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Planets: Terrestrial


Terrestrial planet, or rocky planet,

is a planet composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun.

The terms “terrestrial planet” and “telluric planet” are derived from Latin words for Earth (Terra and Tellus), as these planets are, in terms of composition, “Earth-like”.

Terrestrial planets have a solid planetary surface, making them substantially different from the usually larger gas giants, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states.

{adapted from Wikipedia, Terrestrial Planet }


Many large moon exist in our solar system, which are made of the same materials as terrestrial plants:


Possible evidence of life on Mars

Do Mars Rover Photos Show Potential Signs of Ancient Life?
Johnny Bontemps, Astrobiology Magazine | January 07, 2015

rock bed at the Gillespie Lake outcrop on Mars displays potential signs of ancient microbial sedimentary structures

rock bed at the Gillespie Lake outcrop on Mars displays potential signs of ancient microbial sedimentary structures

A careful study of images taken by the NASA rover Curiosity has revealed intriguing similarities between ancient sedimentary rocks on Mars and structures shaped by microbes on Earth. The findings suggest, but do not prove, that life may have existed earlier on the Red Planet.

The photos were taken as the Mars rover Curiosity drove through the Gillespie Lake outcrop in Yellowknife Bay, a dry lakebed that underwent seasonal flooding billions of years ago. Mars and Earth shared a similar early history. The Red Planet was a much warmer and wetter world back then.

On Earth, carpet-like colonies of microbes trap and rearrange sediments in shallow bodies of water such as lakes and coastal areas, forming distinctive features that fossilize over time. These structures, known as microbially-induced sedimentary structures (or MISS), are found in shallow water settings all over the world and in ancient rocks spanning Earth’s history.

Nora Noffke, a geobiologist at Old Dominion University in Virginia, has spent the past 20 years studying these microbial structures. Last year, she reported the discovery of MISS that are 3.48 billion years old in the Western Australia’s Dresser Formation, making them potentially the oldest signs of life on Earth.

An overlay of sketch on a Mars photograph from above to assist in the identification of the structures on the rock bed surface used in a study by geobiologist Nora Noffke in the journal Astrobiology. The study suggests, but does not prove, potential signs of ancient life on the Red Planet. Credit: Noffke (2105

An overlay of sketch on a Mars photograph from above to assist in the identification of the structures on the rock bed surface used in a study by geobiologist Nora Noffke in the journal Astrobiology. The study suggests, but does not prove, potential signs of ancient life on the Red Planet.
Credit: Noffke (2105

In a paper published online last month in the journal Astrobiology (the print version comes out this week), Noffke details the striking morphological similarities between Martian sedimentary structures in the Gillespie Lake outcrop (which is at most 3.7 billion years old) and microbial structures on Earth.

Possible liquid Water on Mars today

There is some evidence of (very salty) liquid water flowing the surface of Mars today.

Image from NASA’s Mars Reconnaissance Orbiter (MRO)

However, these RSL (recurring slope lineae)  may be the result of granular flows like sand and dust. “The study is based on observations made with the High Resolution Imaging Science Experiment camera on the Mars Reconnaissance Orbiter. The RSL have been perplexing scientists since their discovery. Thousands recur during the warmest season on Mars each year, growing longer and darker until they fade in winter. They’re found on steep, rocky slopes on the darkest areas of Mars: the equator, the northern plains, the southern mid-latitudes. Researchers studied 151 RSL at 10 sites and found all ended at similar points, no matter the length of the slope. If liquid had been involved, there would be longer streaks of liquid on longer slopes. But a closer study of the streaks revealed that they behave just like dry grains of sand on active dunes, all settling at the same “angle of repose.” “We’ve shown that RSL are likely granular flows, which changes our assessment of what they mean for flowing liquid water on Mars and points to formation processes with little or no liquid,”

Flows of ‘water’ on Mars may actually be sand, new study reveals, Ashley Strickland, CNN, 11/21/17

Source: Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water. Colin M. Dundas et al. Nature Geoscience, 2017

Did the Viking probes detect life?

The two Viking Mars landers each carried four types of biological experiments to the surface of Mars in the late 1970s. These were the first Mars landers to carry out experiments to look for biosignatures of microbial life on Mars. …The Labeled Release (LR) experiment is the one that gave the most promise for the exobiologists…. Initially, according to a 1976 paper by Levin and Patricia Ann Straat the results were inconclusive [no life discovered]

…In a 2002 paper published by Joseph Miller, he speculates that recorded delays in the system’s chemical reactions point to biological activity similar to the circadian rhythm previously observed in terrestrial cyanobacteria.

In April 2012, an international team including Levin and Straat published a peer reviewed paper suggesting the detection of “extant microbial life on Mars”, based on mathematical speculation through cluster analysis of the Labeled Release experiments of the 1976 Viking Mission.


Life on Mars Found by NASA’s Viking Mission? New analysis suggests robots discovered microbes in 1976. Ker Than, National Geographic News


After running Viking’s LR data through a mathematical test designed to separate biological signals from non-biological signals, Miller’s team believes that the LR experiments did indeed find signs of microbial life in Martian soil.

[…They] used a technique called cluster analysis, which groups together similar-looking data sets. “We just plugged all the [Viking experimental and control] data in and said, Let the cluster analysis sort it,” Miller said. “What happened was, we found two clusters: One cluster constituted the two active experiments on Viking and the other cluster was the five control experiments.”…

The team concedes, however, that this finding by itself isn’t enough to prove that there’s life on Mars. “It just says there’s a big difference between the active experiments and the controls, and that Viking’s active experiments sorted with terrestrial biology and the controls sorted with non-biological phenomena,” Miller said.

Still, the new findings are consistent with a previous study published by Miller, in which his team found signs of a Martian circadian rhythm in the Viking LR experiment results. Circadian rhythms are internal clocks found in every known life-form—including microbes—that help control biological processes… On Earth this clock is set to a 24-hour cycle, but on Mars it would be about 24.7 hours—the length of a Martian day.

In his previous work, Miller noticed that the LR experiment’s radiation measurements varied with the time of day on Mars. “If you look closely, you could see that the [radioactive-gas measurement] was going up during the day and coming down at night. … The oscillations had a period of 24.66 hours just about on the nose,” Miller said. “That is basically a circadian rhythm, and we think circadian rhythms are a good signal for life.


The Grand Texture of Planets

By Caleb A. Scharf , March 30, 2015

In an idle moment, while staring at a set of solar system data, it occurred to me that it might be interesting to display a set of planetary surfaces on an equal footing, where the overall texture of these worlds was visible (although topography is probably a more accurate word on these scales) – and decided it was worth sharing as a brief piece of solar system trivia.

In the images below I’ve collected a set of planetary maps with the same cylindrical projection, and with their grayscale adjusted to a similar range. I’ve cheated a bit by including Ceres, Europa, Ganymede, and Titan because I wanted a comparison across a wider range of surface environments. I’ve also cheated by using some data that is based on radar reflectivity (Titan, Venus).

For a first pass I simply scaled the x-axis to be the same for all objects (the equatorial circumference). Below these individual images is one where I merged the maps by scaling the x-axis according to the true physical equatorial circumference. Since these are cylindrical projections that’s a slightly odd thing to do – but it does give an idea of the relative scale of each body.

What is, I think, quite striking, is how different each map is – there are very specific characteristics to each world. Even Ceres, which you might say looks a little like Mercury, reveals its diminutive size by virtue of its cratering – the scale of most of the stuff that has hit Ceres is clearly much larger compared to the size of this world than most of the stuff that has hit Mercury (and the impactors on both should share a common size distribution).

The Earth displays a clear dichotomy between continental crust and oceanic basins – and while this may be exaggerated in these particular datasets, there’s no doubt that Earth would always stand out when compared to its planetary siblings.

Mars, for example, looks more warty than crusty. Venus is perhaps the most similar in character, but a bit wrinkly, while Ganymede definitely has ‘plate’ like coverage, but on this moon the regions look fragmented and cracked rather than raised or embossed.

In the last panel, with the worlds gathered together, I think Earth’s richness of form really shows – even in dull gray it appears sculpted to a different degree.

Here’s the gallery: Stepping outwards from the Sun, first up is Mercury as seen by NASA’s Messenger probe.

Next comes Venus, this is a map shaded to indicate topography rather than being a pure radar reflectivity map.

And now an odd-looking world, a topographic map stripped of atmosphere and surface water. I’ve also made a horizontal flip, in order to try to see the Earth through a stranger’s eyes.

Mars rounds out the terrestrial worlds.

Little Ceres – the map is not yet very high resolution or complete, but it displays a distinct character.

Europa is up next, ice-coated and very different than a rocky surface.

The grand Jovian satellite Ganymede has a truly striking surface patterning.

And finally, here is much of the surface of Titan, recorded as a radar reflectivity map, showing dark zones of liquid hydrocarbons.

Putting all of these together, scaling by the x-axis and re-arranging, I got a nice mosaic. The sheer grandeur of the Earth is very apparent.

Caleb A. ScharfAbout the Author: Caleb Scharf is the director of Columbia University’s multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His books include Gravity’s Engines (2012) and The Copernicus Complex (2014) (both from Scientific American / Farrar, Straus and Giroux.) F

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