1. Mars
1.1. Natural Planetwide Disaster
1.1.1. Asteroid - Planetoid Impact
1.1.2. Mars shrinks - somehow
1.1.3. Massive Solar Flare wipes out the entire fleet
1.1.4. Solar Flare also disrupts any attempts to avert planetoid Impact
1.1.5. Massive Comet Bombardment
1.2. What Could So Radically Alter Mars?
1.2.1. Where would that massive chunk of mass go
1.2.2. Interesting Impact - big enough object with right trajectory, like a billiard ball what we call Mars now is a planet from another Solar System
1.2.3. A Huge impact would explode leaving Mars and the rest of the junk goes into the asteroid belt
2. Venus
2.1. Nukes Environmental Rape Ensuing Greenhouse Heatstroke
3. Alternative Earth
3.1. Mass Hysteria Gangnam Style
3.1.1. Infinite Jest - Neurovirus or Neurotoxin that leaves people transfixed or wildly aggressive AKA Firefly story
3.1.2. Hysterical Laughter and Rage
3.2. Accidental Weeding Out of Vital Evolutionary Traits
3.2.1. Fear Instinct No Longer Strong Enough
3.2.1.1. Really Smart in Numerous Ways but Stupid About Caution
3.2.1.2. Human Predators Start an Evolutionary Escalation
3.2.1.3. Human Predators wipe out a huge portion of the population except for a highly adept (and fear based) subgroup
3.2.2. Metameme social cult worm that changes value systems and is self adaptive to move from group to group - total psychological takeover under a new value system
3.3. Parasitic Hostile Takeover
3.3.1. Somehow this parasite survives on Venus and is somehow brought back on a probe
3.3.1.1. Would need to come up with a really inventive probe that can withstand the heat
3.3.2. Havoc ensues
3.3.2.1. The funnier and crazier the better