The eternal chicken or egg question — did Mars seed life on earth or did earth seed life on the Red Planet? — is utter astrobiological “nonsense.”
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Ehrenfreund says that there are two major arguments against “panspermia” (or the notion that microbial life could spread from one planetary body to another).
The first is simply that both ultraviolet radiation from the sun and galactic cosmic rays would likely destroy microbial life in the unprotected vacuum of space. The second is that, even if such life survived a journey from Mars to earth, among other factors, its survival would also likely depend on entry through a roiling, young planetary atmosphere and adaptation to its new home.
Thus, Ehrenfreund views any “We are Martians” scenario as “highly unlikely.”
So, did early life here get a crucial boost from complex molecules from beyond our forming solar system, or even from our young solar nebula itself?
Ehrenfreund says that although molecules like hydrogen cyanide, formaldehyde and water all form in the interstellar medium, prebiotic chemistry really involves replication and structure.
“I don’t think this happens anywhere in [free-floating] space itself,” said Ehrenfreund.
Even so, to date, some 180 different molecular species have been detected in space.
In their gas phase, Polycyclic Aromatic Hydrocarbons (PAHs) which on earth encompass everything from naphthalene, the active ingredient in mothballs, to chimney soot, pine tar, even the “char” residue found on backyard grills, appear to be ubiquitous throughout the universe.
Some estimates are that PAHs make up some 15 percent of the cosmos’ total carbon supply. But how important are they for our own chemistry on earth?
Ehrenfreund says PAHs, as well as solid macromolecules, did make it to earth’s surface, because they were more stable and abundant and may have decayed into smaller subunits incorporated into primitive protocells.
But she says small biomolecules such as amino acids and sugars are fragile and likely were easily destroyed by radiation and high temperatures prevalent on the young earth.
Even so, Ehrenfreund says complex molecules and gases which form in the interstellar medium will be included in the solar nebula, as such, they are precursors for prebiotic chemistry that may have contributed to life on earth.
Ehrenfreund says that via meteoritic analysis, there is now ample evidence that processes involving water on asteroids can form new organic compounds such as amino acids.
She points out that an incredible amount of material, including a tiny fraction of amino acids, was delivered through earth’s atmosphere. But Ehrenfreund says most researchers think that amino acids found in meteorites didn’t form in the Interstellar Medium but rather from water processes in the parent body asteroid.
There is still no undisputed detection of an amino acid in the interstellar medium.
“We have the first undebated indications of life at 3.5 billion years ago,” said Ehrenfreund, who notes that the inner solar system’s epoch of asteroidal and cometary Late Heavy Bombardment, ended about 3.9 billion years ago. But even after such an epoch of impactors, she says our young earth was still plagued by a very violent and hostile surface environment.
The biggest mystery is what basic compounds were available that could also assemble under such inhospitable conditions?
“That’s a key point in the origin of life,” said Ehrenfreund. “That’s a crucial point on which we have a real gap.”
Did these ubiquitous PAHs make the crucial difference in prebiotic chemistry for life?
Ehrenfreund notes that even though researchers now realize that complex carbon chemistry is universal, they are much less knowledgeable about what happens to such chemistry after an interstellar cloud collapses and forms a planetary system.
That’s because what’s really important for the origin of life on earth and by rote, other earthlike planets, is what is actually delivered from extraterrestrial sources.
Although there’s a whole subgroup of microbiologists trying to create artificial life in the lab, says Ehrenfreund, none of these researchers are trying to recreate the assembly of protocells in a simulated early earth environment.
“It’s intrinsically very difficult because there’s a lot of debate about what the conditions actually were,” said Ehrenfreund.
And biotech researchers trying to construct protocells for potential use in contemporary medicine have arguably vastly different goals than astrochemists and astrobiologists.
Partly as a result, Ehrenfreund says that although biotech funding is readily available, it’s very difficult to get funding and grants for research into earth’s early prebiotic chemistry.
No one has tried to create a protocell under early earth conditions, she says.
“There are lots of ongoing Mars simulations,” said Ehrenfreund. “But we do much less for simulations of early earth.”
Answering such fundamental questions about life’s origins on earth, she says, will require astronomers, geologists and chemists working with simulation chambers that include fluctuations in early earth changes in temperatures, atmospherics, radiation and hydrothermal conditions.
“We would be able to see which [prebiotic] components could self-assemble and which could not,” said Ehrenfreund. “We could then extrapolate from where those compounds came.”
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