"I'd be surprised if life hasn't gotten to Mars," Ms Worth told BBC News.
"It's beyond the scope of our study. But it seems reasonable that at some point some Earth organisms have made it over there."
It has been shown that tiny creatures can withstand the harsh environment of space. And bacterial spores can be revived after hundreds of millions of years in a dormant state.
But even if a hardy microbe did stow away for all those millennia, it might simply burn up on arrival, or land in inhospitable terrain.
The most habitable places in range of Earth are Europa, Mars and Titan - but while all three have likely held water, it may not have been on offer to visitors.
Europa's oceans are capped by a crust of ice that may be impenetrably thick.
"But it appears regions of the ice sheet sometimes break into large chunks separated by liquid water, which later refreezes," Ms Worth said.
"Any meteorites lying on top of the ice sheet in a region when this occurs would stand a chance of falling through.
"Additionally, the moons are thought to have been significantly warmer in the not-too-distant past."
Moon fossils
On Mars, there is little evidence of flowing water during the last 3.5bn years - the likeliest window for Earth life to arrive.
But what if the reverse trip took place?
The early Martian atmosphere appears to have been warm and wet - prime conditions for the development of life.
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