While we have great slow rovers for robotic Mars exploration, and we had the moon rovers for manned exploration, we need a new kind of robot for lava tube exploration on Mars.
The surface of Mars is exposed to solar cosmic rays and solar storms, which could kill any life with long life spans. There is not a Martian magnetic field to divert the cosmic rays or solar storms. There is good evidence for an ancient lake in Gale Crater currently being explored by the Curiosity Rover, and for sediment deposits. So there will be a search for fossil life, starting at the unicellular level. But their is a possibility that there could be life in Martian lava caves, people want to investigate them.
People who have walked on lava beds or lava tubes know how dangerous they can be. It is difficult to balance using only two feet, and falling can give severe cuts. It would be excessively dangerous for an astronaut to fall, which they do easily even on flat surfaces due to the low gravity and the bulky spacesuit. It is even dangerous for them to grab onto the sharp rocks to stabilize themselves. It is possible that dust will form a base covering, but not enough to remove all dangerous surfaces.
So we need to explore the caves with robots that are not on wheels, but which can walk with stability. Four legs would be stable on a flat surface, but when one leg is lifted at a time to move, it is unstable. So it would need a fifth leg to move, while four held it stable. As I started writing this, the ad before watching PBS on Cassini, I saw a six footed robot.
The greatest danger that such a robot would face is getting a leg stuck in a crevice. So even more legs would be useful to continue a mission. Also, the ability to disconnect the stuck leg, and attach a new one should be possible. This way it could continue its usefulness indefinitely.
Crabs, spiders, and the model that I saw can move very fast with six legs. These robots could also be useful in dangerous situations on earth.
Now to get samples or carry equipment, the robot should have two arms. One arm only works by squeezing objects, not the best in many situations. Finally, the platform for visual and other sensors should not stick out sideways from the center where it might brush against rocks, so it would be mounted centrally, above the hands to direct them, so it would the head.
In a recent science fiction movie, the octopus like alien life forms has seven appendages.
A guiding computer, perhaps similar to self driving cars, would be carried on the robot, with instructions for choosing and examining rocks or potential life forms, and taking samples. The present Rover was not sufficiently sterilized, so it is not allowed near any possible water deposits.
Spacecraft have to be heated to sterilize it, which is hazardous, especially to computers. People are even more of a problem. The mechanical part of a robotic explorer could be sterilized, and the computer, sterilized by other means, could be inserted.
I’m sure many others have figured this out, and I am not claiming originality. But it does show that robotics are best for exploring the lava tubes directly, and possibly encouraging their exploration without astronauts, whose space suits would be contaminated with earth life.