Month: March 2026

ESA’s Mars Express probe has been exploring Mars from orbit for more than twenty years. The way it mapped the surface with its High Resolution Stereo Camera (HRSC) has drastically changed the way we see the Red Planet. In a recent paper, ESA released a series of HRSC images highlighting the heavily cratered Arabia Terra region. The study of Martian craters provides insights into Mars’ geology, meteorology, and its long and turbulent history. The images were generated from the camera’s digital terrain model as well as the nadir and color channels.

The image above shows the Arabia Terra region, a large plain in the southern highlands that is heavily cratered by impactors that have struck the planet over time. The features are labeled (if you click on the image) and can be enlarged. The crater volume is due to Arabia Terra being one of the oldest geological formations on Mars, estimated to be 3.7 to 4.1 billion years old. At this time, geological activity inside Mars ended, causing it to lose its planetary magnetosphere and its atmosphere to be slowly eroded by the solar wind.

*A bird’s eye view of a region in Trouvelot Crater. It shows the dark volcanic deposits covering the crater floor and a light mound visible within these deposits. Photo credit: ESA/DLR/FU Berlin*

Much like the Moon’s airless environment has preserved its craters, Mars’ thin atmosphere has ensured that these craters are well preserved. Some of the craters in the image are filled with dark material, while others are filled with lighter sand and undulating dunes. This suggests that some of this sand was deposited on Mars by dust storms, while other material may have been thrown out by the impacts themselves. Others still show signs of collapsing crater walls and worn rims, also indicating wind-induced erosion.

To the left of Trouvelot Crater is an older, more eroded basin with a completely collapsed wall almost entirely covered in dark rock. This material was formed by the wind into the characteristic undulating structures known as “barchan” dunes, characterized by their crescent-shaped profile. Mars Express photographed these dunes at several locations in the northern lowlands and the large Tharsis volcanic region. The dark material, known as “mafic rock,” is rich in minerals and is often associated with volcanism here on Earth.

*Close-up showing the light hill at top left contrasting with the dark rock. Photo credit: ESA/DLR/FU Berlin*

This again indicates that material thrown out by impacts was blown around by the wind and eventually pulled down along the crater walls. The fact that Trouvelot intersects this crater suggests that it is the younger of the two, and the commonality of craters with dark material suggests that the mechanisms involved are ubiquitous on Mars. Amid the dark material is a light hill, about 20 km long, covered in ridges and grooves. Such mounds have been observed in other locations and suggest that other processes may be at work.

One clue is the minerals observed in these mounds, which suggest they formed in the presence of running water. Whether this is the case remains a matter of scientific debate, and there are several possibilities as to how they could have been deposited by water.

Further reading: ESA

For about a century, scientists have known that the universe is in a state of constant expansion. In honor of the scientists who conclusively demonstrated this, this extension became known as the Hubble constant (or Hubble-Lemaitre constant). Today, scientists use two main techniques to measure the expansion rate: the cosmic microwave background (CMB) and the cosmic distance ladder. The former is based on redshift measurements of the CMB, the relic radiation left over from the Big Bang, while the latter is based on parallax and redshift measurements of variable stars and supernovae (also called “standard candles”).

The only problem is that the two methods do not agree, resulting in what is known as “Hubble tension.” This problem is considered one of the greatest cosmological puzzles facing scientists today. Fortunately, new methods are emerging that could help resolve this “tension” and bring order to the Standard Model of cosmology. In a recent study, a team of astrophysicists, cosmologists and physicists from the University of Illinois and the University of Chicago proposed a new method that exploits tiny ripples in spacetime known as gravitational waves (GWs).

The study was led by Bryce Cousins, an NSF Graduate Research Fellow from the Institute of Gravitation and the Cosmos (IGC) at the University of Illinois Urbana-Champaign. He was joined by several colleagues from the IGC as well as researchers from the Kavli Institute for Cosmological Physics and the Enrico Fermi Institute at the University of Chicago. Their study, “Stochastic Siren: Astrophysical gravitational-wave background measurements of the Hubble Constant,” appeared Jan. 16 in Physical Review Letters.

Scientists hoping to resolve the Hubble tension have proposed several solutions, ranging from early dark energy (EDE) and interactions between dark matter (DM) and neutrinos to the evolving dynamics of dark energy. In recent years, the discovery of gravitational waves has also emerged as a way to resolve the tension by offering a new way to measure cosmic expansion. Originally predicted by Einstein’s theory of general relativity, gravitational waves are waves that arise in the fabric of space-time and are created by the merger of massive objects (neutron stars and/or black holes).

They were first confirmed in 2016 by scientists at the Laser Interferometer Gravitational Wave Observatory (LIGO). Thanks to improved tools and international collaboration, the LIGO-Virgo-KAGRA (LVK) collaboration has discovered more than 300 GW events. During this time, astronomers have found ways to use events to study cosmological phenomena, including measuring the expansion of the cosmos. In current research, the team has found a way to improve these measurements by exploiting the gravitational wave background (GWB) caused by astrophysical collisions that the LVK network is not yet sensitive enough to detect.

They call it the “standard stochastic siren” method because the collisions that form the gravitational wave background occur stochastically. Daniel Holz, UChicago professor and co-author of the study, explained in a UIUC press release:

It’s not every day that you come up with an entirely new tool for cosmology. We show that we can learn about the age and composition of the universe using the background hum of gravitational waves from merging black holes in distant galaxies. This is an exciting and completely new direction, and we look forward to applying our methods to future datasets to help constrain the Hubble constant as well as other important cosmological quantities.

*Artist’s impression of the electromagnetic signal from the merger of two neutron stars. Photo credit: NSF/LIGO/Sonoma State University/A. Simonnet*

As a proof of principle, the team applied their method to recent LVK collaboration data. They found that failure to detect the GWB provides evidence against slow cosmic expansion rates. They then combined their method with measurements of the Hubble constant based on individual black hole collisions to get a more accurate rate. “Because we observe individual black hole collisions, we can determine the frequency of these collisions throughout the universe,” Cousins ​​said. “Based on these rates, we expect there will be many more events that we cannot observe, called the gravitational wave background.”

This showed that at lower values ​​of the Hubble constant, the total volume of space in which collisions occur is smaller. This would mean that the density of object collisions is higher and the strength of the GWB signal increases to a level that current instruments could detect. “This result is very significant – it is important to obtain an independent measurement of the Hubble constant to resolve the current Hubble voltage,” added co-author Nicolás Yunes, the founding director of the Illinois Center for Advanced Studies of the Universe (ICASU). “Our method is an innovative way to improve the accuracy of Hubble constant conclusions using gravitational waves.”

Due to LVK’s improved architecture, scientists believe that the GWB will likely be discovered within the next six years. If this happens, the team’s method could be used to further improve measurements of the Hubble constant. Until then, the stochastic siren method could be used to constrain higher values ​​of the Hubble constant, thereby setting upper limits on the GWB and allowing scientists to study it before a full discovery occurs.

“This should pave the way for future application of this method as we can further increase the sensitivity, better isolate the gravitational wave background and perhaps even detect it,” says Cousins. “By incorporating this information, we expect to obtain better cosmological results and come closer to resolving the Hubble tension.”

Further reading: University of Illinois

Red dwarfs make up the vast majority of stars in the galaxy. Because of their ubiquity, they host the most rocky exoplanets we have found so far – which in turn makes them interesting for astrobiological studies. However, there’s a catch: Astrobiologists aren’t sure whether the light from these stars can actually support oxygen-producing life. A new paper by Giovanni Covone and Amedeo Balbi, available as a preprint on arXiv, suggests that may not be the case – when it comes to starlight, quality is just as important as quantity. And according to their calculations, maintaining Earth-like biospheres around red dwarfs is incredibly difficult.

Their argument is based on the concept of exergy – a measure of the maximum amount of useful work that can be extracted from a radiation field. In other words, it measures the thermodynamic quality of light and not just the raw energy it contains. When measuring the “habitable zone” of stars, astrobiologists typically look at the total number of photons, particularly in the visible light range between 400 and 700 nanometers wavelength.

So what “useful work” does light do on exoplanets? Perhaps the most important is breaking up water. This process, known as “water oxidation,” represents a kinetic bottleneck in the process of photosynthesis and produces the oxygen expected in biosignatures. However, biological systems require a significant amount of kinetic energy to carry out this chemical reaction. And when it comes to providing that energy, red dwarfs have two advantages.

Fraser talks about habitable planets around red dwarfs

Red dwarfs are cool and their light is strongly redshifted into the infrared. Their photons do not concentrate enough energy to reach the threshold required for water splitting. But even those that do have a smaller percentage of their energy that can actually be converted into useful chemical work. This double combination enormously reduces the potential for the formation of oxygen-containing life around red dwarfs. In comparison, the exergy available to drive water oxidation around Sun-like stars is about five times higher.

However, astrobiologists are an optimistic bunch, so their immediate answer to this concern would be: perhaps life around these stars has evolved to adapt to these higher infrared environments. Could they use longer, lower-energy infrared wavelengths under a red dwarf sky? The short answer is “no” because it is the so-called red border. This is the longest wavelength of light that can support photosynthesis. The authors argue that this is not a fixed value, but rather an emergent property determined by a star’s spectrum, the planet’s atmosphere and a targeted chemical reaction – in this case water oxidation.

They estimate that the red limit for red dwarfs is 0.95 µm, while for sun-like stars it is closer to 1.0 µm. In practice, this means that life cannot simply shift its primary absorption bands deeper into the near-infrared to accommodate its less powerful star. Another concern concerns the development of life on one of these planets. Oxygen-deficient bacteria can use infrared light effectively. If they could multiply, they could outcompete oxygen-containing bacteria and the world would never experience a “major oxidation event” equivalent to what happened on Earth. Without abundant oxygen in the atmosphere, multicellular life would be severely limited, if not eliminated entirely.

Fraser has some videos on this topic that show there is an ongoing debate.

When all of this is taken into account, the possibility of life near red dwarfs paints a bleak picture. But let’s not rule it out completely. Currently, Earth’s biosphere is consuming only about three orders of magnitude below maximum thermodynamics – evidence that life itself is extremely inefficient. But even then, the conditions around red dwarfs that would be favorable for life are likely extremely rare. This article proves that our time searching for an oxygen-rich alien forest would be better spent near stars like our sun than chasing the statistical rarity of a thriving biosphere around a red dwarf.

Learn more:

G. Covone & A. Blabi – Photosynthetic exergy I. Thermodynamic limits for planets in the habitable zone

UT – Red dwarfs are too weak to generate complex life

UT – Planets in the habitable zone around red dwarfs are unlikely to host exomoons

UT – New research suggests advanced civilizations are unlikely to exist in red dwarf systems