it.

8.   But they are also tough enough to survive an impact with the Martian surface.

9.   But two kilometers under the Martian surface, say some scientists, is liquid water at a temperature capable of sustaining life.

10.   Even heating and cooling will be a pain on the Martian surface.