Are Martians really Earthlings?

In all the Universe, only Earth is known to be inhabited.

This aerial view of Grand Prismatic Spring in Yellowstone National Park is one of the most iconic hydrothermal features on land in the world. The colors are due to the various organisms living under these extreme conditions, and depend on the amount of sunlight that reaches the various parts of the springs. Hydrothermal fields like this are some of the best candidate locations for life to have first arisen on a young Earth, and may be home to abundant life on a variety of exoplanets.

But even among the Milky Way, billions of other chances exist.

The surfaces of six different worlds in our Solar System, from an asteroid to the Moon to Venus, Mars, Titan, and Earth, showcase a wide diversity of properties and histories. While only Earth is known to contain liquid water rainfall and large cumulations of liquid water on its surface, other worlds have other forms of precipitation and surface liquids, both at present and also in the distant past. Perhaps, long ago, Earth was joined by other worlds or even other planets, such as Mars and Venus, in possessing liquid water and perhaps life on its planetary surface.

The right ingredients and conditions for life to arise are abundant.

This conceptual image shows meteoroids delivering all five of the nucleobases found in life processes to ancient Earth. All the nucleobases used in life processes, A, C, G, T, and U, have now been found in meteorites, along with more than 80 species of amino acids as well: far more than the 22 that are known to be used in life processes here on Earth. Similar processes no doubt happened in stellar systems all throughout most galaxies over the course of cosmic history.

In our Solar System’s early days, at least three worlds were potentially habitable.

The TRAPPIST-1 system contains the most terrestrial-like planets of any stellar system presently known, and is shown scaled to temperature equivalents to our own Solar System. These seven known worlds, however, exist around a low-mass, consistently flaring red dwarf star. It’s plausible that exactly none of them have atmospheres any longer, although JWST will have more to say about that in future years.

Early Venus, Earth, and Mars may have possessed temperate surfaces, organic molecules, and liquid water.

While Mars is known as a frozen, red planet today, it has all the evidence we could ask for of a watery past, lasting for approximately the first 1.5 billion years of the Solar System. Could it have been Earth-like, even to the point of having had life on it, for the first third of our Solar System’s history?

Today, Venus is a hothouse planet, roasted by a runaway greenhouse effect.

Multiple layers of clouds on Venus are responsible for different signatures in different wavelength bands, but all show a consistent picture of a “hothouse” planet dominated by a runaway greenhouse effect.

Mars, meanwhile, is cold and frozen, with its atmosphere stripped away via the solar wind.

Earth (right) has a strong magnetic field to protect it from the Solar Wind. Worlds like Mars (left) or the Moon do not, and they routinely get struck by the energetic particles emitted from the Sun, which continue to strip airborne particles off of those worlds. The solar wind is radiated spherically outward from the Sun, and puts every world in our Solar System at risk of having its atmosphere stripped away. This happened to Mars in the past and could theoretically happen to all the planets with enough solar activity.

For its first ~1.5 billion years, however, Mars possessed surprisingly Earth-like conditions.

Oxbow bends only occur in the final stages of a slowly flowing river’s life, and this one is found on Mars. While many of Mars’s channel-like features originate from a glacial past, there is ample evidence of a history of liquid water on the surface, such as this dried-up riverbed: Nanedi Vallis.

Its watery past is overwhelmingly assured: as orbiters and ground-based rovers show.

The hematite spheres (or ‘Martian blueberries’) as imaged by the Mars Exploration Rover Opportunity. This photograph was taken in the lowlands of Mars, at low elevations, where liquid water is thought to have once covered the now-exposed surface. A watery past is the most favored scenario that led to the formation of these spherules, with very strong evidence coming from the fact that many of the spherules are found attached together, which ought to occur only if they had a watery origin.

The biggest unanswered question remains, “Did Mars ever have life?”

Recurring slope lineae, like this one on the south-facing slope of a crater on the floor of Melas Chasma, have not only been shown to grow over time and then fade away as the Martian landscape fills them in with dust, but are known to be caused by the flowing of briny, liquid water, as the dried-up flows leave new trails of salts behind. In those flows, life processes not only once occurred, but perhaps are still occurring today as dormant organisms are awakened by flowing liquid water.

If so, Martian life may not be of Martian origin.

The first truly successful landers, Viking 1 and 2, returned data and images for years, including providing a controversial signal that may have indicated life’s presence on the red planet. Decades later, we still don’t have the confirmation to know whether that one successful test was a false positive or not, but the bouldery terrain remains a mystery.

Interplanetary objects frequently impact planets, kicking up debris.

A synestia doesn’t just consist of this puffy ring/torus of debris around a joint planetary core, but also rises to high temperatures in excess of 1000 K, causing it to emit substantial amounts of its own infrared radiation, with peaks in different parts of the infrared spectrum dependent on the exact temperature and temperature profile of the system in question. The heat from the early Earth, which may have been just 24,000 km away from the Moon initially, would have played a role in heating the Earth-facing side of the Moon.

Both Earth’s Moon and Mars’s moons arose from such ancient, massive impacts.

Rather than only the two Martian moons we see today, Phobos and Deimos, a collision followed by a circumplanetary disk may have given rise to three moons of Mars, where only two survive today. The idea is that Mars’s once-innermost moon was destroyed and fell back onto Mars long ago. This hypothetical transient moon of Mars, proposed in a 2016 paper, is now the leading idea in the formation of Mars’s moons, and helps explain the enormous differences in topography between Mars’s northern and southern hemispheres.

Today, a fraction of terrestrial meteorites have identifiably Martian origins.

This scanning electron microscope image of a fragment of the Allen Hills 84001 meteorite contains inclusions that resemble simple life found on Earth. Although this sample is thoroughly inconclusive, bombardment of Earth by extraterrestrial objects is a certainty. If they contain dormant or fossilized life, we could discover it via this method.

Conversely, some meteorites on Mars must have originated from Earth.

Winds at speeds up to 100 km/hr travel across the Martian surface. The craters in this image, caused by impacts in Mars’ past, all show different degrees of erosion. Some still have defined outer rims and clear features within them, while others are much smoother and featureless, evidence of old age and erosion. On Earth, a small but significant percentage of our meteorites originate from Mars; it is unknown what fraction of Martian impacts originate from Earth-based rocks, and whether life stowed-away on any of them.

If Mars possessed life, did Earth “seed” it?

Earth, as well as all planets and moons with rocky surfaces, has experienced a large number of collisions from objects of extraterrestrial origin. Any impact that’s massive and energetic enough, in principle as well as in practice, could cause a mass extinction event if we don’t do something to mitigate it. Such an event, with an asteroidal origin, did in fact occur on Earth some ~65 million years ago.

And where did Earth’s life ultimately originate?

The panspermia hypothesis notes that on any world where life arises, impacts will occur, potentially kicking that life up and out of its home world, where it can seed new life on potentially habitable worlds both nearby and also far away in both space and time.

Lessons for life’s cosmic ubiquity may be awaiting us next door: on Mars.

NASA’s Curiosity Mars Rover detected fluctuations in the methane concentration of Mars’s atmosphere seasonally and at specific locations on the surface. This can be explained via either geochemical or biological processes; the evidence is not sufficient to decide at present. However, future missions, such as Mars Sample Return, may enable us to determine whether fossilized, dormant, or active life exists on Mars. Right now, we can only narrow down the physical possibilities; more information is required to determine which pathway accurately reflects our physical reality.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.