Bond investor Bill Gross of Janus Capital Group Inc. said on Thursday the historic returns that investors have reaped for over four decades are over, given the near end of falling rates and tremendous credit expansion.

In his June Investment Outlook, Gross said asset returns and alpha generation have been "materially aided by declines in interest rates, trade globalization, and an enormous expansion of credit - that is debt.

"Those trends are coming to an end if only because in some cases they can go no further," said Gross, who manages the Janus Global Unconstrained Bond Fund with $1.3 billion in assets.

He said that a repeat performance is not only unlikely, "it is impossible unless you are a friend of Elon Musk and you've got the gumption to blast off for Mars. Planet Earth does not offer such opportunities."

Musk, a billionaire entrepreneur, helped found Tesla Motors Inc and PayPal Holdings Inc and launched SpaceX in 2002 with the goal of slashing launch costs to make travel to Mars affordable.

Gross also lambasted central bank policies for distorting Wall Street and Main Street.

"Capitalism has entered a new era in this post-Lehman period due to unimaginable monetary policies and negative structural transitions that pose risk to growth forecasts and the historical linear upward slope of productivity," he said.

Gross said investment returns will be low and risk will be high, and at some point investors must decide that they are in a new era with conditions that demand a different approach.

"Negative durations? Voiding or shorting corporate credit? Buying instead of selling volatility? Staying liquid with large amounts of cash? These are all potential 'negative' carry positions that at some point may capture capital gains or at a minimum preserve principal," Gross said.

Gross said investors must accept some positive carry assets with the least amount of risk. "Sometime soon though, as inappropriate monetary policies and structural headwinds take their toll, those delicious 'carry rich and greasy' French fries will turn cold and rather quickly get tossed into the garbage can."